Cape Verde - Page 11

E.S. Yes, there would have been a lake in the crater. Inflow would be balanced to an extent by sublimation, but only to a small extent, because the crater walls would tend to keep water vapour in. I think that even today non-polar water is most likely to be found near the centre of a crater, which is why I hope one day Opportunity will investigate a central crater dunefield close-up. I think that some central crater dunefields get a seasonal coating of ice because of vapour being retained by the crater walls, and this this accounts e.g. for the odd appearance of the dunefield in Endurance crater when Opportunity was there.

Brian, the water flow was my notion. I don't know that Horton agrees with it, only that he sees something strange in the image. I think it would be great if Oppy could get to within a few meters of this strangeness, to examine the spot where the waterfall would have hit the ground, if there once was one.

Rpage, I agree that "we are seeing layers formed by compacted layers of sediment that formerly had the support of ice that has since been sublimated".

Brian: "It is also tempting to think that the rock may be displaced to an extent, but that may just be an illusion."

Just because the crack is wide now, doesn't mean that the rock on either side of it has been moving apart. I'd say it's wide because flowing water widened it as well as deepening it. The oblique crack is wider at its lowest point than it is at the top. This is because the water flow started small, not enough to cause a big widening. As the crack deepened, the water flow increased, but this increased flow could only widen the bottom of the crack. The top remained relatively narrow.

In reply 201, where I said sublimation I should have said evaporation. I'm getting so used to water on Mars being mostly in the form of ice, I'm forgetting the right terms to use when we're talking about liquid water.

Martin,

that was it, evaporation .
Surely, having water there right now is ulikely. But a couple billions years ago, if flowing water was somewhere, it should have been in the craters.

eS

Martin, no the rock's displacement is not an illusion. See this super 3D. It clearly shows this rock is not aligned with the rest of the cliff face.

This super 3D also shows that the "white stuff" along the left side of the rock is a coating on the rock that crosses the rock layers.

Thanks Hort; Your image suggest the coating may be similar to the fracture filling we have seen on previous narrow cracks and sometimes formed "razorbacks.

Hort that is a brilliant 3D. I had attributed the 'white stuff' to abrasion damage from rocks falling from above (loose rocks are visible on top of the feature and at the bottom). While Ben is probably right I think I would still sit on the fence at this stage.

Horton reply 205: "It clearly shows this rock is not aligned with the rest of the cliff face"

I would say that the entire cliff face is fragmented, and that *every* fragment has slipped at least slightly with respect to its neighbours, so that you'd be hard pressed to find two adjacent areas that align. To the left of the weird rock, and elsewhere, there's even fragments where the stratification is vertical.

I thought Brian was referring to something other than this general misalignment of everything with everything else. The specific weird rock of reply 194 has a near-vertical crack which doesn't extend all the way from top to bottom, so it's still a single rock despite the crack. The crack seems to be widest at its lowest point, and to form something like an outlet there. I thought that this was the weirdness you were trying to point out, the one which makes this rock specifically weird in a landscape that's absolutely crammed with weirdness. And I thought Brian was suggesting that the two sides of this particular crack might be misaligned with one another, even though they're both part of the same continuous rock slab. My apologies if I misunderstood anything.

sol 1576 L2L6(superres)R12x2 of crack in Cape Verde cliff:

OK, I really, really need a rock guy to explain what I am seeing inside the cliff.

What I think I see is original blocks of rubble from the Victoria creation event -- overlayed by the, er, layered stuff.

This would mean at least some - if not all - of the layering occurred after the crater - not before.

Is this picture an argument for impact surge creating the layering?

I'm sooo confused.

Hort; I believe the layering was present before the impact and what you see are dislocated blocks of rubble in a "cave" like cavity which are very common.

My guess is that the inner blocks are just as stratified as the outer ones, but it isn't as obvious, because they haven't been weathered so much.

Some of the most weathered fragments are like stacks of widely-separated thin wafers. There are alternating layers of stuff that weathers very easily and stuff that doesn't. That's why the layers are easy to see almost everywhere, but not in sheltered spots like this one.

I'm with Ben and Martin. The 'rubble' is common to the stratum and is not displaced ejecta (if that is what you were suggesting Hort?). The ejecta level is well above this stratum which was therefore certainly present before the impact. The material within the crack was protected from aeolian erosion which is why the layering is not pronounced.

To me the question of the provenance of the crack is more interesting than the rubble within it. Underground solution transport rather than dessication cracking or impact shock would fit the JPL/Cornell interpretation of the white layer as the top of a water table. Ahh, hope does spring eternal.

Again Hort, your high res 3D are brilliant!!!!

Yes, underground solution transport is the key to understanding Victoria, I reckon. The springs weren't quite eternal, but they were there for a long time.

Here's my scenario. The crater rim was originally higher, and it was more or less equally high everywhere, no bays and promontories. Water from the surrounding area seeped horizontally into the crater, filling it to the level of the water table. Flow continued thereafter at a reduced rate, to balance the evaporation from the crater lake. Eventually the water table level became lower, and the lake became correspondingly smaller.

All this flow caused chemicals to be dissolved out of the rock under the crater rim. This caused slumping. But the slumping didn't occur evenly everywhere, and the unevenness of the process resulted in the first cracking.

Now when water came to a vertical crack it had the option of going down that crack, whereas previously it had had no option but to continue horizontally. The rocks on the "downstream" side of these cracks now became "high and dry". They still contained lots of really soluble material, but there was no longer a flow that could dissolve it. These regions ultimately became the promontories.

The flows that were bypassing the proto-promontories had to go somewhere. They became more and more concentrated in what were to become the bays. As a result, everything that might be dissolved in these areas was dissolved, and they slumped spectacularly.

And that is more or less where we are today, except there's no lake any more. The promontories are clearly seen to be stratified, because the soft soluble material that was dissolved out of the bays is still present in them, and it weathers fairly easily, but is protected by the alternating harder layers. The promontories are seen to be heavily cracked because these cracks are what saved them from slumping to the same extent that the bays slumped.

Martin; I guess what you may be saying is that a single big or multiple big cracks were expanded by groundwater seepage which formed the bays.

At one point I envisioned that ice may have been involved in excavating the bays.
Could this be the last process before the water was all gone?

Ben,
my suspicion, as I said, is that the bays were brought low by slumping, by the dissolving of underground material beneath them by liquid water flows.

When you mention ice, I suppose you mean a layer of ice which covered the entire landscape and which flowed into the crater, carving out the bays. So the removal of material would be a surface phenomenon. Each bay would have been a mini glacial valley. Is that right?

If so, there'd be post-glacial features visible. Things like lateral and terminal moraines, for instance. I don't recognise any of those in the panoramas I've seen.

Ice will have played a part in the development of this area. The crater lake very likely froze before it disappeared. Ice has probably widened some of the cracks in the rocks. But I don't think the ice has been a very powerful force here.

Ben, Martin.
My scenario leans towards the impact occurring after the water table had disappeared. The sandstone making up the cliffs and bays is very friable, evidenced by the lack of significant rubble in the scree below the cliffs. All the erosion we have seen seems consistent with aeolian rather than water or ice effects. I actually wonder whether the bay formations were not kick started by radial cracking from the impact.

If the erosion had continued then we would probably have seen a result similar to Endurance as the promentaries were undercut and weathered back to align with the bays. We did see some mass wasting effects in Endurance with the remnants of rubble piles on the crater slope where residual promentaries seemed to have slumped.

Not as spectacular as the concept of a Lake Victoria fed by bubbling springs I know - but I feel more appropriate to what we see.

Horton, re your 209, There is one surface of the layered bed rock that shows no layering just to the right of your layerless embedded rock. Ben and I discussed this a few pages back and agreed that the layering may be present but not expressed as visible layers at the surface if a certain sort of erosion has not acted on the rock exposing the layering. Maybe the erosion has consisted of a billion years of dust scour where the layers are visible and more recent fracturing where the layers are not visible.

I think that this may be the case because the layering is barely there at all, recording very small differences in density or particle size during a single event. The layered rock of Meridiani in general shows very little tendency to fracture parallel to the layers. These indications may mean that the layers were not built up slowly and sequentially, which would sometimes create weak and visible horizontal contacts between adjacent layers, but rather the layers were created all at once.

Kye, thanke for expressing what was bothering me about the fractures of the rock in the cliff cave. Why would the large block fracture so cleanly in almost perfect planes not related to the original "bedding" plane -- if in fact it is simply a continuation of the same stuff?

Did anyone notice the faint hint of mint -- I mean blue stuff -- on the largest exposed face of the largest block in the cave?

"Did anyone notice the faint hint of mint -- I mean blue stuff -- on the largest exposed face of the largest block in the cave?"

Not before you mentioned it. The random scattering of blue pixels - probably data transmission errors - especially in the LHS image detracts from the more subtle blue of the cave interior. But I can see it now. Thanks for pointing it out. It looks to me like it might be a relatively fresh crop of berries growing on the recently-exposed unweathered surface. Berries may be an important factor in the weathering process.

Bear in mind that UV is only a problem where sunlight reaches the ground directly. In the Southern Hemisphere there'll be lots of south-facing cliffs where, especially where there's overhangs and caves like this one, direct sunlight never penetrates. But there'll be plenty of light from the sky, and reflected off the ground. More than enough for photosynthesis.

It's a pity that the slope leading up to the cave is probably far too steep for the rover to negotiate. I hope that the next generation of Mars exploration devices will have legs instead of wheels, and will be designed to explore any kind of terrain.

Hort,
I did see the blue tinge which is indicative of hematite in a false colour image. This was what prompted my comment on the possibility of solution transport being the genesis of the crack. But it isn't really obvious and could be deposited basaltic dust.

I don't think the fact that the crack cuts across bedding planes or that the rock sheered across those planes is at all significant. There is a nice roadcut of Juriassic sanstone just down the road from me, with a couple of faultlines almost perpendicular to the bedding planes. Not a marker for surge.