Small Pale Red Planet Issue 1 Phase 5

 

The Martian Dichotomy

The most conspicuous feature of Martian surface geology is a sharp contrast, known as the Martian dichotomy, between the rugged southern highlands and the relatively smooth northern basins. The two hemispheres 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 (that is to say, that much of the northern lowlands are equivalent to being below sea level on Earth) .  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. 

 

Topographical Map of Mars:  The low elevations are in blue and green the high elevations are the other colors in this map.  If one followed the blue border on this map that would be equivalent to following the Martian Dichotomy border.

The northern lowlands comprise about one-third of the surface of Mars and are relatively flat, with occasional impact craters. The other two-thirds of the Martian surface are the highlands of the southern hemisphere. The difference in elevation between the hemispheres is dramatic. Because of the density of impact craters, scientists believe the southern hemisphere to be far older than the northern plains.  The heavily cratered southern highlands date back to the period of the Late heavy bombardment. Three major hypotheses have been proposed for the origin of the crustal dichotomy: endogenic (by mantle processes), single impact, or multiple impacts. 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 and that is doubtful. It is probable that all the areas colored blue on the  map above was once covered by an ocean.  That the Hellas Basin in deep purple was an inland sea with great depth was caused by a large impacter.    Therefore, whatever happened in those early years would have been covered by the bottom of the Northern ocean floor and disappeared with little trace covered by the movement of the ocean floor.  However, there are also events that have occurred since then.

Video on the Geological History of Mars: