We're used to erosion being a kind of sand-blasting, things dissolving in and being washed by rain, etc.

Suppose erosion on Mars is purely chemical and the soft rock doesn't have any chemistry left to perform but the harder rock does. Just an idea.

It isn't. We have seen many typical ventifacts and other aeolian landforms.

tty

You are just a little too far north in Minneapolis for extensive aeolian deposits, but if you go down into the loess country in Iowa or into the Nebraska sandhills you'll see what windblown sand can do in just a few hundred thousand years, much less in hundreds of millions or billions of years.

tty

One thing that would prove to be very helpful in understanding Victoria is to review MOC imagery of many other Martian landforms. One of the best resources for this imagery is the Malin (MSSS) captioned images page. The section on craters, for example, give many examples of buried and exhumed craters.

--Bill

That's a good point, Bill, and a good link. They have a huge selection of images that show the diversity of landforms associated with craters of many ages. Not only are there many examples of buried and exhumed craters from all over Mars, there are a boat-load of them in this part of Meridiani Planum. One is "Example a" in my image borrowed from the Edgett paper in a previous post (#34). It was apparently lager than Victoria. Some recent craters seem to be filled with fairly recent aeolian sediment. We don't really know what originally filled Victoria, or even if it really was buried and exhumed for certain. That seems to be one hypothesis that has been published, and for which there seems to be some evidence. I am trying to keep an open mind as we walk this exposure out.

If the crater was filled with relatively recent, wind blown sediment, it could have easily been removed by a later regimen of windier conditions. I think the working hypothesis that Edgett published advocates a fill from pretty ancient times, which probably lithified. Who knows, maybe it was an ancient crater that once held a lake. I am anxious to see more layers, more closely.

I've spent some time going over those MSSS example images and have chanted 'wow, neat' quite a few times tonight. Just think of what we will have in store with the MRO imagery this year!

We may be able to have a better understanding of the burial-exhumation processes with the Victoria study. I'll presume that we will find chemical changes as well as evidence of the induration of the filling sediments. I'm still kicking around the idea of radiation-induced changes from exposure of cosmic rayson newly-exposed rock. I'll suppose that thousands of years of being hit by cosmic rays would tend to disrupt the crystal lattice of minerals in the rock. I need to check the literature and see whatthey found with the lunar samples in that respect.

--Bill

Strictly speaking, the comparisons to the Moon and other bodies are incorrect. The Moon rocks are volcanic. The rocks in Meridiani are likely sedimentary. The surface of Mars could have been subjected to erosion and tectonic cracking prior to the Victoria event, and this is not likely to be the case for any craters on the Moon.

Craters created in eroded sedimentary rocks will look differently from those in basalt and regolith deposited by previous impacts.

Sure, the signs of erosion are obvious in Victoria, but it's possible that its shape was sinuous from the beginning.

It appears to me that Victoria is a rare specimen among craters. I have yet to see a close analog to Victoria elsewhere, including on UMSF. If anyone can provide a link to an image of a similar cape and bay type rim with a large, relatively shallow bowl please post it. I don't see any similarity to the craters discussed earlier on this thread. I also have a tough time believing that wind and wind borne sand are the only factors in this type of selective (and radially symmetric) erosion. Why did the bays erode selectively ahead of the capes? And why is the pattern of bays and capes so neatly repeated along the circumference? I can't figure how wind could accomplish that. The reality may be more complex, perhaps involving radial fracturing and freeze-thaw cycling, perhaps in combination. Back on topic, the sedimentary record, now well laid out in cross section for us around Victoria in some gorgeous scarps (especially in the north east wall) should answer these questions.

My initial impression from the jagged-edge of Victoria was formation caused by an oblique incoming object exploding right before impact.

But I'm not a crater-expert, nor geologist

Nico

MarkL, check out the image in this post which was originally from Pando. It certainly shows differences from Victoria, but does have the cape/bay structure.

Like Mark, I would expect asymmetry in any wind driven erosion, related in some way to the prevailing wind directions.

If I may toss another hypothesis into the pot to explain the lobate craters, it reminds me of some of the lobate hydrodynamic instabilities one sees in models of supernova explosions. Of course the physics, scale, and timescale are all totally different, but someone who knows more about the hydrodynamics of crater formation may have something to say.
Click to view attachment

Steve

Thanks Fred. I now remember seeing that (it's an MOC image) a while ago too and agree that it's similar in many ways although I recall it was a much larger crater. I think it's somewhere west of Victoria, just outside the landing target ellipse but definitely Meridiani. The capes are not as prominent as Victoria's though. It would be interesting to see a Hirise view of it to compare with Vic.

Reply to Markl:
Yes, I do agree that there is some family resemblance of that crater to VC, and would speculate that a similar process produced it. Basically it looks like Vikkie with an extra story or collapse layer. It's intriguing that it also occurs in Meridiani. Meridiani has had a special developmental history, as indicated by its uniquely extensive lag deposits of hematite concretions. Unless other craters of the type can be spotted in different Mars regions, we might hypothesize that there was something about the water history of Meridiani that resulted in both 'blueberry' formation and 'collapse pits' over craters with water-rich fill.

I can't agree that a fresh crater could be formed with such a pronounced and symmetrical bay-cape rim. I would suggest that the scale of bays and capes might in some way reflect the shear strength, or lack of same, in the 'roofing' rock layers. What extent/width of sulfate sandstone can be undermined before it collapses under its own weight?

This hypothesis would be weakened, if not killed outright, if bay-cape craters occurred on the Moon, since the water-rich scenario would have been much less likely. It will also be killed if the upper cabo walls show unequivocal signs of impact metamorphism.

Fire away!

One thing that struck me with the new MRO images was how Emma Dean is clearly a rayed crater. If an impact creates a circular, rayed crater that then erodes, do the rays become bays?

Amend my "purely" then to "predominantly in the formation of Victoria". Sure, the wind has dominated in other times and other places, but now, back to Victoria...

I'm rather doubtful. Rays manifest themselves outside of the rim, and consist of linear piles of ejecta. Nor are they as numerous as Vikky's bays.

It really is enlightening to see how we all are looking at this crater in so many different ways. I am not a crater expert either, so let me admit that up front. When MarkL mentioned that he had "yet to see a close analog to Victoria elsewhere," I decided to find some analogs to post. Well, it really wasn't very easy to find craters exactly like Victoria.

I am going to try to not make this a long post that few will read, but that will be a challenge.

There are a large number of variables that would seem to control the shape of a crater of this size, but I think the scalloped appearance is definitely the result of a later erosive process, and not the result of the primary impact. Bedrock fractures from the impact might have been influential in the later erosion, though. A crater in horizontally layered rocks of variable strength would also be more conducive to such erosion than massive crystalline rocks.

Some have argued that the regular spacing of the capes and bays could not be created by aeolian erosion. All I can say to that is, go to Utah or some other arid western state and look at the various scales of regularly spaced erosion. Of course, everything is not evenly spaced, there are other factors at play, but reguarity is a common feature of erosion. If you want to see regularly spaced highs and lows from Mars, take a look at these horizontal layers eroding in a crater there.
http://www.msss.com/mars_images/moc/10_30_...s/schiaparelli/

Some of the most obvious, regularly spaced landforms created by aeolian erosion are some of the yardangs.
http://www.msss.com/mars_images/moc/themes/WINDEROSION.html

As I mentioned earlier, I am trying to keep an open mind, but how about this image as a potential model for the early creation of bays in an exhuming Martian crater?
http://www.msss.com/mars_images/moc/2005/07/30/

When it comes to interpreting the geomorphology of a crater, I can't but help to think that it is just another escarpment. I still am inclined to believe that Victoria's shape is only a snapshot in time of an old crater that is still being uncovered.

I'll agree with your assessment 101%. I think that the capes and bays are the result of a resistant unit overlying a weaker unit being eroded by the wind. There may be a number of (not known) factors present that contribute to the regular spacing of the capes/bays but I don't think that there is any mysterious explanation. One other example of the scalloped crater form and exhumation is Erebus: Payson is a bay and the "Payson promontory" is a cape.

We have plenty of time to look at many examples as we walk this outcrop. Or, hopefully so.

--Bill

One thing that I noticed (and I've gone looking for other "exhumed" craters before) is that the other examples of exhumed craters show a very curious profile. The "fill" of the crater seems to stand above the surrounding terrain, based on highlights and shadows. Maybe I'm miss-seeing that, but it seems to be consistent.

I understand that Vickie is old and heavily eroded, but I can't see that she's ever been filled. She just doesn't fit the profile of the other exhumed craters I've seen on Mars.

Right... but are there radial fractures in the bedrock as a result of the impact?

Click to view attachment
Here's a crop of the north-east part of the rim. I've marked some areas which seem to be more resistant to erosion. There is also a concave surface overlying them which might well be wind-carved. Notably, these units are lower strata. Editorially, I think this part of the rim is by far the most interesting as it has these beautifully sculpted cliffs and seems to be the source of the bluish-grey fines that seem to have been swept up onto the apron above. So I'm pulling for clockwise around the crater!

Tom, that's an excellent example of two buried craters being exhumed by wind action. In fact, you can see that capes and bays preferentially develop parallel to the prevailing wind direction:



It might be that once the outer rim of the crater is exposed, the wind dynamics over the crater change, such that we get downdrafts over the rim. The downdrafts could dig deeper and farther into the crater interior, eventually removing the crater fill and preferentially eroding the softer layers along the crater wall.

One thing about wind erosion is that it will affect most similar outcrops around it in a similar fashion. If a symmetrical Cape & Bay system of erosion of was featured in many of the craters in the area rather than just one or two, the wind argument might er, hold more water... With the C&B features so large as what we see in Victoria, I would imagine the eroding winds would have noticible impacts on other nearby features, including the ejecta annulus of Victoria itself. Could winds so powerful as to erode those 60+ ft crater wall escarpments also leave almost no visible evidence on the ejecta annulus around Victoria? I would expect some of that ejecta to be seriously eroded away as well.

It's really hard to see the exhumed craters as indentations rather than as hills, but if you can imagine the light source as being from about 10 o'clock, the depressions should resolve themselves.

With the shadows at about 8:00 (see aldo12xu's post above), it's hard to imagine the light source at 10:00. Also, the highlights and shadows should show consistency with highlights and shadows on dunes, which is what really is throwing me off in terms of seeing an exhumed crater as an indentation.

Just a quick comment. I wouldn't descibe the rim of Victoria as being regular or symmetrical. There are numerous capes and bays but some are short others are long, some are wide and some are narrow. There really isn't an obvious pattern to the orientation of the flat faces of most of the capes. The most regular patterns seem to be along the northeast rim and the western edge of the crater.
I suspect that there are many processes involved.

You are correct, this is a complex area and no one explanation will fit. I'm keeping my hypotheses floating until we get more information.

The scallops are regular but irregular. Overall, Victoria crater has a hexagonal shape, which is due to the "fabric" or pattern of fractures, in the bedrock. This was (AFAIK) first noted by Ralph Baldwin in his landmark book "The Measure of the Moon", which came out at a time when impact or volcanic origin of Lunar craters was being debated. Really.

--Bill

Yes, but the lunar "structural grid" is a rather defunct concept, is it not?

-the other Doug

I think what is meant is not snowflake-like geometric symmetry but general symmetry in the patterns of erosion around the rim. The implication (in my view) is that wind, (which tends to have a prevailing direction over geologically significant time periods) could not alone have caused the same type of erosion all around the rim. And the exhumation of a Martian crater is a wind-driven process. So something else is at play. You could make a case for water being the something else.

Diane,
Exhumed craters can have a variety of profiles. If the fill material is harder than the adjacent surroundings (as when fallback melt breccia is harder than sedimentary surrounds), erosion can leave a circular breccia 'platform' above the plain. When the fill material is softer (as when a crater in basalt bedrock fills with breccia and sand, or, as I am hypothesizing for VC, a crater in sedimentary rock fills with a watery slurry) erosion can hollow out the crater if wind speed is adequate. If the hardnesses are comparable the crater may just flatten out and disappear. I think the MOC images show all of these possibilities.

What may be relatively rare is the roofing over by sedimentary 'evaporite' of a frozen slurry lens filling a crater, followed by loss of water from the lens, causing the 'roof' to cave in and erode (a'la Victoria and perhaps the 'double-decker' sister crater we've seen). This case requires a complex sequence of events, appropriately timed so that we observers catch the crater in the process of being 'unroofed'. A few million years from now that process may be over for Vicky, and she won't look so prominent or unusual any more.

If we can imagine, as Edgett and I do, that Victoria formed over 3 billion years ago, then she must have been filled and buried for much of that time, or she would have eroded away eons ago.

I have stayed out the buried crater discussion for a while because it has really confused me when dealing specifically with Victoria crater. After re-reading the Edgett paper again, I remain unconvinced that Victoria's rim is "buried beneath the uppermost strata of the plains". It seems to me that there is a thick blanket of big blocks of ejecta that thins radially (the "apron") burying the strata. This ejecta seems to be made up of blocks of the same strata that we see in the crater walls and the same type of strata that we have seen in other craters and along the route over the plains.

My interpretation would be that the crater formed after the deposition of the thick eolian basaltic sandstones and thinner sulfate/evaporite sandstones. The blocks of ejecta are angular suggesting that they were already indurated, so, in fact, the impact was probably long after deposition, and could have even been during the current period of landscape erosion. However, there are few, if any, indications of blocky ejecta sticking up from the apron indicating that the ejecta blanket itself has undergone a long period of eolian planing and erosion.

I think the scalloped nature of the current crater rim is due to wind erosion and undercutting of the relatively unconsolidated ejecta blanket and some undercutting of the older strata. I can not see how the crater could have been filled with the material that forms the crater walls...some of the loose, dark sand blowing around on the recent landscape and rock falls from the crater walls, yes...

Oops. I meant to say "light source at 8:00", not 10:00.

Still, the apparent hilliness of craters is a well-known optical illusion.

Look at the MRO grayscale image of Victoria. All around the edge of the ejecta blanket, and especially along the northern quadrant. the ejecta blanket looks like it was ploofed ever so gently down upon the etched terrain ripples. Over the ejecta blanket you can see many buried craters.

Otherwise, I'm letting my hypotheses drift til I can pin them down...

--Bill

I just can't get my head around the sequence of what's happened here. Back in the etched terrain, we have bedrock, evaporite layer, then the dunes on top. The dunes are not present on the ejecta blanket. Victoria is many millions of years old. Does that make the dunes older? Surely not - they wouldn't have survived the formation of Victoria. So is the ejecta blanket not really an ejecta blanket at all, but just millions of years of eroded Victoria-dust being blown out and covering the dunes that appeared more recently, the darkest "rays" being the most recent?

Confused Andy

I've long poo-pooed the idea that the Meridiani ripples are static and unchanging over millions of years or more. Look around Oppy and they seem to be active and dynamic. Yet in multiple examples we see the clear appearance that the younger ejecta overlies the older ripples and that the ripples outside the overlap haven't changed. Of course, this is no substitute for 'boots on the ground and shovel in hand' but it can get me to thinking.

Mars is stranger than strange. Perhaps we ought to take up Zen-planetology?

--Bill

Hi-Rise shows enough very small but fresh-ish craters superimposed on the drift-ripples, it's clear they're relatively inactive at the moment. There's not many, but 5 min scanning across the full height color strip showed several.

It may well have been an ejecta blanket once, but is definitely not now. There's no ejecta on the apron. Maybe some under it though but who knows where or how far down. There is clearly an apron/blanket though and that must have come into existence because of the crater so what we see on top it is most likely an accumulation of dust that has come up out of the crater as the walls have eroded. The crater is a significant local source of dust and so this has resulted in an apron that is differentiated from the dunes around it which are formed from regional/global dust sources that are relatively much more sparse. So the dunes and the dusty apron have developed at the same time. That's my theory anyway!

How true that is, Bill, and yet each of us generates ideas about Mars in his (or her) own way. In a way I'm glad that Tim Demko delayed weighing in on this issue, since I doubt I would have had the guts to espouse an idea about Victoria's nature that I knew ran counter to his professional appraisal. Nevertheless, having labored to give birth to this hypothesis, I will hold to it now until I feel it has been definitely falsified by close-up study of Vicky's innards. So far I'm not convinced that the laminated units we can see from a distance in the capes are far displaced from horizontal, or heavily disrupted by impact shock. I feel certain that the Victoria impact would have to have shattered and disorientated any surface rock layers present at the time to a much greater extent than we so far observe. Therefore I maintain that these layers have been deposited after the impact, and formed a roof over the crater fill that is now being eroded away.

I certainly don't pooh-pooh ideas to the contrary, and will be ready to abandon my 'baby' at the first sight of a shatter cone or impact breccia.

Here are four views of the ejecta balnket clipped from the TRA_000873_178 HiRISE image of Victoria. The first two are from the northen quadrant of the ejecta apron, the third is south of Victoria and the fourth is at the eastern edge of Victoria.

It is clear to see that the ejecta was emplaced on top of the longitudinal ripples. The spacing and pattern of the ripples continues underneath the ejecta, which could only occur if the ejecta were deposited over existing ripples. One interesting note is that the ripples are present up to the eastern edge of Victoria, which may or may not be significant.

--Bill

What can I say, Bill? You are obviously convinced.
I would be more likely convinced if you could diagram the "continuence of spacing and pattern" with superimposed lines crossing the "ejecta" deposits.

What I definitely see is a pattern of ripples produced by the same prevailing winds crossing an area of sediment deposits which, in some areas, is sufficiently fine-grained that it forms ripples, and, in other areas, has coarser particles on the surface which resist transport and sculpturing by the wind. The onset of stronger winds would probably change this entire pattern, with the 'undisturbed' areas shrinking away.

I really, really feel that Vicky's "ejecta apron" contains no ejecta from the VC impact at all.

Take care, All, and God bless.
I will be away for a few days watching loved ones get married in the Blue Mountains.
Please don't drive Oppy off a cliff in my absence.

Bill: I think I see what you are saying, but I want to look through the eyes of the rover once more, as we crossed that boundary. My recollection was that we drove onto a surface that resembled the Half Pipe formation with a much thinner covering of the layered ripples.

Tonight I was amazed by the sol 953 pancams that came down today of Duck Bay. I think they may give us a clearer view of the layers we see in the bay and the capes, and perhaps some insight into the processes that created them. I kind of got sidetracked by a few things tonight, so this markup has some imperfections, but I managed to put this together to demonstrate my current thoughts. To me, it appears as if there is a window in Duck Bay where we can see some relatively in-place bedrock. (between the yellow lines) It appears that there is a thin veneer of junk covering the upper slope of the bay, and the lower slopes as well. What is the upper contact? What is the junk?
Click to view attachment

Relatively in-place bedrock = future cape?

Tom, I noted that in-place bedrock (or rather, the arcuate area at the upper edge) when we arrived at Duck Bay. I don't recall what I said about it, but it is an important puzzle piece.

Oppy entered the ejecta apron at the two "halfpipe formation" exposures near Beagle crater which are more the exception than the rule here. So our first on-the-ground views of that contact are atypical. I don't have a DSL connection or a high-RAM 'puter so I don't have the 250mB full-size image. What I'd like to see are smallish 25-50 mB clips from that full size image showing the path between Beagle and Victoria, and of the route between Erebus and Victoria, and of the Erebus area. I've seen bits and pieces of portions of these area of interest, but nothing centered on them (subtle hint...)...

I may be wrong about the ejecta blanket contact, but this is my initial observation. We can be looking at this part of Victoria's stratigraphy during the conjunction hiatus.

--Bill

But what's to stop the layers we see being exposed here originally lying perhaps a hundred metres (?) away from the original edge of a smaller Victoria crater, which has eroded out to the diameter we see it today?

A 500m crater is about a half-megaton. Would an evaporite layer, perhaps already covered by a few metres of rock and fragments, necessarily be highly disrupted 350 metres away from the impact point?

That scenario would neatly account for no crater rim, clean(er) layers in the current exposed sections, an ejecta blanket which doesn't look like an ejecta blanket, and many metres of crater-crud in the bowl.

Andy

And that is why we are seeing the (paleo) ripples under the ejecta blanket right up to the present rim of Victoria. The original rim is eroding back and when the crater was formed and the ripples were buried with ejecta they were far enough back so that the peak particle velocity (ground vibrations) was low enough to not disrupt the ripples.

--Bill

Back to the thin white line we've seen around the rim. Here's a crop of one of the northeast bays. It seems reasonable to infer this unit was level when deposited (if we are right that it is sedimentary). This image seems to show some uplift toward the centre of the crater. This would fit with a standard crater formation model I think. There are a few implications of this if it's right. The main one is that this bay would be beyond the original crater rim since the uplift occurs outside the rim during the formation event.
Click to view attachment

I don't think we can infer from that overhead shot that we're seeing uplift, since there's no height info in that image. Remember that the surface outside the rim tends to get lower towards the tips of the capes, so you'd expect that kind of view from above.

It's better to look at images taken from the same height as the stratigraphy, elsewhere on the rim. Here's the same bay (D5) from the Duck Bay pan:
Click to view attachment
From this angle, the "white" line looks slightly higher at the far end of the bay, away from the centre of the crater. However, I don't think we can attach much significance to that since we're not perfectly level with bay D5 here but apparently somewhat higher.

Most of the eastern rim we can see looks similar - the white line slices across bays and capes pretty much horizontally as viewed from Duck Bay.

That depends on what type of rock the light-toned unit is. If it is evaporite, then we can assume that it was horizontal when laid down. If it is aeolian, then it followed whatever the topography at the time was. At any rate, if it is the same unit, here is our marker bed.

At any rate, I wouldn't worry about it dipping inward on the capes. Since they are pre-slump features I would expect them to be tilting inward.

I'm waiting for the biggie pan from Beacon...

--Bill

Yes, or perhaps a past cape that has been eroded back. It is going to be difficult to work out the geomorphic history of this crater until Opportunity takes the road trip around at least part of the circumference to fill in more of the diverse details. We can see wonderful detail in the nearby pancams, but I think much detail is lost in the distance.

The "thin white line" has been fascinating us ever since we first glimpsed it on the opposite side. It is the most consistently recognizable layer visible across most of the crater. The thickness of rock above that contact seems be quite variable. I have been trying to correlate it around the crater to our present location, where we should see it most clearly.

So, where is it in Duck Bay? My best guess at this point is that it may be the layer I identified in my last image simply as the "Upper contact," and that most of the material that elsewhere lies above that contact has been removed by erosion. This is the highest side of this crater. Might the material above that contact be the ejecta layer?

One thing I had not appreciated until I looked at at the anaglyph, is that that bedrock section in Duck Bay does stick out a bit, like a small cape. And pre-arrival when I first labeled the capes on the MOC image I even labeled it - P5.

James