[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?

[Guest_BruceMoomaw_*]

The big problem with that -- again -- is the failure of Mars Express to find any hydrated iron oxides anywhere on the northern plains, except for a few -- very tiny -- patches. Unless there's some atmospheric process that destroys Fe oxyhydroxides that are exposed to solar UV on the Martian surface for long periods of time, this simply rules out any really prolonged exposure of the plains materials to liquid water.

Now, it is a fact that very short exposure to liquid water -- of the sort that created the huge funoff channels on Mars -- does not seem to produce such Fe compounds; at any rate, we aren't finding them in any large amounts in those channels. But I would think that, for such mud to flow all the way into the northernmost parts of the northern plains, the sediment WOULD have to be exposed to liquid water for prolonged periods.

By the way, I just remembered the name of the scientist who believes the northern plains to have been filled entirely by windblown dust: Conway Leovy. (The little man who sits in a corner of my skull and keeps track of names for me has been goofing off more and more often in recent years; I really must have a word with him.) As soon as I dig up my records of Leovy's views, I'll have more to say on all this.