Martian dichotomy

All three regions have been studied extensively because they contain landforms believed to have been produced by the movement of ice[12][13] or paleoshorelines questioned as formed by volcanic erosion.

[15] 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.

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.

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.

The elliptical shape of the Borealis basin contributed to the northern single impact hypothesis[20][21] as a revision of the original theory[22] published in 1984.

[11] Initially, the estimated size of the impacting body required for this scenario was Moon-sized,[25][26] but more recent research favour a smaller, 500-750 km-radius projectile.

The roughly circular shape of the lowland could then be attributed to plume-like first-order overturn which could occur in the process of rapid core formation.

There is evidence for internally driven tectonic events in the vicinity of the lowland area that clearly occurred at the end of the early bombardment phase.

Some of the evidence is the abundance of extensive fracturing and igneous activity of late Noachian to early Hesperian age.

A counter argument to the endogenic hypothesis is the possibility of those tectonic events occurring in the Borealis Basin due to the post-impact weakening of the crust.

In order to further support the endogenic origin hypothesis geologic evidence of faulting and flexing of the crust prior to the end of the primordial bombardment is needed.

The multiple-impact hypothesis is also statistically unfavorable because it is unlikely that multiple impact basins occur and overlap primarily in the northern hemisphere.

The atmosphere of Mars varies significantly between Northern and Southern hemispheres, both for reasons related and unrelated to the geographic dichotomy.

[35] When combined with the greater seasonal range of the Southern hemisphere (see above), this results in "the striking north-south hemispherical asymmetries of the atmospheric and residual ice cap inventories of Mars water", "as well as the current north-south asymmetry of the seasonal ice cap albedos".