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Geomorphology of Gale Crater, Rock on!
ngunn
post Sep 26 2012, 10:22 PM
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I'd like a discussion thread about the geology detatched from the time limits of current MSL threads. We had a 'Geomorphology of Cape York' thread that attracted a lot of interesting posts. How about 'Geomorphology of Gale Crater'? I have one or two ideas but many more questions, and I'd like to post them in a longer-running thread away from the day to day imaging discussion. Any other takers?

For starters, does anybody have a contour map of this place like the one at Meridiani with 5m intervals?


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Eyesonmars
post Sep 27 2012, 12:42 AM
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I'm a little confused about the layers on Mt. Sharp. Could one of you geology types set me straight?

a) the thinking is that the mound is a remnant of the vast sediment that once filled gale crater
correct?

cool.gif If so, wouldn't most if not all of the sedimentary layers be flat since no tectonic activity has occured?

Perhaps it is a trick of perspective but all of the layers I can see in the buttes and mesas below the discontinuity are uniformily tilted up toward Mt. Sharp.
So I'm wondering if the layers have nothing to do with the original deposition but are an artifact of more recent aeolian erosion.



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Ondaweb
post Sep 27 2012, 01:25 AM
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QUOTE (ngunn @ Sep 26 2012, 05:22 PM) *
How about 'Geomorphology of Gale Crater'?


I like that idea. I, too, have some questions perhaps better posted there.
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Phil Stooke
post Sep 27 2012, 01:25 AM
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Sediment settling out of a fluid onto a flat surface might make horizontal layers, but other situations can make tilted layers from the start. In particular, if the surface is already tilted and you start depositing layers of wind-born material (sand, volcanic ash etc.) on it, each layer could follow the slope of the ground underneath it for quite a while until upper layers became more level.

Another possiblity - layers form fairly horizontally over uneven topography. Then over time they gradually compact under their own weight, but more so in areas of deeper sediment fill ("differential compaction"), resulting in deformed layers.

So we can't just assume layers would be horizontal.

Phil



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dvandorn
post Sep 27 2012, 03:16 AM
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One rim of Gale Crater is quite a bit higher above mean than the other, right? Even though it appears to be a regular ringwall kind of structure, not breached nor significantly out of circular. It could be that much of Mt. Sharp was deposited in horizontal layers and the overall ground mass below the entire crater could have tilted before the deflation that exposed the central mound and revealed the horizons we now see as the floor. The entire subsurface table tilting would account for the different heights of the rim between north and south.

As to what could have caused the entire subsurface below Gale to tilt -- well, the Tharsis bulge was responsible for enormous deformations of the crust. Also, if this area of Mars ever went through extensive glaciations, the entire subsurface could have been pushed down by the weight of the glaciers during the deposition of Mt. Sharp's layers, and has since recovered its original elevation and orientation via isostatic rebound.

Finally, if the material that supposedly infilled the entire crater (and has since been deflated) was emplaced by a rapidly moving force, such as the rush of waters or repeated pyroclastic flows from the same vent area, well, that material could have piled up on the far wall and filled back from there. If the force emplacing the materials was consistently from the same vector, you would get layers that are tilted in a sort of compromise between the gravity vector and the emplacement vector.

In other words, there are a lot of ways on Mars that you can get tilted and discontinuous rock beds, you don't have to assume tectonic processes.

-the other Doug


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Explorer1
post Sep 27 2012, 03:20 AM
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Could we figure out a ballpark estimate for the size of the original impactor that formed Gale, or is it too degraded?
There's online simulators but they're for Earth impacts only...
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serpens
post Sep 27 2012, 04:34 AM
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Great idea ngunn.

To appreciate the variations in layering we need to take into account the sheer size of this crater (some 18,000 square kilometres) and the necessary presence of a central uplift which could possibly be a factor in Mt Sharp resisting erosion. Seemingly lots of water early on with aeolian deposition/erosion subsequently. Being on the slope at the edge of the dichotomy there would have been a gravitational gradient towards the north. Couple this with cycles of depositition, variable lithification and differential erosion over billions of years and as impied by dvandorn and Phil, flat layers without variation rather than uneven layering would be the eyebrow raiser.
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ngunn
post Sep 27 2012, 07:39 AM
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(Thanks admin smile.gif)

I have been wondering about the 'high thermal inertia' region that is now in front of us. It looks like it has been somehow scoured clean of loose material. Noting also that it is located ahead of the margin of a presumed alluvial fan, I have been wondering if that 'fan' could actually be the remains of a long-outrun avalanche that formed very rapidly, sending a powerful shock wave ahead of it that blasted the soil off this area.
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jmknapp
post Sep 27 2012, 10:11 AM
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The operative sentence from the Anderson/Bell paper:

QUOTE
Despite interest in Gale Crater as a potential landing site, the origin of the mound remains enigmatic.


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ngunn
post Sep 27 2012, 06:03 PM
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Also in that paper (pp107-8) is discussion of the low-thermal-inertia/high-thermal-inertia fan formations and the nature of the boundary between them. We are approaching the margin of the HTIF Glenelg. We'll soon have some new data to match against the proposed interpretation.
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Chmee
post Sep 27 2012, 08:25 PM
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Great idea for this thread.
Since there is no obvious outlet along the rim wall for water / glaciers etc to have eroded the crater bed to, I lean in favor of the theory that the floor of the crater actually dropped, instead of eroding away (with some later minor depositing which smoothed the floor out). Since Mt Sharp sat atop the old central peak of the original crater, it did not sink like the rest of the crater floor. My two and one-half cents rolleyes.gif
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djellison
post Sep 27 2012, 08:30 PM
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QUOTE (Chmee @ Sep 27 2012, 01:25 PM) *
Since there is no obvious outlet along the rim wall for water / glaciers etc to have eroded the crater bed to,


Why must it have been water? The theories I have seen suggest wind.
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ngunn
post Sep 27 2012, 08:45 PM
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Add to that the possibility of quite a lot of ice in the original crater fill and you have sublimation as another removal mechanism.
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Eyesonmars
post Sep 27 2012, 09:16 PM
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Also note that Curiosity is sitting on or very near the lowest spot on the planet (outside of Hellas).
How did it get that way ? There are far larger craters along the global dichotomy. I suspect that the uniqueness of Mt. Sharp and the fact that it is immediately adjacent to this global low spot ... is not a coincidence
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serpens
post Sep 28 2012, 05:10 AM
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QUOTE (Eyesonmars @ Sep 27 2012, 10:16 PM) *
I suspect that the uniqueness of Mt. Sharp and the fact that it is immediately adjacent to this global low spot ... is not a coincidence


Not unique. Emily did a rather nice presentation on this.

http://www.planetary.org/blogs/emily-lakda.../2011/3144.html
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