Mars

{{Main|Geological history of Mars}}

Scientists have theorized that during the Solar System's formation, Mars was created as the result of a random process of run-away accretion of material from the protoplanetary disk that orbited the Sun. Mars has many distinctive chemical features caused by its position in the Solar System. Elements with comparatively low boiling points, such as chlorine, phosphorus, and sulfur, are much more common on Mars than on Earth; these elements were probably pushed outward by the young Sun's energetic solar wind.

After the formation of the planets, the inner Solar System may have been subjected to the so-called Late Heavy Bombardment. About 60% of the surface of Mars shows a record of impacts from that era, whereas much of the remaining surface is probably underlain by immense impact basins caused by those events. However, more recent modeling has disputed the existence of the Late Heavy Bombardment.{{Cite journal |last1=Nesvorný |first1=David |date=June 2018 |title=Dynamical Evolution of the Early Solar System |journal=Annual Review of Astronomy and Astrophysics |volume=56 |pages=137–174 |doi=10.1146/annurev-astro-081817-052028 |arxiv=1807.06647 |bibcode=2018ARA&A..56..137N}} There is evidence of an enormous impact basin in the Northern Hemisphere of Mars, spanning {{convert|10600|by|8500|km}}, or roughly four times the size of the Moon's South Pole–Aitken basin, which would be the largest impact basin yet discovered if confirmed. It has been hypothesized that the basin was formed when Mars was struck by a Pluto-sized body about four billion years ago. The event, thought to be the cause of the Martian hemispheric dichotomy, created the smooth Borealis basin that covers 40% of the planet.

A 2023 study shows evidence, based on the orbital inclination of Deimos (a small moon of Mars), that Mars may once have had a ring system 3.5 billion years to 4 billion years ago.{{Cite journal |last1=Ćuk |first1=Matija |last2=Minton |first2=David A. |last3=Pouplin |first3=Jennifer L. L. |last4=Wishard |first4=Carlisle |date=16 June 2020 |title=Evidence for a Past Martian Ring from the Orbital Inclination of Deimos |journal=The Astrophysical Journal |volume=896 |issue=2 |pages=L28 |doi=10.3847/2041-8213/ab974f |arxiv=2006.00645 |bibcode=2020ApJ...896L..28C |issn=2041-8213 |doi-access=free}} This ring system may have been formed from a moon, 20 times more massive than Phobos, orbiting Mars billions of years ago; and Phobos would be a remnant of that ring.{{Cite web |author=News Staff |date=4 June 2020 |title=Researchers Find New Evidence that Mars Once Had Massive Ring {{!}} Sci.News |url=https://www.sci.news/space/ancient-mars-ring-08502.html |access-date=7 November 2023 |website=Sci.News: Breaking Science News |archive-date=7 November 2023 |archive-url=https://web.archive.org/web/20231107064634/https://www.sci.news/space/ancient-mars-ring-08502.html |url-status=live}}{{Cite news |date=5 June 2020 |title=Did ancient Mars have rings? |url=https://earthsky.org/space/did-ancient-mars-have-rings-deimos/ |access-date=7 November 2023 |newspaper=Earthsky | Updates on Your Cosmos and World |archive-date=7 November 2023 |archive-url=https://web.archive.org/web/20231107064635/https://earthsky.org/space/did-ancient-mars-have-rings-deimos/ |url-status=live}}

The geological history of Mars can be split into many periods, but the following are the three primary periods:

• Noachian period: Formation of the oldest extant surfaces of Mars, 4.5 to 3.5 billion years ago. Noachian age surfaces are scarred by many large impact craters. The Tharsis bulge, a volcanic upland, is thought to have formed during this period, with extensive flooding by liquid water late in the period. Named after Noachis Terra.{{cite web |title=ESA Science & Technology - The Ages of Mars |url=https://sci.esa.int/web/mars-express/-/55481-the-ages-of-mars |access-date=7 December 2021 |website=sci.esa.int |archive-date=29 August 2023 |archive-url=https://web.archive.org/web/20230829030639/https://sci.esa.int/web/mars-express/-/55481-the-ages-of-mars |url-status=live}}
• Hesperian period: 3.5 to between 3.3 and 2.9 billion years ago. The Hesperian period is marked by the formation of extensive lava plains. Named after Hesperia Planum.
• Amazonian period: between 3.3 and 2.9 billion years ago to the present. Amazonian regions have few meteorite impact craters but are otherwise quite varied. Olympus Mons formed during this period, with lava flows elsewhere on Mars. Named after Amazonis Planitia.

Geological activity is still taking place on Mars. The Athabasca Valles is home to sheet-like lava flows created about 200 million years ago. Water flows in the grabens called the Cerberus Fossae occurred less than 20 million years ago, indicating equally recent volcanic intrusions. The Mars Reconnaissance Orbiter has captured images of avalanches.{{Cite web |last=Russell |first=Patrick |date=3 March 2008 |title=Caught in Action: Avalanches on North Polar Scarps |url=https://hirise.lpl.arizona.edu/PSP_007338_2640 |access-date=28 March 2022 |website=HiRISE Operations Center |archive-date=7 April 2022 |archive-url=https://web.archive.org/web/20220407233021/https://hirise.lpl.arizona.edu/PSP_007338_2640 |url-status=live}}{{Cite web |date=12 August 2020 |title=HiRISE Catches an Avalanche on Mars |url=https://www.jpl.nasa.gov/images/pia24035-hirise-catches-an-avalanche-on-mars |access-date=28 March 2022 |website=NASA Jet Propulsion Laboratory (JPL) |archive-date=1 March 2022 |archive-url=https://web.archive.org/web/20220301110525/https://www.jpl.nasa.gov/images/pia24035-hirise-catches-an-avalanche-on-mars |url-status=live}}

{{Main|Geology of Mars|Composition of Mars}}

File:Terrestrial planet size comparisons-2.jpgs of the Inner Solar System. From left: Mercury, Venus, Earth, the Moon, Mars and Ceres.)]]

Mars is approximately half the diameter of Earth, with a surface area only slightly less than the total area of Earth's dry land. Mars is less dense than Earth, having about 15% of Earth's volume and 11% of Earth's mass, resulting in about 38% of Earth's surface gravity. Mars is the only presently known example of a desert planet, a rocky planet with a surface akin to that of Earth's deserts. The red-orange appearance of the Martian surface is caused by rust. It can look like butterscotch; other common surface colors include golden, brown, tan, and greenish, depending on the minerals present.[http://mars.jpl.nasa.gov/msl/multimedia/videos/index.cfm?v=29&a=2 NASA – Mars in a Minute: Is Mars Really Red?] {{Webarchive|url=https://web.archive.org/web/20140720135450/http://mars.jpl.nasa.gov/msl/multimedia/videos/index.cfm?v=29&a=2 |date=20 July 2014 }} ([http://mars.jpl.nasa.gov/multimedia/videos/movies/miam20111110/miam20111110.pdf Transcript] {{Webarchive|url=https://web.archive.org/web/20151106174558/http://mars.jpl.nasa.gov/multimedia/videos/movies/miam20111110/miam20111110.pdf |date=6 November 2015 }}) {{PD-notice}}

File:Mars Internal Structure (2024).svg

File:GMM-3 Mars Gravity.webm

Like Earth, Mars is differentiated into a dense metallic core overlaid by less dense rocky layers. The outermost layer is the crust, which is on average about {{convert|42-56|km}} thick, with a minimum thickness of {{convert|6|km}} in Isidis Planitia, and a maximum thickness of {{convert|117|km}} in the southern Tharsis plateau. For comparison, Earth's crust averages 27.3 ± 4.8 km in thickness. The most abundant elements in the Martian crust are silicon, oxygen, iron, magnesium, aluminium, calcium, and potassium. Mars is confirmed to be seismically active; in 2019 it was reported that InSight had detected and recorded over 450 marsquakes and related events.

Beneath the crust is a silicate mantle responsible for many of the tectonic and volcanic features on the planet's surface. The upper Martian mantle is a low-velocity zone, where the velocity of seismic waves is lower than surrounding depth intervals. The mantle appears to be rigid down to the depth of about 250 km, giving Mars a very thick lithosphere compared to Earth. Below this the mantle gradually becomes more ductile, and the seismic wave velocity starts to grow again. The Martian mantle does not appear to have a thermally insulating layer analogous to Earth's lower mantle; instead, below 1050 km in depth, it becomes mineralogically similar to Earth's transition zone. At the bottom of the mantle lies a basal liquid silicate layer approximately 150–180 km thick.

Mars's iron and nickel core is completely molten, with no solid inner core. It is around half of Mars's radius, approximately 1650–1675 km, and is enriched in light elements such as sulfur, oxygen, carbon, and hydrogen. The temperature of the core is estimated to be 2000-2400 K,{{cite journal | doi=10.1126/sciadv.adk1087 | title=A thermally conductive Martian core and implications for its dynamo cessation | date=2024 | journal=Science Advances | volume=10 | issue=12 | pmid=38507495 | bibcode=2024SciA...10K1087H | vauthors = Hsieh W, Deschamps F, Tsao Y, Yoshino T, Lin J | pages=eadk1087 | pmc=10954222 }} compared to 5400-6230 K for Earth's solid inner core.

{{Further|Martian soil}}File:PIA17944-MarsCuriosityRover-AfterCrossingDingoGapSanddune-20140209.jpg{{'s}} view of Martian soil and boulders after crossing the "Dingo Gap" sand dune]]

Mars is a terrestrial planet with a surface that consists of minerals containing silicon and oxygen, metals, and other elements that typically make up rock. The Martian surface is primarily composed of tholeiitic basalt, although parts are more silica-rich than typical basalt and may be similar to andesitic rocks on Earth, or silica glass. Regions of low albedo suggest concentrations of plagioclase feldspar, with northern low albedo regions displaying higher than normal concentrations of sheet silicates and high-silicon glass. Parts of the southern highlands include detectable amounts of high-calcium pyroxenes. Localized concentrations of hematite and olivine have been found. Much of the surface is deeply covered by finely grained iron(III) oxide dust.

The Phoenix lander returned data showing Martian soil to be slightly alkaline and containing elements such as magnesium, sodium, potassium and chlorine. These nutrients are found in soils on Earth. They are necessary for growth of plants. Experiments performed by the lander showed that the Martian soil has a basic pH of 7.7, and contains 0.6% perchlorate by weight,{{cite journal |last1=Kounaves |first1=S. P. |year=2010 |title=Wet Chemistry Experiments on the 2007 Phoenix Mars Scout Lander: Data Analysis and Results |journal=J. Geophys. Res. |volume=115 |issue=E3 |pages=E00–E10 |doi=10.1029/2008JE003084 |bibcode=2009JGRE..114.0A19K |s2cid=39418301 |doi-access=free}}{{cite journal |last1=Kounaves |first1=S. P. |year=2010 |title=Soluble Sulfate in the Martian Soil at the Phoenix Landing Site |journal=Geophysical Research Letters |volume=37 |issue=9 |page=L09201 |doi=10.1029/2010GL042613 |bibcode=2010GeoRL..37.9201K |s2cid=12914422}} concentrations that are toxic to humans.{{cite web |url=https://www.space.com/21554-mars-toxic-perchlorate-chemicals.html |title=Toxic Mars: Astronauts Must Deal with Perchlorate on the Red Planet |work=Space.com |first=Leonard |last=David |date=13 June 2013 |access-date=26 November 2018 |archive-date=20 November 2020 |archive-url=https://web.archive.org/web/20201120151522/https://www.space.com/21554-mars-toxic-perchlorate-chemicals.html |url-status=live}}{{cite news |url=https://www.theguardian.com/science/2017/jul/06/mars-covered-in-toxic-chemicals-that-can-wipe-out-living-organisms-tests-reveal |title=Mars covered in toxic chemicals that can wipe out living organisms, tests reveal |work=The Guardian |first=Ian |last=Sample |date=6 July 2017 |access-date=26 November 2018 |archive-date=18 February 2021 |archive-url=https://web.archive.org/web/20210218180154/https://www.theguardian.com/science/2017/jul/06/mars-covered-in-toxic-chemicals-that-can-wipe-out-living-organisms-tests-reveal |url-status=live}}

Streaks are common across Mars and new ones appear frequently on steep slopes of craters, troughs, and valleys. The streaks are dark at first and get lighter with age. The streaks can start in a tiny area, then spread out for hundreds of metres. They have been seen to follow the edges of boulders and other obstacles in their path. The commonly accepted hypotheses include that they are dark underlying layers of soil revealed after avalanches of bright dust or dust devils. Several other explanations have been put forward, including those that involve water or even the growth of organisms.

Environmental radiation levels on the surface are on average 0.64 millisieverts of radiation per day, and significantly less than the radiation of 1.84 millisieverts per day or 22 millirads per day during the flight to and from Mars.{{cite web |last=Williams |first=Matt |date=21 November 2016 |title=How bad is the radiation on Mars? |url=https://phys.org/news/2016-11-bad-mars.html |access-date=9 April 2023 |website=Phys.org |archive-date=4 April 2023 |archive-url=https://web.archive.org/web/20230404151856/https://phys.org/news/2016-11-bad-mars.html |url-status=live}}{{cite web |last=Wall |first=Mike |date=9 December 2013 |title=Radiation on Mars 'Manageable' for Manned Mission, Curiosity Rover Reveals |url=https://www.space.com/23875-mars-radiation-life-manned-mission.html |access-date=9 April 2023 |website=Space.com |archive-date=15 December 2020 |archive-url=https://web.archive.org/web/20201215082045/https://www.space.com/23875-mars-radiation-life-manned-mission.html |url-status=live}} For comparison the radiation levels in low Earth orbit, where Earth's space stations orbit, are around 0.5 millisieverts of radiation per day.{{cite web |date=13 March 2003 |title=Comparison of Martian Radiation Environment with International Space Station |url=https://www.jpl.nasa.gov/images/pia04258-comparison-of-martian-radiation-environment-with-international-space-station |access-date=9 April 2023 |website=NASA Jet Propulsion Laboratory (JPL) |archive-date=9 April 2023 |archive-url=https://web.archive.org/web/20230409210426/https://www.jpl.nasa.gov/images/pia04258-comparison-of-martian-radiation-environment-with-international-space-station |url-status=live}} Hellas Planitia has the lowest surface radiation at about 0.342 millisieverts per day, featuring lava tubes southwest of Hadriacus Mons with potentially levels as low as 0.064 millisieverts per day,{{cite arXiv |last1=Paris |first1=Antonio |last2=Davies |first2=Evan |last3=Tognetti |first3=Laurence |last4=Zahniser |first4=Carly |date=27 April 2020 |title=Prospective Lava Tubes at Hellas Planitia |class=astro-ph.EP |eprint=2004.13156v1}} comparable to radiation levels during flights on Earth.

{{main|Magnetic field of Mars}}

Although Mars has no evidence of a structured global magnetic field, observations show that parts of the planet's crust have been magnetized, suggesting that alternating polarity reversals of its dipole field have occurred in the past. This paleomagnetism of magnetically susceptible minerals is similar to the alternating bands found on Earth's ocean floors. One hypothesis, published in 1999 and re-examined in October 2005 (with the help of the Mars Global Surveyor), is that these bands suggest plate tectonic activity on Mars four billion years ago, before the planetary dynamo ceased to function and the planet's magnetic field faded.

{{anchor|Geography}}

{{Main|Geography of Mars}}

{{further|Areoid}}

{{See also|Category:Surface features of Mars}}

File:Mars.ogv

Although better remembered for mapping the Moon, Johann Heinrich von Mädler and Wilhelm Beer were the first areographers. They began by establishing that most of Mars's surface features were permanent and by more precisely determining the planet's rotation period. In 1840, Mädler combined ten years of observations and drew the first map of Mars.{{cite web |date=2016-10-19 |title=What Mars Maps Got Right (and Wrong) Through Time |url=https://www.nationalgeographic.com/culture/article/planets-maps-exploring-mars-space-science |archive-url=https://web.archive.org/web/20210221031131/https://www.nationalgeographic.com/culture/article/planets-maps-exploring-mars-space-science |url-status=dead |archive-date=21 February 2021 |access-date=2022-01-15 |website=National Geographic |language=en}}

Features on Mars are named from a variety of sources. Albedo features are named for classical mythology. Craters larger than roughly 50 km are named for deceased scientists and writers and others who have contributed to the study of Mars. Smaller craters are named for towns and villages of the world with populations of less than 100,000. Large valleys are named for the word "Mars" or "star" in various languages; smaller valleys are named for rivers.

Large albedo features retain many of the older names but are often updated to reflect new knowledge of the nature of the features. For example, Nix Olympica (the snows of Olympus) has become Olympus Mons (Mount Olympus). The surface of Mars as seen from Earth is divided into two kinds of areas, with differing albedo. The paler plains covered with dust and sand rich in reddish iron oxides were once thought of as Martian "continents" and given names like Arabia Terra (land of Arabia) or Amazonis Planitia (Amazonian plain). The dark features were thought to be seas, hence their names Mare Erythraeum, Mare Sirenum and Aurorae Sinus. The largest dark feature seen from Earth is Syrtis Major Planum.{{Citation |title=Syrtis Major (MC-13) |date=2019 |url=https://www.cambridge.org/core/books/atlas-of-mars/syrtis-major-mc13/A385CD9E498305DA40E50BD83C531340 |work=The Atlas of Mars: Mapping its Geography and Geology |pages=136–139 |editor-last=Tanaka |editor-first=Kenneth L. |place=Cambridge |publisher=Cambridge University Press |doi=10.1017/9781139567428.018 |isbn=978-1-139-56742-8 |s2cid=240843698 |access-date=18 January 2022 |editor2-last=Coles |editor2-first=Kenneth S. |editor3-last=Christensen |editor3-first=Philip R. |archive-date=1 March 2024 |archive-url=https://web.archive.org/web/20240301160859/https://www.cambridge.org/core/books/abs/atlas-of-mars/syrtis-major-mc13/A385CD9E498305DA40E50BD83C531340 |url-status=live}} The permanent northern polar ice cap is named Planum Boreum. The southern cap is called Planum Australe.{{cite web |title=Polar Caps |url=https://marsed.asu.edu/mep/ice/polar-caps |url-status=live |archive-url=https://web.archive.org/web/20210528133135/https://marsed.asu.edu/mep/ice/polar-caps |archive-date=28 May 2021 |access-date=7 December 2021 |website=Mars Education at Arizona State University}}

File:Mars MOLA Topography by Fabio Crameri.png-based topographic map showing highlands (light colors) dominating the Southern Hemisphere of Mars, lowlands (dark colors) the northern. Volcanic plateaus delimit regions of the northern plains, whereas the highlands are punctuated by several large impact basins.]]

Mars's equator is defined by its rotation, but the location of its Prime Meridian was specified, as was Earth's (at Greenwich), by choice of an arbitrary point; Mädler and Beer selected a line for their first maps of Mars in 1830. After the spacecraft Mariner 9 provided extensive imagery of Mars in 1972, a small crater (later called Airy-0), located in the Sinus Meridiani ("Middle Bay" or "Meridian Bay"), was chosen by Merton E. Davies, Harold Masursky, and Gérard de Vaucouleurs for the definition of 0.0° longitude to coincide with the original selection.{{Cite journal |last1=Davies |first1=Merton E. |last2=Berg |first2=Richard A. |date=10 January 1971 |title=A preliminary control net of Mars |url=http://doi.wiley.com/10.1029/JB076i002p00373 |journal=Journal of Geophysical Research |volume=76 |issue=2 |pages=373–393 |doi=10.1029/JB076i002p00373 |bibcode=1971JGR....76..373D |access-date=22 March 2022 |archive-date=1 March 2024 |archive-url=https://web.archive.org/web/20240301160852/https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/JB076i002p00373 |url-status=live}}{{Citation |last1=de Vaucouleurs |first1=Gerard |title=Mariner 9 Areographic Coordinate System |date=1973 |journal=Journal of Geophysical Research |volume=78 |issue=20 |pages=4395–4404 |bibcode=1973JGR....78.4395D |doi=10.1029/JB078i020p04395 |last2=Davies |first2=Merton E. |last3=Sturms |first3=Francis M. Jr. |author-link=Gerard de Vaucouleurs |author2-link=Merton Davies}}

Because Mars has no oceans, and hence no "sea level", a zero-elevation surface had to be selected as a reference level; this is called the areoid of Mars, analogous to the terrestrial geoid.{{cite journal |last1=Ardalan |first1=A. A. |last2=Karimi |first2=R. |last3=Grafarend |first3=E. W. |title=A New Reference Equipotential Surface, and Reference Ellipsoid for the Planet Mars |journal=Earth, Moon, and Planets |volume=106 |issue=1 |year=2009 |pages=1–13 |issn=0167-9295 |doi=10.1007/s11038-009-9342-7 |s2cid=119952798}} Zero altitude was defined by the height at which there is {{convert|610.5|Pa|mbar|abbr=on|lk=on}} of atmospheric pressure. This pressure corresponds to the triple point of water, and it is about 0.6% of the sea level surface pressure on Earth (0.006 atm).

For mapping purposes, the United States Geological Survey divides the surface of Mars into thirty cartographic quadrangles, each named for a classical albedo feature it contains.{{cite web |title=ESA Science & Technology - Using iMars: Viewing Mars Express data of the MC11 quadrangle |url=https://sci.esa.int/web/mars-express/-/59473-using-imars-viewing-mars-express-data-of-the-mc11-quadrangle |access-date=29 December 2021 |website=sci.esa.int |archive-date=29 December 2021 |archive-url=https://web.archive.org/web/20211229234909/https://sci.esa.int/web/mars-express/-/59473-using-imars-viewing-mars-express-data-of-the-mc11-quadrangle |url-status=live}} In April 2023, The New York Times reported an updated global map of Mars based on images from the Hope spacecraft.{{cite news |last=Chang |first=Kenneth |title=New Mars Map Lets You 'See the Whole Planet at Once' - Scientists assembled 3,000 images from an Emirati orbiter to create the prettiest atlas yet of the red planet. |url=https://www.nytimes.com/2023/04/15/science/astronomy-mars-atlas.html |date=15 April 2023 |work=The New York Times |access-date=15 April 2023 |archive-date=16 May 2023 |archive-url=https://web.archive.org/web/20230516125940/https://www.nytimes.com/2023/04/15/science/astronomy-mars-atlas.html |url-status=live}} A related, but much more detailed, global Mars map was released by NASA on 16 April 2023.{{cite news |author=Staff |title=Welcome to Mars! Caltech's Jaw-Dropping, 5.7 Terapixel Virtual Expedition Across the Red Planet |url=https://scitechdaily.com/welcome-to-mars-caltechs-jaw-dropping-5-7-terapixel-virtual-expedition-across-the-red-planet/ |date=16 April 2023 |work=SciTech |access-date=6 April 2023 |archive-date=16 April 2023 |archive-url=https://web.archive.org/web/20230416204703/https://scitechdaily.com/welcome-to-mars-caltechs-jaw-dropping-5-7-terapixel-virtual-expedition-across-the-red-planet/ |url-status=live}}

{{Main|Volcanism on Mars}}

File:Olympus Mons - ESA Mars Express - Flickr - Andrea Luck.png. It is approximately {{convert|550|km|abbr=on}} across.]]

The vast upland region Tharsis contains several massive volcanoes, which include the shield volcano Olympus Mons. The edifice is over {{convert|600|km|mi|abbr=on}} wide.{{cite web |title=Mars Atlas: Olympus Mons |url=https://mars.nasa.gov/gallery/atlas/olympus-mons.html |access-date=30 March 2022 |website=NASA's Mars Exploration Program |archive-date=29 March 2023 |archive-url=https://web.archive.org/web/20230329181023/https://mars.nasa.gov/gallery/atlas/olympus-mons.html |url-status=live}}{{cite journal |last1=Plescia |first1=J. B. |year=2004 |title=Morphometric Properties of Martian Volcanoes |journal=J. Geophys. Res. |volume=109 |issue=E3 |page=E03003 |bibcode=2004JGRE..109.3003P |doi=10.1029/2002JE002031 |doi-access=free}} Because the mountain is so large, with complex structure at its edges, giving a definite height to it is difficult. Its local relief, from the foot of the cliffs which form its northwest margin to its peak, is over {{convert|21|km|mi|abbr=on}}, a little over twice the height of Mauna Kea as measured from its base on the ocean floor. The total elevation change from the plains of Amazonis Planitia, over {{convert|1000|km|mi|abbr=on}} to the northwest, to the summit approaches {{convert|26|km|mi|abbr=on}},{{cite book |last=Comins |first=Neil F. |url=https://books.google.com/books?id=qK_4mNve1DYC&pg=PA148 |title=Discovering the Essential Universe |publisher=W. H. Freeman |year=2012 |isbn=978-1-4292-5519-6 |page=148}} roughly three times the height of Mount Everest, which in comparison stands at just over {{convert|8.8|km}}. Consequently, Olympus Mons is either the tallest or second-tallest mountain in the Solar System; the only known mountain which might be taller is the Rheasilvia peak on the asteroid Vesta, at {{convert|20-25|km|mi|abbr=on}}.{{cite journal |last1=Schenk |first1=P. |date=2012 |title=The Geologically Recent Giant Impact Basins at Vesta's South Pole |journal=Science |volume=336 |issue=6082 |pages=694–697 |bibcode=2012Sci...336..694S |doi=10.1126/science.1223272 |pmid=22582256 |s2cid=206541950}}

The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southern highlands, pitted and cratered by ancient impacts. It is possible that, four billion years ago, the Northern Hemisphere of Mars was struck by an object one-tenth to two-thirds the size of Earth's Moon. If this is the case, the Northern Hemisphere of Mars would be the site of an impact crater {{convert|10600|by|8500|km}} in size, or roughly the area of Europe, Asia, and Australia combined, surpassing Utopia Planitia and the Moon's South Pole–Aitken basin as the largest impact crater in the Solar System.{{cite journal |title=The Borealis basin and the origin of the Martian crustal dichotomy |last1=Andrews-Hanna |first1=Jeffrey C. |journal=Nature |volume=453 |pages=1212–1215 |doi=10.1038/nature07011 |year=2008 |pmid=18580944 |last2=Zuber |first2=Maria T. |author-link2=Maria Zuber |last3=Banerdt |first3=W. Bruce |issue=7199 |bibcode=2008Natur.453.1212A |s2cid=1981671}}{{cite web |last=Choi |first=Charles |date=1 October 2021 |title=Mars: What We Know About the Red Planet |url=https://www.space.com/47-mars-the-red-planet-fourth-planet-from-the-sun.html |access-date=6 January 2022 |website=Space.com |language=en |archive-date=6 January 2022 |archive-url=https://web.archive.org/web/20220106100851/https://www.space.com/47-mars-the-red-planet-fourth-planet-from-the-sun.html |url-status=live}}{{cite web |last=Moskowitz |first=Clara |date=25 June 2008 |title=Huge Impact Created Mars' Split Personality |url=https://www.space.com/5558-huge-impact-created-mars-split-personality.html |access-date=6 January 2022 |website=Space.com |language=en |archive-date=6 January 2022 |archive-url=https://web.archive.org/web/20220106102110/https://www.space.com/5558-huge-impact-created-mars-split-personality.html |url-status=live}}

Mars is scarred by 43,000 impact craters with a diameter of {{convert|5|km}} or greater. The largest exposed crater is Hellas, which is {{convert|2300|km}} wide and {{convert|7000|m|ft}} deep, and is a light albedo feature clearly visible from Earth.{{Cite book |last=Vogt |first=Gregory L. |url=https://link.springer.com/content/pdf/10.1007/978-0-387-75468-0_2.pdf |title=Landscapes of Mars |date=2008 |publisher=Springer |isbn=978-0-387-75467-3 |location=New York, NY |pages=44 |doi=10.1007/978-0-387-75468-0 |access-date=31 March 2022 |archive-date=1 March 2024 |archive-url=https://web.archive.org/web/20240301160105/https://link.springer.com/chapter/10.1007/978-0-387-75468-0_2 |url-status=live}}{{cite web |title=ESA Science & Technology - Craters within the Hellas Basin |url=https://sci.esa.int/web/mars-express/-/55575-craters-within-the-hellas-basin |access-date=2 January 2022 |website=sci.esa.int |archive-date=2 January 2022 |archive-url=https://web.archive.org/web/20220102121425/https://sci.esa.int/web/mars-express/-/55575-craters-within-the-hellas-basin |url-status=live}} There are other notable impact features, such as Argyre, which is around {{convert|1800|km}} in diameter,{{cite web |first=Christine M. |last=Rodrigue |url=https://home.csulb.edu/~rodrigue/mars/FNlecture.html |title=The Geography of Mars |publisher=Home.csulb.edu |date= |access-date=20 February 2022 |archive-date=30 January 2022 |archive-url=https://web.archive.org/web/20220130013414/https://home.csulb.edu/~rodrigue/mars/FNlecture.html |url-status=live}} and Isidis, which is around {{convert|1500|km}} in diameter.{{Cite web |title=41st Lunar and Planetary Science Conference (2010) |url=https://www.lpi.usra.edu/meetings/lpsc2010/pdf/1294.pdf |access-date=31 January 2022 |archive-date=30 January 2022 |archive-url=https://web.archive.org/web/20220130215730/https://www.lpi.usra.edu/meetings/lpsc2010/pdf/1294.pdf |url-status=live}} Due to the smaller mass and size of Mars, the probability of an object colliding with the planet is about half that of Earth. Mars is located closer to the asteroid belt, so it has an increased chance of being struck by materials from that source. Mars is more likely to be struck by short-period comets, i.e., those that lie within the orbit of Jupiter.

Martian craters can{{discuss}} have a morphology that suggests the ground became wet after the meteor impact.

File:Mars Valles Marineris.jpeg, taken by the Viking 1 probe]]

The large canyon, Valles Marineris (Latin for 'Mariner Valleys, also known as Agathodaemon in the old canal maps{{Cite journal |last1=Sagan |first1=Carl |author-link=Carl Sagan |last2=Fox |first2=Paul |date=August 1975 |title=The canals of Mars: An assessment after Mariner 9 |url=https://linkinghub.elsevier.com/retrieve/pii/0019103575900421 |journal=Icarus |volume=25 |issue=4 |pages=602–612 |doi=10.1016/0019-1035(75)90042-1 |bibcode=1975Icar...25..602S |access-date=22 March 2022}}), has a length of {{convert|4000|km}} and a depth of up to {{convert|7|km}}. The length of Valles Marineris is equivalent to the length of Europe and extends across one-fifth the circumference of Mars. By comparison, the Grand Canyon on Earth is only {{convert|446|km}} long and nearly {{convert|2|km}} deep. Valles Marineris was formed due to the swelling of the Tharsis area, which caused the crust in the area of Valles Marineris to collapse. In 2012, it was proposed that Valles Marineris is not just a graben, but a plate boundary where {{convert|150|km}} of transverse motion has occurred, making Mars a planet with possibly a two-tectonic plate arrangement.{{cite web |url=http://newsroom.ucla.edu/portal/ucla/ucla-scientist-discovers-plate-237303.aspx |title=UCLA scientist discovers plate tectonics on Mars |publisher=UCLA |date=9 August 2012 |access-date=13 August 2012 |last=Wolpert |first=Stuart |archive-url=https://web.archive.org/web/20120812215548/http://newsroom.ucla.edu/portal/ucla/ucla-scientist-discovers-plate-237303.aspx |archive-date=12 August 2012 |url-status=dead}}{{cite journal |last=Lin |first=An |title=Structural analysis of the Valles Marineris fault zone: Possible evidence for large-scale strike-slip faulting on Mars |journal=Lithosphere |volume=4 |issue=4 |pages=286–330 |date=4 June 2012 |doi=10.1130/L192.1 |bibcode=2012Lsphe...4..286Y |doi-access=free}}

Images from the Thermal Emission Imaging System (THEMIS) aboard NASA's Mars Odyssey orbiter have revealed seven possible cave entrances on the flanks of the volcano Arsia Mons. The caves, named after loved ones of their discoverers, are collectively known as the "seven sisters". Cave entrances measure from {{convert|100|to|252|m}} wide and they are estimated to be at least {{convert|73|to|96|m}} deep. Because light does not reach the floor of most of the caves, they may extend much deeper than these lower estimates and widen below the surface. "Dena" is the only exception; its floor is visible and was measured to be {{convert|130|m}} deep. The interiors of these caverns may be protected from micrometeoroids, UV radiation, solar flares and high energy particles that bombard the planet's surface.{{cite web |title=The Caves of Mars {{!}} U.S. Geological Survey |url=https://www.usgs.gov/news/caves-mars |access-date=12 January 2022 |publisher=USGS |archive-date=31 December 2021 |archive-url=https://web.archive.org/web/20211231032913/http://www.usgs.gov/news/caves-mars |url-status=live}}

{{Main|Atmosphere of Mars}}

File:Mars - Atmosphere and Cassini Crater - Hope Mission, Orbit 19 (52252929420).png

Mars lost its magnetosphere 4 billion years ago, possibly because of numerous asteroid strikes,{{cite magazine |url=https://www.wired.com/2011/01/mars-dynamo-death/ |title=Multiple Asteroid Strikes May Have Killed Mars's Magnetic Field |date=20 January 2011 |access-date=30 March 2022 |magazine=Wired |last1=Grossman |first1=Lisa |archive-date=30 December 2013 |archive-url=https://web.archive.org/web/20131230034219/http://www.wired.com/wiredscience/2011/01/mars-dynamo-death/ |url-status=live}} so the solar wind interacts directly with the Martian ionosphere, lowering the atmospheric density by stripping away atoms from the outer layer.{{Cite journal |last=Jakosky |first=Bruce M. |date=1 April 2022 |title=How did Mars lose its atmosphere and water? |journal=Physics Today |volume=75 |issue=4 |pages=62–63 |doi=10.1063/PT.3.4988 |bibcode=2022PhT....75d..62J |s2cid=247882540 |issn=0031-9228 |doi-access=free}} Both Mars Global Surveyor and Mars Express have detected ionized atmospheric particles trailing off into space behind Mars, and this atmospheric loss is being studied by the MAVEN orbiter. Compared to Earth, the atmosphere of Mars is quite rarefied. Atmospheric pressure on the surface today ranges from a low of {{convert|30|Pa|psi|abbr=on|lk=on}} on Olympus Mons to over {{convert|1155|Pa|psi|abbr=on}} in Hellas Planitia, with a mean pressure at the surface level of {{convert|600|Pa|psi|abbr=on}}. The highest atmospheric density on Mars is equal to that found {{convert|35|km}} above Earth's surface. The resulting mean surface pressure is only 0.6% of Earth's {{convert|101.3|kPa|psi|abbr=on}}. The scale height of the atmosphere is about {{convert|10.8|km}}, which is higher than Earth's {{convert|6|km}}, because the surface gravity of Mars is only about 38% of Earth's.{{cite web |title=Mars Facts {{!}} All About Mars |url=https://mars.nasa.gov/all-about-mars/facts |access-date=27 December 2021 |website=NASA's Mars Exploration Program |date=10 November 2017 |language=en |archive-date=10 October 2023 |archive-url=https://web.archive.org/web/20231010080933/https://mars.nasa.gov/all-about-mars/facts/ |url-status=live}}

The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water.{{cite journal |title=Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover |volume=341 |issue=6143 |pages=263–266 |journal=Science |date=19 July 2013 |doi=10.1126/science.1237966 |pmid=23869014 |last1=Mahaffy |first1=P. R. |bibcode=2013Sci...341..263M |s2cid=206548973}} The atmosphere is quite dusty, containing particulates about 1.5 μm in diameter which give the Martian sky a tawny color when seen from the surface. It may take on a pink hue due to iron oxide particles suspended in it. The concentration of methane in the Martian atmosphere fluctuates from about 0.24 ppb during the northern winter to about 0.65 ppb during the summer.{{cite news |url=https://www.theguardian.com/science/2018/jun/07/nasa-mars-rover-finds-organic-matter-in-ancient-lake-bed |title=Nasa Mars rover finds organic matter in ancient lake bed |work=The Guardian |first=Ian |last=Sample |date=7 June 2018 |access-date=12 June 2018 |archive-date=18 June 2018 |archive-url=https://web.archive.org/web/20180618030502/https://www.theguardian.com/science/2018/jun/07/nasa-mars-rover-finds-organic-matter-in-ancient-lake-bed |url-status=live}} Estimates of its lifetime range from 0.6 to 4 years, so its presence indicates that an active source of the gas must be present. Methane is most likely produced by non-biological process such as serpentinization involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars, although it could also be produced by Martian life.{{Cite journal |last1=Webster |first1=Christopher R. |last2=Mahaffy |first2=Paul R. |last3=Pla-Garcia |first3=Jorge |last4=Rafkin |first4=Scot C. R. |last5=Moores |first5=John E. |last6=Atreya |first6=Sushil K. |last7=Flesch |first7=Gregory J. |last8=Malespin |first8=Charles A. |last9=Teinturier |first9=Samuel M. |last10=Kalucha |first10=Hemani |last11=Smith |first11=Christina L. |date=June 2021 |title=Day-night differences in Mars methane suggest nighttime containment at Gale crater |journal=Astronomy & Astrophysics |volume=650 |pages=A166 |doi=10.1051/0004-6361/202040030 |bibcode=2021A&A...650A.166W |s2cid=236365559 |issn=0004-6361 |doi-access=free}}File:PIA18613-MarsMAVEN-Atmosphere-3UV-Views-20141014.jpg on Mars (carbon, oxygen, and hydrogen) by MAVEN in UV{{cite web |last1=Jones |first1=Nancy |last2=Steigerwald |first2=Bill |last3=Brown |first3=Dwayne |last4=Webster |first4=Guy |date=14 October 2014 |title=NASA Mission Provides Its First Look at Martian Upper Atmosphere |url=http://www.jpl.nasa.gov/news/news.php?release=2014-351 |access-date=15 October 2014 |publisher=NASA |archive-date=19 October 2014 |archive-url=https://web.archive.org/web/20141019184946/http://www.jpl.nasa.gov/news/news.php?release=2014-351 |url-status=live}} {{PD-notice}}|300x300px]]

Compared to Earth, its higher concentration of atmospheric CO₂ and lower surface pressure may be why sound is attenuated more on Mars, where natural sources are rare apart from the wind. Using acoustic recordings collected by the Perseverance rover, researchers concluded that the speed of sound there is approximately 240 m/s for frequencies below 240 Hz, and 250 m/s for those above.{{Cite journal |last=Wright |first=Katherine |date=22 March 2022 |title=Sound Speed Measured on Mars |url=https://physics.aps.org/articles/v15/43 |journal=Physics |volume=15 |page=43 |doi=10.1103/Physics.15.43 |bibcode=2022PhyOJ..15...43W |s2cid=247720720 |doi-access=free |access-date=6 April 2022 |archive-date=12 April 2022 |archive-url=https://web.archive.org/web/20220412151942/https://physics.aps.org/articles/v15/43 |url-status=live}}{{Cite journal |last1=Maurice |first1=S. |last2=Chide |first2=B. |last3=Murdoch |first3=N. |last4=Lorenz |first4=R. D. |last5=Mimoun |first5=D. |last6=Wiens |first6=R. C. |last7=Stott |first7=A. |last8=Jacob |first8=X. |last9=Bertrand |first9=T. |last10=Montmessin |first10=F. |last11=Lanza |first11=N. L. |date=1 April 2022 |title=In situ recording of Mars soundscape |journal=Nature |volume=605 |issue=7911 |pages=653–658 |doi=10.1038/s41586-022-04679-0 |pmid=35364602 |pmc=9132769 |bibcode=2022Natur.605..653M |s2cid=247865804 |issn=0028-0836}}

Auroras have been detected on Mars.{{cite web |title=In an ultraviolet glow, auroras on Mars spotted by UAE orbiter |url=https://www.nbcnews.com/science/space/ultraviolet-glow-auroras-mars-spotted-uae-orbiter-rcna1356 |first=Denise |last=Chow |access-date=7 December 2021 |date=7 December 2021 |website=NBC News |language=en |archive-date=7 December 2021 |archive-url=https://web.archive.org/web/20211207221852/https://www.nbcnews.com/science/space/ultraviolet-glow-auroras-mars-spotted-uae-orbiter-rcna1356 |url-status=live}}{{cite web |title=Auroras on Mars – NASA Science |url=https://science.nasa.gov/science-news/science-at-nasa/2015/11may_aurorasonmars/ |website=science.nasa.gov |date=10 May 2015 |access-date=12 May 2015 |archive-date=14 May 2015 |archive-url=https://web.archive.org/web/20150514060513/http://science.nasa.gov/science-news/science-at-nasa/2015/11may_aurorasonmars/ |url-status=live}} {{PD-notice}}{{cite web |last1=Brown |first1=Dwayne |last2=Neal-Jones |first2=Nancy |last3=Steigerwald |first3=Bill |last4=Scott |first4=Jim |url=http://www.nasa.gov/press/2015/march/nasa-spacecraft-detects-aurora-and-mysterious-dust-cloud-around-mars |title=NASA Spacecraft Detects Aurora and Mysterious Dust Cloud around Mars |publisher=NASA |id=Release 15-045 |date=18 March 2015 |access-date=18 March 2015 |archive-date=19 March 2015 |archive-url=https://web.archive.org/web/20150319152358/http://www.nasa.gov/press/2015/march/nasa-spacecraft-detects-aurora-and-mysterious-dust-cloud-around-mars/ |url-status=live}} {{PD-notice}} Because Mars lacks a global magnetic field, the types and distribution of auroras there differ from those on Earth;{{Cite journal |last1=Deighan |first1=J. |last2=Jain |first2=S. K. |last3=Chaffin |first3=M. S. |last4=Fang |first4=X. |last5=Halekas |first5=J. S. |last6=Clarke |first6=J. T. |last7=Schneider |first7=N. M. |last8=Stewart |first8=A. I. F. |last9=Chaufray |first9=J.-Y. |last10=Evans |first10=J. S. |last11=Stevens |first11=M. H. |date=October 2018 |title=Discovery of a proton aurora at Mars |url=http://www.nature.com/articles/s41550-018-0538-5 |journal=Nature Astronomy |volume=2 |issue=10 |pages=802–807 |doi=10.1038/s41550-018-0538-5 |bibcode=2018NatAs...2..802D |s2cid=105560692 |issn=2397-3366 |access-date=5 April 2022 |archive-date=22 May 2022 |archive-url=https://web.archive.org/web/20220522145256/https://www.nature.com/articles/s41550-018-0538-5 |url-status=live}} rather than being mostly restricted to polar regions as is the case on Earth, a Martian aurora can encompass the planet.{{Cite journal |last1=Schneider |first1=N. M. |last2=Jain |first2=S. K. |last3=Deighan |first3=J. |last4=Nasr |first4=C. R. |last5=Brain |first5=D. A. |last6=Larson |first6=D. |last7=Lillis |first7=R. |last8=Rahmati |first8=Ali |last9=Halekas |first9=J. S. |last10=Lee |first10=C. O. |last11=Chaffin |first11=M. S. |date=16 August 2018 |title=Global Aurora on Mars During the September 2017 Space Weather Event |journal=Geophysical Research Letters |volume=45 |issue=15 |pages=7391–7398 |doi=10.1029/2018GL077772 |bibcode=2018GeoRL..45.7391S |s2cid=115149852 |doi-access=free |hdl=10150/631256 |hdl-access=free}} In September 2017, NASA reported radiation levels on the surface of the planet Mars were temporarily doubled, and were associated with an aurora 25 times brighter than any observed earlier, due to a massive, and unexpected, solar storm in the middle of the month.{{cite web |url=https://www.nasa.gov/feature/jpl/large-solar-storm-sparks-global-aurora-and-doubles-radiation-levels-on-the-martian-surface |title=Large Solar Storm Sparks Global Aurora and Doubles Radiation Levels on the Martian Surface |publisher=NASA |first1=Guy |last1=Webster |first2=Nancy |last2=Neal-Jones |first3=Jim |last3=Scott |first4=Deb |last4=Schmid |first5=Laurie |last5=Cantillo |first6=Dwayne |last6=Brown |date=29 September 2017 |access-date=9 January 2018 |archive-date=1 October 2017 |archive-url=https://web.archive.org/web/20171001181055/https://www.nasa.gov/feature/jpl/large-solar-storm-sparks-global-aurora-and-doubles-radiation-levels-on-the-martian-surface/ |url-status=dead}} {{PD-notice}}

{{Main|Climate of Mars}}

Mars has seasons, alternating between its northern and southern hemispheres, similar to on Earth. Additionally the orbit of Mars has, compared to Earth's, a large eccentricity and approaches perihelion when it is summer in its southern hemisphere and winter in its northern, and aphelion when it is winter in its southern hemisphere and summer in its northern. As a result, the seasons in its southern hemisphere are more extreme and the seasons in its northern are milder than would otherwise be the case. The summer temperatures in the south can be warmer than the equivalent summer temperatures in the north by up to {{convert|30|C-change}}.

Martian surface temperatures vary from lows of about {{convert|-110|C|F}} to highs of up to {{convert|35|C|F}} in equatorial summer. The wide range in temperatures is due to the thin atmosphere which cannot store much solar heat, the low atmospheric pressure (about 1% that of the atmosphere of Earth), and the low thermal inertia of Martian soil. The planet is 1.52 times as far from the Sun as Earth, resulting in just 43% of the amount of sunlight.{{cite web |last=Hille |first=Karl |date=18 September 2015 |title=The Fact and Fiction of Martian Dust Storms |url=http://www.nasa.gov/feature/goddard/the-fact-and-fiction-of-martian-dust-storms |access-date=25 December 2021 |website=NASA |archive-date=2 March 2016 |archive-url=https://web.archive.org/web/20160302231423/http://www.nasa.gov/feature/goddard/the-fact-and-fiction-of-martian-dust-storms/ |url-status=live}}

File:Mars Before and During Global Dust Storm.jpg

Mars has the largest dust storms in the Solar System, reaching speeds of over {{convert|100|mph|km/h|order=flip|abbr=on}}. These can vary from a storm over a small area, to gigantic storms that cover the entire planet. They tend to occur when Mars is closest to the Sun, and have been shown to increase global temperature.

File:PIA22546-Mars-AnnualCO2ice-N&SPoles-20180806.gif (not water ice) coverage on the north (left) and south (right) poles of Mars between northern and southern summer.]]

Seasons also produce dry ice covering polar ice caps.{{cn|date=February 2025}}

{{Main|Water on Mars}}

File:PIA26046-Mars-BuriedIceMaps-20231026.jpg

File:Perspective view of Korolev crater.jpg, seen here in 3D projection.]]

While Mars contains water in larger amounts, most of it is dust covered water ice at the Martian polar ice caps.{{Cite journal |last1=Holt |first1=John W. |last2=Safaeinili |first2=Ali |last3=Plaut |first3=Jeffrey J. |last4=Head |first4=James W. |last5=Phillips |first5=Roger J. |last6=Seu |first6=Roberto |last7=Kempf |first7=Scott D. |last8=Choudhary |first8=Prateek |last9=Young |first9=Duncan A. |last10=Putzig |first10=Nathaniel E. |last11=Biccari |first11=Daniela |date=21 November 2008 |title=Radar Sounding Evidence for Buried Glaciers in the Southern Mid-Latitudes of Mars |url=https://www.science.org/doi/10.1126/science.1164246 |journal=Science |volume=322 |issue=5905 |pages=1235–1238 |bibcode=2008Sci...322.1235H |doi=10.1126/science.1164246 |issn=0036-8075 |jstor=20145331 |pmid=19023078 |s2cid=36614186 |hdl=11573/67950 |access-date=22 April 2022 |archive-date=22 April 2022 |archive-url=https://web.archive.org/web/20220422162701/https://www.science.org/doi/10.1126/science.1164246 |url-status=live}}{{cite web |title=Polar Caps |url=https://marsed.asu.edu/mep/ice/polar-caps |access-date=6 January 2022 |website=Mars Education at Arizona State University |archive-date=28 May 2021 |archive-url=https://web.archive.org/web/20210528133135/https://marsed.asu.edu/mep/ice/polar-caps |url-status=live}}

The volume of water ice in the south polar ice cap, if melted, would be enough to cover most of the surface of the planet with a depth of {{convert|11|m}}.

Water in its liquid form cannot prevail on the surface of Mars due to the low atmospheric pressure on Mars, which is less than 1% that of Earth,{{cite web |title=NASA – NASA Rover Finds Clues to Changes in Mars' Atmosphere |url=http://www.nasa.gov/mission_pages/msl/news/msl20121102.html |publisher=NASA |access-date=19 October 2014 |archive-date=26 December 2018 |archive-url=https://web.archive.org/web/20181226060801/https://www.nasa.gov/mission_pages/msl/news/msl20121102.html%20 |url-status=dead}} {{PD-notice}} only at the lowest of elevations pressure and temperature is high enough for water being able to be liquid for short periods.

Water in the atmosphere is small, but enough to produce larger clouds of water ice and different cases of snow and frost, often mixed with snow of carbon dioxide dry ice.

Landforms visible on Mars strongly suggest that liquid water has existed on the planet's surface. Huge linear swathes of scoured ground, known as outflow channels, cut across the surface in about 25 places. These are thought to be a record of erosion caused by the catastrophic release of water from subsurface aquifers, though some of these structures have been hypothesized to result from the action of glaciers or lava. One of the larger examples, Ma'adim Vallis, is {{convert|700|km}} long, much greater than the Grand Canyon, with a width of {{convert|20|km}} and a depth of {{convert|2|km}} in places. It is thought to have been carved by flowing water early in Mars's history. The youngest of these channels is thought to have formed only a few million years ago.

Elsewhere, particularly on the oldest areas of the Martian surface, finer-scale, dendritic networks of valleys are spread across significant proportions of the landscape. Features of these valleys and their distribution strongly imply that they were carved by runoff resulting from precipitation in early Mars history. Subsurface water flow and groundwater sapping may play important subsidiary roles in some networks, but precipitation was probably the root cause of the incision in almost all cases.

Along craters and canyon walls, there are thousands of features that appear similar to terrestrial gullies. The gullies tend to be in the highlands of the Southern Hemisphere and face the Equator; all are poleward of 30° latitude. A number of authors have suggested that their formation process involves liquid water, probably from melting ice, although others have argued for formation mechanisms involving carbon dioxide frost or the movement of dry dust. No partially degraded gullies have formed by weathering and no superimposed impact craters have been observed, indicating that these are young features, possibly still active. Other geological features, such as deltas and alluvial fans preserved in craters, are further evidence for warmer, wetter conditions at an interval or intervals in earlier Mars history. Such conditions necessarily require the widespread presence of crater lakes across a large proportion of the surface, for which there is independent mineralogical, sedimentological and geomorphological evidence. Further evidence that liquid water once existed on the surface of Mars comes from the detection of specific minerals such as hematite and goethite, both of which sometimes form in the presence of water.

{{Main|Chronology of discoveries of water on Mars}}

In 2004, Opportunity detected the mineral jarosite. This forms only in the presence of acidic water, showing that water once existed on Mars.{{Cite journal |last1=Elwood Madden |first1=M. E. |last2=Bodnar |first2=R. J. |last3=Rimstidt |first3=J. D. |date=October 2004 |title=Jarosite as an indicator of water-limited chemical weathering on Mars |url=http://www.nature.com/articles/nature02971 |journal=Nature |volume=431 |issue=7010 |pages=821–823 |doi=10.1038/nature02971 |issn=0028-0836 |pmid=15483605 |s2cid=10965423 |access-date=22 April 2022 |archive-date=3 March 2022 |archive-url=https://web.archive.org/web/20220303070003/https://www.nature.com/articles/nature02971 |url-status=live}} The Spirit rover found concentrated deposits of silica in 2007 that indicated wet conditions in the past, and in December 2011, the mineral gypsum, which also forms in the presence of water, was found on the surface by NASA's Mars rover Opportunity.{{Cite web |date=10 December 2007 |title=Mars Rover Investigates Signs of Steamy Martian Past |url=https://www.jpl.nasa.gov/news/mars-rover-investigates-signs-of-steamy-martian-past |access-date=5 April 2022 |website=NASA Jet Propulsion Laboratory (JPL) |archive-date=6 July 2022 |archive-url=https://web.archive.org/web/20220706160839/https://www.jpl.nasa.gov/news/mars-rover-investigates-signs-of-steamy-martian-past |url-status=live}} It is estimated that the amount of water in the upper mantle of Mars, represented by hydroxyl ions contained within Martian minerals, is equal to or greater than that of Earth at 50–300 parts per million of water, which is enough to cover the entire planet to a depth of {{convert|200|-|1000|m}}.{{Cite journal |last1=McCubbin |first1=Francis M. |last2=Hauri |first2=Erik H. |last3=Elardo |first3=Stephen M. |last4=Vander Kaaden |first4=Kathleen E. |last5=Wang |first5=Jianhua |last6=Shearer |first6=Charles K. |date=August 2012 |title=Hydrous melting of the martian mantle produced both depleted and enriched shergottites |url=http://pubs.geoscienceworld.org/geology/article/40/8/683/130947/Hydrous-melting-of-the-martian-mantle-produced |journal=Geology |volume=40 |issue=8 |pages=683–686 |bibcode=2012Geo....40..683M |doi=10.1130/G33242.1 |issn=1943-2682}}

On 18 March 2013, NASA reported evidence from instruments on the Curiosity rover of mineral hydration, likely hydrated calcium sulfate, in several rock samples including the broken fragments of "Tintina" rock and "Sutton Inlier" rock as well as in veins and nodules in other rocks like "Knorr" rock and "Wernicke" rock.{{cite web |last1=Webster |first1=Guy |last2=Brown |first2=Dwayne |date=18 March 2013 |title=Curiosity Mars Rover Sees Trend in Water Presence |url=http://mars.jpl.nasa.gov/msl/news/whatsnew/index.cfm?FuseAction=ShowNews&NewsID=1446 |url-status=dead |archive-url=https://web.archive.org/web/20130424111259/http://mars.jpl.nasa.gov/msl/news/whatsnew/index.cfm?FuseAction=ShowNews&NewsID=1446 |archive-date=24 April 2013 |access-date=20 March 2013 |publisher=NASA}} {{PD-notice}}{{cite news |last=Rincon |first=Paul |date=19 March 2013 |title=Curiosity breaks rock to reveal dazzling white interior |work=BBC News |publisher=BBC |url=https://www.bbc.co.uk/news/science-environment-21340279 |access-date=19 March 2013 |archive-date=8 March 2021 |archive-url=https://web.archive.org/web/20210308051408/https://www.bbc.co.uk/news/science-environment-21340279 |url-status=live}} Analysis using the rover's DAN instrument provided evidence of subsurface water, amounting to as much as 4% water content, down to a depth of {{convert|60|cm}}, during the rover's traverse from the Bradbury Landing site to the Yellowknife Bay area in the Glenelg terrain. In September 2015, NASA announced that they had found strong evidence of hydrated brine flows in recurring slope lineae, based on spectrometer readings of the darkened areas of slopes.{{cite web |date=28 September 2015 |title=NASA Confirms Evidence That Liquid Water Flows on Today's Mars |url=http://www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars |access-date=28 September 2015 |publisher=NASA |archive-date=28 September 2015 |archive-url=https://web.archive.org/web/20150928161524/http://www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars/ |url-status=live}} {{PD-notice}}{{cite web |last=Drake |first=Nadia |date=28 September 2015 |title=NASA Finds 'Definitive' Liquid Water on Mars |url=http://news.nationalgeographic.com/2015/09/150928-mars-liquid-water-confirmed-surface-streaks-space-astronomy/ |archive-url=https://web.archive.org/web/20150930194303/http://news.nationalgeographic.com/2015/09/150928-mars-liquid-water-confirmed-surface-streaks-space-astronomy/ |url-status=dead |archive-date=30 September 2015 |access-date=29 September 2015 |website=National Geographic News}}{{cite journal |last1=Ojha |first1=L. |last2=Wilhelm |first2=M. B. |last3=Murchie |first3=S. L. |last4=McEwen |first4=A. S. |last5=Wray |first5=J. J. |last6=Hanley |first6=J. |last7=Massé |first7=M. |last8=Chojnacki |first8=M. |year=2015 |title=Spectral evidence for hydrated salts in recurring slope lineae on Mars |journal=Nature Geoscience |volume=8 |issue=11 |pages=829–832 |bibcode=2015NatGe...8..829O |doi=10.1038/ngeo2546 |s2cid=59152931}} These streaks flow downhill in Martian summer, when the temperature is above −23 °C, and freeze at lower temperatures.{{cite web |last=Moskowitz |first=Clara |title=Water Flows on Mars Today, NASA Announces |url=http://www.scientificamerican.com/article/water-flows-on-mars-today-nasa-announces/ |access-date=29 September 2015 |website=Scientific American |archive-date=15 May 2021 |archive-url=https://web.archive.org/web/20210515071020/https://www.scientificamerican.com/article/water-flows-on-mars-today-nasa-announces/ |url-status=live}} These observations supported earlier hypotheses, based on timing of formation and their rate of growth, that these dark streaks resulted from water flowing just below the surface.{{cite journal |last1=McEwen |first1=Alfred |last2=Lujendra |first2=Ojha |last3=Dundas |first3=Colin |last4=Mattson |first4=Sarah |last5=Bryne |first5=S |last6=Wray |first6=J |last7=Cull |first7=Selby |last8=Murchie |first8=Scott |last9=Thomas |first9=Nicholas |last10=Gulick |first10=Virginia |date=5 August 2011 |title=Seasonal Flows on Warm Martian Slopes |url=https://www.science.org/doi/10.1126/science.1204816 |journal=Science |volume=333 |issue=6043 |pages=740–743 |bibcode=2011Sci...333..740M |doi=10.1126/science.1204816 |pmid=21817049 |s2cid=10460581 |archive-url=https://web.archive.org/web/20150929112931/https://sciencescape.org/paper/21817049 |archive-date=29 September 2015 |access-date=28 September 2015}} However, later work suggested that the lineae may be dry, granular flows instead, with at most a limited role for water in initiating the process.{{Cite journal |last1=Dundas |first1=Colin M. |last2=McEwen |first2=Alfred S. |last3=Chojnacki |first3=Matthew |last4=Milazzo |first4=Moses P. |last5=Byrne |first5=Shane |last6=McElwaine |first6=Jim N. |last7=Urso |first7=Anna |date=December 2017 |title=Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water |url=https://www.nature.com/articles/s41561-017-0012-5 |journal=Nature Geoscience |volume=10 |issue=12 |pages=903–907 |bibcode=2017NatGe..10..903D |doi=10.1038/s41561-017-0012-5 |issn=1752-0908 |s2cid=24606098 |hdl-access=free |hdl=10150/627918 |access-date=3 April 2022 |archive-date=22 November 2017 |archive-url=https://web.archive.org/web/20171122143221/https://www.nature.com/articles/s41561-017-0012-5 |url-status=live}} A definitive conclusion about the presence, extent, and role of liquid water on the Martian surface remains elusive.{{Cite journal |last=Schorghofer |first=Norbert |date=12 February 2020 |title=Mars: Quantitative Evaluation of Crocus Melting behind Boulders |journal=The Astrophysical Journal |volume=890 |issue=1 |pages=49 |bibcode=2020ApJ...890...49S |doi=10.3847/1538-4357/ab612f |issn=1538-4357 |s2cid=213701664 |doi-access=free}}{{Cite journal |last=Wray |first=James J. |date=30 May 2021 |title=Contemporary Liquid Water on Mars? |url=https://www.annualreviews.org/doi/10.1146/annurev-earth-072420-071823 |journal=Annual Review of Earth and Planetary Sciences |volume=49 |issue=1 |pages=141–171 |bibcode=2021AREPS..49..141W |doi=10.1146/annurev-earth-072420-071823 |issn=0084-6597 |s2cid=229425641 |access-date=3 April 2022 |archive-date=3 May 2021 |archive-url=https://web.archive.org/web/20210503080828/https://www.annualreviews.org/doi/10.1146/annurev-earth-072420-071823 |url-status=live}}

Researchers suspect much of the low northern plains of the planet were covered with an ocean hundreds of meters deep, though this theory remains controversial. In March 2015, scientists stated that such an ocean might have been the size of Earth's Arctic Ocean. This finding was derived from the ratio of protium to deuterium in the modern Martian atmosphere compared to that ratio on Earth. The amount of Martian deuterium (D/H = 9.3 ± 1.7 10⁻⁴) is five to seven times the amount on Earth (D/H = 1.56 10⁻⁴), suggesting that ancient Mars had significantly higher levels of water. Results from the Curiosity rover had previously found a high ratio of deuterium in Gale Crater, though not significantly high enough to suggest the former presence of an ocean. Other scientists caution that these results have not been confirmed, and point out that Martian climate models have not yet shown that the planet was warm enough in the past to support bodies of liquid water.{{cite news |last=Kaufman |first=Marc |date=5 March 2015 |title=Mars Had an Ocean, Scientists Say, Pointing to New Data |work=The New York Times |url=https://www.nytimes.com/2015/03/06/science/mars-had-an-ocean-scientists-say-pointing-to-new-data.html |access-date=5 March 2015 |archive-date=7 March 2020 |archive-url=https://web.archive.org/web/20200307000937/https://www.nytimes.com/2015/03/06/science/mars-had-an-ocean-scientists-say-pointing-to-new-data.html |url-status=live}} Near the northern polar cap is the {{convert|81.4|km}} wide Korolev Crater, which the Mars Express orbiter found to be filled with approximately {{convert|2200|km3}} of water ice.{{cite news |last1=Sample |first1=Ian |date=21 December 2018 |title=Mars Express beams back images of ice-filled Korolev crater |newspaper=The Guardian |url=https://www.theguardian.com/science/2018/dec/21/mars-express-beams-back-images-of-ice-filled-korolev-crater |access-date=21 December 2018 |archive-date=8 February 2020 |archive-url=https://web.archive.org/web/20200208045902/https://www.theguardian.com/science/2018/dec/21/mars-express-beams-back-images-of-ice-filled-korolev-crater |url-status=live}}

In November 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior (which is 12,100 cubic kilometers{{Cite web |date=29 October 2010 |title=EPA; Great Lakes; Physical Facts |url=http://www.epa.gov/glnpo/physfacts.html |archive-url=https://web.archive.org/web/20101029215637/http://www.epa.gov/glnpo/physfacts.html |archive-date=29 October 2010 |access-date=15 February 2023}}).{{cite web |date=22 November 2016 |title=Mars Ice Deposit Holds as Much Water as Lake Superior |url=http://www.jpl.nasa.gov/news/news.php?release=2016-299 |access-date=23 November 2016 |publisher=NASA |archive-date=26 December 2018 |archive-url=https://web.archive.org/web/20181226060807/https://www.jpl.nasa.gov/news/ |url-status=live}} {{PD-notice}}{{cite web |author=Staff |date=22 November 2016 |title=Scalloped Terrain Led to Finding of Buried Ice on Mars |url=http://photojournal.jpl.nasa.gov/catalog/PIA21136 |access-date=23 November 2016 |publisher=NASA |archive-date=26 December 2018 |archive-url=https://web.archive.org/web/20181226060804/https://photojournal.jpl.nasa.gov/catalog/PIA21136%20 |url-status=live}} {{PD-notice}} During observations from 2018 through 2021, the ExoMars Trace Gas Orbiter spotted indications of water, probably subsurface ice, in the Valles Marineris canyon system.{{Cite journal |last1=Mitrofanov |first1=I. |last2=Malakhov |first2=A. |last3=Djachkova |first3=M. |last4=Golovin |first4=D. |last5=Litvak |first5=M. |last6=Mokrousov |first6=M. |last7=Sanin |first7=A. |last8=Svedhem |first8=H. |last9=Zelenyi |first9=L. |date=March 2022 |title=The evidence for unusually high hydrogen abundances in the central part of Valles Marineris on Mars |journal=Icarus |volume=374 |pages=114805 |bibcode=2022Icar..37414805M |doi=10.1016/j.icarus.2021.114805 |s2cid=244449654 |doi-access=free}}

{{Main|Orbit of Mars}}

{{See also|Timekeeping on Mars}}{{Anchor|Viewing from Earth|Orbit and rotation}}File:Solar system orrery inner planets.gif

Mars's average distance from the Sun is roughly {{convert|143|e6mi|e6km|abbr=unit|order=flip}}, and its orbital period is 687 (Earth) days. The solar day (or sol) on Mars is only slightly longer than an Earth day: 24 hours, 39 minutes, and 35.244 seconds.{{cite book |last1=Badescu |first1=Viorel |url=https://books.google.com/books?id=BnPE37Ms5awC&pg=PA600 |title=Mars: Prospective Energy and Material Resources |date=2009 |publisher=Springer Science & Business Media |isbn=978-3-642-03629-3 |edition=illustrated |page=600 |access-date=20 May 2016 |archive-date=4 March 2023 |archive-url=https://web.archive.org/web/20230304004803/https://books.google.com/books?id=BnPE37Ms5awC&pg=PA600 |url-status=live}} A Martian year is equal to 1.8809 Earth years, or 1 year, 320 days, and 18.2 hours. The gravitational potential difference and thus the delta-v needed to transfer between Mars and Earth is the second lowest for Earth.{{cite journal |last1=Petropoulos |first1=Anastassios E. |last2=Longuski |first2=James M. |last3=Bonfiglio |first3=Eugene P. |year=2000 |title=Trajectories to Jupiter via Gravity Assists from Venus, Earth, and Mars |journal=Journal of Spacecraft and Rockets |publisher=American Institute of Aeronautics and Astronautics (AIAA) |volume=37 |issue=6 |pages=776–783 |bibcode=2000JSpRo..37..776P |doi=10.2514/2.3650 |issn=0022-4650}}{{cite web |last=Taylor |first=Chris |date=9 July 2020 |title=Welcome to Cloud City: The case for going to Venus, not Mars |url=https://mashable.com/feature/venus-mars-space-exploration |access-date=21 October 2022 |website=Mashable |archive-date=21 October 2022 |archive-url=https://web.archive.org/web/20221021222623/https://mashable.com/feature/venus-mars-space-exploration |url-status=live}}

The axial tilt of Mars is 25.19° relative to its orbital plane, which is similar to the axial tilt of Earth. As a result, Mars has seasons like Earth, though on Mars they are nearly twice as long because its orbital period is that much longer. In the present day, the orientation of the north pole of Mars is close to the star Deneb.

Mars has a relatively pronounced orbital eccentricity of about 0.09; of the seven other planets in the Solar System, only Mercury has a larger orbital eccentricity. It is known that in the past, Mars has had a much more circular orbit. At one point, 1.35 million Earth years ago, Mars had an eccentricity of roughly 0.002, much less than that of Earth today. Mars's cycle of eccentricity is 96,000 Earth years compared to Earth's cycle of 100,000 years.

Mars has its closest approach to Earth (opposition) in a synodic period of 779.94 days. It should not be confused with Mars conjunction, where the Earth and Mars are at opposite sides of the Solar System and form a straight line crossing the Sun. The average time between the successive oppositions of Mars, its synodic period, is 780 days; but the number of days between successive oppositions can range from 764 to 812. The distance at close approach varies between about {{convert|54|and|103|e6km|e6mi|abbr=unit}} due to the planets' elliptical orbits, which causes comparable variation in angular size. At their furthest Mars and Earth can be as far as {{convert|401|e6km|e6mi|abbr=unit}} apart.{{cite web |last1=Tillman |first1=Nola Taylor |last2=Dobrijevic |first2=Daisy |title=How Long Does It Take to Get to Mars? |website=Space.com |date=2022-01-20 |url=https://www.space.com/24701-how-long-does-it-take-to-get-to-mars.html#:~:text=The%20two%20planets%20are%20farthest,miles%20(225%20million%20km). |access-date=2024-08-23}} Mars comes into opposition from Earth every 2.1 years. The planets come into opposition near Mars's perihelion in 2003, 2018 and 2035, with the 2020 and 2033 events being particularly close to perihelic opposition.{{cite web |title=Mars Opposition |url=https://mars.nasa.gov/all-about-mars/night-sky/opposition |access-date=7 December 2021 |website=NASA's Mars Exploration Program |date=10 November 2017 |archive-date=5 October 2023 |archive-url=https://web.archive.org/web/20231005222312/https://mars.nasa.gov/all-about-mars/night-sky/opposition/ |url-status=live}}{{cite web |date=5 October 2021 |title=Why is Mars sometimes bright and sometimes faint? |url=https://earthsky.org/astronomy-essentials/why-is-mars-sometimes-bright-and-sometimes-faint/ |access-date=7 December 2021 |website=EarthSky |archive-date=7 December 2021 |archive-url=https://web.archive.org/web/20211207122120/https://earthsky.org/astronomy-essentials/why-is-mars-sometimes-bright-and-sometimes-faint/ |url-status=live}}{{Cite web |date=3 November 2005 |title=Close encounter: Mars at opposition |url=https://www.spacetelescope.org/images/opo0534l/ |access-date=1 April 2022 |website=ESA/Hubble |archive-date=10 September 2015 |archive-url=https://web.archive.org/web/20150910011439/http://www.spacetelescope.org/images/opo0534l/ |url-status=live}}

File:Mars - 2020 Opposition (crop).jpg

The mean apparent magnitude of Mars is +0.71 with a standard deviation of 1.05. Because the orbit of Mars is eccentric, the magnitude at opposition from the Sun can range from about −3.0 to −1.4. The minimum brightness is magnitude +1.86 when the planet is near aphelion and in conjunction with the Sun. At its brightest, Mars (along with Jupiter) is second only to Venus in apparent brightness. Mars usually appears distinctly yellow, orange, or red. When farthest away from Earth, it is more than seven times farther away than when it is closest. Mars is usually close enough for particularly good viewing once or twice at 15-year or 17-year intervals.{{cite web |title=Mars Close Approach |url=https://mars.nasa.gov/all-about-mars/night-sky/close-approach |access-date=18 January 2022 |website=NASA's Mars Exploration Program |date=10 November 2017 |archive-date=8 November 2019 |archive-url=https://web.archive.org/web/20191108232313/https://mars.nasa.gov/all-about-mars/night-sky/close-approach/ |url-status=live}} Optical ground-based telescopes are typically limited to resolving features about {{convert|300|km}} across when Earth and Mars are closest because of Earth's atmosphere.{{cite web |title=Slide 2 Earth Telescope View of Mars |url=http://www.lpi.usra.edu/publications/slidesets/redplanet2/slide_2.html |work=The Red Planet: A Survey of Mars |publisher=Lunar and Planetary Institute |access-date=28 November 2011 |archive-date=18 May 2021 |archive-url=https://web.archive.org/web/20210518203239/https://www.lpi.usra.edu/publications/slidesets/redplanet2/slide_2.html |url-status=live}}

As Mars approaches opposition, it begins a period of retrograde motion, which means it will appear to move backwards in a looping curve with respect to the background stars. This retrograde motion lasts for about 72 days, and Mars reaches its peak apparent brightness in the middle of this interval.

{{Main|Moons of Mars|Phobos (moon)|Deimos (moon)}}

{{Multiple image

| align = right

| direction = horizontal

| total_width = 350

| image1 = Phobos colour 2008.jpg

| caption1 = Enhanced-color HiRISE image of Phobos, showing a series of mostly parallel grooves and crater chains, with Stickney crater at right

| image2 = Deimos-MRO.jpg

| caption2 = Enhanced-color HiRISE image of Deimos (not to scale), showing its smooth blanket of regolith

}}

Mars has two relatively small (compared to Earth's) natural moons, Phobos (about {{convert|22|km}} in diameter) and Deimos (about {{convert|12|km}} in diameter), which orbit close to the planet. The origin of both moons is unclear, although a popular theory states that they were asteroids captured into Martian orbit.

Both satellites were discovered in 1877 by Asaph Hall and were named after the characters Phobos (the deity of panic and fear) and Deimos (the deity of terror and dread), twins from Greek mythology who accompanied their father Ares, god of war, into battle.{{Cite web |title=Planetary Names |url=https://planetarynames.wr.usgs.gov/Page/Planets#MartianSystem |access-date=30 May 2022 |website=planetarynames.wr.usgs.gov |archive-date=30 December 2023 |archive-url=https://web.archive.org/web/20231230164721/https://planetarynames.wr.usgs.gov/Page/Planets#MartianSystem |url-status=live}} Mars was the Roman equivalent to Ares. In modern Greek, the planet retains its ancient name Ares (Aris: Άρης).

From the surface of Mars, the motions of Phobos and Deimos appear different from that of the Earth's satellite, the Moon. Phobos rises in the west, sets in the east, and rises again in just 11 hours. Deimos, being only just outside synchronous orbit{{Snd}}where the orbital period would match the planet's period of rotation{{Snd}}rises as expected in the east, but slowly. Because the orbit of Phobos is below a synchronous altitude, tidal forces from Mars are gradually lowering its orbit. In about 50 million years, it could either crash into Mars's surface or break up into a ring structure around the planet.{{cite web |title=Phobos |url=https://solarsystem.nasa.gov/moons/mars-moons/phobos/in-depth |access-date=12 January 2022 |date=19 December 2019 |website=NASA Solar System Exploration |archive-date=12 January 2022 |archive-url=https://web.archive.org/web/20220112004526/https://solarsystem.nasa.gov/moons/mars-moons/phobos/in-depth/ |url-status=live}}

The origin of the two satellites is not well understood. Their low albedo and carbonaceous chondrite composition have been regarded as similar to asteroids, supporting a capture theory. The unstable orbit of Phobos would seem to point toward a relatively recent capture. But both have circular orbits near the equator, which is unusual for captured objects, and the required capture dynamics are complex. Accretion early in the history of Mars is plausible, but would not account for a composition resembling asteroids rather than Mars itself, if that is confirmed.{{cite web |title=Explaining the Birth of the Martian Moons |date=23 September 2016 |url=https://aasnova.org/2016/09/23/explaining-the-birth-of-the-martian-moons/ |access-date=13 December 2021 |language=en-US |website=AAS Nova |publisher=American Astronomical Society |archive-date=13 December 2021 |archive-url=https://web.archive.org/web/20211213022350/https://aasnova.org/2016/09/23/explaining-the-birth-of-the-martian-moons/ |url-status=live}} Mars may have yet-undiscovered moons, smaller than {{convert|50|to|100|m}} in diameter, and a dust ring is predicted to exist between Phobos and Deimos.{{cite conference |last1=Adler |first1=M. |last2=Owen |first2=W. |last3=Riedel |first3=J. |date=June 2012 |title=Use of MRO Optical Navigation Camera to Prepare for Mars Sample Return |url=http://www.lpi.usra.edu/meetings/marsconcepts2012/pdf/4337.pdf |conference=Concepts and Approaches for Mars Exploration. 12–14 June 2012. Houston, Texas. |issue=1679 |at=4337 |bibcode=2012LPICo1679.4337A |access-date=28 August 2012 |archive-date=26 December 2018 |archive-url=https://web.archive.org/web/20181226060810/https://www.lpi.usra.edu/meetings/marsconcepts2012/pdf/4337.pdf%20 |url-status=live}}

A third possibility for their origin as satellites of Mars is the involvement of a third body or a type of impact disruption. More-recent lines of evidence for Phobos having a highly porous interior, and suggesting a composition containing mainly phyllosilicates and other minerals known from Mars, point toward an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit, similar to the prevailing theory for the origin of Earth's satellite. Although the visible and near-infrared (VNIR) spectra of the moons of Mars resemble those of outer-belt asteroids, the thermal infrared spectra of Phobos are reported to be inconsistent with chondrites of any class. It is also possible that Phobos and Deimos were fragments of an older moon, formed by debris from a large impact on Mars, and then destroyed by a more recent impact upon the satellite.{{cite journal |title=Dynamical evidence for Phobos and Deimos as remnants of a disrupted common progenitor |doi=10.1038/s41550-021-01306-2 |journal=Nature Astronomy |date=22 February 2021 |volume=5 |pages=539–543 |first1=Amirhossein |last1=Bagheri |first2=Amir |last2=Khan |first3=Michael |last3=Efroimsky |first4=Mikhail |last4=Kruglyakov |first5=Domenico |last5=Giardini |issue=6 |bibcode=2021NatAs...5..539B |s2cid=233924981}}

{{Main|Exploration of Mars|History of Mars observation}}

The history of observations of Mars is marked by oppositions of Mars when the planet is closest to Earth and hence is most easily visible, which occur every couple of years. Even more notable are the perihelic oppositions of Mars, which are distinguished because Mars is close to perihelion, making it even closer to Earth.

The ancient Sumerians named Mars Nergal, the god of war and plague. During Sumerian times, Nergal was a minor deity of little significance, but, during later times, his main cult center was the city of Nineveh.{{cite book |last1=Rabkin |first1=Eric S. |url=https://books.google.com/books?id=a2QP30zybNkC&pg=PA11 |title=Mars: A Tour of the Human Imagination |date=2005 |publisher=Praeger |isbn=978-0-275-98719-0 |location=Westport, Connecticut |pages=9–11}} In Mesopotamian texts, Mars is referred to as the "star of judgement of the fate of the dead".{{cite book |last1=Thompson |first1=Henry O. |url=https://books.google.com/books?id=kc0UAAAAIAAJ&pg=PA125 |title=Mekal: The God of Beth-Shan |date=1970 |publisher=E. J. Brill |location=Leiden |page=125}} The existence of Mars as a wandering object in the night sky was also recorded by the ancient Egyptian astronomers and, by 1534 BCE, they were familiar with the retrograde motion of the planet. By the period of the Neo-Babylonian Empire, the Babylonian astronomers were making regular records of the positions of the planets and systematic observations of their behavior. For Mars, they knew that the planet made 37 synodic periods, or 42 circuits of the zodiac, every 79 years. They invented arithmetic methods for making minor corrections to the predicted positions of the planets. In Ancient Greece, the planet was known as {{langx|grc|Πυρόεις|label=none}}.{{cite book |last=Cicero |first=Marcus Tullius |url=https://archive.org/details/marcitulliicicer00cice |title=De Natura Deorum |publisher=Methuen |year=1896 |location=London |translator=Francis Brooks |trans-title=On the Nature of the Gods}} Commonly, the Greek name for the planet now referred to as Mars, was Ares. It was the Romans who named the planet Mars, for their god of war, often represented by the sword and shield of the planet's namesake.{{Cite web |last=NASA |date=9 October 2022 |title=All About Mars |url=https://mars.nasa.gov/allaboutmars/mystique/history/early/ |website=mars.nasa.gov |access-date=10 October 2022 |archive-date=10 October 2022 |archive-url=https://web.archive.org/web/20221010032729/https://mars.nasa.gov/allaboutmars/mystique/history/early/ |url-status=live}}

In the fourth century BCE, Aristotle noted that Mars disappeared behind the Moon during an occultation, indicating that the planet was farther away.{{Cite journal |last=Stephenson |first=F. Richard |date=November 2000 |title=A Lunar Occultation of Mars Observed by Aristotle |journal=Journal for the History of Astronomy |volume=31 |issue=4 |pages=342–344 |bibcode=2000JHA....31..342S |doi=10.1177/002182860003100405 |issn=0021-8286 |s2cid=125518456 }} Ptolemy, a Greek living in Alexandria, attempted to address the problem of the orbital motion of Mars. Ptolemy's model and his collective work on astronomy was presented in the multi-volume collection later called the Almagest (from the Arabic for "greatest"), which became the authoritative treatise on Western astronomy for the next fourteen centuries.{{citation |last=McCluskey |first=S. C. |title=Astronomies and Cultures in Early Medieval Europe |pages=20–21 |year=1998 |publisher=Cambridge University Press |isbn=978-0-521-77852-7}} Literature from ancient China confirms that Mars was known by Chinese astronomers by no later than the fourth century BCE. In the East Asian cultures, Mars is traditionally referred to as the "fire star" based on the Wuxing system.{{cite book |last=de Groot |first=Jan Jakob Maria |title=Religion in China – Universism: A Key to the Study of Taoism and Confucianism |date=1912 |publisher=G. P. Putnam's Sons |series=American Lectures on the History of Religions, volume 10 |page=300 |chapter=Fung Shui |oclc=491180 |chapter-url=https://books.google.com/books?id=ZAaP7dyjCrAC&pg=PA300 |access-date=5 January 2016}}{{cite book |last=Crump |first=Thomas |url=https://books.google.com/books?id=mk3vxD3Ib_sC&pg=PT43 |title=The Japanese Numbers Game: The Use and Understanding of Numbers in Modern Japan |date=1992 |publisher=Routledge |isbn=978-0-415-05609-0 |series=Nissan Institute/Routledge Japanese Studies Series |pages=39–40}}{{cite book |last=Hulbert |first=Homer Bezaleel |url=https://archive.org/details/passingkorea01hulbgoog |title=The Passing of Korea |date=1909 |publisher=Doubleday, Page & Company |page=[https://archive.org/details/passingkorea01hulbgoog/page/n538 426] |oclc=26986808 |orig-year=1906}}

During the seventeenth century A.D., Tycho Brahe measured the diurnal parallax of Mars that Johannes Kepler used to make a preliminary calculation of the relative distance to the planet. From Brahe's observations of Mars, Kepler deduced that the planet orbited the Sun not in a circle, but in an ellipse. Moreover, Kepler showed that Mars sped up as it approached the Sun and slowed down as it moved farther away, in a manner that later physicists would explain as a consequence of the conservation of angular momentum.{{Cite book |last1=Frautschi |first1=Steven C. |title=The Mechanical Universe: Mechanics and Heat |title-link=The Mechanical Universe |last2=Olenick |first2=Richard P. |last3=Apostol |first3=Tom M. |last4=Goodstein |first4=David L. |date=2007 |publisher=Cambridge University Press |isbn=978-0-521-71590-4 |edition=Advanced |location=Cambridge [Cambridgeshire] |oclc=227002144 |author-link=Steven Frautschi |author-link3=Tom M. Apostol |author-link4=David L. Goodstein}}{{Rp|pages=433–437}} When the telescope became available, the diurnal parallax of Mars was again measured in an effort to determine the Sun-Earth distance. This was first performed by Giovanni Domenico Cassini in 1672. The early parallax measurements were hampered by the quality of the instruments. The only occultation of Mars by Venus observed was that of 13 October 1590, seen by Michael Maestlin at Heidelberg. In 1610, Mars was viewed by Italian astronomer Galileo Galilei, who was first to see it via telescope. The first person to draw a map of Mars that displayed any terrain features was the Dutch astronomer Christiaan Huygens.

File:Lowell - Mars (1894) - Huyghens' drawing of the Syrtis Major, Nov. 28, 1659.jpg, observed on the 28th of November 1659.{{cite web | last=Huygens | first=Christiaan | title=I. (Manuscrit K). 1657-1659., Oeuvres complètes. Tome XV. Observations astronomiques, Christiaan Huygens | publisher=DBNL | url=https://www.dbnl.org/tekst/huyg003oeuv15_01/huyg003oeuv15_01_0002.php | language=nl | access-date=April 15, 2025}} The feature is probably Syrtis Major Planum. Huygens calculated from its observation the rotation of Mars.]]

{{Main|Martian canals}}

File:Mars_-_MEC-1_Prototype._LOC_2013593160.jpg, showing canals snaking through the Martian landscape. At the time, the existence of canals was still highly controversial as no close-up pictures of Mars had been taken (until Mariner 4's flyby in 1965).]]

By the 19th century, the resolution of telescopes reached a level sufficient for surface features to be identified. On 5 September 1877, a perihelic opposition to Mars occurred. The Italian astronomer Giovanni Schiaparelli used a {{convert|22|cm|adj=on}} telescope in Milan to help produce the first detailed map of Mars. These maps notably contained features he called canali, which, with the possible exception of the natural canyon Valles Marineris, were later shown to be an optical illusion. These canali were supposedly long, straight lines on the surface of Mars, to which he gave names of famous rivers on Earth. His term, which means "channels" or "grooves", was popularly mistranslated in English as "canals".{{cite web |last=Milner |first=Richard |date=6 October 2011 |title=Tracing the Canals of Mars: An Astronomer's Obsession |url=https://www.space.com/13197-mars-canals-water-history-lowell.html |access-date=25 December 2021 |website=Space.com |archive-date=25 December 2021 |archive-url=https://web.archive.org/web/20211225043755/https://www.space.com/13197-mars-canals-water-history-lowell.html |url-status=live}}

Influenced by the observations, the orientalist Percival Lowell founded an observatory which had 30- and 45-centimetre (12- and 18-in) telescopes. The observatory was used for the exploration of Mars during the last good opportunity in 1894, and the following less favorable oppositions. He published several books on Mars and life on the planet, which had a great influence on the public.{{cite news |last=Dunlap |first=David W. |date=1 October 2015 |title=Life on Mars? You Read It Here First. |work=The New York Times |url=https://www.nytimes.com/2015/09/30/insider/life-on-mars-you-read-it-here-first.html |access-date=1 October 2015 |archive-date=2 October 2015 |archive-url=https://web.archive.org/web/20151002192143/https://www.nytimes.com/2015/09/30/insider/life-on-mars-you-read-it-here-first.html |url-status=live}} The canali were independently observed by other astronomers, like Henri Joseph Perrotin and Louis Thollon in Nice, using one of the largest telescopes of that time.

The seasonal changes (consisting of the diminishing of the polar caps and the dark areas formed during Martian summers) in combination with the canals led to speculation about life on Mars, and it was a long-held belief that Mars contained vast seas and vegetation. As bigger telescopes were used, fewer long, straight canali were observed. During observations in 1909 by Antoniadi with an {{convert|84|cm|adj=on}} telescope, irregular patterns were observed, but no canali were seen.

File:M04 12.jpg,{{cite web | title=Attempt to Take Pictures of Black Space Scrubbed | website=NASA Jet Propulsion Laboratory (JPL) | date=August 21, 1965 | url=https://www.jpl.nasa.gov/news/attempt-to-take-pictures-of-black-space-scrubbed/ | access-date=April 15, 2025}} the second spacecraft to reach Mars and the first to transmit data (15 July 1965). The data disproved Mars being hospitable and having canals.]]

Dozens of crewless spacecraft, including orbiters, landers, and rovers, have been sent to Mars by the Soviet Union, the United States, Europe, India, the United Arab Emirates, and China to study the planet's surface, climate, and geology.{{cite web |date=29 July 2020 |title=Why we explore Mars—and what decades of missions have revealed |url=https://www.nationalgeographic.com/science/article/mars-exploration-article |archive-url=https://web.archive.org/web/20210218212120/https://www.nationalgeographic.com/science/article/mars-exploration-article |url-status=dead |archive-date=18 February 2021 |access-date=7 December 2021 |website=National Geographic |first=Nadia |last=Drake |author-link=Nadia Drake |language=en}} NASA's Mariner 4 was the first spacecraft to visit Mars; launched on 28 November 1964, it made its closest approach to the planet on 15 July 1965. Mariner 4 detected the weak Martian radiation belt, measured at about 0.1% that of Earth, and captured the first images of another planet from deep space.{{cite web |url=https://solarsystem.nasa.gov/missions/mariner-04/in-depth |title=Mariner 04 |website=NASA Solar System Exploration |date=20 December 2017 |access-date=9 February 2020 |quote=The Mariner 4 mission, the second of two Mars flyby attempts launched in 1964 by NASA, was one of the great early successes of the agency, and indeed the Space Age, returning the very first photos of another planet from deep space. |url-status=live |archive-url=https://web.archive.org/web/20200803192306/https://solarsystem.nasa.gov/missions/mariner-04/in-depth/ |archive-date=3 August 2020}} {{PD-notice}} {{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1964-077A |title=Spacecraft – Details - Mariner 4 |website=NASA – NSSDCA |access-date=9 February 2020 |quote=Mariner 4...represented the first successful flyby of the planet Mars, returning the first pictures of the martian surface. These represented the first images of another planet ever returned from deep space. |archive-date=4 September 2018 |archive-url=https://web.archive.org/web/20180904174225/https://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1964-077A |url-status=live}} {{PD-notice}}

Once spacecraft visited the planet during NASA's Mariner missions in the 1960s and 1970s, many previous concepts of Mars were radically broken. After the results of the Viking life-detection experiments, the hypothesis of a dead planet was generally accepted. The data from Mariner 9 and Viking allowed better maps of Mars to be made, and the Mars Global Surveyor mission, which launched in 1996 and operated until late 2006, produced complete, extremely detailed maps of the Martian topography, magnetic field and surface minerals. These maps are available online at websites including Google Mars. Both the Mars Reconnaissance Orbiter and Mars Express continued exploring with new instruments and supporting lander missions. NASA provides two online tools: Mars Trek, which provides visualizations of the planet using data from 50 years of exploration, and Experience Curiosity, which simulates traveling on Mars in 3-D with Curiosity.{{cite web |title=New Online Tools Bring NASA's Journey to Mars to a New Generation |url=http://www.nasa.gov/press-release/new-online-tools-bring-nasa-s-journey-to-mars-to-new-generation-of-explorers |access-date=5 August 2015 |date=5 August 2015 |website=NASA |archive-date=7 August 2015 |archive-url=https://web.archive.org/web/20150807234318/http://www.nasa.gov/press-release/new-online-tools-bring-nasa-s-journey-to-mars-to-new-generation-of-explorers/ |url-status=dead}}{{cite magazine |url=https://www.wired.co.uk/article/nasa-mars-trek |title=Explore the Red Planet with Nasa's Mars Trek |magazine=Wired UK |date=10 July 2015 |access-date=31 March 2022 |first=Daniel |last=Culpan |archive-date=31 March 2022 |archive-url=https://web.archive.org/web/20220331183510/https://www.wired.co.uk/article/nasa-mars-trek |url-status=live}}

File:Mars 2020 selfie containing both perseverance rover and ingenuity.gif of Perseverance rover and Ingenuity helicopter (left) at Wright Brothers Field, 2021]]

{{As of|2023}}, Mars is host to ten functioning spacecraft. Eight are in orbit: 2001 Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, MAVEN, ExoMars Trace Gas Orbiter, the Hope orbiter, and the Tianwen-1 orbiter.{{Cite web |last=Strickland |first=Ashley |date=12 February 2021 |title=Meet the orbiters that help rovers on Mars talk to Earth |url=https://www.cnn.com/2021/02/12/world/nasa-maven-mro-mars-orbiters-scn/index.html |access-date=22 March 2022 |website=CNN |archive-date=22 March 2022 |archive-url=https://web.archive.org/web/20220322041827/https://www.cnn.com/2021/02/12/world/nasa-maven-mro-mars-orbiters-scn/index.html |url-status=live}}{{Cite news |last=Hill |first=Tanya |title=As new probes reach Mars, here's what we know so far from trips to the red planet |url=http://theconversation.com/as-new-probes-reach-mars-heres-what-we-know-so-far-from-trips-to-the-red-planet-153791 |access-date=22 March 2022 |work=The Conversation |date=9 February 2021 |archive-date=16 February 2022 |archive-url=https://web.archive.org/web/20220216192112/https://theconversation.com/as-new-probes-reach-mars-heres-what-we-know-so-far-from-trips-to-the-red-planet-153791 |url-status=live}} Another two are on the surface: the Mars Science Laboratory Curiosity rover and the Perseverance rover.{{cite news |last1=Myers |first1=Steven Lee |last2=Chang |first2=Kenneth |title=China's Mars Rover Mission Lands on the Red Planet |url=https://www.nytimes.com/2021/05/14/science/china-mars.html |archive-url=https://ghostarchive.org/archive/20211228/https://www.nytimes.com/2021/05/14/science/china-mars.html |archive-date=28 December 2021 |url-access=limited |access-date=15 May 2021 |work=The New York Times |date=14 May 2021}}{{cbignore}}

Planned missions to Mars include:

• NASA's EscaPADE spacecraft, planned to launch in 2025.{{Cite tweet |number=1827079568470995106 |user=blueorigin |title=#NewGlenn's inaugural mission aims to send @NASA's ESCAPADE to Mars, with a launch date no earlier than October 13.}}
• The Rosalind Franklin rover mission, designed to search for evidence of past life, which was intended to be launched in 2018 but has been repeatedly delayed, with a launch date pushed to 2028 at the earliest.{{Cite web |date=2 May 2016 |title=Second ExoMars mission moves to next launch opportunity in 2020 |url=https://www.esa.int/Newsroom/Press_Releases/Second_ExoMars_mission_moves_to_next_launch_opportunity_in_2020 |access-date=24 March 2022 |website=European Space Agency |archive-date=19 March 2022 |archive-url=https://archive.today/20220319064613/https://www.esa.int/Newsroom/Press_Releases/Second_ExoMars_mission_moves_to_next_launch_opportunity_in_2020 |url-status=live}}{{Cite web |date=12 March 2020 |title=ExoMars to take off for the Red Planet in 2022 |url=https://www.esa.int/Newsroom/Press_Releases/ExoMars_to_take_off_for_the_Red_Planet_in_2022 |access-date=24 March 2022 |website=European Space Agency |archive-date=19 March 2022 |archive-url=https://web.archive.org/web/20220319062237/https://www.esa.int/Newsroom/Press_Releases/ExoMars_to_take_off_for_the_Red_Planet_in_2022 |url-status=live}}{{Cite news |last=Amos |first=Jonathan |date=17 March 2022 |title=Joint Europe-Russia Mars rover project is parked |language=en-GB |work=BBC News |url=https://www.bbc.com/news/science-environment-60782932 |access-date=24 March 2022 |archive-date=6 April 2022 |archive-url=https://web.archive.org/web/20220406171653/https://www.bbc.com/news/science-environment-60782932 |url-status=live}} The project was restarted in 2024 with additional funding.{{Cite web |title=A saga for Rosalind Franklin – To Mars and back |url=https://blogs.esa.int/to-mars-and-back/2024/05/09/a-saga-for-rosalind-franklin/ |access-date=13 May 2024 |language=en-US}}
• A current concept for a joint NASA-ESA mission to return samples from Mars would launch in 2026.{{cite web |url=https://www.planetary.org/articles/nasa-esa-latest-msr-plan |title=NASA, ESA Officials Outline Latest Mars Sample Return Plans |website=planetary.org |access-date=9 September 2019 |date=13 August 2019 |archive-date=4 August 2020 |archive-url=https://web.archive.org/web/20200804091735/https://www.planetary.org/articles/nasa-esa-latest-msr-plan |url-status=live}}{{cite web |title=Mars Sample Return Campaign |url=https://mars.nasa.gov/msr/ |access-date=31 January 2022 |website=mars.nasa.gov |language=en |archive-date=15 June 2022 |archive-url=https://web.archive.org/web/20220615020149/https://mars.nasa.gov/msr/ |url-status=live}}
• China's Tianwen-3, a sample return mission, scheduled to launch in 2030.

{{As of|2024|February}}, debris from these types of missions has reached over seven tons. Most of it consists of crashed and inactive spacecraft as well as discarded components.{{Cite web |last=Kilicpublished |first=Cagri |date=28 September 2022 |title=Mars is littered with 15,694 pounds of human trash from 50 years of robotic exploration |url=https://www.space.com/mars-littered-with-human-trash |access-date=4 April 2024 |website=Space.com |language=en}}{{Cite web |title=Humans have already dumped 7 tonnes of junk on Mars, map reveals |url=https://www.wionews.com/science/map-shows-human-made-debris-piling-up-on-mars-over-seven-tonnes-of-space-junk-lying-around-red-planet-686201 |access-date=4 April 2024 |website=WION |date=2 February 2024 |language=en-us}}

In April 2024, NASA selected several companies to begin studies on providing commercial services to further enable robotic science on Mars. Key areas include establishing telecommunications, payload delivery and surface imaging.{{Cite web |title=NASA Selects Commercial Service Studies to Enable Mars Robotic Science |url=https://www.jpl.nasa.gov/news/nasa-selects-commercial-service-studies-to-enable-mars-robotic-science |access-date=1 May 2024 |website=NASA Jet Propulsion Laboratory (JPL) |language=en-US}}

{{Main|1 = Life on Mars}}

File:MarsCuriosityRover-Drilling-Sol170++-2.jpg rover's robotic arm showing drill in place, February 2013]]

During the late 19th century, it was widely accepted in the astronomical community that Mars had life-supporting qualities, including the presence of oxygen and water. However, in 1894 W. W. Campbell at Lick Observatory observed the planet and found that "if water vapor or oxygen occur in the atmosphere of Mars it is in quantities too small to be detected by spectroscopes then available". That observation contradicted many of the measurements of the time and was not widely accepted. Campbell and V. M. Slipher repeated the study in 1909 using better instruments, but with the same results. It was not until the findings were confirmed by W. S. Adams in 1925 that the myth of the Earth-like habitability of Mars was finally broken. However, even in the 1960s, articles were published on Martian biology, putting aside explanations other than life for the seasonal changes on Mars.

The current understanding of planetary habitability{{Snd}}the ability of a world to develop environmental conditions favorable to the emergence of life{{Snd}}favors planets that have liquid water on their surface. Most often this requires the orbit of a planet to lie within the habitable zone, which for the Sun is estimated to extend from within the orbit of Earth to about that of Mars.{{cite journal |last1=Kopparapu |first1=Ravi Kumar |last2=Ramirez |first2=Ramses |last3=Kasting |first3=James F. |last4=Eymet |first4=Vincent |last5=Robinson |first5=Tyler D. |last6=Mahadevan |first6=Suvrath |last7=Terrien |first7=Ryan C. |last8=Domagal-Goldman |first8=Shawn |last9=Meadows |first9=Victoria |last10=Deshpande |first10=Rohit |date=2013 |title=Habitable Zones Around Main-Sequence Stars: New Estimates |journal=The Astrophysical Journal |volume=765 |issue=2 |pages=131 |arxiv=1301.6674 |bibcode=2013ApJ...765..131K |doi=10.1088/0004-637X/765/2/131 |s2cid=76651902}} During perihelion, Mars dips inside this region, but Mars's thin (low-pressure) atmosphere prevents liquid water from existing over large regions for extended periods. The past flow of liquid water demonstrates the planet's potential for habitability. Recent evidence has suggested that any water on the Martian surface may have been too salty and acidic to support regular terrestrial life.

The environmental conditions on Mars are a challenge to sustaining organic life: the planet has little heat transfer across its surface, it has poor insulation against bombardment by the solar wind due to the absence of a magnetosphere and has insufficient atmospheric pressure to retain water in a liquid form (water instead sublimes to a gaseous state). Mars is nearly, or perhaps totally, geologically dead; the end of volcanic activity has apparently stopped the recycling of chemicals and minerals between the surface and interior of the planet.

Evidence suggests that the planet was once significantly more habitable than it is today, but whether living organisms ever existed there remains unknown. The Viking probes of the mid-1970s carried experiments designed to detect microorganisms in Martian soil at their respective landing sites and had positive results, including a temporary increase in {{chem2|CO2}} production on exposure to water and nutrients. This sign of life was later disputed by scientists, resulting in a continuing debate, with NASA scientist Gilbert Levin asserting that Viking may have found life.{{cite web |last=Chang |first=Kenneth |date=4 August 2021 |title=Gilbert V. Levin, Who Said He Found Signs of Life on Mars, Dies at 97 |url=https://www.nytimes.com/2021/08/04/science/space/gilbert-v-levin-dead.html |access-date=4 August 2021 |work=The New York Times |archive-date=4 August 2021 |archive-url=https://web.archive.org/web/20210804185707/https://www.nytimes.com/2021/08/04/science/space/gilbert-v-levin-dead.html |url-status=live}} A 2014 analysis of Martian meteorite EETA79001 found chlorate, perchlorate, and nitrate ions in sufficiently high concentrations to suggest that they are widespread on Mars. UV and X-ray radiation would turn chlorate and perchlorate ions into other, highly reactive oxychlorines, indicating that any organic molecules would have to be buried under the surface to survive.{{cite journal |last1=Kounaves |first1=S. P. |year=2014 |title=Evidence of martian perchlorate, chlorate, and nitrate in Mars meteorite EETA79001: implications for oxidants and organics |journal=Icarus |volume=229 |pages=206–213 |bibcode=2014Icar..229..206K |doi=10.1016/j.icarus.2013.11.012}}

Small quantities of methane and formaldehyde detected by Mars orbiters are both claimed to be possible evidence for life, as these chemical compounds would quickly break down in the Martian atmosphere. Alternatively, these compounds may instead be replenished by volcanic or other geological means, such as serpentinite. Impact glass, formed by the impact of meteors, which on Earth can preserve signs of life, has also been found on the surface of the impact craters on Mars.{{cite news |last=Nickel |first=Mark |date=18 April 2014 |title=Impact glass stores biodata for millions of years |publisher=Brown University |url=https://news.brown.edu/articles/2014/04/impactglass |access-date=9 June 2015 |archive-date=17 June 2015 |archive-url=https://web.archive.org/web/20150617122630/https://news.brown.edu/articles/2014/04/impactglass |url-status=live}}{{cite journal |last1=Schultz |first1=P. H. |last2=Harris |first2=R. Scott |last3=Clemett |first3=S. J. |last4=Thomas-Keprta |first4=K. L. |last5=Zárate |first5=M. |date=June 2014 |title=Preserved flora and organics in impact melt breccias |journal=Geology |volume=42 |issue=6 |pages=515–518 |bibcode=2014Geo....42..515S |doi=10.1130/G35343.1 |s2cid=39019154 |hdl-access=free |hdl=2060/20140013110}} Likewise, the glass in impact craters on Mars could have preserved signs of life, if life existed at the site.{{cite press release |url=http://www.nasa.gov/press-release/nasa-spacecraft-detects-impact-glass-on-surface-of-mars |title=NASA Spacecraft Detects Impact Glass on Surface of Mars |publisher=NASA |first1=Dwayne |last1=Brown |first2=Guy |last2=Webster |first3=Kevin |last3=Stacey |date=8 June 2015 |access-date=9 June 2015 |archive-date=9 June 2015 |archive-url=https://web.archive.org/web/20150609132637/http://www.nasa.gov/press-release/nasa-spacecraft-detects-impact-glass-on-surface-of-mars/ |url-status=live}} {{PD-notice}}{{cite news |last=Stacey |first=Kevin |date=8 June 2015 |title=Martian glass: Window into possible past life? |publisher=Brown University |url=https://news.brown.edu/articles/2015/06/marsglass |access-date=9 June 2015 |archive-date=11 June 2015 |archive-url=https://web.archive.org/web/20150611003248/http://news.brown.edu/articles/2015/06/marsglass |url-status=live}}{{cite news |last=Temming |first=Maria |date=12 June 2015 |title=Exotic Glass Could Help Unravel Mysteries of Mars |work=Scientific American |url=http://www.scientificamerican.com/article/exotic-glass-could-help-unravel-mysteries-of-mars/ |access-date=15 June 2015 |archive-date=15 June 2015 |archive-url=https://web.archive.org/web/20150615010829/http://www.scientificamerican.com/article/exotic-glass-could-help-unravel-mysteries-of-mars/ |url-status=live}}

The Cheyava Falls rock discovered on Mars in June 2024 has been designated by NASA as a "potential biosignature" and was core sampled by the Perseverance rover for possible return to Earth and further examination. Although highly intriguing, no definitive final determination on a biological or abiotic origin of this rock can be made with the data currently available.

File:NextSTEP_mars-soil-excavation.jpg system concept for autonomous robotic excavation and processing of Mars soil to extract water for use in exploration missions.]]

Several plans for a human mission to Mars have been proposed throughout the 20th and 21st centuries, but none have come to fruition. The NASA Authorization Act of 2017 directed NASA to study the feasibility of a crewed Mars mission in the early 2030s; the resulting report eventually concluded that this would be unfeasible.{{cite web |date=21 March 2017 |title=S.442 - National Aeronautics and Space Administration Transition Authorization Act of 2017 |url=https://www.congress.gov/bill/115th-congress/senate-bill/442/text |access-date=29 March 2022 |website=congress.gov |archive-date=30 March 2022 |archive-url=https://web.archive.org/web/20220330021740/https://www.congress.gov/bill/115th-congress/senate-bill/442/text |url-status=live}}{{cite web |last=Foust |first=Jeff |date=18 April 2019 |title=Independent report concludes 2033 human Mars mission is not feasible |url=https://spacenews.com/independent-report-concludes-2033-human-mars-mission-is-not-feasible/ |access-date=29 March 2022 |website=Space News |archive-date=22 August 2020 |archive-url=https://perma-archives.org/warc/20200822230855/https://spacenews.com/independent-report-concludes-2033-human-mars-mission-is-not-feasible/ |url-status=live}} In addition, in 2021, China was planning to send a crewed Mars mission in 2033.{{cite web |date=23 June 2021 |title=China plans its first crewed mission to Mars in 2033 |url=https://www.reuters.com/business/aerospace-defense/china-plans-its-first-crewed-mission-mars-2033-2021-06-24/ |access-date=20 December 2021 |publisher=Reuters |archive-date=21 December 2021 |archive-url=https://web.archive.org/web/20211221031133/https://www.reuters.com/business/aerospace-defense/china-plans-its-first-crewed-mission-mars-2033-2021-06-24/ |url-status=live}} Privately held companies such as SpaceX have also proposed plans to send humans to Mars, with the eventual goal to settle on the planet.{{Cite journal |last=Musk |first=Elon |date=1 March 2018 |title=Making Life Multi-Planetary |url=https://www.liebertpub.com/doi/10.1089/space.2018.29013.emu |journal=New Space |volume=6 |issue=1 |pages=2–11 |bibcode=2018NewSp...6....2M |doi=10.1089/space.2018.29013.emu |issn=2168-0256 |access-date=27 August 2022 |archive-date=29 June 2019 |archive-url=https://web.archive.org/web/20190629143329/https://www.liebertpub.com/doi/10.1089/space.2018.29013.emu |url-status=live}} As of 2024, SpaceX has proceeded with the development of the Starship launch vehicle with the goal of Mars colonization. In plans shared with the company in April 2024, Elon Musk envisions the beginning of a Mars colony within the next twenty years. This enabled by the planned mass manufacturing of Starship and initially sustained by resupply from Earth, and in situ resource utilization on Mars, until the Mars colony reaches full self sustainability.{{Cite web |author1=Howell |first=Elizabeth |date=13 April 2024 |title=SpaceX's giant Starship will be 500 feet tall for Mars missions, Elon Musk says (video) |url=https://www.space.com/spacex-starship-500-feet-tall-mars-missions-elon-musk |access-date=20 April 2024 |website=Space.com |language=en}} Any future human mission to Mars will likely take place within the optimal Mars launch window, which occurs every 26 months. The moon Phobos has been proposed as an anchor point for a space elevator.{{Cite journal |last=Weinstein |first=Leonard M. |date=2003 |title=Space Colonization Using Space-Elevators from Phobos |url=https://space.nss.org/wp-content/uploads/2003-Space-Colonization-Using-Space-Elevators-From-Phobos.pdf |journal=AIP Conference Proceedings |location=Albuquerque, New Mexico (USA) |publisher=AIP |volume=654 |pages=1227–1235 |bibcode=2003AIPC..654.1227W |doi=10.1063/1.1541423 |s2cid=1661518 |hdl=2060/20030065879 |access-date=6 December 2022 |archive-date=19 January 2023 |archive-url=https://web.archive.org/web/20230119073347/https://space.nss.org/wp-content/uploads/2003-Space-Colonization-Using-Space-Elevators-From-Phobos.pdf |url-status=live}} Besides national space agencies and space companies, there are groups such as the Mars Society{{Cite news |last=Bichell |first=Rae Ellen |date=6 July 2017 |title=To Prepare For Mars Settlement, Simulated Missions Explore Utah's Desert |url=https://www.npr.org/2017/07/06/522060920/to-prepare-for-mars-settlement-simulated-missions-explore-utahs-desert |url-status=live |archive-url=https://web.archive.org/web/20221231181009/https://www.npr.org/2017/07/06/522060920/to-prepare-for-mars-settlement-simulated-missions-explore-utahs-desert |archive-date=31 December 2022 |access-date=31 December 2022 |work=NPR |language=en}} and The Planetary Society{{Cite web |last=Boyle |first=Alan |date=29 September 2015 |title=Destination Phobos: 'Humans Orbiting Mars' report goes public |url=https://www.geekwire.com/2015/destination-phobos-humans-orbiting-mars-report-goes-public/ |website=GeekWire}} that advocates for human missions to Mars.

{{Main|Mars in culture|Mars in fiction}}

{{see also|Planets in astrology#Mars}}

File:War-of-the-worlds-tripod.jpg by H. G. Wells, 1897, depicts an invasion of Earth by fictional Martians.]]

Mars is named after the Roman god of war (Greek Ares), but was also associated with the demi-god Heracles (Roman Hercules) by ancient Greek astronomers, as detailed by Aristotle.[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. This association between Mars and war dates back at least to Babylonian astronomy, in which the planet was named for the god Nergal, deity of war and destruction.{{Cite book |last=Koch |first=Ulla Susanne |url=https://books.google.com/books?id=8QiwAqGlmAQC |title=Mesopotamian Astrology: An Introduction to Babylonian and Assyrian Celestial Divination |date=1995 |publisher=Museum Tusculanum Press |isbn=978-87-7289-287-0 |pages=128–129 |language=en}}{{Cite journal |last=Cecilia |first=Ludovica |date=6 November 2019 |title=A Late Composition Dedicated to Nergal |url=https://www.degruyter.com/document/doi/10.1515/aofo-2019-0014/html |journal=Altorientalische Forschungen |volume=46 |issue=2 |pages=204–213 |doi=10.1515/aofo-2019-0014 |hdl=1871.1/f23ff882-1539-4906-bc08-049906f8d505 |s2cid=208269607 |issn=2196-6761 |hdl-access=free |access-date=22 March 2022 |archive-date=22 March 2022 |archive-url=https://web.archive.org/web/20220322014922/https://www.degruyter.com/document/doi/10.1515/aofo-2019-0014/html |url-status=live}} It persisted into modern times, as exemplified by Gustav Holst's orchestral suite The Planets, whose famous first movement labels Mars "the bringer of war".{{Cite journal |last=Reid |first=James |date=2011 |title=An Astronomer's Guide to Holst's The Planets |url=https://skyandtelescope.org/wp-content/uploads/Reid_on_Holst.pdf |journal=Sky and Telescope |volume=121 |issue=1 |pages=66 |bibcode=2011S&T...121a..66R |access-date=22 March 2022 |archive-date=22 March 2022 |archive-url=https://web.archive.org/web/20220322014920/https://skyandtelescope.org/wp-content/uploads/Reid_on_Holst.pdf |url-status=live}} The planet's symbol, a circle with a spear pointing out to the upper right, is also used as a symbol for the male gender.{{cite web |title=Solar System Symbols |url=https://solarsystem.nasa.gov/resources/680/solar-system-symbols |access-date=7 December 2021 |website=NASA Solar System Exploration |date=30 January 2018 |archive-date=20 December 2021 |archive-url=https://web.archive.org/web/20211220171351/https://solarsystem.nasa.gov/resources/680/solar-system-symbols/ |url-status=live}} The symbol dates from at least the 11th century, though a possible predecessor has been found in the Greek Oxyrhynchus Papyri.

{{cite book |title=Astronomical papyri from Oxyrhynchus |last=Jones |first=Alexander |date=1999 |pages=62–63 |publisher=American Philosophical Society |isbn=978-0-87169-233-7 |url=https://books.google.com/books?id=8MokzymQ43IC}}

The idea that Mars was populated by intelligent Martians became widespread in the late 19th century. Schiaparelli's "canali" observations combined with Percival Lowell's books on the subject put forward the standard notion of a planet that was a drying, cooling, dying world with ancient civilizations constructing irrigation works.{{cite web |last1=Eschner |first1=Kat |title=The Bizarre Beliefs of Astronomer Percival Lowell |url=https://www.smithsonianmag.com/smart-news/bizarre-beliefs-astronomer-percival-lowell-180962432/ |access-date=25 December 2021 |website=Smithsonian Magazine |language=en |archive-date=25 December 2021 |archive-url=https://web.archive.org/web/20211225111323/https://www.smithsonianmag.com/smart-news/bizarre-beliefs-astronomer-percival-lowell-180962432/ |url-status=live}} Many other observations and proclamations by notable personalities added to what has been termed "Mars Fever". High-resolution mapping of the surface of Mars revealed no artifacts of habitation, but pseudoscientific speculation about intelligent life on Mars still continues. Reminiscent of the canali observations, these speculations are based on small scale features perceived in the spacecraft images, such as "pyramids" and the "Face on Mars".{{Cite book |last=Plait |first=Philip C. |title=Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing 'Hoax' |title-link=Bad Astronomy |date=2002 |publisher=Wiley |isbn=0-471-40976-6 |location=New York |pages=233–234 |oclc=48885221 |author-link=Phil Plait}} In his book Cosmos, planetary astronomer Carl Sagan wrote: "Mars has become a kind of mythic arena onto which we have projected our Earthly hopes and fears."

The depiction of Mars in fiction has been stimulated by its dramatic red color and by nineteenth-century scientific speculations that its surface conditions might support not just life but intelligent life. This gave way to many science fiction stories involving these concepts, such as H. G. Wells's The War of the Worlds, in which Martians seek to escape their dying planet by invading Earth; Ray Bradbury's The Martian Chronicles, in which human explorers accidentally destroy a Martian civilization; as well as Edgar Rice Burroughs's series Barsoom, C. S. Lewis's novel Out of the Silent Planet (1938), and a number of Robert A. Heinlein stories before the mid-sixties. Since then, depictions of Martians have also extended to animation. A comic figure of an intelligent Martian, Marvin the Martian, appeared in Haredevil Hare (1948) as a character in the Looney Tunes animated cartoons of Warner Brothers, and has continued as part of popular culture to the present. After the Mariner and Viking spacecraft had returned pictures of Mars as a lifeless and canal-less world, these ideas about Mars were abandoned; for many science-fiction authors, the new discoveries initially seemed like a constraint, but eventually the post-Viking knowledge of Mars became itself a source of inspiration for works like Kim Stanley Robinson's Mars trilogy.{{Cite book |last=Crossley |first=Robert |url=https://books.google.com/books?id=v3TDEDfEPdEC |title=Imagining Mars: A Literary History |date=3 January 2011 |publisher=Wesleyan University Press |isbn=978-0-8195-7105-2 |pages=xiii-xiv |language=en}}

• Astronomy on Mars
• {{Annotated link|Outline of Mars}}
• {{Annotated link|List of missions to Mars}}
• {{Annotated link|Magnetic field of Mars}}
• {{Annotated link|Mineralogy of Mars}}

{{reflist |group=caption}}

{{notelist |notes=

{{efn | name=best-fit ellipsoid | Best-fit ellipsoid}}

}}

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{{cite journal |journal=Journal of Geophysical Research |date=7 May 2005 |last=Heldmann |first=Jennifer L. |title=Formation of Martian gullies by the action of liquid water flowing under current Martian environmental conditions |url=http://daleandersen.seti.org/Dale_Andersen/Science_articles_files/Heldmann%20et%20al.2005.pdf |volume=110 |issue=E5 |page=Eo5004 |doi=10.1029/2004JE002261 |bibcode=2005JGRE..110.5004H |access-date=17 September 2008 |hdl=2060/20050169988 |citeseerx=10.1.1.596.4087 |s2cid=1578727 |archive-date=1 October 2008 |archive-url=https://web.archive.org/web/20081001162643/http://daleandersen.seti.org/Dale_Andersen/Science_articles_files/Heldmann%20et%20al.2005.pdf |url-status=dead}} 'conditions such as now occur on Mars, outside of the temperature-pressure stability regime of liquid water'... 'Liquid water is typically stable at the lowest elevations and at low latitudes on the planet because the atmospheric pressure is greater than the vapor pressure of water and surface temperatures in equatorial regions can reach 273 K for parts of the day [Haberle et al.., 2001]'

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{{cite web |first=David |last=Williams |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html |title=Mars Fact Sheet |publisher=NASA Goddard Space Flight Center |year=2018 |access-date=22 March 2020 |archive-url=https://web.archive.org/web/20200317184127/https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html |archive-date=17 March 2020}}; Mean Anomaly (deg) 19.412 = (Mean Longitude (deg) 355.45332) – (Longitude of perihelion (deg) 336.04084) {{PD-notice}}

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{{cite journal |last1=Stähler |first1=Simon C. |last2=Khan |first2=Amir |last3=Banerdt |first3=W. Bruce |last4=Lognonné |first4=Philippe |last5=Giardini |first5=Domenico |last6=Ceylan |first6=Savas |last7=Drilleau |first7=Mélanie |last8=Duran |first8=A. Cecilia |last9=Garcia |first9=Raphaël F. |last10=Huang |first10=Quancheng |last11=Kim |first11=Doyeon |date=23 July 2021 |title=Seismic detection of the martian core |url=https://research-information.bris.ac.uk/en/publications/c5448d92-a902-4519-8836-910067ffd838 |journal=Science |volume=373 |issue=6553 |pages=443–448 |bibcode=2021Sci...373..443S |doi=10.1126/science.abi7730 |pmid=34437118 |last34=Clinton |last36=van Driel |first35=Nikolaj |last35=Dahmen |first34=John |first12=Vedran |first33=Éric |last33=Beucler |last37=Gudkova |first32=Géraldine |last32=Zenhäusern |first36=Martin |last40=Plasman |first37=Tamara |last38=Horleston |first38=Anna |last39=Pike |first39=W. Thomas |last31=Schmelzbach |first40=Matthieu |last41=Smrekar |first41=Suzanne E. |hdl=20.500.11850/498074 |s2cid=236179579 |first31=Cédric |first30=Attilio |last13=Samuel |last17=Stutzmann |last20=Charalambous |first19=Daniele |last19=Antonangeli |first18=Zongbo |last18=Xu |first17=Éléonore |first16=David |last21=Davis |last16=Sollberger |first15=Nicholas |last15=Schmerr |first14=Martin |last14=Schimmel |first13=Henri |first20=Constantinos |first21=Paul M. |last30=Rivoldini |first26=Angela G. |first29=Ana-Catalina |last29=Plesa |first28=Clément |last28=Perrin |first27=Mark P. |last27=Panning |last26=Marusiak |last22=Irving |first25=Ross |last25=Maguire |last12=Lekic |last24=Knapmeyer |first23=Taichi |last23=Kawamura |first22=Jessica C. E. |first24=Martin |hdl-access=free |access-date=17 October 2021 |archive-date=1 March 2024 |archive-url=https://web.archive.org/web/20240301160849/https://research-information.bris.ac.uk/en/publications/seismic-detection-of-the-martian-core |url-status=live}}

{{cite journal |last=van der Lee |first=Suzan |title=Deep Mars is surprisingly soft |url=https://www.nature.com/articles/d41586-023-03151-x |date=25 October 2023 |journal=Nature |volume=622 |issue=7984 |pages=699–700 |access-date=12 March 2024 |archive-date=6 December 2023 |archive-url=https://web.archive.org/web/20231206093019/https://www.nature.com/articles/d41586-023-03151-x |url-status=live |doi=10.1038/d41586-023-03151-x |pmid=37880433 |bibcode=2023Natur.622..699V}}

{{cite journal |last=Witze |first=Alexandra |title=Mars has a surprise layer of molten rock inside |url=https://www.nature.com/articles/d41586-023-03271-4 |date=25 October 2023 |journal=Nature |volume=623 |issue=7985 |page=20 |access-date=12 March 2024 |archive-date=9 January 2024 |archive-url=https://web.archive.org/web/20240109052301/https://www.nature.com/articles/d41586-023-03271-4 |url-status=live |doi=10.1038/d41586-023-03271-4 |pmid=37880531 |bibcode=2023Natur.623...20W}}

{{cite journal |title=Global Crustal Thickness Revealed by Surface Waves Orbiting Mars |journal=Geophysical Research Letters |volume=50 |issue=12 |date=June 2023 |bibcode=2023GeoRL..5003482K |doi=10.1029/2023GL103482 |last1=Kim |first1=D. |last2=Duran |first2=C. |last3=Giardini |first3=D. |last4=Plesa |first4=A. -C. |last5=Stähler |first5=S. C. |last6=Boehm |first6=C. |last7=Lekić |first7=V. |last8=McLennan |first8=S. M. |last9=Ceylan |first9=S. |last10=Clinton |first10=J. F. |last11=Davis |first11=P. |last12=Khan |first12=A. |last13=Knapmeyer-Endrun |first13=B. |last14=Panning |first14=M. P. |last15=Wieczorek |first15=M. |last16=Lognonné |first16=P. |last17=Banerdt |first17=W. B. |hdl=20.500.11850/621318 |hdl-access=free}}

{{cite journal |title=InSight Constraints on the Global Character of the Martian Crust |journal=Journal of Geophysical Research: Planets |volume=127 |issue=5 |date=May 2022 |bibcode=2022JGRE..12707298W |doi=10.1029/2022JE007298 |last1=Wieczorek |first1=Mark A. |last2=Broquet |first2=Adrien |last3=McLennan |first3=Scott M. |last4=Rivoldini |first4=Attilio |last5=Golombek |first5=Matthew |last6=Antonangeli |first6=Daniele |last7=Beghein |first7=Caroline |last8=Giardini |first8=Domenico |last9=Gudkova |first9=Tamara |last10=Gyalay |first10=Szilárd |last11=Johnson |first11=Catherine L. |last12=Joshi |first12=Rakshit |last13=Kim |first13=Doyeon |last14=King |first14=Scott D. |last15=Knapmeyer-Endrun |first15=Brigitte |last16=Lognonné |first16=Philippe |last17=Michaut |first17=Chloé |last18=Mittelholz |first18=Anna |last19=Nimmo |first19=Francis |last20=Ojha |first20=Lujendra |last21=Panning |first21=Mark P. |last22=Plesa |first22=Ana-Catalina |last23=Siegler |first23=Matthew A. |last24=Smrekar |first24=Suzanne E. |last25=Spohn |first25=Tilman |last26=Banerdt |first26=W. Bruce |hdl=10919/110830 |hdl-access=free}}

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• {{Cite book |last1=Weinersmith |first1=Kelly |author-link1=Kelly Weinersmith |url=https://books.google.com/books?id=ImmvEAAAQBAJ |title=A city on Mars: can we settle space, should we settle space, and have we really thought this through? |last2=Weinersmith |first2=Zach |author-link2=Zach Weinersmith |publisher=Penguin Press |year=2023 |isbn=978-1-9848-8172-4 |location=New York}}
• {{Cite book |last=Shindell |first=Matthew |url=https://books.google.com/books?id=qSi4EAAAQBAJ |title=For the love of Mars: a human history of the red planet |date=2023 |publisher=The University of Chicago Press |isbn=978-0-226-82189-4 |location=Chicago; London}}

• [https://trek.nasa.gov/mars/ Mars Trek], an integrated map browser of maps and datasets for Mars
• [http://www.google.com/mars/ Google Mars] and [https://www.google.com/maps/space/mars/ Google Mars 3D], interactive maps of the planet
• [https://www.cnn.com/2023/07/15/world/nasa-mariner-4-photo-mars-anniversary-scn/index.html First TV image of Mars (15 July 1965)], CNN News; 15 July 2023

{{Mars}}

{{Geography of Mars}}

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