Mercury (planet)#Spin–orbit resonance

Historically, humans knew Mercury by different names depending on whether it was an evening star or a morning star. By about 350 BC, the ancient Greeks had realized the two stars were one. They knew the planet as {{Lang|grc|Στίλβων|italic=no}} {{Lang|grc-latn|Stilbōn}}, meaning "twinkling", and {{Lang|grc|Ἑρμής|italic=no}} {{Lang|grc-latn|Hermēs}}, for its fleeting motion,{{LSJ|sti/lbwn|Στίλβων}}, {{LSJ|*(ermh{{=}}s|Ἑρμῆς|ref}}. a name that is retained in modern Greek ({{Lang|el|Ερμής|italic=no}} {{Lang|el-latn|Ermis}}).{{cite web |url=http://www.greek-names.info/greek-names-of-the-planets/ |title=Greek Names of the Planets |access-date=July 14, 2012 |quote=Ermis is the Greek name of the planet Mercury, which is the closest planet to the Sun. It is named after the Greek God of commerce, Ermis or Hermes, who was also the messenger of the Ancient Greek gods. |date=April 25, 2010 |archive-date=May 9, 2010 |archive-url=https://web.archive.org/web/20100509164917/http://www.greek-names.info/greek-names-of-the-planets/ |url-status=live }} See also the Greek article about the planet. The Romans named the planet after the swift-footed Roman messenger god, Mercury (Latin {{Lang|la|Mercurius}}), whom they equated with the Greek Hermes, because it moves across the sky faster than any other planet,{{cite book |title=The Voyage of Mariner 10 – Mission to Venus and Mercury |last1=Dunne |first1=James A. |last2=Burgess |first2=Eric |chapter-url=https://history.nasa.gov/SP-424/ch1.htm |publisher=NASA History Office |date=1978 |chapter=Chapter One |url=https://history.nasa.gov/SP-424/ |access-date=July 12, 2017 |archive-date=November 17, 2017 |archive-url=https://web.archive.org/web/20171117190025/https://history.nasa.gov/SP-424/ |url-status=dead }}{{cite book |first=Eugène Michel |last=Antoniadi |others=Translated from French by Moore, Patrick |date=1974 |title=The Planet Mercury |publisher=Keith Reid Ltd |location=Shaldon, Devon |pages=9–11 |isbn=978-0-904094-02-2}} though some associated the planet with Apollo instead, as detailed by Pliny the Elder.[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0063%3Aalphabetic+letter%3DP%3Aentry+group%3D4%3Aentry%3Dplanetae-cn "Planetae"], in Dictionary of Greek and Roman Antiquities, pp. 922, 923. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus; a Christian cross was added in the 16th century:File:Mercury symbol (fixed width).svg.{{cite book |title=Astronomy: A Textbook |first=John Charles |last=Duncan |date=1946 |publisher=Harper & Brothers |page=125 |quote=The symbol for Mercury represents the Caduceus, a wand with two serpents twined around it, which was carried by the messenger of the gods.}}{{cite book |last=Jones |first=Alexander |url=https://books.google.com/books?id=8MokzymQ43IC&pg=PA62 |title=Astronomical papyri from Oxyrhynchus |date=1999 |isbn=9780871692337 |pages=62–63 |publisher=American Philosophical Society |quote=It is now possible to trace the medieval symbols for at least four of the five planets to forms that occur in some of the latest papyrus horoscopes ([ P.Oxy. ] 4272, 4274, 4275 [...]). Mercury's is a stylized caduceus. |access-date=March 19, 2023 |archive-date=April 11, 2023 |archive-url=https://web.archive.org/web/20230411141114/https://books.google.com/books?id=8MokzymQ43IC&pg=PA62 |url-status=live }}

File:Terrestrial planet size comp 2024.png planetary-mass objects beside the Sun, arranged by the order of their orbits outward from the Sun (from left: Mercury, Venus,

Earth, the Moon, Mars and Ceres)]]

Mercury is one of four terrestrial planets in the Solar System, which means it is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of {{convert|2439.7|km}}. Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material.

File:Mercury with magnetic field.svg

Mercury appears to have a solid silicate crust and mantle overlying a solid, metallic outer core layer, a deeper liquid core layer, and a solid inner core.{{cite web |url=https://www.nasa.gov/mission_pages/messenger/media/PressConf20120321.html |title=MESSENGER Provides New Look at Mercury's Surprising Core and Landscape Curiosities |publisher=NASA |editor-first=Tricia |editor-last=Talbert |date=March 21, 2012 |access-date=April 20, 2018 |archive-date=January 12, 2019 |archive-url=https://web.archive.org/web/20190112170032/https://www.nasa.gov/mission_pages/messenger/media/PressConf20120321.html |url-status=dead }}{{Cite web |url=https://news.agu.org/press-release/scientists-find-evidence-mercury-has-a-solid-inner-core/ |title=Scientists find evidence Mercury has a solid inner core |format=Press release |date=April 17, 2023 |last1=Genova |first1=Antonio |display-authors=et al |website=AGU Newsroom |language=en-US |access-date=April 17, 2019 |archive-date=April 17, 2019 |archive-url=https://web.archive.org/web/20190417162031/https://news.agu.org/press-release/scientists-find-evidence-mercury-has-a-solid-inner-core/ |url-status=live }} The composition of the iron-rich core remains uncertain, but it likely contains nickel, silicon and perhaps sulfur and carbon, plus trace amounts of other elements.{{cite book | chapter=The Chemical Composition of Mercury | last1=Nittler | first1=Larry R. | last2=Chabot | first2=Nancy L. | last3=Grove | first3=Timothy L. | last4=Peplowski | first4=Patrick N. | title=Mercury: The View after MESSENGER | editor1-first=Sean C. | editor1-last=Solomon | editor2-first=Larry R. | editor2-last=Nittler | editor3-first=Brian J. | editor3-last=Anderson | isbn=9781316650684 | series=Cambridge Planetary Science Book Series | publication-place=Cambridge, UK | publisher=Cambridge University Press | year=2018 | pages=30–51 | doi=10.1017/9781316650684.003 | arxiv=1712.02187 | bibcode=2018mvam.book...30N | s2cid=119021137 }} The planet's density is the second highest in the Solar System at 5.427 g/cm³, only slightly less than Earth's density of 5.515 g/cm³. If the effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3 g/cm³ versus Earth's 4.4 g/cm³.{{cite web |date=May 8, 2003 |url=https://astrogeology.usgs.gov/Projects/BrowseTheGeologicSolarSystem/MercuryBack.html |title=Mercury |publisher=US Geological Survey |access-date=November 26, 2006 |archive-url=https://web.archive.org/web/20060929091534/http://astrogeology.usgs.gov/Projects/BrowseTheGeologicSolarSystem/MercuryBack.html |archive-date=September 29, 2006 |url-status=dead }} Mercury's density can be used to infer details of its inner structure. Although Earth's high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.{{cite journal |title=On the Internal Structures of Mercury and Venus |last=Lyttleton |first=Raymond A. |author-link=Raymond Lyttleton |journal=Astrophysics and Space Science |volume=5 |issue=1 |pages=18–35 |year=1969 |doi=10.1007/BF00653933 |bibcode=1969Ap&SS...5...18L |s2cid=122572625 }}

The radius of Mercury's core is estimated to be {{convert|2020|±|30|km|mi|abbr=on}}, based on interior models constrained to be consistent with a moment of inertia factor of {{val|0.346|0.014}}. Hence, Mercury's core occupies about 57% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that Mercury has a molten core.{{cite news |first=Lauren |last=Gold |title=Mercury has molten core, Cornell researcher shows |date=May 3, 2007 |work=Chronicle |publisher=Cornell University |url=http://www.news.cornell.edu/stories/May07/margot.mercury.html |access-date=May 12, 2008 |archive-date=June 17, 2012 |archive-url=https://web.archive.org/web/20120617123129/http://www.news.cornell.edu/stories/May07/margot.mercury.html |url-status=live }}{{cite news |last=Finley |first=Dave |date=May 3, 2007 |title=Mercury's Core Molten, Radar Study Shows |publisher=National Radio Astronomy Observatory |url=http://www.nrao.edu/pr/2007/mercury/ |access-date=May 12, 2008 |archive-date=May 3, 2012 |archive-url=https://web.archive.org/web/20120503202505/http://www.nrao.edu/pr/2007/mercury/ |url-status=live }} The mantle-crust layer is in total {{convert|420|km|mi|abbr=on}} thick.{{cite journal |last1=Hauck |first1=Steven A. |display-authors=etal |title=The curious case of Mercury's internal structure |journal=Journal of Geophysical Research: Planets |date=May 6, 2013 |volume=118 |issue=6 |pages=1204–1220 |doi=10.1002/jgre.20091 |bibcode=2013JGRE..118.1204H |s2cid=17668886 |url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/jgre.20091 |access-date=June 5, 2023 |hdl=1721.1/85633 |hdl-access=free |archive-date=June 5, 2023 |archive-url=https://web.archive.org/web/20230605175115/https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/jgre.20091 |url-status=live }} Projections differ as to the size of the crust specifically; data from the {{nowrap|Mariner 10}} and MESSENGER probes suggests a thickness of {{convert|35|km|mi|abbr=on}}, whereas an Airy isostacy model suggests a thickness of {{convert|26|±|11|km|mi|abbr=on}}.{{Cite book |last1=Solomon |first1=Sean C. |url=https://books.google.com/books?id=4o92DwAAQBAJ |title=Mercury: The View after MESSENGER |last2=Nittler |first2=Larry R. |last3=Anderson |first3=Brian J. |date=December 20, 2018 |publisher=Cambridge University Press |isbn=978-1-107-15445-2 |pages=534 |language=en |access-date=November 19, 2022 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162217/https://books.google.com/books?id=4o92DwAAQBAJ |url-status=live }}{{cite journal | title=A thin, dense crust for Mercury | last=Sori | first=Michael M. | journal=Earth and Planetary Science Letters | volume=489 | pages=92–99 | date=May 2018 | doi=10.1016/j.epsl.2018.02.033 | bibcode=2018E&PSL.489...92S | doi-access=free }} One distinctive feature of Mercury's surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is thought that these were formed as Mercury's core and mantle cooled and contracted at a time when the crust had already solidified.{{cite journal |title=Lobate Thrust Scarps and the Thickness of Mercury's Lithosphere |last1=Schenk |first1=Paul M. |last2=Melosh |first2=H. Jay |author-link2=H. Jay Melosh |journal=Abstracts of the 25th Lunar and Planetary Science Conference |volume=1994 |pages=1994LPI....25.1203S |bibcode=1994LPI....25.1203S |date=March 1994 }}{{cite conference | last1=Watters | first1=T. R. | first2=F. | last2=Nimmo | first3=M. S. | last3=Robinson | title=Chronology of Lobate Scarp Thrust Faults and the Mechanical Structure of Mercury's Lithosphere | conference=Lunar and Planetary Science Conference | page=1886 | year=2004 | bibcode= 2004LPI....35.1886W }}{{cite journal | journal=Geology | date=November 1998 | volume=26 | issue=11 | pages=991–994 | title=Topography of lobate scarps on Mercury; new constraints on the planet's contraction | first1=Thomas R. | last1=Watters | first2=Mark S. | last2=Robinson | first3=Anthony C. | last3=Cook | doi=10.1130/0091-7613(1998)026<0991:TOLSOM>2.3.CO;2 | bibcode=1998Geo....26..991W }}

Mercury's core has a higher iron content than that of any other planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal–silicate ratio similar to common chondrite meteorites, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass. Early in the Solar System's history, Mercury may have been struck by a planetesimal of approximately {{frac|1|6}} Mercury's mass and several thousand kilometers across. The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component. A similar process, known as the giant impact hypothesis, has been proposed to explain the formation of Earth's Moon.

Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. It would initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K and possibly even as high as 10,000 K.{{cite journal |title=The partial volatilization of Mercury |last=Cameron |first=Alastair G. W. |author-link=Alastair G. W. Cameron |journal=Icarus |volume=64 |issue=2 |pages=285–294 |year=1985 |doi=10.1016/0019-1035(85)90091-0 |bibcode=1985Icar...64..285C}} Much of Mercury's surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" that could have been carried away by the solar wind. A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material and not gathered by Mercury.{{cite journal |title=Iron/silicate fractionation and the origin of Mercury |last=Weidenschilling |first=Stuart J. |journal=Icarus |volume=35 |issue=1 |pages=99–111 |year=1987 |doi=10.1016/0019-1035(78)90064-7 |bibcode=1978Icar...35...99W}}

Each hypothesis predicts a different surface composition, and two space missions have been tasked with making observations of this composition. The first MESSENGER, which ended in 2015, found higher-than-expected potassium and sulfur levels on the surface, suggesting that the giant impact hypothesis and vaporization of the crust and mantle did not occur because said potassium and sulfur would have been driven off by the extreme heat of these events.{{cite news |url=https://www.csmonitor.com/Science/2011/0929/Messenger-s-message-from-Mercury-Time-to-rewrite-the-textbooks |title=Messenger's message from Mercury: Time to rewrite the textbooks |work=The Christian Science Monitor |first=Mark |last=Sappenfield |date=September 29, 2011 |access-date=August 21, 2017 |archive-date=August 21, 2017 |archive-url=https://web.archive.org/web/20170821214604/https://www.csmonitor.com/Science/2011/0929/Messenger-s-message-from-Mercury-Time-to-rewrite-the-textbooks |url-status=live }} BepiColombo, which will arrive at Mercury in 2025, will make observations to test these hypotheses.{{cite web |url=http://sci.esa.int/bepicolombo/ |title=BepiColombo |series=Science & Technology |publisher=European Space Agency |access-date=April 7, 2008 |archive-date=March 6, 2018 |archive-url=https://web.archive.org/web/20180306193632/http://sci.esa.int/bepicolombo/ |url-status=live }} The findings so far would seem to favor the third hypothesis; however, further analysis of the data is needed.{{cite news |url=https://www.chemistryworld.com/news/messenger-sheds-light-on-mercurys-formation/3002463.article |title=Messenger sheds light on Mercury's formation |work=Chemistry World |first=Jon |last=Cartwright |date=September 30, 2011 |access-date=August 21, 2017 |archive-date=August 6, 2017 |archive-url=https://web.archive.org/web/20170806063258/https://www.chemistryworld.com/news/messenger-sheds-light-on-mercurys-formation/3002463.article |url-status=live }}

{{Main|Geology of Mercury}}

{{multiple image

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| footer = Mercury rendered with Blender with data from NASA and the USGS

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Mercury's surface is similar in appearance to that of the Moon, showing extensive mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. It is more heterogeneous than the surface of Mars or the Moon, both of which contain significant stretches of similar geology, such as maria and plateaus. Albedo features are areas of markedly different reflectivity, which include impact craters, the resulting ejecta, and ray systems. Larger albedo features correspond to higher reflectivity plains.{{cite conference | title=Albedo Features of Mercury | last1=Hughes | first1=E. T. | last2=Vaughan | first2=W. M. | conference=43rd Lunar and Planetary Science Conference, held March 19–23, 2012 at The Woodlands, Texas | volume=1659 | id=2151 | date=March 2012 | bibcode=2012LPI....43.2151H }} Mercury has "wrinkle-ridges" (dorsa), Moon-like highlands, mountains (montes), plains (planitiae), escarpments (rupes), and valleys (valles).{{cite web |last=Blue |first=Jennifer |date=April 11, 2008 |url=http://planetarynames.wr.usgs.gov/ |title=Gazetteer of Planetary Nomenclature |publisher=US Geological Survey |access-date=April 11, 2008 |archive-date=April 12, 2012 |archive-url=https://web.archive.org/web/20120412082057/http://planetarynames.wr.usgs.gov/ |url-status=live }}{{cite book |title=The Voyage of Mariner 10 – Mission to Venus and Mercury |last1=Dunne |first1=James A. |last2=Burgess |first2=Eric |author-link2=Eric Burgess |chapter-url=https://history.nasa.gov/SP-424/ch7.htm |publisher=NASA History Office |date=1978 |chapter=Chapter Seven |url=https://history.nasa.gov/SP-424/ |access-date=May 28, 2008 |archive-date=November 17, 2017 |archive-url=https://web.archive.org/web/20171117190025/https://history.nasa.gov/SP-424/ |url-status=dead }}

File:Unmasking the Secrets of Mercury.jpg spectrum scan of Mercury's surface by MESSENGER]]

The planet's mantle is chemically heterogeneous, suggesting the planet went through a magma ocean phase early in its history. Crystallization of minerals and convective overturn resulted in a layered, chemically heterogeneous crust with large-scale variations in chemical composition observed on the surface. The crust is low in iron but high in sulfur, resulting from the stronger early chemically reducing conditions than is found on other terrestrial planets. The surface is dominated by iron-poor pyroxene and olivine, as represented by enstatite and forsterite, respectively, along with sodium-rich plagioclase and minerals of mixed magnesium, calcium, and iron-sulfide. The less reflective regions of the crust are high in carbon, most likely in the form of graphite.{{cite journal | title=The Surface Composition of Mercury | first1=Larry R. | last1=Nittler | first2=Shoshana Z. | last2=Weider | journal=Elements | year=2019 | volume=15 | issue=1 | pages=33–38 | doi=10.2138/gselements.15.1.33 | bibcode=2019Eleme..15...33N | s2cid=135051680 }}{{cite journal | title=The Role of Reducing Conditions in Building Mercury | first1=Camille | last1=Cartier | first2=Bernard J. | last2=Wood | journal=Elements | volume=15 | number=1 | pages=39–45 | date=February 2019 | doi=10.2138/gselements.15.1.39 | bibcode=2019Eleme..15...39C | s2cid=135268415 }}

Names for features on Mercury come from a variety of sources and are set according to the IAU planetary nomenclature system. Names coming from people are limited to the deceased. Craters are named for artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field. Ridges, or dorsa, are named for scientists who have contributed to the study of Mercury. Depressions or fossae are named for works of architecture. Montes are named for the word "hot" in a variety of languages. Plains or planitiae are named for Mercury in various languages. Escarpments or rupēs are named for ships of scientific expeditions. Valleys or valles are named for abandoned cities, towns, or settlements of antiquity.{{cite web |url=http://planetarynames.wr.usgs.gov/Page/Categories |title=Categories for Naming Features on Planets and Satellites |publisher=US Geological Survey |access-date=August 20, 2011 |archive-date=July 8, 2014 |archive-url=https://web.archive.org/web/20140708063522/http://planetarynames.wr.usgs.gov/Page/Categories |url-status=live }}

File:PIA19421-Mercury-Craters-MunchSanderPoe-20150416.jpg (left), Sander (center), and Poe (right) amid volcanic plains (orange) near Caloris Basin]]

Mercury was heavily bombarded by comets and asteroids during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the Late Heavy Bombardment that ended 3.8 billion years ago.{{cite journal |last=Strom |first=Robert G. |year=1979 |volume=24 |issue=1 |title=Mercury: a post-Mariner assessment |journal=Space Science Reviews |pages=3–70 |bibcode=1979SSRv...24....3S |doi=10.1007/BF00221842 |s2cid=122563809 }} Mercury received impacts over its entire surface during this period of intense crater formation, facilitated by the lack of any atmosphere to slow impactors down.{{cite journal |last1=Broadfoot |first1=A. Lyle |first2=Shailendra |last2=Kumar |first3=Michael J. S. |last3=Belton |author-link3=Michael J. Belton |first4=Michael B. |last4=McElroy |author-link4=Michael McElroy (scientist) |title=Mercury's Atmosphere from Mariner 10: Preliminary Results |journal=Science |volume=185 |issue=4146 |date=July 12, 1974 |pages=166–169 |doi=10.1126/science.185.4146.166 |pmid=17810510 |bibcode=1974Sci...185..166B|s2cid=7790470 }} During this time Mercury was volcanically active; basins were filled by magma, producing smooth plains similar to the maria found on the Moon.{{cite book | date=1997 | doi=10.3133/i2596 | title=Geology of the solar system | series=IMAP 2596 | publisher=U.S. Geological Survey }}{{cite journal |last1=Head |first1=James W. |author-link1=James W. Head |last2=Solomon |first2=Sean C. |author-link2=Sean Solomon |title=Tectonic Evolution of the Terrestrial Planets |journal=Science |year=1981 |volume=213 |issue=4503 |pages=62–76 |doi=10.1126/science.213.4503.62 |pmid=17741171 |bibcode=1981Sci...213...62H |hdl=2060/20020090713 |url=http://www.planetary.brown.edu/pdfs/323.pdf |citeseerx=10.1.1.715.4402 |access-date=October 25, 2017 |archive-date=July 21, 2018 |archive-url=https://web.archive.org/web/20180721153426/http://www.planetary.brown.edu/pdfs/323.pdf |url-status=dead }} One of the most unusual craters is Apollodorus, or "the Spider", which hosts a series of radiating troughs extending outwards from its impact site.{{cite web |title=Scientists see Mercury in a new light |url=https://www.sciencedaily.com/releases/2008/02/080201093149.htm |website=Science Daily |date=February 28, 2008 |access-date=April 7, 2008 |archive-date=December 5, 2020 |archive-url=https://web.archive.org/web/20201205202019/https://www.sciencedaily.com/releases/2008/02/080201093149.htm |url-status=live }}

Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins hundreds of kilometers across. They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants. Mercurian craters differ subtly from lunar craters in that the area blanketed by their ejecta is much smaller, a consequence of Mercury's stronger surface gravity.{{cite journal |first=Paul D. |last=Spudis |author-link=Paul Spudis |title=The Geological History of Mercury |journal=Workshop on Mercury: Space Environment, Surface, and Interior, Chicago |issue=1097 |year=2001 |page=100 |bibcode=2001mses.conf..100S}} According to International Astronomical Union rules, each new crater must be named after an artist who was famous for more than fifty years, and dead for more than three years, before the date the crater is named.

{{multiple image |direction=horizontal |align=right |total_width=400

|image1=The Mighty Caloris (PIA19213).png | caption1=Overhead view of Caloris Basin

|image2=PIA19450-PlanetMercury-CalorisBasin-20150501.jpg | caption2=Perspective view of Caloris Basin – high (red); low (blue)

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The largest known crater is Caloris Planitia, or Caloris Basin, with a diameter of {{convert|1550|km|mi|abbr=on}}.{{cite news |url=https://www.newscientist.com/article/dn13257-bizarre-spider-scar-found-on-mercurys-surface.html |title=Bizarre spider scar found on Mercury's surface |date=January 30, 2008 |publisher=NewScientist.com news service |first=David |last=Shiga |access-date=September 4, 2017 |archive-date=December 10, 2014 |archive-url=https://web.archive.org/web/20141210213025/http://www.newscientist.com/article/dn13257-bizarre-spider-scar-found-on-mercurys-surface.html |url-status=live }} The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric mountainous ring ~{{convert|2|km|mi|abbr=on}} tall surrounding the impact crater. The floor of the Caloris Basin is filled by a geologically distinct flat plain, broken up by ridges and fractures in a roughly polygonal pattern. It is not clear whether they were volcanic lava flows induced by the impact or a large sheet of impact melt.

At the antipode of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain". One hypothesis for its origin is that shock waves generated during the Caloris impact traveled around Mercury, converging at the basin's antipode (180 degrees away). The resulting high stresses fractured the surface.{{cite journal |last1=Schultz |first1=Peter H. |author-link1=Peter H. Schultz |last2=Gault |first2=Donald E. |year=1975 |title=Seismic effects from major basin formations on the moon and Mercury |journal=Earth, Moon, and Planets |volume=12 |issue=2 |pages=159–175 |doi=10.1007/BF00577875 |bibcode=1975Moon...12..159S|s2cid=121225801 }} Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin's antipode.{{cite journal |last1=Wieczorek |first1=Mark A. |last2=Zuber |first2=Maria T. |author-link2=Maria Zuber |title=A Serenitatis origin for the Imbrian grooves and South Pole-Aitken thorium anomaly |journal=Journal of Geophysical Research |year=2001 |volume=106 |issue=E11 |pages=27853–27864 |url=http://www.agu.org/pubs/crossref/2001/2000JE001384.shtml |access-date=May 12, 2008 |doi=10.1029/2000JE001384 |bibcode=2001JGR...10627853W |doi-access=free |archive-date=May 12, 2011 |archive-url=https://web.archive.org/web/20110512152936/http://www.agu.org/pubs/crossref/2001/2000JE001384.shtml |url-status=live }}

File:EW1027346412Gnomap.png

Overall, 46 impact basins have been identified.{{cite journal | title=Large impact basins on Mercury: Global distribution, characteristics, and modification history from MESSENGER orbital data | last1=Fassett | first1=Caleb I. | last2=Head | first2=James W. | last3=Baker | first3=David M. H. | last4=Zuber | first4=Maria T. | last5=Smith | first5=David E. | last6=Neumann | first6=Gregory A. | last7=Solomon | first7=Sean C. | last8=Klimczak | first8=Christian | last9=Strom | first9=Robert G. | last10=Chapman | first10=Clark R. | last11=Prockter | first11=Louise M. | last12=Phillips | first12=Roger J. | last13=Oberst | first13=Jürgen | last14=Preusker | first14=Frank | journal=Journal of Geophysical Research | volume=117 | id=E00L08 | date=October 2012 | at=15 pp. | doi=10.1029/2012JE004154 | bibcode=2012JGRE..117.0L08F | doi-access=free }} A notable basin is the {{convert|400|km|mi|abbr=on|adj=mid}}-wide, multi-ring Tolstoj Basin that has an ejecta blanket extending up to {{convert|500|km|mi|abbr=on}} from its rim and a floor that has been filled by smooth plains materials. Beethoven Basin has a similar-sized ejecta blanket and a {{convert|625|km|mi|abbr=on|adj=mid}}-diameter rim. Like the Moon, the surface of Mercury has likely incurred the effects of space weathering processes, including solar wind and micrometeorite impacts.{{cite journal |title=Albedo of Immature Mercurian Crustal Materials: Evidence for the Presence of Ferrous Iron |journal=Lunar and Planetary Science |volume=39 |issue=1391 |year=2008 |page=1750 |last1=Denevi |first1=Brett W. |author-link1=Brett Denevi |last2=Robinson |first2=Mark S. |bibcode=2008LPI....39.1750D}}

There are two geologically distinct plains regions on Mercury. Gently rolling, hilly plains in the regions between craters are Mercury's oldest visible surfaces, predating the heavily cratered terrain. These inter-crater plains appear to have obliterated many earlier craters, and show a general paucity of smaller craters below about {{convert|30|km|mi|abbr=on}} in diameter.

Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to lunar maria. Unlike lunar maria, the smooth plains of Mercury have the same albedo as the older inter-crater plains. Despite a lack of unequivocally volcanic characteristics, the localization and rounded, lobate shape of these plains strongly support volcanic origins. All the smooth plains of Mercury formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket.

An unusual feature of Mercury's surface is the numerous compression folds, or rupes, that crisscross the plains. These exist on the Moon, but are much more prominent on Mercury.{{cite journal | title=Wrinkle ridges on Mercury and the Moon within and outside of mascons | last1=Schleicher | first1=Lisa S. | last2=Watters | first2=Thomas R. | last3=Martin | first3=Aaron J. | last4=Banks | first4=Maria E. | journal=Icarus | volume=331 | pages=226–237 | date=October 2019 | doi=10.1016/j.icarus.2019.04.013 | bibcode=2019Icar..331..226S | s2cid=150072193 }} As Mercury's interior cooled, it contracted and its surface began to deform, creating wrinkle ridges and lobate scarps associated with thrust faults. The scarps can reach lengths of {{convert|1000|km|mi|abbr=on}} and heights of {{convert|3|km|mi|abbr=on}}.{{cite web |url=http://www.space.com/34199-earthquakes-rock-mercury-today.html |title=Mercuryquakes May Currently Shake Up the Tiny Planet |last=Choi |first=Charles Q. |date=September 26, 2016 |website=Space.com |access-date=September 28, 2016 |archive-date=September 28, 2016 |archive-url=https://web.archive.org/web/20160928040406/http://www.space.com/34199-earthquakes-rock-mercury-today.html |url-status=live }} These compressional features can be seen on top of other features, such as craters and smooth plains, indicating they are more recent.{{cite journal |last=Dzurisin |first=Daniel |date=October 10, 1978 |title=The tectonic and volcanic history of Mercury as inferred from studies of scarps, ridges, troughs, and other lineaments |journal=Journal of Geophysical Research |volume=83 |issue=B10 |pages=4883–4906 |bibcode=1978JGR....83.4883D |doi=10.1029/JB083iB10p04883}} Mapping of the features has suggested a total shrinkage of Mercury's radius in the range of ~{{convert|1–7|km|mi|abbr=on}}.{{cite journal |last1=Watters |first1=Thomas R. |last2=Daud |first2=Katie |last3=Banks |first3=Maria E. |last4=Selvans |first4=Michelle M. |last5=Chapman |first5=Clark R. |last6=Ernst |first6=Carolyn M. |title=Recent tectonic activity on Mercury revealed by small thrust fault scarps |journal=Nature Geoscience |date=September 26, 2016 |doi=10.1038/ngeo2814 |volume=9 |issue=10 |pages=743–747 |bibcode=2016NatGe...9..743W}} Most activity along the major thrust systems probably ended about 3.6–3.7 billion years ago.{{cite journal | title=Dating long thrust systems on Mercury: New clues on the thermal evolution of the planet | first1=L. | last1=Giacomini | first2=M. | last2=Massironi | first3=V. | last3=Galluzzi | first4=S. | last4=Ferrari | first5=P. | last5=Palumbo | journal=Geoscience Frontiers | volume=11 | issue=3 | date=May 2020 | pages=855–870 | doi=10.1016/j.gsf.2019.09.005 | bibcode=2020GeoFr..11..855G | s2cid=210298205 | doi-access=free }} Small-scale thrust fault scarps have been found, tens of meters in height and with lengths in the range of a few kilometers, that appear to be less than 50 million years old, indicating that compression of the interior and consequent surface geological activity continue to the present.

File:Picasso crater.png—the large arc-shaped pit located on the eastern side of its floor is postulated to have formed when subsurface magma subsided or drained, causing the surface to collapse into the resulting void.]]

There is evidence for pyroclastic flows on Mercury from low-profile shield volcanoes.{{cite journal |title=Explosive volcanic eruptions on Mercury: Eruption conditions, magma volatile content, and implications for interior volatile abundances |journal=Earth and Planetary Science Letters |date=August 15, 2009 |last1=Kerber |first1=Laura |last2=Head |first2=James W. |last3=Solomon |first3=Sean C. |last4=Murchie |first4=Scott L. |last5=Blewett |first5=David T. | volume=285 | issue=3–4 | pages=263–271 | doi=10.1016/j.epsl.2009.04.037 | bibcode=2009E&PSL.285..263K }}{{cite journal |title=Flood Volcanism in the Northern High Latitudes of Mercury Revealed by MESSENGER |journal=Science |date=September 30, 2011 |last1=Head |first1=James W. |last2=Chapman |first2=Clark R. |last3=Strom |first3=Robert G. |last4=Fassett |first4=Caleb I. |last5=Denevi |first5=Brett W. |volume=333 |issue=6051 |pages=1853–1856 |doi=10.1126/science.1211997 |bibcode=2011Sci...333.1853H |pmid=21960625 |s2cid=7651992 |url=https://authors.library.caltech.edu/72395/2/Head.SOM.pdf |access-date=August 20, 2019 |archive-date=July 19, 2018 |archive-url=https://web.archive.org/web/20180719031049/https://authors.library.caltech.edu/72395/2/Head.SOM.pdf |url-status=live }}{{cite journal |last1=Thomas |first1=Rebecca J. |last2=Rothery |first2=David A. |last3=Conway |first3=Susan J. |last4=Anand |first4=Mahesh |title=Long-lived explosive volcanism on Mercury |journal=Geophysical Research Letters |date=September 16, 2014 |volume=41 |issue=17 |pages=6084–6092 |doi=10.1002/2014GL061224 |bibcode=2014GeoRL..41.6084T |s2cid=54683272 |url=http://oro.open.ac.uk/40782/ |access-date=July 19, 2017 |archive-date=August 22, 2017 |archive-url=https://web.archive.org/web/20170822012357/http://oro.open.ac.uk/40782/ |url-status=live }} Fifty-one pyroclastic deposits have been identified,{{cite journal |title=Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data |journal=Journal of Geophysical Research |date=March 2014 |last1=Groudge |first1=Timothy A. |last2=Head |first2=James W. |doi=10.1002/2013JE004480 |volume=119 |issue=3 |pages=635–658 |bibcode=2014JGRE..119..635G |s2cid=14393394 |url=http://www.planetary.brown.edu/pdfs/4334.pdf |access-date=August 25, 2019 |archive-date=July 18, 2019 |archive-url=https://web.archive.org/web/20190718161242/http://www.planetary.brown.edu/pdfs/4334.pdf |url-status=dead }} where 90% of them are found within impact craters. A study of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval.

A "rimless depression" inside the southwest rim of the Caloris Basin consists of at least nine overlapping volcanic vents, each individually up to {{convert|8|km|mi|abbr=on}} in diameter. It is thus a "compound volcano".{{cite journal |title=Prolonged eruptive history of a compound volcano on Mercury: Volcanic and tectonic implications |journal=Earth and Planetary Science Letters |date=January 1, 2014 |last1=Rothery |first1=David A. |last2=Thomas |first2=Rebeca J. |last3=Kerber |first3=Laura |volume=385 |pages=59–67 |bibcode=2014E&PSL.385...59R |doi=10.1016/j.epsl.2013.10.023 |url=http://oro.open.ac.uk/38842/1/Rothery2.pdf |access-date=August 20, 2019 |archive-date=March 6, 2020 |archive-url=https://web.archive.org/web/20200306081432/http://oro.open.ac.uk/38842/1/Rothery2.pdf |url-status=live }} The vent floors are at least {{convert|1|km|mi|abbr=on}} below their brinks and they bear a closer resemblance to volcanic craters sculpted by explosive eruptions or modified by collapse into void spaces created by magma withdrawal back down into a conduit. Scientists could not quantify the age of the volcanic complex system but reported that it could be on the order of a billion years.

{{Main|Atmosphere of Mercury}}

File:North pole of Mercury -- NASA.jpg

The surface temperature of Mercury ranges from {{Convert|100 to 700|K|C F}}.{{cite book |last=Prockter |first=Louise |title=Ice in the Solar System |publisher=Johns Hopkins APL Technical Digest |volume=26 |issue=2 |date=2005 |url=https://www.jhuapl.edu/content/techdigest/pdf/V26-N02/26-02-Prockter.pdf |access-date=July 27, 2009 |archive-date=September 24, 2021 |archive-url=https://web.archive.org/web/20210924085243/https://www.jhuapl.edu/Content/techdigest/pdf/V26-N02/26-02-Prockter.pdf |url-status=live }} It never rises above 180 K at the poles, due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. At perihelion, the equatorial subsolar point is located at latitude 0°W or 180°W, and it climbs to a temperature of about {{val|700|u=K}}. During aphelion, this occurs at 90° or 270°W and reaches only {{val|550|u=K}}.{{cite book |first=John S. |last=Lewis |date=2004 |title=Physics and Chemistry of the Solar System |page=463 |edition=2nd |publisher=Academic Press |isbn=978-0-12-446744-6}} On the dark side of the planet, temperatures average {{val|110|u=K}}.{{cite journal |last1=Murdock |first1=Thomas L. |last2=Ney |first2=Edward P. |title=Mercury: The Dark-Side Temperature |journal=Science |year=1970 |volume=170 |issue=3957 |pages=535–537 |doi=10.1126/science.170.3957.535 |pmid=17799708 |bibcode=1970Sci...170..535M|s2cid=38824994 }} The intensity of sunlight on Mercury's surface ranges between 4.59 and 10.61 times the solar constant (1,370 W·m⁻²).{{cite book |title=Physics and Chemistry of the Solar System |last=Lewis |first=John S. |publisher=Academic Press |date=2004 |url=https://books.google.com/books?id=ERpMjmR1ErYC&pg=RA1-PA461 |access-date=June 3, 2008 |isbn=978-0-12-446744-6 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162200/https://books.google.com/books?id=ERpMjmR1ErYC&pg=RA1-PA461 |url-status=live }}

Although daylight temperatures at the surface of Mercury are generally extremely high, observations strongly suggest that ice (frozen water) exists on Mercury. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102 K, far lower than the global average.{{cite journal |last1=Ingersoll |first1=Andrew P. |last2=Svitek |first2=Tomas |last3=Murray |first3=Bruce C. |title=Stability of polar frosts in spherical bowl-shaped craters on the Moon, Mercury, and Mars |journal=Icarus |volume=100 |issue=1 |pages=40–47 |year=1992 |bibcode=1992Icar..100...40I |doi=10.1016/0019-1035(92)90016-Z}} This creates a cold trap where ice can accumulate. Water ice strongly reflects radar, and observations by the 70-meter Goldstone Solar System Radar and the VLA in the early 1990s revealed that there are patches of high radar reflection near the poles.{{cite journal |last1=Slade |first1=Martin A. |last2=Butler |first2=Bryan J. |last3=Muhleman |first3=Duane O. |year=1992 |title=Mercury radar imaging – Evidence for polar ice |journal=Science |volume=258 |issue=5082 |pages=635–640 |doi=10.1126/science.258.5082.635 |pmid=17748898 |bibcode=1992Sci...258..635S|s2cid=34009087 }} Although ice was not the only possible cause of these reflective regions, astronomers thought it to be the most likely explanation.{{cite web |last=Williams |first=David R. |date=June 2, 2005 |url=http://nssdc.gsfc.nasa.gov/planetary/ice/ice_mercury.html |title=Ice on Mercury |publisher=NASA Goddard Space Flight Center |access-date=May 23, 2008 |archive-date=January 31, 2011 |archive-url=https://web.archive.org/web/20110131225129/http://nssdc.gsfc.nasa.gov/planetary/ice/ice_mercury.html |url-status=live }} The presence of water ice was confirmed using MESSENGER images of craters at the north pole.{{cite news |url=https://www.nytimes.com/2012/11/30/science/space/mercury-home-to-ice-messenger-spacecraft-findings-suggest.html |title=On Closest Planet to the Sun, NASA Finds Lots of Ice |work=The New York Times |first=Kenneth |last=Chang |date=November 29, 2012 |page=A3 |archive-date=November 29, 2012 |archive-url=https://web.archive.org/web/20121129194012/http://www.nytimes.com/2012/11/30/science/space/mercury-home-to-ice-messenger-spacecraft-findings-suggest.html |url-status=live |quote=Sean C. Solomon, the principal investigator for MESSENGER, said there was enough ice there to encase Washington, D.C., in a frozen block two and a half miles deep.}}

The icy crater regions are estimated to contain about 10¹⁴–10¹⁵ kg of ice,{{cite journal |last1=Rawlins |first1=Katherine |last2=Moses |first2=Julianne I. |last3=Zahnle |first3=Kevin J. |author-link3=Kevin J. Zahnle |title=Exogenic Sources of Water for Mercury's Polar Ice |journal=Bulletin of the American Astronomical Society |year=1995 |volume=27 |bibcode=1995DPS....27.2112R |page=1117}} and may be covered by a layer of regolith that inhibits sublimation.{{cite journal |last1=Harmon |first1=John K. |last2=Perillat |first2=Phil J. |last3=Slade |first3=Martin A. |title=High-Resolution Radar Imaging of Mercury's North Pole |journal=Icarus |volume=149 |issue=1 |pages=1–15 |year=2001 |doi=10.1006/icar.2000.6544 |bibcode=2001Icar..149....1H}} By comparison, the Antarctic ice sheet on Earth has a mass of about 4{{e|18}} kg, and Mars's south polar cap contains about 10¹⁶ kg of water. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet's interior and deposition by impacts of comets.

Mercury is too small and hot for its gravity to retain any significant atmosphere over long periods of time; it does have a tenuous surface-bounded exosphere{{cite journal |last1=Domingue |first1=Deborah L. |last2=Koehn |first2=Patrick L. |display-authors=2 |last3=Killen |first3=Rosemary M. |last4=Sprague |first4=Ann L. |last5=Sarantos |first5=Menelaos |last6=Cheng |first6=Andrew F. |last7=Bradley |first7=Eric T. |last8=McClintock |first8=William E. |title=Mercury's Atmosphere: A Surface-Bounded Exosphere |journal=Space Science Reviews |volume=131 |issue=1–4 |pages=161–186 |year=2009 |doi=10.1007/s11214-007-9260-9 |bibcode=2007SSRv..131..161D |s2cid=121301247 |name-list-style=vanc}} at a surface pressure of less than approximately 0.5 nPa (0.005 picobars). It includes hydrogen, helium, oxygen, sodium, calcium, potassium, magnesium, silicon, and hydroxide, among others. This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen atoms and helium atoms probably come from the solar wind, diffusing into Mercury's magnetosphere before later escaping back into space. The radioactive decay of elements within Mercury's crust is another source of helium, as well as sodium and potassium. Water vapor is present, released by a combination of processes such as comets striking its surface, sputtering creating water out of hydrogen from the solar wind and oxygen from rock, and sublimation from reservoirs of water ice in the permanently shadowed polar craters. The detection of high amounts of water-related ions like O⁺, OH⁻, and H₃O⁺ was a surprise.{{cite book |editor-first=Faith |editor-last=Vilas |editor-first2=Clark R. |editor-last2=Chapman |editor-first3=Mildred |editor-last3=Shapley Matthews |last1=Hunten |first1=Donald M. |last2=Shemansky |first2=Donald Eugene |last3=Morgan |first3=Thomas Hunt |date=1988 |publisher=University of Arizona Press |isbn=978-0-8165-1085-6 |chapter=The Mercury atmosphere |title=Mercury |chapter-url=https://www.researchgate.net/publication/23869200_The_Mercury_atmosphere |url=https://uapress.arizona.edu/book/mercury |access-date=February 19, 2020 |archive-date=February 19, 2020 |archive-url=https://web.archive.org/web/20200219213720/https://uapress.arizona.edu/book/mercury |url-status=live }}{{cite news |first=Emily |last=Lakdawalla |date=July 3, 2008 |title=MESSENGER Scientists "Astonished" to Find Water in Mercury's Thin Atmosphere |publisher=The Planetary Society |url=https://www.planetary.org/blogs/emily-lakdawalla/2008/0703_MESSENGER_Scientists_Astonished_to.html |access-date=May 18, 2009 |archive-date=April 4, 2017 |archive-url=https://web.archive.org/web/20170404043436/http://www.planetary.org/blogs/emily-lakdawalla/2008/0703_MESSENGER_Scientists_Astonished_to.html |url-status=live }} Because of the quantities of these ions that were detected in Mercury's space environment, scientists surmise that these molecules were blasted from the surface or exosphere by the solar wind.{{cite journal |last1=Zurbuchen |first1=Thomas H. |last2=Raines |first2=Jim M. |display-authors=2 |last3=Gloeckler |first3=George |last4=Krimigis |first4=Stamatios M. |last5=Slavin |first5=James A. |last6=Koehn |first6=Patrick L. |last7=Killen |first7=Rosemary M. |last8=Sprague |first8=Ann L. |last9=McNutt Jr. |first9=Ralph L. |last10=Solomon |first10=Sean C. |title=MESSENGER Observations of the Composition of Mercury's Ionized Exosphere and Plasma Environment |journal=Science |volume=321 |issue=5885 |pages=90–92 |year=2008 |doi=10.1126/science.1159314 |pmid=18599777 |bibcode=2008Sci...321...90Z |s2cid=206513512 |name-list-style=vanc}}{{cite news |publisher=University of Michigan |date=June 30, 2008 |title=Instrument Shows What Planet Mercury Is Made Of |url=http://newswise.com/articles/view/542209/ |access-date=May 18, 2009 |archive-date=March 22, 2012 |archive-url=https://web.archive.org/web/20120322021728/http://newswise.com/articles/view/542209/ |url-status=live }}

Sodium, potassium, and calcium were discovered in the atmosphere during the 1980s–1990s, and are thought to result primarily from the vaporization of surface rock struck by micrometeorite impacts{{cite journal |last1=Killen |first1=Rosemary |title=Processes that Promote and Deplete the Exosphere of Mercury |year=2007 |journal=Space Science Reviews |volume=132 |issue=2–4 |pages=433–509 |doi=10.1007/s11214-007-9232-0 |ref=Killen2007 |bibcode=2007SSRv..132..433K |last2=Cremonese |first2=Gabrielle |display-authors=2 |last3=Lammer |first3=Helmut |last4=Orsini |first4=Stefano |last5=Potter |first5=Andrew E. |last6=Sprague |first6=Ann L. |last7=Wurz |first7=Peter |last8=Khodachenko |first8=Maxim L. |last9=Lichtenegger |first9=Herbert I. M. |s2cid=121944553 |url=https://boris.unibe.ch/25351/ |access-date=October 16, 2022 |archive-date=October 9, 2022 |archive-url=https://web.archive.org/web/20221009053004/https://boris.unibe.ch/25351/ |url-status=live }} including presently from Comet Encke.{{cite journal |first1=Rosemary M. |last1=Killen |first2=Joseph M. |last2=Hahn |title=Impact Vaporization as a Possible Source of Mercury's Calcium Exosphere |journal=Icarus |date=December 10, 2014 |doi=10.1016/j.icarus.2014.11.035 |bibcode=2015Icar..250..230K |volume=250 |pages=230–237 |hdl=2060/20150010116}} In 2008, magnesium was discovered by MESSENGER.{{cite journal |last1=McClintock |first1=William E. |last2=Vervack |first2=Ronald J. |last3=Bradley |first3=E. Todd |last4=Killen |first4=Rosemary M. |last5=Mouawad |first5=Nelly |last6=Sprague |first6=Ann L. |last7=Burger |first7=Matthew H. |last8=Solomon |first8=Sean C. |last9=Izenberg |first9=Noam R. |title=MESSENGER Observations of Mercury's Exosphere: Detection of Magnesium and Distribution of Constituents |journal=Science |year=2009 |volume=324 |doi=10.1126/science.1172525 |pages=610–613 |bibcode=2009Sci...324..610M |pmid=19407195 |issue=5927 |s2cid=5578520 |display-authors=2 }} Studies indicate that, at times, sodium emissions are localized at points that correspond to the planet's magnetic poles. This would indicate an interaction between the magnetosphere and the planet's surface.

According to NASA, Mercury is not a suitable planet for Earth-like life. It has a surface boundary exosphere instead of a layered atmosphere, extreme temperatures, and high solar radiation. It is unlikely that any living beings can withstand those conditions.{{cite web|url= https://solarsystem.nasa.gov/planets/mercury/in-depth/|title= Mercury|date= October 19, 2021|publisher= NASA|accessdate= July 4, 2022|archive-date= July 5, 2022|archive-url= https://web.archive.org/web/20220705191357/https://solarsystem.nasa.gov/planets/mercury/in-depth/|url-status= live}} Some parts of the subsurface of Mercury may have been habitable, and perhaps life forms, albeit likely primitive microorganisms, may have existed on the planet.{{cite news |last=Hall |first=Shannon |title=Life on the Planet Mercury? 'It's Not Completely Nuts' – A new explanation for the rocky world's jumbled landscape opens a possibility that it could have had ingredients for habitability. |url=https://www.nytimes.com/2020/03/24/science/mercury-life-water.html |archive-url=https://web.archive.org/web/20200324150021/https://www.nytimes.com/2020/03/24/science/mercury-life-water.html |archive-date=March 24, 2020 |url-access=subscription |url-status=live |date=March 24, 2020 |work=The New York Times |access-date=March 26, 2020 }}{{cite journal | last1=Rodriguez | first1=J. Alexis P. | last2=Leonard | first2=Gregory J. | last3=Kargel | first3=Jeffrey S. | last4=Domingue | first4=Deborah | last5=Berman | first5=Daniel C. | last6=Banks | first6=Maria | last7=Zarroca | first7=Mario | last8=Linares | first8=Rogelio | last9=Marchi | first9=Simone | last10=Baker | first10=Victor R. | last11=Webster | first11=Kevin D. | last12=Sykes | first12=Mark |title=The Chaotic Terrains of Mercury Reveal a History of Planetary Volatile Retention and Loss in the Innermost Solar System |date=March 16, 2020 |journal=Scientific Reports |volume=10 |issue=4737 |page=4737 |doi=10.1038/s41598-020-59885-5 |pmid=32179758 |pmc=7075900 |bibcode=2020NatSR..10.4737R }}{{cite news | title=Vast Collapsed Terrains on Mercury Might be Windows Into Ancient – Possibly Habitable – Volatile-Rich Materials | work=Planetary Science Institute | date=March 16, 2020 | url=https://www.psi.edu/news/mercurychaos | access-date=August 27, 2022 | archive-date=August 28, 2022 | archive-url=https://web.archive.org/web/20220828041010/https://www.psi.edu/news/mercurychaos | url-status=live }}

{{Main|Mercury's magnetic field}}

File:Mercury Magnetic Field NASA.jpg

Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field. According to measurements taken by {{nowrap|Mariner 10}}, it is about 1.1% the strength of Earth's. The magnetic-field strength at Mercury's equator is about {{nowrap|300 nT}}.{{cite book |title=Astronomy: The Solar System and Beyond |first=Michael A. |last=Seeds |date=2004 |isbn=978-0-534-42111-3 |publisher=Brooks Cole |edition=4th}}{{cite web |last=Williams |first=David R. |date=January 6, 2005 |url=http://nssdc.gsfc.nasa.gov/planetary/planetfact.html |title=Planetary Fact Sheets |publisher=NASA National Space Science Data Center |access-date=August 10, 2006 |archive-date=September 25, 2008 |archive-url=https://web.archive.org/web/20080925071832/http://nssdc.gsfc.nasa.gov/planetary/planetfact.html |url-status=live }} Like that of Earth, Mercury's magnetic field is dipolar{{cite book |first1=J. Kelly |last1=Beatty |last2=Petersen |first2=Carolyn Collins |last3=Chaikin |first3=Andrew |title=The New Solar System |date=1999 |publisher=Cambridge University Press |isbn=978-0-521-64587-4}} and nearly aligned with the planet's spin axis (10° dipolar tilt, compared to 11° for Earth).{{cite web |date=January 30, 2008 |url=https://messenger.jhuapl.edu/Explore/Science-Images-Database/gallery-image-152.html |title=Mercury's Internal Magnetic Field |publisher=NASA |access-date=April 21, 2021 |archive-date=April 21, 2021 |archive-url=https://web.archive.org/web/20210421163118/https://messenger.jhuapl.edu/Explore/Science-Images-Database/gallery-image-152.html |url-status=live }} Measurements from both the {{nowrap|Mariner 10}} and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable.

It is likely that this magnetic field is generated by a dynamo effect, in a manner similar to the magnetic field of Earth.{{cite web |last=Gold |first=Lauren |date=May 3, 2007 |url=http://www.news.cornell.edu/stories/May07/margot.mercury.html |title=Mercury has molten core, Cornell researcher shows |publisher=Cornell University |access-date=April 7, 2008 |archive-date=June 17, 2012 |archive-url=https://web.archive.org/web/20120617123129/http://www.news.cornell.edu/stories/May07/margot.mercury.html |url-status=live }}{{cite journal |last=Christensen |first=Ulrich R. |title=A deep dynamo generating Mercury's magnetic field |journal=Nature |year=2006 |volume=444 |pages=1056–1058 |doi=10.1038/nature05342 |pmid=17183319 |issue=7122 |bibcode=2006Natur.444.1056C |s2cid=4342216 |url=https://resolver.sub.uni-goettingen.de/purl?gro-2/65564 |access-date=October 29, 2023 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162201/https://publications.goettingen-research-online.de/handle/2/65564 |url-status=live }} This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong tidal heating effects caused by the planet's high orbital eccentricity would serve to keep part of the core in the liquid state necessary for this dynamo effect.{{cite journal |last1=Spohn |first1=Tilman |last2=Sohl |first2=Frank |last3=Wieczerkowski |first3=Karin |last4=Conzelmann |first4=Vera |title=The interior structure of Mercury: what we know, what we expect from BepiColombo |journal=Planetary and Space Science |volume=49 |issue=14–15 |pages=1561–1570 |doi=10.1016/S0032-0633(01)00093-9 |bibcode=2001P&SS...49.1561S |year=2001 }}{{cite journal | title=The tides of Mercury and possible implications for its interior structure | last1=Padovan | first1=Sebastiano | last2=Margot | first2=Jean-Luc | last3=Hauck | first3=Steven A. | last4=Moore | first4=William B. | last5=Solomon | first5=Sean C. | journal=Journal of Geophysical Research: Planets | volume=119 | issue=4 | pages=850–866 | date=April 2014 | doi=10.1002/2013JE004459 | bibcode=2014JGRE..119..850P | s2cid=56282397 }}

Mercury's magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, though small enough to fit within Earth, is strong enough to trap solar wind plasma. This contributes to the space weathering of the planet's surface. Observations taken by the {{nowrap|Mariner 10}} spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts of energetic particles in the planet's magnetotail indicate a dynamic quality to the planet's magnetosphere.

During its second flyby of the planet on October 6, 2008, MESSENGER discovered that Mercury's magnetic field can be extremely "leaky". The spacecraft encountered magnetic "tornadoes"—twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space—that were up to {{nowrap|800 km}} wide or a third of the radius of the planet. These twisted magnetic flux tubes, technically known as flux transfer events, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface via magnetic reconnection. This also occurs in Earth's magnetic field. The MESSENGER observations showed the reconnection rate was ten times higher at Mercury, but its proximity to the Sun only accounts for about a third of the reconnection rate observed by MESSENGER.{{cite web |first=Bill |last=Steigerwald |date=June 2, 2009 |title=Magnetic Tornadoes Could Liberate Mercury's Tenuous Atmosphere |publisher=NASA Goddard Space Flight Center |url=http://www.nasa.gov/mission_pages/messenger/multimedia/magnetic_tornadoes.html |access-date=July 18, 2009 |archive-date=May 18, 2012 |archive-url=https://web.archive.org/web/20120518035510/http://www.nasa.gov/mission_pages/messenger/multimedia/magnetic_tornadoes.html |url-status=dead }}

{{multiple image |direction=horizontal |align=right |total_width=400

|image1=ThePlanets Orbits Mercury PolarView.svg |caption1=Orbit of Mercury (2006)

|image2=Mercuryorbitsolarsystem.gif |caption2=Animation of Mercury's and Earth's revolution around the Sun

}}

Mercury has the most eccentric orbit of all the planets in the Solar System; its eccentricity is 0.21 with its distance from the Sun ranging from {{convert|46000000|to|70000000|km|mi|abbr=on}}. It takes 87.969 Earth days to complete an orbit. The diagram illustrates the effects of the eccentricity, showing Mercury's orbit overlaid with a circular orbit having the same semi-major axis. Mercury's higher velocity when it is near perihelion is clear from the greater distance it covers in each 5-day interval. In the diagram, the varying distance of Mercury to the Sun is represented by the size of the planet, which is inversely proportional to Mercury's distance from the Sun.

This varying distance to the Sun leads to Mercury's surface being flexed by tidal bulges raised by the Sun that are about 17 times stronger than the Moon's on Earth.{{cite journal |last1=Van Hoolst |first1=Tim |last2=Jacobs |first2=Carla |year=2003 |title=Mercury's tides and interior structure |journal=Journal of Geophysical Research |volume=108 |issue=E11 |page=7 |doi=10.1029/2003JE002126 |bibcode=2003JGRE..108.5121V|doi-access=free }} Combined with a 3:2 spin–orbit resonance of the planet's rotation around its axis, it also results in complex variations of the surface temperature.{{cite book |first1=Robert G. |last1=Strom |last2=Sprague |first2=Ann L. |date=2003 |title=Exploring Mercury: the iron planet |publisher=Springer |isbn=978-1-85233-731-5 |url=https://archive.org/details/exploringmercury00stro }} The resonance makes a single solar day (the length between two meridian transits of the Sun) on Mercury last exactly two Mercury years, or about 176 Earth days.{{cite web |title=Space Topics: Compare the Planets: Mercury, Venus, Earth, The Moon, and Mars |publisher=Planetary Society |url=http://www.planetary.org/explore/topics/compare_the_planets/terrestrial.html |access-date=April 12, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20110728044444/http://www.planetary.org/explore/topics/compare_the_planets/terrestrial.html |archive-date=July 28, 2011}}

Mercury's orbit is inclined by 7 degrees to the plane of Earth's orbit (the ecliptic), the largest of all eight known solar planets. As a result, transits of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between Earth and the Sun, which is in May or November. This occurs about every seven years on average.{{cite web |last=Espenak |first=Fred |date=April 21, 2005 |url=http://eclipse.gsfc.nasa.gov/transit/catalog/MercuryCatalog.html |title=Transits of Mercury |publisher=NASA/Goddard Space Flight Center |access-date=May 20, 2008 |archive-date=August 29, 2015 |archive-url=https://archive.today/20150829155710/http://eclipse.gsfc.nasa.gov/transit/catalog/MercuryCatalog.html |url-status=live }}

Mercury's axial tilt is almost zero, with the best measured value as low as 0.027 degrees.{{cite journal |last1=Margot |first1=J. L. |display-authors=4 |last2=Peale |first2=S. J. |last3=Jurgens |first3=R. F. |last4=Slade |first4=M. A. |last5=Holin |first5=I. V. |title=Large Longitude Libration of Mercury Reveals a Molten Core |journal=Science |year=2007 |volume=316 |pages=710–714 |doi=10.1126/science.1140514 |bibcode=2007Sci...316..710M |pmid=17478713 |issue=5825|s2cid=8863681 }} This is significantly smaller than that of Jupiter, which has the second smallest axial tilt of all planets at 3.1 degrees. This means that to an observer at Mercury's poles, the center of the Sun never rises more than 2.1 arcminutes above the horizon. By comparison, the angular size of the Sun as seen from Mercury ranges from {{fraction|1|1|4}} to 2 degrees across.{{cite book | title=From The Sun To The Stars | first=James B. | last=Kaler | author-link=James B. Kaler | year=2016 | page=56 | isbn=9789813143265 | publisher=World Scientific Publishing Company | url=https://books.google.com/books?id=ZYv4DAAAQBAJ&pg=PA56 | access-date=October 25, 2023 | archive-date=October 31, 2023 | archive-url=https://web.archive.org/web/20231031140355/https://books.google.com/books?id=ZYv4DAAAQBAJ&pg=PA56 | url-status=live }}

At certain points on Mercury's surface, an observer would be able to see the Sun peek up a little more than two-thirds of the way over the horizon, then reverse and set before rising again, all within the same Mercurian day.{{efn|name=angular}} This is because approximately four Earth days before perihelion, Mercury's angular orbital velocity equals its angular rotational velocity so that the Sun's apparent motion ceases; closer to perihelion, Mercury's angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a retrograde direction. Four Earth days after perihelion, the Sun's normal apparent motion resumes. A similar effect would have occurred if Mercury had been in synchronous rotation: the alternating gain and loss of rotation over a revolution would have caused a libration of 23.65° in longitude.{{cite book |title=Popular Astronomy: A Review of Astronomy and Allied Sciences |url=https://books.google.com/books?id=ePc-AQAAIAAJ |year=1896 |publisher=Goodsell Observatory of Carleton College |quote=although in the case of Venus the libration in longitude due to the eccentricity of the orbit amounts to only 47' on either side of the mean position, in the case of Mercury it amounts to 23° 39' |access-date=December 24, 2016 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162153/https://books.google.com/books?id=ePc-AQAAIAAJ |url-status=live }}

For the same reason, there are two points on Mercury's equator, 180 degrees apart in longitude, at either of which, around perihelion in alternate Mercurian years (once a Mercurian day), the Sun passes overhead, then reverses its apparent motion and passes overhead again, then reverses a second time and passes overhead a third time, taking a total of about 16 Earth-days for this entire process. In the other alternate Mercurian years, the same thing happens at the other of these two points. The amplitude of the retrograde motion is small, so the overall effect is that, for two or three weeks, the Sun is almost stationary overhead, and is at its most brilliant because Mercury is at perihelion, its closest to the Sun. This prolonged exposure to the Sun at its brightest makes these two points the hottest places on Mercury. Maximum temperature occurs when the Sun is at an angle of about 25 degrees past noon due to diurnal temperature lag, at 0.4 Mercury days and 0.8 Mercury years past sunrise.{{cite web |last=Seligman |first=C. |title=The Rotation of Mercury |url=https://cseligman.com/text/planets/mercuryrot.htm |publisher=cseligman.com |at=NASA Flash animation |access-date=July 31, 2019 |archive-date=August 6, 2019 |archive-url=https://web.archive.org/web/20190806213722/http://cseligman.com/text/planets/mercuryrot.htm |url-status=live }} Conversely, there are two other points on the equator, 90 degrees of longitude apart from the first ones, where the Sun passes overhead only when the planet is at aphelion in alternate years, when the apparent motion of the Sun in Mercury's sky is relatively rapid. These points, which are the ones on the equator where the apparent retrograde motion of the Sun happens when it is crossing the horizon as described in the preceding paragraph, receive much less solar heat than the first ones described above.{{cite journal | title=On the Variations in the Insolation at Mercury Resulting from Oscillations of the Orbital Eccentricity | last=van Hemerlrijck | first=E. | journal=The Moon and the Planets | volume=29 | issue=1 | pages=83–93 | date=August 1983 | doi=10.1007/BF00928377 | bibcode=1983M&P....29...83V | s2cid=122761699 }}

Mercury attains an inferior conjunction (nearest approach to Earth) every 116 Earth days on average, but this interval can range from 105 days to 129 days due to the planet's eccentric orbit. Mercury can come as near as {{Convert|82200000|km|AU e6mi|abbr=in}} to Earth, and that is slowly declining: The next approach to within {{Convert|82100000|km|e6mi|abbr=unit|sigfig=2}} is in 2679, and to within {{Convert|82000000|km|e6mi|abbr=unit}} in 4487, but it will not be closer to Earth than {{Convert|80000000|km|e6mi|abbr=unit}} until 28,622.Mercury Closest Approaches to Earth generated with:
1. [http://chemistry.unina.it/~alvitagl/solex/ Solex 10]  {{Webarchive|url=https://web.archive.org/web/20081220235836/http://chemistry.unina.it/~alvitagl/solex/ |date=December 20, 2008 }} ([http://home.surewest.net/kheider/astro/SolexMerc.txt Text Output file] {{webarchive|url=https://web.archive.org/web/20120309120624/http://home.surewest.net/kheider/astro/SolexMerc.txt |date=March 9, 2012 }})
2. [http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1235936812 Gravity Simulator charts] {{Webarchive|url=https://web.archive.org/web/20140912091426/http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1235936812 |date=September 12, 2014 }}
3. [http://home.surewest.net/kheider/astro/Mercury.txt JPL Horizons 1950–2200]  {{webarchive|url=https://web.archive.org/web/20151106172707/http://home.surewest.net/kheider/astro/Mercury.txt |date=November 6, 2015 }} {{noprint|(3 sources are provided to address original research concerns and to support general long-term trends) }} Its period of retrograde motion as seen from Earth can vary from 8 to 15 days on either side of an inferior conjunction. This large range arises from the planet's high orbital eccentricity. Essentially, because Mercury is closest to the Sun, when taking an average over time, Mercury is most often the closest planet to the Earth,{{cite journal | title=Venus is not Earth's closest neighbor | journal=Physics Today | publisher=AIP Publishing | date=March 12, 2019 | issue=3 | issn=1945-0699 | doi=10.1063/pt.6.3.20190312a | page=30593| bibcode=2019PhT..2019c0593. | s2cid=241077611 }}{{cite web |last1=Harford |first1=Tim |title=BBC Radio 4 – More or Less, Sugar, Outdoors Play and Planets |url=https://www.bbc.co.uk/programmes/m0001y9p |publisher=BBC |date=January 11, 2019 |quote=Oliver Hawkins, more or less alumnus and statistical legend, wrote some code for us, which calculated which planet was closest to the Earth on each day for the past 50 years, and then sent the results to David A. Rothery, professor of planetary geosciences at the Open University. |access-date=January 12, 2019 |archive-date=January 12, 2019 |archive-url=https://web.archive.org/web/20190112044935/https://www.bbc.co.uk/programmes/m0001y9p |url-status=live }} and—in that measure—it is the closest planet to each of the other planets in the Solar System.{{cite journal |last1=Stockman |first1=Tom |last2=Monroe |first2=Gabriel |last3=Cordner |first3=Samuel |title=Venus is not Earth's closest neighbor |journal=Physics Today |date=March 12, 2019 |issue=3 |page=30593 |doi=10.1063/PT.6.3.20190312a|bibcode=2019PhT..2019c0593. |s2cid=241077611 }}{{cite AV media |last1=Stockman |first1=Tom |date=March 7, 2019 |title=Mercury is the closest planet to all seven other planets |medium=video |url=https://www.youtube.com/watch?v=GDgbVIqGADQ | archive-url=https://ghostarchive.org/varchive/youtube/20211028/GDgbVIqGADQ| archive-date=October 28, 2021|access-date=May 29, 2019 |via=YouTube }}{{cbignore}}{{Citation|title=🌍 Which Planet is the Closest?| date=October 30, 2019 |url=https://www.youtube.com/watch?v=SumDHcnCRuU| archive-url=https://ghostarchive.org/varchive/youtube/20211028/SumDHcnCRuU| archive-date=October 28, 2021|language=en|access-date=July 22, 2021}}{{cbignore}}{{efn|In astronomical literature, the term "closest planets" often means "the two planets that approach each other most closely". In other words, the orbits of the two planets approach each other most closely. However, this does not mean that the two planets are closest over a long period of time. For example, essentially because Mercury is closer to the Sun than Venus, Mercury spends more time in proximity to Earth; it could, therefore, be said that Mercury is the planet that is "closest to Earth when averaged over time". However, it turns out that using this time-average definition of 'closeness', Mercury can be the "closest planet" to all other planets in the solar system.}}

The longitude convention for Mercury puts the zero of longitude at one of the two hottest points on the surface, as described above. However, when this area was first visited, by {{nowrap|Mariner 10}}, this zero meridian was in darkness, so it was impossible to select a feature on the surface to define the exact position of the meridian. Therefore, a small crater further west was chosen, called Hun Kal, which provides the exact reference point for measuring longitude.{{cite journal | last=Davies | first=M. E. | title=Surface Coordinates and Cartography of Mercury | journal=Journal of Geophysical Research | volume=80 | issue=B17 | pages=2417–2430 | date=June 10, 1975 | doi=10.1029/JB080i017p02417 | bibcode=1975JGR....80.2417D }}{{cite book | last1=Davies | first1=M. E. | first2=S. E. | last2=Dwornik | first3=D. E. | last3=Gault | first4=R. G. | last4=Strom | title=NASA Atlas of Mercury | publisher=NASA Scientific and Technical Information Office | date=1978 }} The center of Hun Kal defines the 20° west meridian. A 1970 International Astronomical Union resolution suggests that longitudes be measured positively in the westerly direction on Mercury.{{cite web |url=https://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table1.html |access-date=October 22, 2009 |title=USGS Astrogeology: Rotation and pole position for the Sun and planets (IAU WGCCRE) |archive-url=https://web.archive.org/web/20111024101856/http://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table1.html |archive-date=October 24, 2011 |url-status=dead}} The two hottest places on the equator are therefore at longitudes 0° W and 180° W, and the coolest points on the equator are at longitudes 90° W and 270° W. However, the MESSENGER project uses an east-positive convention.{{cite journal |last1=Archinal |first1=Brent A. |display-authors=4 |last2=A'Hearn |first2=Michael F. |last3=Bowell |first3=Edward L. |last4=Conrad |first4=Albert R. |last5=Consolmagno |first5=Guy J. |last6=Courtin |first6=Régis |last7=Fukushima |first7=Toshio |last8=Hestroffer |first8=Daniel |last9=Hilton |first9=James L. |last10=Krasinsky |first10=George A. |last11=Neumann |first11=Gregory A. |last12=Oberst |first12=Jürgen |last13=Seidelmann |first13=P. Kenneth |last14=Stooke |first14=Philip J. |last15=Tholen |first15=David J. |last16=Thomas |first16=Peter C. |last17=Williams |first17=Iwan P. |title=Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009 |journal=Celestial Mechanics and Dynamical Astronomy |volume=109 |issue=2 |year=2010 |pages=101–135 |issn=0923-2958 |doi=10.1007/s10569-010-9320-4 |bibcode=2011CeMDA.109..101A|s2cid=189842666 }}

File:Mercury's orbital resonance.svg

For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and always keeping the same face directed towards the Sun, in the same way that the same side of the Moon always faces Earth. Radar observations in 1965 proved that the planet has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun. The eccentricity of Mercury's orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly stationary in Mercury's sky.{{cite journal |last1=Liu |first1=Han-Shou |last2=O'Keefe |first2=John A. |title=Theory of Rotation for the Planet Mercury |journal=Science |year=1965 |volume=150 |issue=3704 |page=1717 |doi=10.1126/science.150.3704.1717 |pmid=17768871 |bibcode=1965Sci...150.1717L|s2cid=45608770 }}

The 3:2 resonant tidal locking is stabilized by the variance of the tidal force along Mercury's eccentric orbit, acting on a permanent dipole component of Mercury's mass distribution. In a circular orbit there is no such variance, so the only resonance stabilized in such an orbit is at 1:1 (e.g., Earth–Moon), when the tidal force, stretching a body along the "center-body" line, exerts a torque that aligns the body's axis of least inertia (the "longest" axis, and the axis of the aforementioned dipole) to always point at the center. However, with noticeable eccentricity, like that of Mercury's orbit, the tidal force has a maximum at perihelion and therefore stabilizes resonances, like 3:2, ensuring that the planet points its axis of least inertia roughly at the Sun when passing through perihelion.

The original reason astronomers thought it was synchronously locked was that, whenever Mercury was best placed for observation, it was always nearly at the same point in its 3:2 resonance, hence showing the same face. This is because, coincidentally, Mercury's rotation period is almost exactly half of its synodic period with respect to Earth. Due to Mercury's 3:2 spin-orbit resonance, a solar day lasts about 176 Earth days. A sidereal day (the period of rotation) lasts about 58.7 Earth days.

Simulations indicate that the orbital eccentricity of Mercury varies chaotically from nearly zero (circular) to more than 0.45 over millions of years due to perturbations from the other planets.{{cite journal |last1=Correia |first1=Alexandre C. M. |last2=Laskar |first2=Jacques |title=Mercury's capture into the 3/2 spin-orbit resonance including the effect of core–mantle friction |journal=Icarus |year=2009 |doi=10.1016/j.icarus.2008.12.034 |arxiv=0901.1843 |volume=201 |issue=1 |pages=1–11 |bibcode=2009Icar..201....1C|s2cid=14778204 }} This was thought to explain Mercury's 3:2 spin-orbit resonance (rather than the more usual 1:1), because this state is more likely to arise during a period of high eccentricity.{{cite journal |last1=Correia |first1=Alexandre C. M. |last2=Laskar |first2=Jacques |year=2004 |title=Mercury's capture into the 3/2 spin-orbit resonance as a result of its chaotic dynamics |journal=Nature |volume=429 |pages=848–850 |doi=10.1038/nature02609 |pmid=15215857 |issue=6994 |bibcode=2004Natur.429..848C|s2cid=9289925 }} However, accurate modeling based on a realistic model of tidal response has demonstrated that Mercury was captured into the 3:2 spin-orbit state at a very early stage of its history, within 20 (more likely, 10) million years after its formation.{{Cite journal |bibcode=2014Icar..241...26N |last1=Noyelles |first1=B. |last2=Frouard |first2=J. |last3=Makarov |first3=V. V. |last4=Efroimsky |first4=M. |name-list-style=amp |title=Spin-orbit evolution of Mercury revisited |journal=Icarus |pages=26–44 |year=2014 |volume=241 |issue=2014 |doi=10.1016/j.icarus.2014.05.045 |arxiv=1307.0136|s2cid=53690707 }}

Numerical simulations show that a future secular orbital resonant interaction with the perihelion of Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where there is a 1% chance that the orbit will be destabilized in the next five billion years. If this happens, Mercury may fall into the Sun, collide with Venus, be ejected from the Solar System, or even disrupt the rest of the inner Solar System.{{cite journal |last=Laskar |first=Jacques |date=March 18, 2008 |title=Chaotic diffusion in the Solar System |journal=Icarus |volume=196 |issue=1 |pages=1–15 |bibcode=2008Icar..196....1L |doi=10.1016/j.icarus.2008.02.017 |arxiv=0802.3371|s2cid=11586168 }}{{cite journal |last1=Laskar |first1=Jacques |last2=Gastineau |first2=Mickaël |date=June 11, 2009 |title=Existence of collisional trajectories of Mercury, Mars and Venus with the Earth |journal=Nature |volume=459 |issue=7248 |pages=817–819 |doi=10.1038/nature08096 |bibcode=2009Natur.459..817L |pmid=19516336|s2cid=4416436 }}

{{Main|Perihelion precession of Mercury}}

File:Drehung der Apsidenlinie light.svg of Mercury's orbit]]

In 1859, the French mathematician and astronomer Urbain Le Verrier reported that the slow precession of Mercury's orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets. He suggested, among possible explanations, that another planet (or perhaps instead a series of smaller "corpuscules") might exist in an orbit even closer to the Sun than that of Mercury, to account for this perturbation.{{cite journal | last=Le Verrier | first=Urbain | year=1859 | language=French | url=https://archive.org/stream/comptesrendusheb49acad#page/378/mode/2up | title=Lettre de M. Le Verrier à M. Faye sur la théorie de Mercure et sur le mouvement du périhélie de cette planète | journal=Comptes rendus hebdomadaires des séances de l'Académie des sciences | publication-place=Paris | volume=49 | pages=379–383 }} (At p. 383 in the same volume Le Verrier's report is followed by another, from Faye, enthusiastically recommending to astronomers to search for a previously undetected intra-mercurial object.) Other explanations considered included a slight oblateness of the Sun. The success of the search for Neptune based on its perturbations of the orbit of Uranus led astronomers to place faith in this possible explanation, and the hypothetical planet was named Vulcan, but no such planet was ever found.{{cite book |first1=Richard |last1=Baum |last2=Sheehan |first2=William |title=In Search of Planet Vulcan, The Ghost in Newton's Clockwork Machine |date=1997 |isbn=978-0-306-45567-4 |publisher=Plenum Press |location=New York |url-access=registration |url=https://archive.org/details/insearchofplanet0000baum }}

The observed perihelion precession of Mercury is 5,600 arcseconds (1.5556°) per century relative to Earth, or {{val|574.10|0.65|u=arcseconds}} per century{{cite journal |first=Gerald M. |last=Clemence |title=The Relativity Effect in Planetary Motions |journal=Reviews of Modern Physics |volume=19 |issue=4 |pages=361–364 |year=1947 |doi=10.1103/RevModPhys.19.361 |bibcode=1947RvMP...19..361C}} relative to the inertial ICRF. Newtonian mechanics, taking into account all the effects from the other planets and including 0.0254 arcseconds per century due to the oblateness of the Sun, predicts a precession of 5,557 arcseconds (1.5436°) per century relative to Earth, or {{val|531.63|0.69|u=arcseconds}} per century relative to ICRF. In the early 20th century, Albert Einstein's general theory of relativity provided the explanation for the observed precession, by formalizing gravitation as being mediated by the curvature of spacetime. The effect is small: just {{val|42.980|0.001|u=arcseconds}} per century (or 0.43 arcsecond per year, or 0.1035 arcsecond per orbital period) for Mercury; it therefore requires a little over 12.5 million orbits, or 3 million years, for a full excess turn. Similar, but much smaller, effects exist for other Solar System bodies: 8.6247 arcseconds per century for Venus, 3.8387 for Earth, 1.351 for Mars, and 10.05 for 1566 Icarus.{{cite journal |last=Gilvarry |first=John J. |title=Relativity Precession of the Asteroid Icarus |journal=Physical Review |year=1953 |volume=89 |issue=5 |page=1046 |doi=10.1103/PhysRev.89.1046 |bibcode=1953PhRv...89.1046G}}{{cite web |first=Kevin |last=Brown |url=http://www.mathpages.com/rr/s6-02/6-02.htm |title=6.2 Anomalous Precession |website=Reflections on Relativity |publisher=MathPages |access-date=May 22, 2008 |archive-date=August 3, 2019 |archive-url=https://web.archive.org/web/20190803235349/https://www.mathpages.com/rr/s6-02/6-02.htm |url-status=live }}

File:Mercury.jpg

Mercury's apparent magnitude is calculated to vary between −2.48 (brighter than Sirius) around superior conjunction and +7.25 (below the limit of naked-eye visibility) around inferior conjunction. The mean apparent magnitude is 0.23 while the standard deviation of 1.78 is the largest of any planet. The mean apparent magnitude at superior conjunction is −1.89 while that at inferior conjunction is +5.93. Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the Sun's glare for much of the time. Mercury can be observed for only a brief period during either morning or evening twilight.{{cite book |last=Menzel |first=Donald H. |author-link=Donald Howard Menzel |title=A Field Guide to the Stars and Planets |date=1964 |series=The Peterson Field Guide Series |location=Boston |publisher=Houghton Mifflin Co. |pages=292–293}}

Ground-based telescope observations of Mercury reveal only an illuminated partial disk with limited detail. The Hubble Space Telescope cannot observe Mercury at all, due to safety procedures that prevent its pointing too close to the Sun.{{cite journal |last1=Baumgardner |first1=Jeffrey |last2=Mendillo |first2=Michael |last3=Wilson |first3=Jody K. |title=A Digital High-Definition Imaging System for Spectral Studies of Extended Planetary Atmospheres. I. Initial Results in White Light Showing Features on the Hemisphere of Mercury Unimaged by Mariner 10 |journal=The Astronomical Journal |year=2000 |volume=119 |issue=5 |pages=2458–2464 |doi=10.1086/301323 |bibcode=2000AJ....119.2458B|s2cid=17361371 |doi-access=free }} Because the shift of 0.15 revolutions of Earth in a Mercurian year makes up a seven-Mercurian-year cycle (0.15 × 7 ≈ 1.0), in the seventh Mercurian year, Mercury follows almost exactly (earlier by 7 days) the sequence of phenomena it showed seven Mercurian years before.

Like the Moon and Venus, Mercury exhibits phases as seen from Earth. It is "new" at inferior conjunction and "full" at superior conjunction. The planet is rendered invisible from Earth on both of these occasions because of its being obscured by the Sun, except at its new phase during a transit. Mercury is technically brightest as seen from Earth when it is at a full phase. Although Mercury is farthest from Earth when it is full, the greater illuminated area that is visible and the opposition brightness surge more than compensates for the distance. The opposite is true for Venus, which appears brightest when it is a crescent, because it is much closer to Earth than when gibbous.{{cite web | last=Espenak | first=Fred | year=1996 | url=http://eclipse.gsfc.nasa.gov/TYPE/venus2.html | title=NASA Reference Publication 1349; Venus: Twelve year planetary ephemeris, 1995–2006 | website=Twelve Year Planetary Ephemeris Directory | publisher=NASA | access-date=May 24, 2008 | archive-date=August 17, 2000 | archive-url=https://web.archive.org/web/20000817181616/http://sunearth.gsfc.nasa.gov/eclipse/TYPE/venus2.html | url-status=live }}

File:PIA19247-Mercury-NPolarRegion-Messenger20150316.jpg

File:Mercury Venus Moon over San Jose 08 Jan 2024.png with Venus and the Moon.]]

Mercury is best observed at the first and last quarter, although they are phases of lesser brightness. The first and last quarter phases occur at greatest elongation east and west of the Sun, respectively. At both of these times, Mercury's separation from the Sun ranges anywhere from 17.9° at perihelion to 27.8° at aphelion.{{cite web |title=Mercury Chaser's Calculator |publisher=Fourmilab Switzerland |first=John |last=Walker |url=http://www.fourmilab.ch/images/3planets/elongation.html |access-date=May 29, 2008 |archive-date=August 2, 2009 |archive-url=https://web.archive.org/web/20090802110601/http://www.fourmilab.ch/images/3planets/elongation.html |url-status=live }} (look at 1964 and 2013){{cite web |title=Mercury Elongation and Distance |url=http://home.surewest.net/kheider/astro/Mercury.txt |access-date=May 30, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20130511073623/http://home.surewest.net/kheider/astro/Mercury.txt |archive-date=May 11, 2013 }} – Numbers generated using the Solar System Dynamics Group, [http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=1 Horizons On-Line Ephemeris System] {{Webarchive|url=https://web.archive.org/web/20150707172324/http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=1 |date=July 7, 2015 }} At greatest western elongation, Mercury rises at its earliest before sunrise, and at greatest eastern elongation, it sets at its latest after sunset.{{cite book |title=Observer's Handbook 2007 |editor-first=Patrick |editor-last=Kelly |publisher=Royal Astronomical Society of Canada |date=2007 |isbn=978-0-9738109-3-6 |url=https://archive.org/details/observershandboo00raji_0 }}

File:PIA19422-Mercury-CarnegieRupes-MDIS-MLA-20150416.jpg, a tectonic landform—high terrain (red); low (blue).]]

Mercury is more often and easily visible from the Southern Hemisphere than from the Northern. This is because Mercury's maximum western elongation occurs only during early autumn in the Southern Hemisphere, whereas its greatest eastern elongation happens only during late winter in the Southern Hemisphere. In both of these cases, the angle at which the planet's orbit intersects the horizon is maximized, allowing it to rise several hours before sunrise in the former instance and not set until several hours after sundown in the latter from southern mid-latitudes, such as Argentina and South Africa.

An alternate method for viewing Mercury involves observing the planet with a telescope during daylight hours when conditions are clear, ideally when it is at its greatest elongation. This allows the planet to be found easily, even when using telescopes with {{Convert|8|cm|abbr = on}} apertures. However, great care must be taken to obstruct the Sun from sight because of the extreme risk for eye damage.{{cite journal | title=Finding Venus or Mercury in daylight | last=Curtis | first=A. C. | journal=Journal of the British Astronomical Association | volume=82 | pages=438–439 | date=October 1972 | bibcode=1972JBAA...82..438C }} This method bypasses the limitation of twilight observing when the ecliptic is located at a low elevation (e.g. on autumn evenings). The planet is higher in the sky and less atmospheric effects affect the view of the planet. Mercury can be viewed as close as 4° to the Sun near superior conjunction when it is almost at its brightest.

Mercury can, like several other planets and the brightest stars, be seen during a total solar eclipse.{{cite web |date=January 22, 2003 |title=Total Solar Eclipse of 2006 March 29 |publisher=Department of Physics at Fizik Bolumu in Turkey |first=Tunç |last=Tezel |url=http://www.physics.metu.edu.tr/~aat/TSE2006/TSE2006.html |access-date=May 24, 2008 |archive-date=September 12, 2016 |archive-url=https://web.archive.org/web/20160912153809/http://www.physics.metu.edu.tr/~aat/TSE2006/TSE2006.html |url-status=live }}

File:Mercury-bonatti.png

The earliest known recorded observations of Mercury are from the MUL.APIN tablets. These observations were most likely made by an Assyrian astronomer around the 14th century BC.{{cite journal |title=The Latitude and Epoch for the Origin of the Astronomical Lore in MUL.APIN |first=Bradley E. |last=Schaefer |journal=American Astronomical Society Meeting 210, #42.05 |year=2007 |volume=38 |bibcode=2007AAS...210.4205S |page=157}} The cuneiform name used to designate Mercury on the MUL.APIN tablets is transcribed as UDU.IDIM.GU\U₄.UD ("the jumping planet").{{efn|name=Cuneiform MUL}}{{cite journal |first1=Hermann |last1=Hunger |last2=Pingree |first2=David |title=MUL.APIN: An Astronomical Compendium in Cuneiform |journal=Archiv für Orientforschung |volume=24 |year=1989 |page=146}} Babylonian records of Mercury date back to the 1st millennium BC. The Babylonians called the planet Nabu after the messenger to the gods in their mythology.{{cite web |year=2008 |url=http://btc.montana.edu/messenger/elusive_planet/ancient_cultures_2.php |title=MESSENGER: Mercury and Ancient Cultures |publisher=NASA JPL |access-date=April 7, 2008 |archive-date=July 23, 2012 |archive-url=https://web.archive.org/web/20120723001225/http://www.messenger-education.org/elusive_planet/ancient_cultures_2.php |url-status=live }}

The Greco-Egyptian{{Cite book|title=A History of Greek Mathematics|last=Heath|first=Sir Thomas|publisher=Clarendon Press |year=1921|location=Oxford|pages=vii, 273|volume=II|url=https://archive.org/stream/historyofgreekma029268mbp/historyofgreekma029268mbp_djvu.txt}} astronomer Ptolemy wrote about the possibility of planetary transits across the face of the Sun in his work Planetary Hypotheses. He suggested that no transits had been observed either because planets such as Mercury were too small to see, or because transits were too infrequent.{{cite journal |last=Goldstein |first=Bernard R. |title=The Pre-telescopic Treatment of the Phases and Apparent Size of Venus |journal=Journal for the History of Astronomy |page=1 |year=1996 |bibcode=1996JHA....27....1G |volume=27|doi=10.1177/002182869602700101 |s2cid=117218196 }}

File:Shatir500.jpg's model for the appearances of Mercury, showing the multiplication of epicycles using the Tusi couple, thus eliminating the Ptolemaic eccentrics and equant.]]

In ancient China, Mercury was known as "the Hour Star" (Chen-xing {{lang|zh|辰星}}). It was associated with the direction north and the phase of water in the Five Phases system of metaphysics.{{cite book |first1=David H. |last1=Kelley |last2=Milone |first2=E. F. |last3=Aveni |first3=Anthony F. |date=2004 |title=Exploring Ancient Skies: An Encyclopedic Survey of Archaeoastronomy |publisher=Birkhäuser |isbn=978-0-387-95310-6}} Modern Chinese, Korean, Japanese and Vietnamese cultures refer to the planet literally as the "water star" ({{lang|zh|水星}}), based on the Five elements.{{cite book |first=Jan Jakob Maria |last=De Groot |year=1912 |chapter=Religion in China: universism. a key to the study of Taoism and Confucianism |title=American lectures on the history of religions |volume=10 |page=300 |publisher=G. P. Putnam's Sons |chapter-url=https://books.google.com/books?id=ZAaP7dyjCrAC&pg=PA300 |access-date=January 8, 2010 |archive-date=February 26, 2024 |archive-url=https://web.archive.org/web/20240226150305/https://books.google.com/books?id=ZAaP7dyjCrAC&pg=PA300#v=onepage&q&f=false |url-status=live }}{{cite book |first=Thomas |last=Crump |year=1992 |title=The Japanese numbers game: the use and understanding of numbers in modern Japan |pages=39–40 |publisher=Routledge |isbn=978-0-415-05609-0}}{{cite book |first=Homer Bezaleel |last=Hulbert |year=1909 |title=The passing of Korea |page=[https://archive.org/details/passingkorea01hulbgoog/page/n538 426] |publisher=Doubleday, Page & company |url=https://archive.org/details/passingkorea01hulbgoog |access-date=January 8, 2010}} Hindu mythology used the name Budha for Mercury, and this god was thought to preside over Wednesday.{{cite book |first1=R. M. |last1=Pujari |last2=Kolhe |first2=Pradeep |last3=Kumar |first3=N. R. |date=2006 |title=Pride of India: A Glimpse Into India's Scientific Heritage |publisher=Samskrita Bharati |isbn=978-81-87276-27-2}} The god Odin (or Woden) of Germanic paganism was associated with the planet Mercury and Wednesday.{{cite book |first=Michael E. |last=Bakich |date=2000 |title=The Cambridge Planetary Handbook |publisher=Cambridge University Press |isbn=978-0-521-63280-5 |url=https://archive.org/details/cambridgeplaneta00baki }} The Maya may have represented Mercury as an owl (or possibly four owls; two for the morning aspect and two for the evening) that served as a messenger to the underworld.{{cite book |first=Susan |last=Milbrath |date=1999 |title=Star Gods of the Maya: Astronomy in Art, Folklore and Calendars |publisher=University of Texas Press |isbn=978-0-292-75226-9}} Mercury was sometimes known as Stilbon (Greek: Στίλβων) meaning 'the shining, glittering'.{{Cite web |title=Cicero: De Natura Deorum II |url=https://www.thelatinlibrary.com/cicero/nd2.shtml#51 |access-date=2025-02-13 |website=www.thelatinlibrary.com |language=Latin |quote=Huic autem proximum inferiorem orbem tenet PuroeiV, quae stella Martis appellatur, eaque quattuor et viginti mensibus sex, ut opinor, diebus minus eundem lustrat orbem quem duae superiores, infra hanc autem stella Mercuri est (ea Stilbwn appellatur a Graecis), quae anno fere vertenti signiferum lustrat orbem neque a sole longius umquam unius signi intervallo discedit tum antevertens tum subsequens. |trans-quote=But next to this is the lower planet, Puroei, which is called the star of Mars, and it surveys the same planet in twenty-four months and six days, as I believe, less than the two upper planets. Below this is the star of Mercury (it is called Stilbon by the Greeks), which surveys the planet in about a year, and never departs from the sun further than one sign, both in its preceding and following directions.}}

In medieval Islamic astronomy, the Andalusian astronomer Abū Ishāq Ibrāhīm al-Zarqālī in the 11th century described the deferent of Mercury's geocentric orbit as being oval, like an egg or a pignon, although this insight did not influence his astronomical theory or his astronomical calculations.{{cite journal |last1=Samsó |first1=Julio |last2=Mielgo |first2=Honorino |bibcode=1994JHA....25..289S |title=Ibn al-Zarqālluh on Mercury |journal=Journal for the History of Astronomy |volume=25 |issue=4 |year=1994 |pages=289–96 [292]|doi=10.1177/002182869402500403 |s2cid=118108131 }}{{cite journal |first=Willy |last=Hartner |title=The Mercury Horoscope of Marcantonio Michiel of Venice |journal=Vistas in Astronomy |volume=1 |issue=1 |year=1955 |pages=84–138 |bibcode=1955VA......1...84H |doi=10.1016/0083-6656(55)90016-7}} at pp. 118–122. In the 12th century, Ibn Bajjah observed "two planets as black spots on the face of the Sun", which was later suggested as the transit of Mercury and/or Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.{{cite conference |title=History of oriental astronomy: proceedings of the joint discussion-17 at the 23rd General Assembly of the International Astronomical Union, organised by the Commission 41 (History of Astronomy), held in Kyoto, August 25–26, 1997 |first=S. M. Razaullah |last=Ansari |publisher=Springer Science+Business Media |year=2002 |isbn=1-4020-0657-8 |page=137}} Most such medieval reports of transits were later taken as observations of sunspots.{{cite journal |last=Goldstein |first=Bernard R. |year=1969 |title=Some Medieval Reports of Venus and Mercury Transits |journal=Centaurus |volume=14 |issue=1 |pages=49–59 |doi=10.1111/j.1600-0498.1969.tb00135.x |bibcode=1969Cent...14...49G}}

In India, the Kerala school astronomer Nilakantha Somayaji in the 15th century developed a partially heliocentric planetary model in which Mercury orbits the Sun, which in turn orbits Earth, similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century.{{cite journal |last1=Ramasubramanian |first1=K. |last2=Srinivas |first2=M. S. |last3=Sriram |first3=M. S. |title=Modification of the Earlier Indian Planetary Theory by the Kerala Astronomers (c. 1500 AD) and the Implied Heliocentric Picture of Planetary Motion |journal=Current Science |volume=66 |year=1994 |pages=784–790 |url=http://www.physics.iitm.ac.in/~labs/amp/kerala-astronomy.pdf |access-date=April 23, 2010 |archive-url=https://web.archive.org/web/20101223145939/http://www.physics.iitm.ac.in/~labs/amp/kerala-astronomy.pdf |archive-date=December 23, 2010 |url-status=dead }}

The first telescopic observations of Mercury were made by Thomas Harriot and Galileo from 1610. In 1612, Simon Marius observed the brightness of Mercury varied with the planet's orbital position and concluded it had phases "in the same way as Venus and the Moon".{{cite book |last=Gaab |first=Hans |date=2018 |title=Simon Marius and His Research |url=https://books.google.com/books?id=miaeDwAAQBAJ |publisher=Springer |page=256 |isbn=978-3-319-92620-9 |quote=Marius noted in the dedication from June 30, 1612, in the Prognosticon auf 1613 "that Mercury is illuminated by the Sun in the same way as the Venus and the Moon" and reports his observations of the brightness. |access-date=March 22, 2023 |archive-date=April 11, 2023 |archive-url=https://web.archive.org/web/20230411141118/https://books.google.com/books?id=miaeDwAAQBAJ |url-status=live }} In 1631, Pierre Gassendi made the first telescopic observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by Johannes Kepler. In 1639, Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited the Sun.

A rare event in astronomy is the passage of one planet in front of another (occultation), as seen from Earth. Mercury and Venus occult each other every few centuries, and the event of May 28, 1737, is the only one historically observed, having been seen by John Bevis at the Royal Greenwich Observatory.{{cite journal |last1=Sinnott |first1=Roger W. |author-link=Roger W. Sinnott |last2=Meeus |first2=Jean |author-link2=Jean Meeus |year=1986 |title=John Bevis and a Rare Occultation |journal=Sky and Telescope |volume=72 |page=220 |bibcode=1986S&T....72..220S}} The next occultation of Mercury by Venus will be on December 3, 2133.{{cite book |first=Timothy |last=Ferris |date=2003 |title=Seeing in the Dark: How Amateur Astronomers |publisher=Simon and Schuster |isbn=978-0-684-86580-5}}

The difficulties inherent in observing Mercury meant that it was far less studied than the other planets. In 1800, Johann Schröter made observations of surface features, claiming to have observed {{Convert|20|km|4 = -high|adj = mid}} mountains. Friedrich Bessel used Schröter's drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°.{{cite journal |last1=Colombo |first1=Giuseppe |author-link1=Giuseppe Colombo |last2=Shapiro |first2=Irwin I. |author-link2=Irwin I. Shapiro |title=The Rotation of the Planet Mercury |journal=SAO Special Report #188R |bibcode=1965SAOSR.188.....C |volume=188 |pages=188 |date=November 1965}} In the 1880s, Giovanni Schiaparelli mapped the planet more accurately, and suggested that Mercury's rotational period was 88 days, the same as its orbital period due to tidal locking.{{cite journal |last=Holden |first=Edward S. |year=1890 |title=Announcement of the Discovery of the Rotation Period of Mercury [by Professor Schiaparelli] |journal=Publications of the Astronomical Society of the Pacific |volume=2 |issue=7 |page=79 |bibcode=1890PASP....2...79H |doi=10.1086/120099|s2cid=122095054 |doi-access=free }} This phenomenon is known as synchronous rotation. The effort to map the surface of Mercury was continued by Eugenios Antoniadi, who published a book in 1934 that included both maps and his own observations. Many of the planet's surface features, particularly the albedo features, take their names from Antoniadi's map.{{cite book | url=https://history.nasa.gov/SP-423/sp423.htm | title=Atlas of Mercury | publisher=NASA Office of Space Sciences | first1=Merton E. | last1=Davies | first2=Stephen E. | last2=Dwornik | first3=Donald E. | last3=Gault | first4=Robert G. | last4=Strom | date=1978 | chapter=Surface Mapping | chapter-url=https://history.nasa.gov/SP-423/surface.htm | access-date=May 28, 2008 | archive-date=October 9, 2019 | archive-url=https://web.archive.org/web/20191009033507/https://history.nasa.gov/SP-423/sp423.htm | url-status=live }}

In June 1962, Soviet scientists at the Institute of Radio-engineering and Electronics of the USSR Academy of Sciences, led by Vladimir Kotelnikov, became the first to bounce a radar signal off Mercury and receive it, starting radar observations of the planet.{{cite journal |first1=John V. |last1=Evans |author-link=John V. Evans (astronomer) |last2=Brockelman |first2=Richard A. |last3=Henry |first3=John C. |last4=Hyde |first4=Gerald M. |last5=Kraft |first5=Leon G. |last6=Reid |first6=Wyatt A. |last7=Smith |first7=W. W. |title=Radio Echo Observations of Venus and Mercury at 23 cm Wavelength |year=1965 |journal=Astronomical Journal |volume=70 |bibcode=1965AJ.....70..486E |pages=487–500 |doi=10.1086/109772}}{{cite book |last=Moore |first=Patrick |title=The Data Book of Astronomy |page=483 |date=2000 |publisher=CRC Press |location=New York |url=https://books.google.com/books?id=fDDpBwAAQBAJ |isbn=978-0-7503-0620-1 |access-date=February 27, 2018 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162216/https://books.google.com/books?id=fDDpBwAAQBAJ |url-status=live }}{{cite book |title=To See the Unseen: A History of Planetary Radar Astronomy |url=https://archive.org/details/toseeunseenhis00butr |first=Andrew J. |last=Butrica |publisher=NASA History Office, Washington D.C. |date=1996 |chapter=Chapter 5 |chapter-url=https://history.nasa.gov/SP-4218/ch5.htm |isbn=978-0-16-048578-7 }} Three years later, radar observations by Americans Gordon H. Pettengill and Rolf B. Dyce, using the {{Convert|300|m|yd|4=-wide|adj=mid}} Arecibo radio telescope in Puerto Rico, showed conclusively that the planet's rotational period was about 59 days.{{cite journal |last1=Pettengill |first1=Gordon H. |last2=Dyce |first2=Rolf B. |title=A Radar Determination of the Rotation of the Planet Mercury |journal=Nature |volume=206 |issue=1240 |pages=451–2 |year=1965 |doi=10.1038/2061240a0 |bibcode=1965Natur.206Q1240P|s2cid=31525579 }}{{cite web | url=http://scienceworld.wolfram.com/astronomy/Mercury.html | title=Mercury | website=Eric Weisstein's World of Astronomy | publisher=Wolfram Research | access-date=April 18, 2021 | archive-date=November 6, 2015 | archive-url=https://web.archive.org/web/20151106173823/http://scienceworld.wolfram.com/astronomy/Mercury.html | url-status=live }} The theory that Mercury's rotation was synchronous had become widely held, and it was a surprise to astronomers when these radio observations were announced. If Mercury were tidally locked, its dark face would be extremely cold, but measurements of radio emission revealed that it was much hotter than expected. Astronomers were reluctant to drop the synchronous rotation theory and proposed alternative mechanisms such as powerful heat-distributing winds to explain the observations.{{cite book |first1=Bruce C. |last1=Murray |last2=Burgess |first2=Eric |date=1977 |title=Flight to Mercury |publisher=Columbia University Press |isbn=978-0-231-03996-3 |url=https://archive.org/details/flighttomercury00bruc }}

In 1965, Italian astronomer Giuseppe Colombo noted that the rotation value was about two-thirds of Mercury's orbital period, and proposed that the planet's orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance.{{cite journal |last=Colombo |first=Giuseppe |title=Rotational Period of the Planet Mercury |journal=Nature |volume=208 |issue=5010 |page=575 |year=1965 |doi=10.1038/208575a0 |bibcode=1965Natur.208..575C|s2cid=4213296 |doi-access=free }} Data from {{nowrap|Mariner 10}} subsequently confirmed this view.{{cite web |year=1976 |last=Davies |first=Merton E. |display-authors=etal |url=https://history.nasa.gov/SP-423/mariner.htm |title=Mariner 10 Mission and Spacecraft |website=SP-423 Atlas of Mercury |publisher=NASA JPL |access-date=April 7, 2008 |archive-date=June 24, 2012 |archive-url=https://web.archive.org/web/20120624000448/http://history.nasa.gov/SP-423/mariner.htm |url-status=live }} This means that Schiaparelli's and Antoniadi's maps were not "wrong". Instead, the astronomers saw the same features during every second orbit and recorded them, but disregarded those seen in the meantime, when Mercury's other face was toward the Sun, because the orbital geometry meant that these observations were made under poor viewing conditions.

Ground-based optical observations did not shed much further light on Mercury, but radio astronomers using interferometry at microwave wavelengths, a technique that enables removal of the solar radiation, were able to discern physical and chemical characteristics of the subsurface layers to a depth of several meters.{{cite thesis | last=Golden | first=Leslie M. | title=A Microwave Interferometric Study of the Subsurface of the Planet Mercury |year=1977 | publisher=University of California, Berkeley | bibcode=1977PhDT.........9G }}{{cite journal |last1=Mitchell |first1=David L. |last2=De Pater |first2=Imke |title=Microwave Imaging of Mercury's Thermal Emission at Wavelengths from 0.3 to 20.5 cm (1994) |journal=Icarus |volume=110 |issue=1 |pages=2–32 |doi=10.1006/icar.1994.1105 |bibcode=1994Icar..110....2M |year=1994}} Not until the first space probe flew past Mercury did many of its most fundamental morphological properties become known. Moreover, technological advances have led to improved ground-based observations. In 2000, high-resolution lucky imaging observations were conducted by the Mount Wilson Observatory {{Convert|1.5|m|ft|abbr=out|adj=mid}} Hale telescope. They provided the first views that resolved surface features on the parts of Mercury that were not imaged in the {{nowrap|Mariner 10}} mission.{{cite journal |last1=Dantowitz |first1=Ronald F. |last2=Teare |first2=Scott W. |last3=Kozubal |first3=Marek J. |title=Ground-based High-Resolution Imaging of Mercury |journal=Astronomical Journal |volume=119 |issue=4 |pages=2455–2457 |year=2000 |bibcode=2000AJ....119.2455D |doi=10.1086/301328|s2cid=121483006 |doi-access=free }} Most of the planet has been mapped by the Arecibo radar telescope, with {{Convert|5|km|abbr = on|adj = on}} resolution, including polar deposits in shadowed craters of what may be water ice.{{cite journal |title=Mercury: Radar images of the equatorial and midlatitude zones |journal=Icarus |volume=187 |issue=2 |pages=374–405 |year=2007 |bibcode=2007Icar..187..374H |doi=10.1016/j.icarus.2006.09.026 |last1=Harmon |first1=John K. |first2=Martin A. |last2=Slade |first3=Bryan J. |last3=Butler |first4=James W. |last4=Head III |first5=Melissa S. |last5=Rice |first6=Donald B. |last6=Campbell}}

File:Transit Of Mercury, May 9th, 2016.png|Transit of Mercury. Mercury is visible as a black dot below and to the left of center. The dark area above the center of the solar disk is a sunspot.

File:Planet Elongation.jpg|Elongation is the angle between the Sun and the planet, with Earth as the reference point. Mercury appears close to the Sun.

File:PIA19411-Mercury-WaterIce-Radar-MDIS-Messenger-20150416.jpg|Water ice (yellow) at Mercury's north polar region

{{Main|Exploration of Mercury}}

File:MESSENGER Assembly.jpg

File:PIA18389-MarsCuriosityRover-MercuryTransitsSun-20140603.gif as viewed by the Mars rover Curiosity (June 3, 2014).{{cite web |last=Webster |first=Guy |title=Mercury Passes in Front of the Sun, as Seen From Mars |url=http://www.jpl.nasa.gov/news/news.php?release=2014-183 |date=June 10, 2014 |website=NASA |access-date=June 10, 2014 |archive-date=February 15, 2020 |archive-url=https://web.archive.org/web/20200215091810/https://www.jpl.nasa.gov/news/news.php?release=2014-183 |url-status=live }}]]

Reaching Mercury from Earth poses significant technical challenges, because it orbits so much closer to the Sun than Earth. A Mercury-bound spacecraft launched from Earth must travel over {{Convert|91|e6km|e6mi|abbr = off}} into the Sun's gravitational potential well. Mercury has an orbital speed of {{Convert|47.4|km/s|abbr = on}}, whereas Earth's orbital speed is {{Convert|29.8|km/s|abbr = on}}.{{cite web | first=David R. | last=Williams | date=October 21, 2019 | title=Planetary Fact Sheet – Metric | publisher=NASA | url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/ | access-date=April 20, 2021 | archive-date=July 19, 2012 | archive-url=https://archive.today/20120719082605/http://nssdc.gsfc.nasa.gov/planetary/factsheet/ | url-status=live }} Therefore, the spacecraft must make a larger change in velocity (delta-v) to get to Mercury and then enter orbit,{{cite book | title=Inner Solar System: Prospective Energy and Material Resources | first=Kris | last=Zacny | isbn=9783319195698 | date=July 2, 2015 | publisher=Springer International Publishing | page=154 | url=https://books.google.com/books?id=ZrAYCgAAQBAJ&pg=PA154 | access-date=March 19, 2023 | archive-date=April 11, 2023 | archive-url=https://web.archive.org/web/20230411141121/https://books.google.com/books?id=ZrAYCgAAQBAJ&pg=PA154 | url-status=live }} as compared to the delta-v required for, say, Mars planetary missions.

The potential energy liberated by moving down the Sun's potential well becomes kinetic energy, requiring a delta-v change to do anything other than pass by Mercury. Some portion of this delta-v budget can be provided from a gravity assist during one or more fly-bys of Venus.{{cite journal | title=Hybrid Algorithm for Multiple Gravity-Assist and Impulsive Delta-V Maneuvers | first1=Sam | last1=Wagner | first2=Bong | last2=Wie | journal=Journal of Guidance, Control, and Dynamics | volume=38 | issue=11 | pages=2096–2107 | date=November 2015 | bibcode=2015JGCD...38.2096W | doi=10.2514/1.G000874 }} To land safely or enter a stable orbit the spacecraft would rely entirely on rocket motors. Aerobraking is ruled out because Mercury has a negligible atmosphere. A trip to Mercury requires more rocket fuel than that required to escape the Solar System completely. As a result, only three space probes have visited it so far.{{cite web | url=https://www.jpl.nasa.gov/education/images/pdf/ss-mercury.pdf | title=Mercury | publisher=NASA Jet Propulsion Laboratory | date=May 5, 2008 | access-date=April 26, 2021 | archive-date=February 9, 2017 | archive-url=https://web.archive.org/web/20170209135719/http://www.jpl.nasa.gov/education/images/pdf/ss-mercury.pdf | url-status=live }} A proposed alternative approach would use a solar sail to attain a Mercury-synchronous orbit around the Sun.{{cite journal |last1=Leipold |first1=Manfred E. |last2=Seboldt |first2=W. |last3=Lingner |first3=Stephan |last4=Borg |first4=Erik |last5=Herrmann |first5=Axel Siegfried |last6=Pabsch |first6=Arno |last7=Wagner |first7=O. |last8=Brückner |first8=Johannes |title=Mercury sun-synchronous polar orbiter with a solar sail |year=1996 |journal=Acta Astronautica |volume=39 |issue=1 |pages=143–151 |doi=10.1016/S0094-5765(96)00131-2 |bibcode=1996AcAau..39..143L}}

{{clear}}

{{Main|Mariner 10}}

File:Mariner 10.jpg

The first spacecraft to visit Mercury was NASA's {{nowrap|Mariner 10}} (1974–1975). The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury, making it both the first spacecraft to use this gravitational "slingshot" effect and the first NASA mission to visit multiple planets.{{cite book |title=The Voyage of Mariner 10 – Mission to Venus and Mercury |last1=Dunne |first1=James A. |last2=Burgess |first2=Eric |name-list-style=amp |chapter-url=https://history.nasa.gov/SP-424/ch4.htm |publisher=NASA History Office |date=1978 |chapter=Chapter Four |url=https://history.nasa.gov/SP-424/ |access-date=May 28, 2008 |archive-date=November 17, 2017 |archive-url=https://web.archive.org/web/20171117190025/https://history.nasa.gov/SP-424/ |url-status=dead }} {{nowrap|Mariner 10}} provided the first close-up images of Mercury's surface, which immediately showed its heavily cratered nature, and revealed many other types of geological features, such as the giant scarps that were later ascribed to the effect of the planet shrinking slightly as its iron core cools.{{cite web |date=October 1976 |first=Tony |last=Phillips |url=http://www.nasa.gov/vision/universe/solarsystem/20oct_transitofmercury.html |title=NASA 2006 Transit of Mercury |website=SP-423 Atlas of Mercury |publisher=NASA |access-date=April 7, 2008 |archive-date=March 25, 2008 |archive-url=https://web.archive.org/web/20080325090336/http://www.nasa.gov/vision/universe/solarsystem/20oct_transitofmercury.html |url-status=dead }} Unfortunately, the same face of the planet was lit at each of {{nowrap|Mariner 10{{'s}}}} close approaches. This made close observation of both sides of the planet impossible,{{cite web |url=http://sci.esa.int/bepicolombo/56015-missions-to-mercury/ |title=BepiColumbo – Background Science |publisher=European Space Agency |access-date=June 18, 2017 |archive-date=July 1, 2017 |archive-url=https://web.archive.org/web/20170701093446/http://sci.esa.int/bepicolombo/56015-missions-to-mercury/ |url-status=live }} and resulted in the mapping of less than 45% of the planet's surface.{{cite news |url=https://www.usatoday.com/tech/news/2004-08-16-mercury-may-shrink_x.htm |title=MESSENGER to test theory of shrinking Mercury |newspaper=USA Today |first=Tariq |last=Malik |date=August 16, 2004 |access-date=May 23, 2008 |archive-date=July 26, 2011 |archive-url=https://web.archive.org/web/20110726231423/http://www.usatoday.com/tech/news/2004-08-16-mercury-may-shrink_x.htm |url-status=live }}

The spacecraft made three close approaches to Mercury, the closest of which took it to within {{Convert|327|km|abbr=on}} of the surface.{{cite book |url=https://history.nasa.gov/SP-423/sp423.htm |title=Atlas of Mercury |publisher=NASA Office of Space Sciences |first=Merton E. |last=Davies |display-authors=etal |date=1978 |chapter=Mariner 10 Mission and Spacecraft |chapter-url=https://history.nasa.gov/SP-423/mariner.htm |access-date=May 30, 2008 |name-list-style=vanc |archive-date=October 9, 2019 |archive-url=https://web.archive.org/web/20191009033507/https://history.nasa.gov/SP-423/sp423.htm |url-status=live }} At the first close approach, instruments detected a magnetic field, to the great surprise of planetary geologists—Mercury's rotation was expected to be much too slow to generate a significant dynamo effect. The second close approach was primarily used for imaging, but at the third approach, extensive magnetic data were obtained. The data revealed that the planet's magnetic field is much like Earth's, which deflects the solar wind around the planet. For many years after the {{nowrap|Mariner 10}} encounters, the origin of Mercury's magnetic field remained the subject of several competing theories.{{cite journal |last=Ness |first=Norman F. |year=1978 |title=Mercury – Magnetic field and interior |journal=Space Science Reviews |volume=21 |issue=5 |pages=527–553 |bibcode=1978SSRv...21..527N |doi=10.1007/BF00240907|s2cid=120025983 }}{{cite journal |last1=Aharonson |first1=Oded |last2=Zuber |first2=Maria T |last3=Solomon |first3=Sean C |title=Crustal remanence in an internally magnetized non-uniform shell: a possible source for Mercury's magnetic field? |journal=Earth and Planetary Science Letters |year=2004 |volume=218 |issue=3–4 |pages=261–268 |doi=10.1016/S0012-821X(03)00682-4 |bibcode=2004E&PSL.218..261A}}

On March 24, 1975, just eight days after its final close approach, {{nowrap|Mariner 10}} ran out of fuel. Because its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut down.{{cite book |title=The Voyage of Mariner 10 – Mission to Venus and Mercury |last1=Dunne |first1=James A. |last2=Burgess |first2=Eric |name-list-style=amp |chapter-url=https://history.nasa.gov/SP-424/ch8.htm |publisher=NASA History Office |date=1978 |chapter=Chapter Eight |url=https://history.nasa.gov/SP-424/ |access-date=July 12, 2017 |archive-date=November 17, 2017 |archive-url=https://web.archive.org/web/20171117190025/https://history.nasa.gov/SP-424/ |url-status=dead }} {{nowrap|Mariner 10}} is thought to be still orbiting the Sun, passing close to Mercury every few months.{{cite web |date=April 2, 2008 |first=Ed |last=Grayzeck |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1973-085A |title=Mariner 10 |website=NSSDC Master Catalog |publisher=NASA |access-date=April 7, 2008 |archive-date=September 8, 2018 |archive-url=https://web.archive.org/web/20180908063158/https://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1973-085A |url-status=live }}

{{Main|MESSENGER}}

File:Details of MESSENGER's Impact Location.jpg

A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004. It made a fly-by of Earth in August 2005, and of Venus in October 2006 and June 2007 to place it onto the correct trajectory to reach an orbit around Mercury.{{cite web |year=2005 |url=http://www.spaceref.com/news/viewsr.html?pid=18956 |title=MESSENGER Engine Burn Puts Spacecraft on Track for Venus |publisher=SpaceRef.com |access-date=March 2, 2006}} A first fly-by of Mercury occurred on January 14, 2008, a second on October 6, 2008,{{cite web |url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=115 |title=Countdown to MESSENGER's Closest Approach with Mercury |date=January 14, 2008 |publisher=Johns Hopkins University Applied Physics Laboratory |access-date=May 30, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20130513080731/http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=115 |archive-date=May 13, 2013}} and a third on September 29, 2009.{{cite web |title=MESSENGER Gains Critical Gravity Assist for Mercury Orbital Observations |url=http://messenger.jhuapl.edu/news_room/details.php?id=136 |date=September 30, 2009 |publisher=MESSENGER Mission News |access-date=September 30, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20130510175510/http://messenger.jhuapl.edu/news_room/details.php?id=136 |archive-date=May 10, 2013}} Most of the hemisphere not imaged by {{nowrap|Mariner 10}} was mapped during these fly-bys. The probe successfully entered an elliptical orbit around the planet on March 18, 2011. The first orbital image of Mercury was obtained on March 29, 2011. The probe finished a one-year mapping mission, and then entered a one-year extended mission into 2013. In addition to continued observations and mapping of Mercury, MESSENGER observed the 2012 solar maximum.{{cite news | url=http://www.upi.com/Top_News/US/2011/11/15/NASA-extends-spacecrafts-Mercury-mission/UPI-55131321408343/ | title=NASA extends spacecraft's Mercury mission | work=United Press International | date=November 15, 2011 | access-date=November 16, 2011 | archive-date=May 31, 2013 | archive-url=https://web.archive.org/web/20130531103344/http://www.upi.com/Top_News/US/2011/11/15/NASA-extends-spacecrafts-Mercury-mission/UPI-55131321408343/ | url-status=live }}

File:MercuryTopo.png The mission was designed to clear up six key issues: Mercury's high density, its geological history, the nature of its magnetic field, the structure of its core, whether it has ice at its poles, and where its tenuous atmosphere comes from. To this end, the probe carried imaging devices that gathered much-higher-resolution images of much more of Mercury than {{nowrap|Mariner 10}}, assorted spectrometers to determine the abundances of elements in the crust, and magnetometers and devices to measure velocities of charged particles. Measurements of changes in the probe's orbital velocity were expected to be used to infer details of the planet's interior structure.{{cite web |url=http://messenger.jhuapl.edu/Resources/Resources/MESSENGER_FS22811_PRESS.PDF |title=MESSENGER: Fact Sheet |publisher=Applied Physics Laboratory |date=February 2011 |access-date=August 21, 2017 |archive-date=August 22, 2017 |archive-url=https://web.archive.org/web/20170822012453/http://messenger.jhuapl.edu/Resources/Resources/MESSENGER_FS22811_PRESS.PDF |url-status=live }} MESSENGER{{'s}} final maneuver was on April 24, 2015, and it crashed into Mercury's surface on April 30, 2015.{{cite news |last=Wall |first=Mike |url=http://www.space.com/28948-messenger-mercury-probe-final-days.html |title=NASA Mercury Probe Trying to Survive for Another Month |work=Space.com |date=March 29, 2015 |access-date=April 4, 2015 |archive-date=April 3, 2019 |archive-url=https://web.archive.org/web/20190403102620/https://www.space.com/28948-messenger-mercury-probe-final-days.html |url-status=live }}{{cite news |last=Chang |first=Kenneth |title=NASA's Messenger Mission Is Set to Crash Into Mercury |url=https://www.nytimes.com/2015/04/28/science/nasas-messenger-mission-is-set-to-crash-into-mercury.html |archive-url=https://web.archive.org/web/20150429030725/http://www.nytimes.com/2015/04/28/science/nasas-messenger-mission-is-set-to-crash-into-mercury.html |archive-date=April 29, 2015 |url-access=subscription |url-status=live |date=April 27, 2015 |work=The New York Times |access-date=April 27, 2015}}{{cite news |last=Corum |first=Jonathan |title=Messenger's Collision Course With Mercury |url=https://www.nytimes.com/interactive/2015/04/30/science/space/messenger-collides-with-mercury.html |date=April 30, 2015 |work=The New York Times |access-date=April 30, 2015 |archive-date=March 31, 2019 |archive-url=https://web.archive.org/web/20190331205609/https://www.nytimes.com/interactive/2015/04/30/science/space/messenger-collides-with-mercury.html |url-status=live }} The spacecraft's impact with Mercury occurred at 3:26:01 p.m. EDT on April 30, 2015, leaving a crater estimated to be {{convert|16|m|ft|abbr=on}} in diameter.{{cite web |date=June 3, 2015 |title=Best Determination of MESSENGER's Impact Location |url=https://messenger.jhuapl.edu/Explore/Science-Images-Database/gallery-image-1605.html |archive-url=https://web.archive.org/web/20230411141108/https://messenger.jhuapl.edu/Explore/Science-Images-Database/gallery-image-1605.html |archive-date=April 11, 2023 |access-date=October 6, 2023 |website=messenger.jhuapl.edu |publisher=Johns Hopkins Applied Physics Laboratory}}

{{Main|BepiColombo}}

The European Space Agency and the Japanese Space Agency developed and launched a joint mission called BepiColombo, which will orbit Mercury with two probes: one to map the planet and the other to study its magnetosphere.{{cite web |title=ESA gives go-ahead to build BepiColombo |date=February 26, 2007 |publisher=European Space Agency |url=http://www.esa.int/esaSC/SEMC8XBE8YE_index_0.html |access-date=May 29, 2008 |archive-date=March 31, 2008 |archive-url=https://web.archive.org/web/20080331042423/http://www.esa.int/esaSC/SEMC8XBE8YE_index_0.html |url-status=live }} Launched on October 20, 2018, BepiColombo is expected to reach Mercury in 2025.{{cite web |title=BepiColombo Fact Sheet |url=http://sci.esa.int/bepicolombo/47346-fact-sheet/ |publisher=European Space Agency |access-date=December 19, 2016 |date=December 1, 2016 |archive-date=May 20, 2016 |archive-url=http://arquivo.pt/wayback/20160520023104/http://sci.esa.int/bepicolombo/47346%2Dfact%2Dsheet/ |url-status=live }} It will release a magnetometer probe into an elliptical orbit, then chemical rockets will fire to deposit the mapper probe into a circular orbit. Both probes will operate for one terrestrial year. The mapper probe carries an array of spectrometers similar to those on MESSENGER, and will study the planet at many different wavelengths including infrared, ultraviolet, X-ray and gamma ray.{{cite web |title=Objectives |publisher=European Space Agency |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31350 |date=February 21, 2006 |access-date=May 29, 2008 |archive-date=September 28, 2006 |archive-url=https://web.archive.org/web/20060928055507/http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31350 |url-status=live }} BepiColombo conducted the first of its six planned Mercury flybys on October 1, 2021,{{cite web|url=https://www.nasaspaceflight.com/2021/10/bepicolombo-first-mercury-flyby/|title=BepiColombo completes first Mercury flyby, science provides insight into planet's unique environment|website=NASA Spaceflight|date=October 24, 2021|accessdate=October 8, 2022|last1=Warren|first1=Haygen|archive-date=October 8, 2022|archive-url=https://web.archive.org/web/20221008121521/https://www.nasaspaceflight.com/2021/10/bepicolombo-first-mercury-flyby/|url-status=live}} and the sixth was completed on January 9, 2025. The spacecraft will enter the planet's orbit in 2026.{{cite web |last=Fullbrook |first=Danny |date=January 9, 2025 |title=New Images of Mercury Captured by UK Spacecraft |website=BBC |url=https://www.bbc.com/news/articles/cx2v2r1jm7go |access-date=January 10, 2025 }}

{{div col|colwidth=20em}}

• Astronomy on Mercury
• Colonization of Mercury
• Mercury in astrology
• Mercury in fiction
• Outline of Mercury (planet)

{{div col end}}

{{reflist |group=lower-alpha |refs=

{{efn |name=Cuneiform MUL

| Some sources precede the cuneiform transcription with "MUL". "MUL" is a cuneiform sign that was used in the Sumerian language to designate a star or planet, but it is not considered part of the actual name. The "4" is a reference number in the Sumero–Akkadian transliteration system to designate which of several syllables a certain cuneiform sign is most likely designating.

}}

{{efn |name=angular

| The Sun's total angular displacement during its apparent retrograde motion as seen from the surface of Mercury is ~1.23°, while the Sun's angular diameter when the apparent retrograde motion begins and ends is ~1.71°, increasing to ~1.73° at perihelion (midway through the retrograde motion).

}}

}}

{{clear}}

{{reflist |colwidth=30em |refs=

{{cite news |last=Ritzel |first=Rebecca |title=Ballet isn't rocket science, but the two aren't mutually exclusive, either |newspaper=The Washington Post |location=Washington, D.C., United States |date=December 20, 2012 |url=https://www.washingtonpost.com/entertainment/theater_dance/ballet-isnt-rocket-science-but-the-twoarentmutually-exclusive-either/2012/12/20/83fae9d6-3e2b-11e2-ae43-cf491b837f7b_story.html |access-date=December 22, 2012 |archive-date=December 23, 2012 |archive-url=https://web.archive.org/web/20121223125341/http://www.washingtonpost.com/entertainment/theater_dance/ballet-isnt-rocket-science-but-the-twoarentmutually-exclusive-either/2012/12/20/83fae9d6-3e2b-11e2-ae43-cf491b837f7b_story.html |url-status=live }}

{{cite web |date=April 7, 2008 |first=Donald K. |last=Yeomans |url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%271%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%272000-01-01%27&STOP_TIME=%272000-01-02%27&STEP_SIZE=%27200%20years%27&CENTER=%27@Sun%27&OUT_UNITS=%27AU-D%27 |title=HORIZONS Web-Interface for Mercury Major Body |publisher=JPL Horizons On-Line Ephemeris System |access-date=April 7, 2008 |archive-date=August 18, 2023 |archive-url=https://web.archive.org/web/20230818120425/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%271%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%272000-01-01%27&STOP_TIME=%272000-01-02%27&STEP_SIZE=%27200%20years%27&CENTER=%27%40Sun%27&OUT_UNITS=%27AU-D%27 |url-status=live }} – Select "Ephemeris Type: Orbital Elements", "Time Span: 2000-01-01 12:00 to 2000-01-02". ("Target Body: Mercury" and "Center: Sun" should be defaulted to.) Results are instantaneous osculating values at the precise J2000 epoch.

{{cite journal

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{{cite journal |title=Collisional stripping of Mercury's mantle |last1=Benz |first1=W. |last2=Slattery |first2=W. L. |last3=Cameron |first3=Alastair G. W. |journal=Icarus |volume=74 |issue=3 |pages=516–528 |year=1988 |doi=10.1016/0019-1035(88)90118-2 |bibcode=1988Icar...74..516B |url=https://zenodo.org/record/1253898 |access-date=August 25, 2019 |archive-date=September 5, 2019 |archive-url=https://web.archive.org/web/20190905034903/https://zenodo.org/record/1253898 |url-status=live }}

{{cite journal |first=Jefferson |last=Morris |date=November 10, 2008 |title=Laser Altimetry |journal=Aviation Week & Space Technology |volume=169 |issue=18 |page=18 |quote=Mercury's crust is more analogous to a marbled cake than a layered cake.}}

{{cite conference |last1=Wagner |first1=Roland J. |last2=Wolf |first2=Ursula |last3=Ivanov |first3=Boris A. |last4=Neukum |first4=Gerhard |title=Application of an Updated Impact Cratering Chronology Model to Mercury' s Time-Stratigraphic System |work=Workshop on Mercury: Space Environment, Surface, and Interior. Proceedings of a workshop held at The Field Museum. |date=October 4–5, 2001 |location=Chicago, IL |publisher=Lunar and Planetary Science Institute |page=106 |bibcode=2001mses.conf..106W}}

{{cite book |last=Biswas |first=Sukumar |date=2000 |title=Cosmic Perspectives in Space Physics |publisher=Springer |series=Astrophysics and Space Science Library |page=176 |isbn=978-0-7923-5813-8}}

{{cite arXiv|title=The spherical bolometric albedo for planet Mercury |first1=Anthony |last1=Mallama |year=2017 |eprint=1703.02670 |class=astro-ph.EP }}

{{cite journal |last1=Mallama |first1=Anthony |last2=Wang |first2=Dennis |last3=Howard |first3=Russell A. |title=Photometry of Mercury from SOHO/LASCO and Earth |journal=Icarus |volume=155 |issue=2 |pages=253–264 |year=2002 |doi=10.1006/icar.2001.6723 |bibcode=2002Icar..155..253M}}

{{cite journal |last=Mallama |first=Anthony |title=Planetary magnitudes |journal=Sky and Telescope |volume=121 |issue=1 |pages=51–56 |year=2011}}

{{cite journal |last1=Margot |first1=Jean-Luc |last2=Peale |first2=Stanton J. |last3=Solomon |first3=Sean C. |last4=Hauck |first4=Steven A. |last5=Ghigo |first5=Frank D. |last6=Jurgens |first6=Raymond F. |last7=Yseboodt |first7=Marie |last8=Giorgini |first8=Jon D. |last9=Padovan |first9=Sebastiano|last10=Campbell|first10=Donald B. |title=Mercury's moment of inertia from spin and gravity data |journal=Journal of Geophysical Research: Planets |volume=117 |issue=E12 |year=2012 |pages=n/a |issn=0148-0227 |doi=10.1029/2012JE004161 |bibcode=2012JGRE..117.0L09M|citeseerx=10.1.1.676.5383 |s2cid=22408219 }}

{{cite journal |last1=Mazarico |first1=Erwan |last2=Genova |first2=Antonio |last3=Goossens |first3=Sander |last4=Lemoine |first4=Frank G. |last5=Neumann |first5=Gregory A. |last6=Zuber |first6=Maria T. |last7=Smith |first7=David E. |last8=Solomon |first8=Sean C. |title=The gravity field, orientation, and ephemeris of Mercury from MESSENGER observations after three years in orbit |journal=Journal of Geophysical Research: Planets |volume=119 |issue=12 |year=2014 |pages=2417–2436 |issn=2169-9097 |doi=10.1002/2014JE004675 |hdl=1721.1/97927 |bibcode=2014JGRE..119.2417M |s2cid=42430050 |url=https://iris.uniroma1.it/bitstream/11573/1242426/2/Mazarico_the-gravity_2014.pdf |access-date=August 25, 2019 |archive-date=September 29, 2021 |archive-url=https://web.archive.org/web/20210929073931/https://iris.uniroma1.it/retrieve/handle/11573/1242426/1160748/Mazarico_the-gravity_2014.pdf |url-status=live }}

{{cite journal |doi=10.1002/2014GL062487 |title=Thickness of the crust of Mercury from geoid-to-topography ratios |journal=Geophysical Research Letters |volume=42 |issue=4 |pages=1029 |year=2015 |last1=Padovan |first1=Sebastiano |last2=Wieczorek |first2=Mark A. |last3=Margot |first3=Jean-Luc |last4=Tosi |first4=Nicola |last5=Solomon |first5=Sean C. |bibcode=2015GeoRL..42.1029P |s2cid=31442257 |url=http://www.escholarship.org/uc/item/403071pz |doi-access=free |access-date=December 15, 2018 |archive-date=February 12, 2019 |archive-url=https://web.archive.org/web/20190212190428/https://escholarship.org/uc/item/403071pz |url-status=live }}

{{cite journal |last1=Colombo |first1=Giuseppe |author-link1=Giuseppe Colombo|last2=Shapiro |first2=Irwin I. |author-link2=Irwin I. Shapiro|title=The rotation of the planet Mercury |journal=Astrophysical Journal |year=1966 |volume=145 |page=296 |doi=10.1086/148762 |bibcode=1966ApJ...145..296C|doi-access=free }}

{{cite journal |title=Computing apparent planetary magnitudes for The Astronomical Almanac |journal=Astronomy and Computing |first1=Anthony |last1=Mallama |first2=James L. |last2=Hilton |volume=25 |pages=10–24 |date=October 2018 |doi=10.1016/j.ascom.2018.08.002 |bibcode=2018A&C....25...10M |arxiv=1808.01973|s2cid=69912809 }}

{{cite journal |last1=Hauck |first1=Steven A. |last2=Margot |first2=Jean-Luc |last3=Solomon |first3=Sean C. |last4=Phillips |first4=Roger J. |last5=Johnson |first5=Catherine L. |last6=Lemoine |first6=Frank G. |last7=Mazarico |first7=Erwan |last8=McCoy |first8=Timothy J. |last9=Padovan |first9=Sebastiano |last10=Peale |first10=Stanton J. |last11=Perry |first11=Mark E. |last12=Smith |first12=David E. |last13=Zuber |first13=Maria T. |title=The curious case of Mercury's internal structure|journal=Journal of Geophysical Research: Planets |year=2013 |volume=118 |issue=6 |pages=1204–1220 |doi=10.1002/jgre.20091|bibcode=2013JGRE..118.1204H |hdl=1721.1/85633 |s2cid=17668886 |hdl-access=free }}

}}

{{Spoken Wikipedia|En-Mercury(Planet).ogg|date=January 16, 2008}}

{{Sister project links|display=Mercury|voy=no|commonscat=yes|d=y|wikt=Mercury|m=no|mw=no|species=no|n=no|v=Solar System, technical/Mercury|s=Category:Planetary_astronomy}}

• {{cite book |url=https://history.nasa.gov/SP-423/sp423.htm |title=Atlas of Mercury |publisher=NASA |date=1978 |id=SP-423}}
• [http://planetarynames.wr.usgs.gov/Page/MERCURY/target Mercury nomenclature] and [https://web.archive.org/web/20130217190331/http://planetarynames.wr.usgs.gov/Page/mercuryQuadMap map with feature names] from the USGS/IAU Gazetteer of Planetary Nomenclature
• [http://messenger-act.actgate.com/msgr_public_released/react_quickmap.html Equirectangular map of Mercury] {{Webarchive|url=https://web.archive.org/web/20160520025929/http://messenger-act.actgate.com/msgr_public_released/react_quickmap.html |date=May 20, 2016 }} by Applied Coherent Technology Corp
• [https://www.google.com/maps/space/mercury/ 3D globe of Mercury] by Google
• [http://solarviews.com/eng/mercury.htm Mercury] at Solarviews.com
• [http://www.astronomycast.com/2007/08/episode-49-mercury/ Mercury] by Astronomy Cast
• [http://messenger.jhuapl.edu/ MESSENGER mission web site]
• [http://www.esa.int/bepicolombo BepiColombo mission web site]

{{Mercury (planet)|state=expanded}}

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