More Structure of Victoria Crater

I thought we could use a thread for general discussion of the structure of the crater again. I am still intrigued by the question "What's on top?".


I see a number of structures that I will just call "hoods" to avoid any implication about how they form. This image shows the top of the slope at Duck Bay. There is a rough line in the slope about half a meter below the crest. Near the center of the image this "contact" or discontinuity seems to rise over a very large rock. Further to the left the contact rises over a smaller rock where it forms a "hood", a thin layer above the rock and molded to it (perhaps) that protrudes slightly in front of the rock as if it were more resistant to erosion.

http://marsrovers.jpl.nasa.gov/gallery/all/1/p/952/1P212696323EFF76EKP2388R1M1.JPG

Notice also in the image above, a thinner layer at the plain's surface of uniform material, appearing fine-textured, perhaps 20 cm thick. I am now pretty convinced that this material is continuous "bedrock" that overlies the breccia immediately below. I do not understand how it formed. I sure wish scientists would take an interest.

Getting back to "hoods", there are two more clear ones in this image of the slope-top south-west of Bottomless Bay. Look near the slope crest just left of center where two adjacent rocks have little caps or "hoods":

http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1034/1P219973173EFF77TUP2443L7M1.JPG

Here are perhaps up to a dozen more "hoods" in this image of the top of the cliff at Bottomless Bay. In most of these cases the underlying rock "mold" (?) is missing. These "hoods" and all the others I think I see are within half a meter of the plain's surface:

http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1034/1P219973052EFF77TUP2443L7M1.JPG

There are more examples of hoods scattered along the rim. I think that these hoods may be structures that have formed within the breccia as a result of proximity to the plain's surface, long before their exposure in the eroding crater wall. The "half-meter contact" in the Duck Bay image (first above) is at about the maximum depth that would be reached by diurnal warming. I think that these hoods are yet another chemically formed structure like the fracture fills and rinds.

Here is another image of a continuous top rock layer on Victoria's rim:

http://marsrovers.jpl.nasa.gov/gallery/all/1/p/970/1P214302068EFF76POP2400R2M1.JPG

I am now fairly convinced that most of the top layer that forms the plain's surface in the image above is rock not soil. At the bottom of this top layer where it meets coarser material, in the distance in this image, there seems to be a thin layer of protruding material more resistant than that above or below it. That material, at least, is clearly rock and clearly not breccia, so what the heck is it?

Here is that image of the Duck Bay slope again:

http://marsrovers.jpl.nasa.gov/gallery/all/1/p/952/1P212696323EFF76EKP2388R1M1.JPG

A uniform layer about 20 cm thick, that appears fine-textured, is seen in-section at the top of the slope. It is obvious across most of the image except to the far left. I think that this material is rock. In this image much of the "contact" between the top layer and the coarser material below shows a thin shadow line, as if the top layer is slightly undercut. The top uniform layer is likely rock not soil or it could not be undercut without collapsing. Again, this material is not breccia, so what is it and how has it been emplaced above the breccia?

Kye: Interesting comments about the hoods and surface ledge.
We know there has been erosion of the ejecta blanket because many of the rocks at the top of the cliffs show they have been worn smooth to a near-horizontal plane.

The hoods appear to be associated with a ledge forming disconformity that is not planar but seperates ejecta material.

If this harder layer were associated with ground water it would be planar and I doubt it can represent the boundary between two ejecta layers, so I agree it must represent a time? boundary where subsurface chemical precipitation occurred .

I agree that the thin, possibly discontinuous due to erosion,, layer at the surface is essentially rock .
My first thought is a caliche like analogy.
Caliche layers do develop below the surface
and I have read a paper where the French
believe ALH 84001 could be altered caliche.

Both of these features are representative of relativly recent activity which makes me wonder if Mars isn't in a warming cycle.

As a newbie commenter, but a long time observer in these pages, I still have this nagging question in the back of my brain ... how can you have such a rugged, large boulder, shallow subsurface with what looks like a fairly flat surface expression? I see large boulders which seems to be conveniently flat on their surface side ... not just one, but many. I can't picture anything I've seen in the field like that.

fizzbyn: I think they have been ground smooth by some erosive process.
Personally I think moving ice was involved because the surface is so planar but then why wouldn't the ice have moved the loose rocks a bit.
It could just be eons of abrasion by the wind since we really don't know how hard those boulders are but IMO that wouldn't produce such an even plain.
Whatever it was, large areas are ground flat like a terazzo floor.

Ben, it is encouraging to me that you agree about a couple of my observations/speculations: that we may be seeing evidence of past near-surface chemical activity in the exposed "hoods" and near-horizontal contacts and that the uniform top layer at Duck Bay is plausibly solid rock. That last idea is very subversive, don't you know, because almost any explanation of how the rock cap formed above the breccia would require a major change of direction for Mars science. I have been trying to explain it as a thin impact-surge deposit, but I am now wondering if something like caliche might be closer. Water and dissolved minerals might come from ice or hydrated minerals below the surface, or water could come from the atmosphere as frost or vapour. Perhaps there are climatic cycles, daily, seasonal or longer that move water up and down through the near subsurface.

fizzbyn, Ben, About the flattening of boulders: We see a lot of surfaces made up of both rock and soil that have been strangely flattened. Here is an example from the raised rim of Beagle Crater:

http://marsrovers.jpl.nasa.gov/gallery/all/1/p/901/1P208170254EFF74ZTP2291R2M1.JPG

The surface is not entirely planar but a similar smoothing seems to have happened to Beagle's ejecta as to Victoria's slopes and the general plains surface. I am really stumped for an explanation of how this smoothing process works. The classic explanation of erosion by aeolian sand-scouring seems like it might work, but I have questions: How do the harder rock and softer soil end up being scoured to the same level? Sand-scour erosion sometimes picks out and removes slightly softer rock, forming pits in harder rock. Slow dust deposition might fill the inter-block spaces with soil, but when the rare superstorm comes and sand flies why doesn't it quickly erode the inter-block soil and then start rounding the corners on the rock blocks, especially the windward corners? Similarly, how is the very sharp corner where the slope meets the plain at Victoria kept from rounding-off? Winds blowing up or down the slope would surely attack that corner. If sand-scour smoothed Victoria's slopes I would expect to see grooves cut into the slope-crest at right-angles to the rim. Instead, the slope-crest is crisp and pristine. Wierd. I really think we have a surprise coming when we figure out how erosion works on Mars.

I have suggested the following several times.
Aeolian erosion acting over a long time is the main process in creating the even flat surfaces. However I think that at times the surface has behaved like a sol or colloid, i.e. the finer material partially liquefies and the boulders 'float' in this for short periods. This may seem odd but would explain ephemeral fluid flow. Also the fine grained sediment rims around some of the blocks that we see everywhere (sometimes referred to here as 'microchannels') are formed due to small upward movement of fluid as blocks settle. I have even suggested that the balloon imprints in Eagle also show evidence of shortlived upward fluid movement. Think of jumping on a solid sandy beach in the intertidal zone. The effect is similar.

"Ventifacts are any bedrock surface or stone that has been abraded or shaped by wind-blown sediment in a process similar to sand blasting.

Yardangs are streamlined wind-eroded ridges commonly found in deserts. "

[Link]


Marsman : Are you suggesting the flat surfaces are an advanced stage of "ventifaction" a process that has not had time to have been reached on earth?

The rocks could be poorly cemented and soft enough to be easily worn down by wind born, abrasive sand grains.

Kye: I definitely think the near-surface layers now at Spirit meet the criteria of caliche including the cementation of the granular material we have seen recently.

Newboy: I think the secondary filling we saw in the microchannels also supports your
idea of the fluid movement.
All of this is very common in modern sabkhas.

Just thought I would point out some more of the rock "hoods" that seem to have formed all along the rim. I guess that they could be weathering rinds that have formed from the top outside of the breccia pieces, or lithified soil or dust mantles that have formed against the top of the breccia pieces. Either way they are now exposed by erosion in-section as long thin protruding structures that parallel the top surfaces of the rocks near the surface of the plains. I see about 6 more examples here:

http://qt.exploratorium.edu/mars/opportunity/pancam/2007-02-05/1P222292283EFF78BPP2350R2M1.JPG

There are many more imaged rock hoods that I have not pointed out. They are a regular feature at the top of the breccia layer. It seems that they never occur lower on the slope and never along the downhill side of the rocks. It seems likely to me that these rind-like structures are present at this depth all across the annulus, not just here where they have been exposed.

Kye: Give me your take on the reply 9 dark streaks. Moving uphill or down?

Marsman : Are you suggesting the flat surfaces are an advanced stage of "ventifaction" a process that has not had time to have been reached on earth?

Given enough time, Ben, I think that it is possible.

Very Respectfully,

marsman

Marsman: My question was to see if you agreed. Glad you do because I see it that way.

Ben re reply 10 which was "Kye: Give me your take on the reply 9 dark streaks. Moving uphill or down?"

I don't know what those dark patches on the slope are getting up to to Ben. They are sure intriguing, because from the HiRise view it looks like there may be dark features in this quarter of the crater that we haven't seen anywhere else, and this might be one of the first examples from close range. To me the dark patch looks like it could be made up of pebble-sized particles, but it could also be clumps of finer material. This similar dark patch from the next bay west seems to be associated with a steeper pitch of slope and perhaps a particular stratum underneath the breccia layer. To me these dark patches look like they may be related to the chemistry of the underlying rock, that is, they are not just aeolian deposits on top, though they may be that in part as well.

http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1073/1P223438275EFF78W4P2360R2M1.JPG

super 3D panorama ( L257-L2 1x2 ) of Cape Desire:




The super 3D shows a dark area in shadow that seems more than shadow under a long overhanging "contact" area.

( I shutter every time I look at the slope of Cape Desire. Anyone "up" for a "Special effects" placement of Oppy on this cape (maybe 3D?)?

Great Image Hort; On earth, that dark layer would be Swallows (muddobbers)nests.
Here I think it represents an angular unconformity which represents a period of erosion between deposition of the lighter material above and the darker below.
If this is an old erosional surface the dark stuff could be lag material that accumulated on the older surface and has now been exposed in the cliff.

I mentioned this before and will try again.
I don't see the massive beds just above the talus slope on the left end of the far point.
It looks like jumbled ejecta all the way down to the bottom of the the cliff.

Is this evidence of a pre-ejecta surface that had topographic relief or later post-ejecta faulting?

Ben, re 15, I think that the jumble you see is somehow an illusion of distance. I don't think we ever got a good look at that cliff because it faces out into the crater. Now Oppy is far enough around the circumference to see it looking back but too far away to resolve the layering. That cliff is around the corner in this image, I think:

http://marsrovers.jpl.nasa.gov/gallery/all/1/n/1034/1N219979148EFF77UNP0745R0M1.JPG

The up-to-date route map on Unmanned Spaceflight is a good place to start with these puzzles.

Kye: I am not totally convinced that it isn't thicker ejecta .
How do you explain the thinning of the ejecta layer as we proceed around the crater rim? It looks like it may be completely missing a short distance in front of us.
Has subsequent uplift allowed this edge to be more eroded or could the pre-impact surface have had undulations which now show up as thick and thin areas under the plain.

Ben, I think that the rim ejecta layer does taper down to nothing or almost nothing on some of the capes, but never appears below the pre-impact surface marked by the extensive bright layer.

That might not be so. Right after the impact the crater "walls" might (should?) have been covered with breccia at the angle of repose, but I think that all that breccia has been removed and the crater has expanded outward beyond the original walls. There is no certainty of this simple situation but I would have to see a pretty clear counter-example to change my mind. As you say there could have been a lot of relief in the old surface, even an earlier crater.

The scene of fractured rock and linear depressions in the foreground of this image is very similar overall to exposed plains bedrock outcrops. I interpret the foreground outcrop as a large piece of impact breccia now fractured in-place into smaller pieces. Upturned bedding layers are visible running away from the viewer.

The fractures seem to run vertically into the rock much as the plains outcrop fractures do, but here they would be sometimes near-parallel to the bedding planes instead of crossing them. I think that the fractures in both cases have formed in relation to the surface environment unaffected by the bedding planes. The fractures always form running inward, away from the exposed surface.

http://qt.exploratorium.edu/mars/opportunity/navcam/2007-02-27/1N225845772EFF798DP0705L0M1.JPG