Geomorphology of Gale Crater, Rock on! Options

[ngunn]

That's fine for HTIF and HP, but going on Anderson and Bell's map there should be no LTIF at our current location. They have the LTIF mapped to the north of the HTIF while we are to the south of it, hence my query.

http://martianchronicles.files.wordpress.c.../09/figure7.jpg

[JRehling]

I've been thinking that the landing site was HP, having missed the fan, as you say, to the north. But our confusion is, I think, owing to the inherent complexity:

The distinctions we're talking about are not necessarily visible or even rigorously meaningful. Thermal inertia is a property that can vary from place to place on the basis of any combination of changes in composition or fine-scale morphology in potentially-wicked interaction between the visible surface and the near subsurface. Maybe MSL landed outside the area that Anderson and Bell colored as "fan" on their map, but is nonetheless in an area where the fan material is present, but in combination with other stuff so as to give it a different thermal inertial. In fact, there's no logical disconnect between these labels: "high thermal", "fan unit", "hummocky", and "plains" are potentially overlapping in any combination because they four different kinds of property.

I think MSL missed the region that A&B labeled as being the fan, but may in fact have some of that fan material all around, in some fraction, anyway.

On a very similar theme, I was surprised, having read A&B carefully, how difficult I find it to see the units on Mt. Sharp, which seem apparently in the B&W images taken from orbit, in the images from MSL. There are many possible reasons for this, including the viewing geometry, the image properties (such as gamma), my lack of field geography savvy, etc.

A&B did a good job of imposing some logic and order on Gale, but in both the MSL landing site and the distant views of Mt. Sharp, things seem a little more chaotic up-close.

[elakdawalla]

I was having the same issue you were in seeing the units on Mt. Sharp until I realized that most of the interesting stuff -- the clays and sulfates -- is actually in a trough at the base of Mt. Sharp and mostly not visible from where the rover landed.

Working out some comparisons of the A&B units to images from HiRISE and pointing out locations on the landing site panorama has been on my list of blog entries to write for a long time, but it's a big project and I haven't made much progress yet.

[ngunn]

(Replying to JR) All good points. I agree that the views from the ground are taking us into a post-A&B era. But the meeting of three distinct terrain types at Glenelg is most clearly seen in the orbital images so it didn't take Curiosity to show us that.

High thermal inertia is, I think, indicative mainly of a lack of loose cover over the bedrock. In Anderson and Bell that is identified with a particular rock unit, but why would one particular type of rock preferentially remain clear of debris? I think ithe HTI
disribution may be controlled more by the geographical context of the removal process and the ease of removability of whatever material used to cover the bedrock.

EDIT
Emily: good luck with that project - I look forward to seeing the results

[stewjack]

In the conclusion section of the abstact for Anderson and Bell III: Mars 5, 76-128, 2010 open access paper,
there is the following sentence -"Some layers in the mound are traceable for >10 km, suggesting that a
spring mound origin is unlikely."

My understanding of that would be that Mt Sharp was not cemented together by underground (upwelling) mineral water flows during,
I guess, - the period when Gale crater was buried in sediment. Because earlier it is said "The rim of Gale Crater is dissected by
fluvial channels, all of which flow into the crater with no obvious outlet." As well as, I guess, that hot springs would be variable in flow,
time and location? After doing some Googling apparently hot spring can create mounds using nothing but precipitated minerals. However;
I don't now how that would relate to the 10 km layers. Would be enough to say that a mound with many layers wasn't created by hot springs?
Or am I completely misunderstanding what the sentence is trying to communicate?

[serpens]

...... a very nice graph showing exactly why we don't see the basalt yet. It's close, but MSL will need to drive towards the mount to find an exposed trough all the way down to the basalt unit. (In many places the dark dunes are actually covering the basalt floor...)

Most of what we see on Mars has a basaltic provenance but as implied by djellison I think that we could be a little more careful in our use of the word. The Anderson and Bell paper refers to the Basal Unit. Basal is by (USGS) definition located at the bottom of a geological unit which in this case I would think is the bottom of the post impact crater fill sequence. I am not sure why the reference to basalt crept in. The final crater floor would have been made up of allogenic breccias and impact melt and I guess this is the Basal Unit referred to by Anderson and Bell. At least that is what I assumed when I first read the paper, which on re-reading, only mentions basalt once in reference to the makeup of dunes. That final crater fill would be pretty deep and beneath that would be fractured pre impact material that, given the size of the impactor, would probably have been subject to a degree of impact metamorphism.

[dvandorn]

Yep, Mars' surface is primarily basaltic, no doubt. And like the Moon, much of the original crust has been highly brecciated by the Late Heavy Bombardment (the "event" which likely resulted in the Gale impact, among tens of thousands of other impacts of similar size).

Analysis of basalts, where they were emplaced, would give us a nice feel for what was happening in Mars' mantle while the majority of the basaltic eruptions occurred and the basalt was emplaced on the surface. Sort of a snapshot of the mantle during the period(s) of heavy volcanism. However, it is the alterations and re-depositions of that basaltic set of "building blocks" that tell us about the climate and conditions on the surface after the basalts were originally emplaced.

So... Gale is not a good place at all to survey variations in directly emplaced basalt flows. The occasional unaltered chunks of basalt lying on Gale's floor were likely transported from somewhere else (be it a few kilometers to hundreds of kilometers from where a rock might rest right now). It is, however, a wonderful place to look at the history of re-deposition and alteration of rock beds (and even deflation of covering beds), much of which (it seems to me) has to have happened when the alteration, deposition and deflation processes that went on were far more active than they are now.

Since one of the main purposes of Curiosity is to try and characterize those processes (because those processes, once understood, then highly constrain the climate and environment in which they occurred), Gale is a very good place. Precisely because this is a place where we can study the history of those processes and try to understand them.

-the other Doug

[Zelenyikot]

I think that rock high thermal inertia is a lava stream, from a volcano about which I wrote earlier. Ancient eruption caused a wave of a lava which became HTI. After eruption were weaker and began only water flows from the melted glaciers. So appeared the alluvial fan.
This hypothesis is hasty, but it seems to me logical.
Hope we discover soon.

[JRehling]

Replying to stewjack, re: 10 km layer seemingly disproving a spring origin:

I remember encountering this passage for the first time. My interpretation was that a layer which extends 10 km and remains roughly constant in altitude indicates, if sedimentary, a massive reservoir of water filling the crater like a lake, whereas a spring would have a small origin and would not supply adequate water to create a level surface across such a great area. In fact, that seems like a profound understatement, although I suppose that depends entirely on how large the volume of a "spring" may be.

Since we only see the edges, I suppose, also that you could have a level visible edge at some distance away from and below the source of the spring (as the edge of, say, Olympus Mons is far away from, but below, the vent, and is nonetheless relatively level), but then Olympus Mons is hardly a "spring."

[stewjack]

Replying to stewjack, re: 10 km layer seemingly disproving a spring origin:
I remember encountering this passage for the first time. My interpretation was that a layer which extends 10 km and remains roughly constant in altitude indicates, if sedimentary, a massive reservoir of water filling the crater like a lake, whereas a spring would have a small origin and would not supply adequate water to create a level surface across such a great area.

Thanks! I bet that sentence took a while to construct. Leaving out "roughly constant in altitude," avoids discussion of sloped layers.

[serpens]

There are a couple of other reasons why attributing Mount Sharp as a spring mound will not hold water. The lower half of the mound transitions from phyllosillicates to sulphates but the upper half of the mound is a aeolian deposition. So a spring would not explain Mt Sharp. Further, if this was a spring mound then we are considering a huge volume of water - probably enough to fill the crater given the size of Mt Sharp, which would have almost certainly have resulted in a breach of the Northern crater wall. No such breach exists. Well that's my take anyway.

[nprev]

Is there any real evidence that Mt Sharp is anything but a typical central crater peak, albeit with modified surface units due to subsequent environmental variations?

If that's true then the areas of interest are these modifications and the processes that made them, not the mountain's origin.

[ngunn]

The peak is too big and other similar size craters nearby don't have them so it's definitely atypical, probably the most extreme example of its kind on Mars.

I'm thinking about the possibility that Gale crater once had a much higher northern rim, at least as high as the top of the horizontal beds on Mt Sharp. If it formed at the edge of a frozen ocean maybe the north rim was largely composed of ice which has gone now.

[serpens]

nprev. I'm with you in that Mount Sharp probably has a central uplift core, but the bulk of the mountain is sedimentary. Have a look at a couple of the complex Lunar craters such as Maunder to get an idea of the relative size of a pretty much pristine central uplift.

The puzzle (and I deliberately avoid the word mystery) is why the sediment ended up as a central mound. I have difficulty accepting the explanation that the crater was overfilled to the height of (or greater than) Mt Sharp and then excavated, despite the credentials and credibility of the proposers. That hypothesis requires that the sediment that must have covered the rest of the crater and the surrounding area was totally removed while that on Mount Sharp was significantly more resistant. I'm backing a shallow crater lake for the phyllosilicates and a vortexing effect for the remainder. I don't have the smarts to model something so complex so take the last as being accompanied by wild guestures from the depths of an armchair.

[dvandorn]

If I don't say this as smoothly as I might otherwise, please forgive me. The thought racing around my brain delves into areas of physics about which I'm not completely confident.

First, it has struck me that dust devils form more easily on Mars than they do here on Earth. Considering how thin the air is and how cold the overall environment is, you would think there would be more energy available on Earth for such vortex formation than on Mars.

But, I says to myself -- Mars spins around its axis at roughly the same speed as Earth spins about her own axis. But Mars is significantly smaller. Its surface is rather closer to the center of rotation than is ours.

Would this not, based on conservation of angular momentum, mean that the coriolis force would be noticeably stronger on Mars? The spinning skater spins faster and faster as her arms are drawn towards her, and on Mars the difference in rotational speed between me and the spot 10 meters to the north or south is greater than at the same distance on Earth. And, if I understand the coriolis force correctly, it is this difference in rotational speed that drives everything from typhoons to dust devils to the swirl of water running down the drain.

So -- if I'm reading this right and the coriolis force on Mars is noticeably greater than on Earth, encouraging a lot more atmospheric vortex formation, how would this affect simple aeolian erosion patterns on an early Mars with a much thicker atmosphere than now?

Consider that in 6mb air pressure a modern Martian dust devil can pick up and entrain a pretty impressive mass of dust and pebbles. This process keeps much of the Martian surface swept clean of the ubiquitous orange-brown-yellow dust, the darker gray rock beds thus exposed forming the dark markings visible in telescopic images of Mars for more than a century.

How much more erosive would a thicker atmosphere be, if an increased coriolis force makes it tend to form vorteces at every opportunity?

This relates to the previous posts thus -- imagine Gale crater nearly filled with some form of fill. Then imagine a racetrack wind pattern running around inside the crater walls, breaking up into hordes of large dust devils which, due to the thicker air, are able to pick up tons of material and toss it high into the air?

You'd have a pretty dusty atmosphere all the time (which would tend to cool the surface, I imagine), but such a wind pattern might be able to deflate an *awful* lot of material out of a crater in a pretty short time, at least in geologic terms.

Maybe it was such a dust devil breakout phase that deflated a lot of crater fill on Mars?

-the other Doug