[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?

[edstrick]

Herbert Frey has been the scientist most doggedly pursuing the origin of the hemisphere dichotomy since Viking. Evidence at the border between highlands and lowlands is badly eroded and degraded, while the plains "bedrock" is deeply buried.

MOLA discovered (what I've called ever since) as "Ghost Craters", where a faint circular topographic rise surrounds a topographic depression. They clearly are impact craters entirely buried under plains, but the plains have deformed over the craters, seemingly compacting slightly, less over the crater rims, more where the plains are deeper than average over the crater centers. That's my take at least. MARSIS seems to be reporting that there is an entirely MOLA invisible population of craters in addition to the ghosts that have absolutely no noticable expression in the topographic data.

I increasingly doubt there was ever a true polar ocean, but the idea of a deposit of global northern lava plains with an absolute lack of mare-like lava flow features or wrinkle ridges or small eruptive centers or ANYTHING just leaves me unconvinced that lava was responsible.

The arm-waving idea I've had for some good fraction of a decade is that the Vastitas Borealis plains are a complex of ice-and-dust-and-sand sheets. Dump a humongous amount of watery mud or muddy water out of catastrophic flood events, you may spread the goop out onto polar lowlands as a "15 day" or "15 hour" ocean that freezes up quick. Maybe multiple layers, maybe interbedded with regional dust deposits or dessication crusts, drifted across by sands or cemented silts.

Marsis may not show it's structure well because it's resolution of 150 meters is coarse, but Sharad may be just the peeler to get beneath the skin of this rather tough apple! <where did that similie come from?>