Caloris Planitia

File:MESSENGER first photo of unseen side of mercury cropped to Caloris.jpg{{'}}s first image of the unseen side of Mercury from a distance of about {{convert|27000|km|mi|-3}} cropped to highlight Caloris. The rim is hard to discern as the Sun is directly overhead, preventing shadows.]]

Caloris was discovered on images taken by the Mariner 10 probe in 1974. Its name was suggested by Brian O'Leary, astronaut and member of the Mariner 10 imagery team. It was situated on the terminator—the line dividing the daytime and nighttime hemispheres—at the time the probe passed by, and so half of the crater could not be imaged. Later, on January 15, 2008, one of the first photos of the planet taken by the MESSENGER probe revealed the crater in its entirety.

The basin was initially estimated to be about {{convert|810|mi|km|abbr=on}} in diameter, though this was increased to {{convert|960|mi|km|abbr=on}} based on subsequent images taken by MESSENGER. It is ringed by mountains up to {{convert|2|km|mi|abbr=on}} high. Inside the crater walls, the floor of the crater is filled by lava plains,{{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=16 September 2014|volume=41|issue=17|pages=6084–6092|doi=10.1002/2014GL061224|bibcode = 2014GeoRL..41.6084T |doi-access=free|url=https://hal-insu.archives-ouvertes.fr/insu-02276045/file/HAL_Thomas_et_al-2014-Geophysical_Research_Letters.pdf}} similar to the maria of the Moon. These plains are superposed by explosive vents associated with pyroclastic material. Outside the walls, material ejected in the impact which created the basin extends for {{convert|1,000|km|mi|abbr=on}}, and concentric rings surround the crater.

In the center of the basin is a region containing numerous radial troughs that appear to be extensional faults, with an unrelated {{convert|40|km|mi|abbr=on}} crater, Apollodorus, located near the center of the pattern. The exact cause of this pattern of troughs is not currently known. The feature is named Pantheon Fossae.[http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=182 Mercury's First Fossae] {{webarchive|url=https://web.archive.org/web/20140714162830/http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=182 |date=2014-07-14 }}. MESSENGER. May 5, 2008. Accessed on July 13, 2009.

File:Caloris Basin comparison.jpg

The impacting body is estimated to have been at least 100 km (62 miles) in diameter.{{cite web |url=http://www.universetoday.com/34568/caloris-basin/ |author=Coffey, Jerry |title=Caloris Basin |publisher=Universe Today |date=July 9, 2009 |access-date=July 1, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120831000433/http://www.universetoday.com/34568/caloris-basin/ |archive-date=August 31, 2012 }}

Bodies in the inner Solar System experienced a heavy bombardment of large rocky bodies in the first billion years or so of the Solar System. The impact that created Caloris must have occurred after most of the heavy bombardment had finished, because fewer impact craters are seen on its floor than exist on comparably-sized regions outside the crater. Similar impact basins on the Moon such as the Mare Imbrium and Mare Orientale are believed to have formed at about the same time, possibly indicating that there was a 'spike' of large impacts towards the end of the heavy bombardment phase of the early Solar System.{{cite journal |last=Gault |first=D. E. |author2=Cassen, P. |author3=Burns, J. A. |author4= Strom, R. G. |year=1977 |title=Mercury |journal=Annual Review of Astronomy and Astrophysics |volume=15 |pages=97–126 |doi=10.1146/annurev.aa.15.090177.000525 |bibcode=1977ARA&A..15...97G}} Based on MESSENGER{{'}}s photographs, Caloris' age has been determined to be between 3.8 and 3.9 billion years.

A gravitational high, also known as a mascon, is centered on Caloris Planitia.[https://photojournal.jpl.nasa.gov/catalog/PIA19285 PIA19285: Gravity Anomalies], Jet Propulsion Laboratory Photojournal Most large impact basins on the moon, such as Mare Imbrium and Mare Crisium, are also the site of mascons.

File:Mercury weird terrain.jpg

File:Mercury's 'Weird Terrain'.jpg

The giant impact believed to have formed Caloris may have had global consequences for the planet. At the exact antipode of the basin is a large area of hilly, grooved terrain, with few small impact craters that are known as chaotic terrain (also "weird terrain").{{cite web| url = https://www.planetary.org/blogs/emily-lakdawalla/2011/3004.html| title = Mercury's Weird Terrain| last = Lakdawalla| first = E.| author-link=Emily Lakdawalla| date = 19 April 2011| publisher = The Planetary Society| access-date= 2020-01-29}} It is thought by some to have been created as seismic waves from the impact converged on the opposite side of the planet.{{cite journal |last=Schultz |first=P. H. |author2=Gault, D. E. |year=1975 |title=Seismic effects from major basin formations on the moon and Mercury |journal=The Moon |volume=12 |issue= 2|pages=159–177 |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

| last=Wieczorek | first=Mark A. |author2=Zuber, Maria T.

| 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=https://hal.archives-ouvertes.fr/hal-02458520/file/Wieczorek%20and%20Zuber%2C%202001a.pdf

| access-date=2008-05-12

| doi=10.1029/2000JE001384 | bibcode=2001JGR...10627853W| doi-access=free}} This hypothetical impact is also believed to have triggered volcanic activity on Mercury, resulting in the formation of smooth plains.{{cite journal |last=Kiefer |first=W. S. |author2=Murray, B. C. |year=1987 |title=The formation of Mercury's smooth plains |journal=Icarus |volume=72 |issue= 3|pages=477–491 |doi=10.1016/0019-1035(87)90046-7 |bibcode=1987Icar...72..477K}} Surrounding Caloris is a series of geologic formations thought to have been produced by the basin's ejecta, collectively called the Caloris Group.

Mercury has a very tenuous and transient atmosphere, containing small amounts of hydrogen and helium captured from the solar wind, as well as heavier elements such as sodium and potassium. These are thought to originate within the planet, being "out-gassed" from beneath its crust. The Caloris basin has been found to be a significant source of sodium and potassium, indicating that the fractures created by the impact facilitate the release of gases from within the planet. The unusual terrain is also a source of these gases.{{cite journal |last=Sprague |first=A. L. |author2=Kozlowski, R. W. H. |author3=Hunten, D. M. |year=1990 |title=Caloris Basin: An Enhanced Source for Potassium in Mercury's Atmosphere |journal=Science |volume=249 |issue=4973 |pages=1140–1142 |doi=10.1126/science.249.4973.1140 |pmid=17831982 |bibcode = 1990Sci...249.1140S |s2cid=36754700 }}

File:Caloris Basin topography JMARS Colorized Shade 2km.jpg|Topographic map of Caloris basin

File:Caloris basin labeled.png|Mosaic of eastern Caloris basin photographed by Mariner 10 in 1974–75.

File:Spider crater on planet mercury.jpg|Pantheon Fossae in the center of Caloris basin, with Apollodorus crater at center of the fossae.

File:PIA19421-Mercury-Craters-MunchSanderPoe-20150416.jpg|Enhanced color image of craters Munch, Sander, and Poe, amid plains of Caloris basin

File:PIA19450-PlanetMercury-CalorisBasin-20150501.jpg|Perspective view of Caloris – high (red); low (blue).

• Geology of Mercury
• Skinakas basin

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Category:Impact craters on Mercury