The large canyon, Valles Marineris

The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southern highlands, pitted and cratered by ancient impacts. Research in 2008 has presented evidence regarding a theory proposed in 1980 postulating that, four billion years ago, the northern hemisphere of Mars was struck by an object one-tenth to two-thirds the size of the Moon. If validated, this would make the northern hemisphere of Mars the site of an impact crater 10,600 km long by 8,500 km wide, or roughly the area of Europe, Asia, and Australia combined, surpassing the South Pole–Aitken basin as the largest impact crater in the Solar System.[15][16]
Mars is scarred by a number of impact craters: a total of 43,000 craters with a diameter of 5 km or greater have been found.[108] The largest confirmed of these is the Hellas impact basin, a light albedo feature clearly visible from Earth.[109] Due to the smaller mass of Mars, the probability of an object colliding with the planet is about half that of the 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 also more likely to be struck by short-period comets, i.e. those that lie within the orbit of Jupiter.[110] In spite of this, there are far fewer craters on Mars compared with the Moon, because the atmosphere of Mars provides protection against small meteors. Some craters have a morphology that suggests the ground became wet after the meteor impacted.[111]
Volcanoes


Top down view of Olympus Mons
Main article: Volcanism on Mars
The shield volcano, Olympus Mons (Mount Olympus) is an extinct volcano in the vast upland region Tharsis, which contains several other large volcanoes. Olympus Mons is over three times the height of Mount Everest, which in comparison stands at just over 8.8 km.[112] It is either the tallest or second tallest mountain in the solar system, depending how you measure, with sources ranging from about 21 to 27 km high.[113][114]
Tectonic sites
The large canyon, Valles Marineris (Latin for Mariner Valleys, also known as Agathadaemon in the old canal maps), has a length of 4,000 km and a depth of up to 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 446 km long and nearly 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 also a plate boundary where 150 km of transverse motion has occurred, making Mars a planet with possibly a two-plate tectonic arrangement

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Other geological features, such as deltas and alluvial fans preserved in craters

Other geological features, such as deltas and alluvial fans preserved in craters, also argue very strongly for warmer, wetter conditions at some interval or intervals in earlier Mars history.[70] Such conditions necessarily require the widespread presence of crater lakes across a large proportion of the surface, for which there is also independent mineralogical, sedimentological and geomorphological evidence.[71] Some authors have even gone so far as to argue that at times in the Martian past, much of the low northern plains of the planet were covered with a true ocean hundreds of meters deep, though this remains controversial.[72]


Composition of "Yellowknife Bay" rocks – rock veins are higher in calcium and sulfur than "Portage" soil – APXS results – Curiosity rover (March, 2013).
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.[73] Some of the evidence believed to indicate ancient water basins and flows has been negated by higher resolution studies by the Mars Reconnaissance Orbiter.[74] In 2004, Opportunity detected the mineral jarosite. This forms only in the presence of acidic water, which demonstrates that water once existed on Mars.[75] More recent evidence for liquid water comes from the finding of the mineral gypsum on the surface by NASA's Mars rover Opportunity in December 2011.[76][77] Additionally, the study leader Francis McCubbin, a planetary scientist at the University of New Mexico in Albuquerque looking at hydroxals in crystalline minerals from Mars states the amount of water in the upper mantle of Mars 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 200 to 1000 meters.[78]
On March 18, 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.[79][80][81] 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 60 cm, in the rover's traverse from the Bradbury Landing site to the Yellowknife Bay area in the Glenelg terrain

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