[X] My Assistant

[Chmee]

What is everyone's thoughts on origin of the smooth low-lands on Mars and why they are so different from the cratered high-lands?

I beleive there are at least three theories:

1. The low-lands are the result of a super-large impact on the Northern hemisphere (during the Noachian age). This impact erased the earlier craters leaving the low, smooth surface seen today.

2. The low lands were cratered just as the highlands were, but later (Amazonian) hemisphere-wide lava flows erased all evidence of the craters.

3. The low lands were cratered just as the highlands were, but an early ocean laid deposits in this low- area, burying the earlier craters.

I am leaning towards both theory 1 and 2. In other words, a very early impact on Mars created the dichotomy we see now on Mars (by basically blasting a large proportion of teh Northern Hemisphere completely away) but later hemisphere-wide lava flows filled in this low lying area.

Thoughts?

[JRehling]

2. The low lands were cratered just as the highlands were, but later (Amazonian) hemisphere-wide lava flows erased all evidence of the craters.

I see little doubt that this is close to the truth, but for the word "lava", which may or may not be fully correct.

My interpretation would be:

It seems clear that early Mars would have been heavily cratered everywhere, with a hemispheric dichotomy already present in the geoid, such that one hemisphere had a lower altitude than the other. That dichotomy is another story.

A crust rich in H2O ice overlaid the entire planet. As volcanic activity heated the entire Tharsis plateau (among other places), the H2O in that area melted and flowed downhill. That would have carried a great deal of rock and sand with it. Lava? Perhaps, if it were hot enough, although that would exclude much H2O from being part of the mix.

Whatever the composition of this mess, it flowed to the lowest portion of the northern basin and filled it in, freezing/solidifying very shortly after it arrived.

Given the composition of the crust now, I suspect that H2O was part of the flow, and thus that it was not "lava"-hot. Alternately, it was lava and the H2O seeped in, somehow, later.