Martian dichotomy

The most conspicuous feature of Mars is a sharp contrast, known as the Martian dichotomy, between the Southern hemisphere and the Northern. The two hemispheres' geography differ in elevation by 1 to 3 km. The average thickness of the Martian crust is 45 km, with 32 km in the northern lowlands region, and 58 km in the southern highlands.

The boundary between the two regions is quite complex in places. One distinctive type of topography is called fretted terrain. It contains mesas, knobs, and flat-floored valleys having walls about a mile high. Around many of the mesas and knobs are lobate debris aprons that have been shown to be rock-covered glaciers.

Many large valleys formed by the lava erupted from the volcanoes of Mars cut through the dichotomy.

The Martian dichotomy boundary includes the regions called Deuteronilus Mensae, Protonilus Mensae, and Nilosyrtis Mensae. All three regions have been studied extensively because they contain landforms believed to have been produced by the movement of ice or paleoshorelines questioned as formed by volcanic erosion.

The northern lowlands comprise about one-third of the surface of Mars and are relatively flat, with as many impact craters as the southern hemisphere. The other two-thirds of the Martian surface are the highlands of the southern hemisphere. The difference in elevation between the hemispheres is dramatic. Three major hypotheses have been proposed for the origin of the crustal dichotomy: endogenic (by mantle processes), single impact, or multiple impact. Both impact-related hypotheses involve processes that could have occurred before the end of the primordial bombardment, implying that the crustal dichotomy has its origins early in the history of Mars.

A single mega-impact would produce a very large, circular depression in the crust. The proposed depression has been named the Borealis Basin. However, most estimations of the shape of the lowlands area produce a shape that in places dramatically deviates from the circular shape. Additional processes could create those deviations from circularity. Also if the proposed Borealis basin is a depression created by an impact, it would make it the largest impact crater in the Solar System. An object that large could have hit Mars sometime during the process of the Solar System accretion.

It is expected that an impact of such magnitude would have produced an ejecta blanket that should be found in areas around the lowland and generate enough heat to form volcanoes. However, if the impact occurred around 4.5 Ga (billion years ago), erosional factors could explain the absence of the ejecta blanket but could not explain the absence of volcanoes. Also, the mega-impact could have scattered a large portion of the debris into outer space and across the southern hemisphere. Geologic evidence of the debris would provide very convincing support for this hypothesis. A 2008 study provided additional research towards the single giant impact theory in the northern hemisphere. In the past tracing of the impact boundaries was complicated by the presence of the Tharsis volcanic rise. The Tharsis volcanic rise buried the proposed dichotomy boundary under 30 km of basalt. The researchers at MIT and Jet Propulsion Lab at CIT have been able to use gravity and topography of Mars to constrain the location of the dichotomy beneath the Tharsis rise, thus creating an elliptical model of the dichotomy boundary. The elliptical shape of the Borealis Basin brought to the Northern Single Impact Hypothesis as a re-edition of the original theory published in 1984. However, this hypothesis has been countered by a new hypothesis of a giant impact to the South Pole of Mars with a lunar sized object that melted the southern hemisphere of Mars, triggered the magnetic field of the planet, and formed the dichotomy upon cooling of the magma ocean. The discovery of twelve volcanic alignments lends evidence to this new hypothesis.

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