Origin of Low-Lands on Mars Options

[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 utter lack of widespread iron oxyhydroxides on the northern plains has already convinced Mars Express' OMEGA team that they are NOT looking at ocean sediments -- and, presumably, also not silt poured gradually into the northern basin by water flows from the southern hemisphere. Lava floods would seem to be possible, although the utter flatness of the plains is still amazing in that case -- they would presumably have to have been ALL filled by a single gigantic one-time flood of lava in that case, which also seems implausible. In short, I don't know what to think of them. (One theorist -- whose name escapes me at the moment -- has proposed that they may actually have been filled up instead with windblown silt during Mars' Noachian days, when it had a dense atmosphere.)

[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?>

[RNeuhaus]

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?

As an example, MEX with its MARSIS had found a big shaped bowl of hundred kilometer under the surface in the northen plains. It is one of many buried craters. I suspect that the northen plains must have many buried craters.

Rodolfo

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?>

It is true.

SHARAD will likely not probe as deeply as MARSIS, but it has ten times the vertical resolution, allowing for opportunities to map detailed subsurface stratigraphy, says Phillips, who is also director of Washington University's McDonnell Center for the Space Sciences.

"I think that SHARAD will be an excellent mapper of the sedimentary layers on Mars, and that will help us better understand the water history of the planet, "Phillips says.


More detail, visit the following URL: Comparision between MARSIS and SHARAD, Two orbiters with ground penetrating radars

Rodolfo

[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.

[tty]

The lowlands might be somewhat similar to the strange "edoma" deposits of northeastern Siberia. This is an ice-rich loessic material that is something inbetween an ordinary soil and a glacier. It is an eolian deposit that is formed only in extremely cold and dry climate during glacial periods. It does not form at the present time and indeed seems to be melting in some areas. Geomorphologically it is recognizable by the existence of "thermokarst", but it is uncertain whether this would be true on Mars.

ttu

[Alex]

No chance that it was due to an ancient ocean of water? I can see shorelines, ancient rivers, sediments like the rivers on earth... Is it so crazy the idea of an ancient ocean in Mars? I do not agree with the theory of the lava flows, there are no signs of those flows anywhere...

[Chmee]

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.
....
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.

From what I hear from everyone so far, is the difference in the height of the southern and northern hemispheres may be a different story than why the hemispheres are rough versus smooth.

Therefore maybe something like this happened: Late during the formation of Mars, a very large impactor created the large variation in altitude between the hemispheres. However, later bombardment created a planet-wide cratered surface similar to what we see now in just the southern highlands.

Then, much much later (during the Amazonian period) the Tharsis bulge was created along with the giant volcanos. This resulted in giant floods of a soil/water mix (more mud than water) from underwater aquafiers put under pressure by the rising land. These mud-floods covered the lower hemisphere erasing the craters and also creating the giant carved channels we see today leading to the northern plains.

Is this about right?

It does seem more plausible than lava, since lava could not be so smooth over such a gigantic area. Also, an ancient standing ocean would have left more evidence of weathering (shorelines, limestone, etc).

[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.

[JRehling]

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.

5 km/hour... 10000 km... --> ~ 75 days.

It would be awfully convenient, but the sheer speed doesn't rule it out.

[dvandorn]

OK -- first off, are you saying that MEX is finding no sign of hydrated iron oxides in the very soils that seem to be giving off a fair amount of hydrogen, as detected by Odyssey, and therefore are assumed to be covering significant ice deposits? Or at least that contain a significant admixture of ice?

So, which set of readings do we believe -- the ones that show a lack of hydrated iron oxides, or the ones that indicate a significant ice presence within much of this region?

If there is ice in close proximity to the surface of the northern portion of the northern hemisphere, how can that ice exist within and under soils that show no signs of hydration? And if the soils were emplaced after the ice was emplaced, then the non-hydrated soils could be covering an ancient sea floor and MEX wouldn't have a clue.

Another point -- is MEX looking at the very surficial layer, which may be dust and soils generated from non-hydrated basalts dating from the Tharsis development, or does it "see" deeper than the very surface layer? Is it possible that non-hydrated dust deposition may have covered any hydrated iron oxides that might still lie rather close to the surface?

And finally, is there any correlation between the places where MEX sees hydrated iron oxides and those places where the erratic magnetic field seems to be shielding the surface from solar UV? If you want to test UV effects, and if you can assume that these erratic magnetic "loops" that form bubbles of partial UV screening are long-lived, then it makes sense to identify these areas as precisely as possible and then compare surface characteristics inside and outside of them...

-the other Doug

[Guest_BruceMoomaw_*]

There's another LPSC abstract that deals with the ice question. They aren't even finding such minerals in the areas where Phoenix will land -- which consist of just a few cm of soil spread over solid ice: http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1706.pdf .

So as the OMEGA team says, Mars has been devoid of any large amount of surface liquid water for a VERY long time. Presumably non-hydrated soils have indeed covered the northern plains for that very long time -- but they must cover any existing hydrated Fe oxides, phyllosilicates, or sulfates very deeply indeed, because all of those are being seen in just a few tiny patches on the Martian surface exposed by the chance patterns of erosion. And Mars Express cannot analyze from orbit any substance buried by more than a fraction of a cm of other material -- I can't think of any instrument that CAN do so except for gamma-ray and neutron spectrometers.

Finally, those local magnetospheric bubbles do absolutely nothing to provide shielding against solar UV (or, for that matter, X-rays and neutrons) -- just against solar charged particles. And their shielding against those isn't all that intense. As far as I know, there is absolutely no correlation between them and the exposed patches of hydrated minerals. (In fact, another LPSC abstract -- which I can't track down at the moment -- compared the highest-intensity magnetic "stripes" found by MGS with local mineral maps made by the various orbiters, and found no compositional correlation with that either. Whatever minerals are responsible for the stripes are also deeply buried.)