Fossil Craters
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wassock
moderator
Shows a section of the polar layered deposits. The layers and top most surface are pretty much void of any craters but the surface at the bottom appears to be heavily cratered aibeit that the craters are fillled in some what. So the surface at the bottom of the pile and the more recently exposed terraces are not. So are the craters at the bottom fossils from a bygone era prior to the deposition of the layers, now revealed like dinosaur footprints in an ancient mud bank?
Posted
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mschwamb
scientist, translator
in response to wassock's comment.
Those potmarks could be the start of swiss cheese terrain
Cheers,
~Meg
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wassock
moderator
I though the Swiss cheese was in the top most layer of the layered deposits. To continue the footprint analogy for a moment consider the bottom most layer as maybe the original, harder surface onto which the layers were deposited. In this image the crater bearing strata has also been eroded leaving half craters.
http://hirise-pds.lpl.arizona.edu/PDS/EXTRAS/RDR/ESP/ORB_023200_023299/ESP_023259_0925/ESP_023259_0925_RED.abrowse.jpgPosted
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Kitharode
moderator
in response to wassock's comment.
I'm inclined to agree with Meg on this one. They don't look right to me as dinasaur craters, but they do fit well with the cheese. Your idea that the 'bottom' surface could be cratered, then covered by the layered deposits, then exposed, may well be correct and may well be on show, but I'm uncomfortable with this being the case in your images.
I look forward to being proved wrong (as usual) but this'll be an interesting discussion methinks. Nice one wassock. 😉
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Kitharode
moderator
in response to Kitharode's comment.
Having said that, if there are craters on the bottom layer and they are being exposed, surely we can gather together the information that would tell us how much depth of erosion there has been to reach them. If so, we could see how much sense the results made.
If there are craters underneath that are not being exposed, then perhaps they act as starting points for the swiss cheese effect. Need to read up on cratering in that region and history of layered deposits.
Do you have lat/long/location for your images?
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Kitharode
moderator
Just one of many pages that seem to show that the south polar region (and probably the pole) have been cratered in the past;
http://cmex.ihmc.us/MarsEssy/age2.htm
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Kitharode
moderator
Thought this might be useful. Zoom out for full picture. The SHARAD instrument was only designed for penetrating about 1km, hence the lack of a basal reading.
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wassock
moderator
Wrt the link it confirms that the older surfaces on mars are more heavily cratered. HiRise info page http://hirise.lpl.arizona.edu/ESP_022839_0925
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Kitharode
moderator
This http://www.uahirise.org/ESP_023174_0945 is useful, but would still like to know location of your original image.
Posted
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wassock
moderator
Sorry kith sent wrong one http://hirise.lpl.arizona.edu/ESP_022839_0930 is the original one
Posted
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Kitharode
moderator
Locating the original image on the nasa thickness mapping image suggests this area is between 2 and 3km thick. The swiss cheese effect appears to be the front runner now. What do you think?
But it's set me off looking for the edges of this layered terrain because your idea of exposed craters has, I think, more chance of being found there. I'd also like to get my hands on a scholarly article describing the deposition rates of these layers. I've seen one, but gotta pay to get it.
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wassock
moderator
I'm sticking to not Swiss cheese, the second image shows similar features which have been eroded, as I understand it the cheese holes can be seen to grow over the course of a season
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wassock
moderator
Kith have you found this one yet? http://www.marsjournal.org/contents/2006/0001/files/kolb_mars_2006_0001.pdf
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Kitharode
moderator
Thanks wassock. Yes I have that paper but haven't read it yet. I'll give it a scan later today.
I'm a little unsure now of what precisely you are talking about. Are you suggesting that the layered deposits have been eroded to reveal old craters on the surface of Mars, or are you suggesting that one/some layeres have been eroded to expose craters on an underlying layered deposit which itself is covering the martian surface?
It hadn't crossed my mind that the layered deposits themselves were vulnerable to crater impacts. Maybe some craters live on the second or third or whatever layer from the top ... !?
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Kitharode
moderator
PS. I've also read that cheese holes are seen to grow. However, I've got a feeling it was more north than south pole area. Could well be wrong there, so confirmation of growth at which poles would be good.
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wassock
moderator
Basically the theory runs thus. In the olden days the surface got hit a lot more often than it does now, actually that is irrelevant. In the beginning the surface was cratered. Then the surface was covered by lots of layers. Then parts of the layers wore away to reveal the surface we started at. Simples. IF they are craters then there's a lot more in the little bit showing at the bottom of the slope than else where indicating that one part is older than the other
Posted
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Kitharode
moderator
I'm happy with the Simples Theory as long as the stress is on "parts of the layers wore away to reveal the surface we started at", that is, a cratered surface. These parts, surely, are at/near the extreme edge of the oldest deposits. Once we move 'up' to a certain depth of layering the erosion (outside of any chasmic formations) would probably not penetrate deeply enough to expose craters, methinks.
That's why I'd like the co-ordinates of your first picture (in your first post) to see where your 'craters' live. I know they look like they're at the 'bottom' of the pile, but I'd like to know where that pile is. I'm still with swiss cheese in that image at the moment, but exposed craters must be around somewhere. Good thread.
PS. I'm not sure that less impacts = irrelevant. Perhaps you mean only for this specific scenario...?
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wassock
moderator
in response to Kitharode's comment.
As far as I can make out this is on the Southern edge of the Promethei Lingua near the Chasma Australe - Been reading that last paper, problem with which is that it is completely devoid of any co-ordinates for where any of the cross sections are.
Old = more impacts is irrelevant in this instance because the layer with the "craters" in is clearly more heavily populated and thus not the same age as the rest of it. There may well also be well cratered layers between the bottom and the top but there's not enough of them showing to tell. And I'm suggesting that the bottom is harder than the layers so when we get down to it what ever process is removing the layers above (which I'm getting the picture of as some sort of slush puppy mix of ice and dust) slows down. Co-ords for the first image are on the HiRise page http://hirise.lpl.arizona.edu/ESP_022839_0930
Been looking at cheese holes and they are not the same!However it could be that I've got the wrong glasses on and what I'm seeing as the bottom is actually the top?
This is the double crater at the bottom of http://hirise-pds.lpl.arizona.edu/PDS/EXTRAS/RDR/ESP/ORB_022800_022899/ESP_022839_0930/ESP_022839_0930_RED.abrowse.jpg as viewed on HiView
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Kitharode
moderator
Your craters are at -87degS and 141degE which puts them, to my eye, in an area with layered depth of 2-3km. http://www.nasa.gov/images/content/171411main_pia09224-thick-annot-516.jpg
I'm thinking that the smooth 'cratered' layer in your image is an extended ledge leading to the edge of the next layer down. On this ledge are the cheese holes / craters. I don't see your craters being on the surface of Mars because they aren't at the bottom of a chasm or close to the layered edge. I don't believe the large number of holes on such an elevated layer would be due to cratering. Therefore, for now, it's cheese ... perhaps, maybe.
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wassock
moderator
OK now I play the old card - if it's a very old surface then it would have a lot more craters then much of the current surface. (top layer)
OK lets try this - yes no questions
these things are in a layer which lies below the layers exposed to the North?
They look like worn craters?
They show the same morphology as cheese holes - sharp edges, often showing one or 2 layers, very smooth inside, growing at a rate that is discernible over a year?
If we accept that they are craters for a moment it's odd that they are only in this bit of the image?
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Kitharode
moderator
Not sure if you're not telling me or asking me no questions ... erm, yes then ??
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wassock
moderator
in response to Kitharode's comment.
Not sure you can give the same answer to all 4 question's
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Kitharode
moderator
Yes/No questions, rather than 'yes (I have) no questions'. That'll work.
These things are in a layer which lies below the layers exposed to the North? YES (but not below those to the south)
They look like worn craters? YES (for the most part)
They show the same morphology as cheese holes - sharp edges, often showing one or 2 layers, very smooth inside, growing at a rate that is discernible over a year? YES (if they do grow)
If we accept that they are craters for a moment it's odd that they are only in this bit of the image? YES (I would agree). So...
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Kitharode
moderator
From Wikipedia re 'swiss cheese'. Last paragraph in particular - circling suns and all that. Nice idea.
"While the north polar cap of Mars has a flat, pitted surface resembling cottage cheese, the south polar cap has larger pits, troughs and flat mesas that give it a Swiss cheese appearance. The upper layer of the Martian south polar residual cap has been eroded into flat-topped mesas with circular depressions.
Observations made by Mars Orbiter Camera in 2001 have showed that the scarps and pit walls of the south polar cap had retreated at an average rate of about 3 meters (10 feet) since 1999. In other words, they were retreating 3 meters per Mars year. In some places on the cap, the scarps retreat less than 3 meters a Mars year, and in others it can retreat as much as 8 meters (26 feet) per Martian year. Over time, south polar pits merge to become plains, mesas turn into buttes, and buttes vanish forever. Since 2001, two additional Mars years have elapsed.
The round shape is probably aided in its formation by the angle of the sun. In the summer, the sun moves around the sky, sometimes for 24 hours each day, just above the horizon. As a result the walls of a round depression will receive more intense sunlight then the floor; the wall will melt far more than the floor. The walls melt and recede, while the floor remains the same".
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wassock
moderator
Found that one earlier, the pic is nice
But as I read it the upper layer has the Swiss cheese pits. Which look nothing like the ones at the bottom methinks
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Kitharode
moderator
Referring again to your original image in first post.
I can see that your craters are on a smooth plateau which is obviously underneath a number of other layers. I'd say a dozen layers lie above your crater plateau. But the plateau itself must be a layered deposit because its about 2km or more above the martian surface (the true bottom of the pile). This doesn't prove that your craters are not craters, only that if they are craters it is a layered deposit crater and not a surface crater.
It would be useful to know what precisely constitutes a layered deposit, especially the top layer. Do the dozen or more layers in your image constitute a dozen layers, or is a single layered deposit itself made up of several/many layers? Perhaps swiss cheese occurs in the topmost exposed layer rather than the top (most recent) layer.
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Kitharode
moderator
Swiss cheese sandwich. Layered above and below:
Posted
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wassock
moderator
PSP_005236_0930 8 September 2007
ESP_022839_0930 11 June 2011
Both the same hence not cheese's
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Kitharode
moderator
That's truly excellent work wassock, really, I'm impressed with that. You make your point very well indeed.
However, I would like to hear your thoughts on 2km high craters, if you're still with craters. We cannot escape the fact that your original craters are way above the martian surface.
Also, your 'classic' swiss cheese image two posts previous seems to be set in a larger area which may be some distance from the layered edges, unlike the cheese above which lies adjacent the the layers edges. I feel this could be significant.
Still cheese for me, but congratulations on a most stimulating area of citizen scientistry. ** 😉 **
Posted
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wassock
moderator
As I read it what ever the cheese holes form in it's mostly ice, the main erosional force appears to be sunshine. I do have some reading somewhere on how the layers formed, just waiting on a rainy day. Lets assume this layer with the apparent craters is of the same stuff as the layers (high ice content) what does an impact crater there look like?
Not too bothered if "my" layer is the bottom or halfway up the stack it just would be kinda neat if they are indeed "fossil" cratersPosted
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Kitharode
moderator
South Polar Layered Deposits - Circular Depression. Also probably not a crater, but certainly a mystery. http://hirise-pds.lpl.arizona.edu/PDS/EXTRAS/RDR/ESP/ORB_012700_012799/ESP_012760_0930/ESP_012760_0930_RED.browse.jpg
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wassock
moderator
So if you had some thick liquidy mud and you dropped a pebble into it what would it look like?
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Kitharode
moderator
Perhaps jshoe will pass by this way and pick up on that. I've played that game before and there are a lot of different effects because there's a lot of variables. Having said that, we never did sloppy stuff with a pebble. In short, I don't know.
I've said that this discussion may relate to this one (see JM's rotated image) http://talk.planetfour.org/#/boards/BPF0000001/discussions/DPF0000dfn?page=1&comment_id=51784ed0de59d9074e00002c
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Kitharode
moderator
This is the big circular depression from earlier: http://hirise-pds.lpl.arizona.edu/PDS/EXTRAS/RDR/ESP/ORB_012700_012799/ESP_012760_0930/ESP_012760_0930_RED.browse.jpg
OK, this may be a little off the wall but it leans in favour of fossil craters existing between layered deposits above the martian surface. In the big circular depression, as in the smaller one above it, we clearly see the remnants of a layered deposit. This layer must have formed after the circular event had occured. Perhaps two or more layers have formed after the event. If an impact crater became buried by layered deposits which themselves eventually become eroded, then the possibility of exposing a circular feature (a fossil crater) does not seem impossible.
Posted
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wassock
moderator
I don't see either as being the same thing Kith. The brown bits are in a different terrain and the big depression is in the top layer of the deposits. The smaller version of the Swiss cheese within the circle might be taken to indicate that it is a newer feature than the surroundings. Looks to me like something melted.
NB there are 3 main surfaces on mars each relating to a different epoch and distinguished in part by the crater density.This from wikapedia (so it musht be true)
The geological history of Mars can be split into many periods, but the following are the three primary periods:[44][45]
Noachian period (named after Noachis Terra): Formation of the oldest extant surfaces of Mars, 4.5 billion years ago to 3.5 billion years ago. Noachian age surfaces are scarred by many large impact craters. The Tharsis bulge, a volcanic upland, is thought to have formed during this period, with extensive flooding by liquid water late in the period.
Hesperian period (named after Hesperia Planum): 3.5 billion years ago to 2.9–3.3 billion years ago. The Hesperian period is marked by the formation of extensive lava plains.
Amazonian period (named after Amazonis Planitia): 2.9–3.3 billion years ago to present. Amazonian regions have few meteorite impact craters, but are otherwise quite varied. Olympus Mons formed during this period, along with lava flows elsewhere on Mars.
Posted
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Kitharode
moderator
I'll go along with brown bits in different terrain, I'm probably wrong there. I'd argue (for the hell of it) against the big depression being in the top layer of deposits. If it was in the top layer, how come we have the remnants of a further layer above it? I'd suggest it became depressed, then covered with layer, then exposed. Why not that?
Thanks for the epoch info. I've been meaning to organise that for ages. Saves me a job Cheers.
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wassock
moderator
in response to Kitharode's comment.
What do you see as overlaying layers?
Posted
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Kitharode
moderator
The remnants of the swiss cheese inside the circular depression is, I believe, overlaying the depression. The swiss cheese effect over the smaller 'crater' is also an overlayer methinks.
Posted
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Kitharode
moderator
PS: "So if you had some thick liquidy mud and you dropped a pebble into it what would it look like?" Were you suggesting this as a possible scenario for the formation of the big circle?
Not impossible. A relatively small rock could form a relatively large circular wave pattern given the right conditions - pebble in a pond type of thing. Temperature would be important, both for forming and 'freezing' the shape, and the rock size and mud thickness would also need to be right. Going back far enough in time might allow this. If the timescales were right then overlayers could form.
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wassock
moderator
in response to Kitharode's comment.
No it's just what it looks like rather than a serious contender for how - more expressly a freeze frame from a slow mo of a drop of milk falling into milk - you get that little blip in the middle. But would only work for a liquid. If the top layers have lots of ice then an impact would melt said Ice but would also splash it all over the shop.
I'm sure I've seen something similar elsewhere (maybe the same one) and the explanation being that something underneath had collapsed,/melted/disappeared.
Can you not see the smaller swiss cheese pattern inside the circle as a younger version of the bigger pattern out side? Or maybe it's a REALLY deep hole and the bottom is a lot further away than the surroundings 😃
Surely the depression is made into the surface with the cheese rather than the cheese has worn away to reveal the depression?Posted
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Kitharode
moderator
We seem to have crossed posts (don't know how). Did you see my reply to your 'overlaying' question?
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wassock
moderator
in response to Kitharode's comment.
Saw that don't see how two different cheese surfaces are viewed as separate overlying layers
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Kitharode
moderator
The article below relates to the image in the opening post of this discussion and to the question; Are these fossil craters? The area on display in that opening image is within one of the areas mentioned in the article below.
We also meet the mysterious 'wire brush' terrain and the 'snakes'. Not my words, but theirs. ** 😃 **
Posted
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wassock
moderator
So now there are snakes on Mars! Back again, just running in again, up to speed soon.
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wassock
moderator
This is a triple impact crater - thought to be the result of what ever it was breaking into 3 just before impact. These are uncommon (but not all that rare) and there seem to be a whole lot of doubles all over the surface away from the polar latitudes.
This is the feature in the very first image in this thread, which I suggest has been beneath the polar layers for a while. It's smaller than the other one (about 500 meters long) but the similarity is strong (says I)
Posted
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wassock
moderator
And back to this one again now that I've finally worked out how to work out positionof the sun (thanks michaelaye)
http://hirise-pds.lpl.arizona.edu/PDS/EXTRAS/RDR/ESP/ORB_022800_022899/ESP_022839_0930/ESP_022839_0930_RED.abrowse.jpg map projected, grey scale
So the sun is in the top Right corner so the layers descend to the pockmarked surface bottom left which would once have been covered by the now eroded layers. Thus the surface at the bottom is old, if the pock marks are the remmnants of craters does the density of them give us an age for the underlying surface and thus for the start of the formation of the spld?
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Kitharode
moderator
Crater size and density can give an idea of the ages of surfaces, but I think only very approximately. This is a good starting point... http://www.msss.com/http/ps/age2.html
I've got to admit that I'm still not sure that they are fossil craters, but I would be more than happy to be proved wrong. 😃
Posted
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wassock
moderator
Try coming at it from the other end. If the bottom layer is actually bedrock then it would likely have craters from the early period of mars, if that surface were covered by layers of ice for a long time and then revealed again by (one of many?) a thaw. If all that be true what would the surface look like?
Posted
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Kitharode
moderator
In short, I can't say what exposed fossil craters would look like. From the links in my previous post we see that martian craters are often different (when new) from craters on, say, the Moon. They seem to form and degrade differently. The SPLD's seem to be fairly unique, so I would expect that their formation and erosion would affect the outcome of what we eventually see, perhaps in ways that I can't even think about.
But your ideas are sound I think. Stuff at the bottom of a layered pile are indeed older than stuff above. If there are craters on the bottom, which is almost certainly the case, there is a good chance that they could eventually be exposed as such. If you can 'age' one surface, you should be able to get a handle on the ages of overlying layers.
Can you say whether or not the fossil craters in your image are on/at the surface? I think they'd need to be beyond/at the extremes of the SPLD's considering the sizes you give (approx 500m is pretty big). Craters impacting the spld's would, I think, probably be much smaller.
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wassock
moderator
in response to Kitharode's comment.
500 meters is the size of the example triple crater I put up the thing under the layers (no more) is a lot smaller but similar shape. The layers form a nice terrace of ledges leading down to a 'flat' layer with the supposed craters in it. I suggest that the bottom layer is different from the layers above as it has not been eroded in the same way and an explanation would be that the bottom is the 'bedrock' of Mars. If it is the ancient surface tgen it is to be expected that it would have been cratered before being covered by the layers. These pock marks look like old craters thus the shaving implement of Occam suggests that is what we're looking at. Ses me!
Posted
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Kitharode
moderator
The latitude of the image is -87 deg. This is less than 200 miles from the south pole. The spld's are 2 or 3km thick at this location so I think it highly unlikely that we are near the 'true' cratered surface of Mars.
There is a high probability that cratering has occured throughout the history of spld formation (as it does today) so it might well be that each and every layer has some cratering on it. However, we would expect the size and number of craters to decrease over time and I find it difficult to accept that 'your' layer would have so many craters on it in such a relatively small area.
We don't know how 'swiss cheese' erosion begins, but the circular/oval nature of the depressions is explained by remembering that at this latitude (-85) the Sun is permanently above the horizon during the hottest part of Mars' orbit, so sublimation/dessication occurs all around the starting point.
Perhaps we need a double-edged sword, rather than a razor ... 😃
Posted
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Kitharode
moderator
PS: Just found this posted on Faceache, sorry, facebook - http://photojournal.jpl.nasa.gov/catalog/PIA18384
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wassock
moderator
in response to Kitharode's comment.
Kith, I just dont see this as cheese. Its at the bottom, far as I have seen cheese occurs on the top layer. Cheese changes with time, over the course of a season some times, these are the same year on year. Cheese has a distinct apearance these dont, there are channels eroded into these, doesnt happen with cheese
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Kitharode
moderator
in response to wassock's comment.
You make a good case against the idea of 'cheese' and I'm willing to accept that it may not be, but my feeling is that it's more related to a 'cheese process' than it is to craters.
Having placed some doubt in my mind with your case against cheese, convince me with your case in favour of craters. 😉
Posted
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mschwamb
scientist, translator
in response to wassock's comment.
I wonder if you're starting to see the Swiss Cheese Terrain .
Cheers,
~Meg
Posted
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p.titchin
in response to Kitharode's comment.
OK, have to respond. Been following this for a year +. The eroded steps show these 'things' to be OLD. I can't see they are the same as 'cheese'. They seem to be at least near the surface of Mars, and like Wassock, I would therefor not be surprised by craters. I struggle to see any similarity with 'Swiss cheese' terrain. Sorry, Kith, looking at the arguments at present put forward,I can't see the links. Meg, why now the Swiss Cheese suggestion again over a year since you first suggested it, but still with no substantiating argument? As a retired old science fossil, for me, Wassock has shown what appear to be craters under the PLD's, and the suggestion is they are evolved like the close surface features called 'swiss cheese' is not viable.Challenge is to show slow change in swiss cheese areas, or seasonal change in Wassocks older surface 'craters' It could be there, but as a 'citizen scientist', I don't have the same access as scientists and moderators! I look foreward to the continueing saga!! 😃 ~Pete
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mschwamb
scientist, translator
in response to p.titchin's comment.
Hi Pete,
I don't even remember I commented on this thread a year ago, but I guess my thinking is the same. It reminded me of the swiss cheese terrain and I commented on the image. I guess I didn't get through the rest of the thread though I thought I had. Apologies for that, I would have added a bit more had I same the discussion yesterday when I posted. If there isn't any evolution over the 4 Martian years than that likely indicates crater. Mainly it was based on the shape of the features in the latest image. The potmarks made me link of these as perhaps the start of new swiss cheese holes. I'm not saying it is definitely the swiss cheese terrain.
I wanted to give some background about myself. I am a planetary scientist and astronomer so I have the general background to work on Mars, but my PhD thesis is on the planetesimals in the outer solar system and I've spent the past few years working also on exoplanets with the Planet Hunters citizen science project. I'm bringing that perspective to the science team. I'm not a Mars expert, I defer to Anya, Michael, and Candy who have worked on Mars for far longer and in much more detail. . I have a lot of experience analyzing citizen science data, and have familiarity with Mars from my grad school courses and with one of my graduate preliminary exam projects mapping the general directions of seasonal fans in Mars Orbiter Camera (MOC) images from Mars Global Surveyor to compare to a global climate model (that did not quite work at that time) of the atmosphere but beyond the science of the fans and the broad strokes of Martian history and surface processes in many ways I'm picking up the details just like you are.
I'm not sure what you mean by access of moderators and scientists, but If there's a science paper you'd like to read and you can't because of a paywall, send me a direct message, and I'll see what I can do.
Cheers,
~Meg
Posted
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p.titchin
in response to mschwamb's comment.
Thanks Meg, I only meant that we tend to have our images for marking and try to follow on from them ,in my case, with very basic computer literacy! I find it difficult to search for other similar images other than searching peoples collections.The fault lies in my limitations, not anything else. Thanks again~Pete.
Posted
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Kitharode
moderator
The main difficulty I have here is understanding why one would believe that these objects are on the 'true' surface of Mars. Yes, they lie beneath a number of overlying layers, but they lie above a much greater number of spld's - about 2km's worth in fact. I'm willing to admit that there may be craters on each and every layer, but I find it hard to imagine so many craters would be formed on this particular relatively recent layer.
Whatever the situation, the idea of fossil craters is quite brilliant. Let's hope they turn out to be real. 😉
Posted
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wassock
moderator
The map showing the spld thickness doesn't appear to go poleward of -87, see link below so we don't know how thick the spld is here, but Kiths figure of a couple km seems probable, maybe on the edge of a 'peak' looking at the map?
This still leaves us looking at some sort of difference between the eroded layers above and the surface with the 'craters' in which itself sits atop more layers.
The layered deposits are thought to be the result of some sort of cyclic seasonality, could this be evidence for a further level of change? So we have conditions which build up a pile of layers a couple km thick and then a (long) period of stasis where the layers neither grow nor shrink. This gives a period for cratering to occur and for a layer of surface dust to accumalate. They the conditions for layering return and more layers get put down on top. When the top most layers later start to erode the layer of material atop the original layers acts as a barrier to further erosion and gives us what we are looking at now?
http://www.nasa.gov/images/content/171460main_pia09224-hires-annot.jpg
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wassock
moderator
The NASA page that the map above comes from is here http://www.nasa.gov/mission_pages/mars/images/pia09224.html
It says this about the layers - "The radar data indicate that the deposit, larger than Texas in area, is more than 3.7 kilometers (2.3 miles) thick in places, and that the material consists of nearly pure water ice with only a small component of dust"
So I'm picturing this as annual (or circadian, or whatever) build up of ice followed by a bit of dust during the summer/warmer spell. This suggests that the spld was laid down a long time ago when the atmosphere was a lot wetter than it is now.
Antartica has lakes of liquid water beneath it's ice (of similar thickness) and on Mars the bottom of a 2000 meter pile of ice could be assumed not to be affected by the pressure at the surface. Thus conditions under the martian ice ought to be similar to those at our South pole, cept the ice may be a lot colder?
NB if the surface with the craters is old and was there in wetter times and is mostly made of water then the suggestion that the channels may be water erosion may merit a bit more looking at.
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wassock
moderator
Just turned this up, the image on the right shows the topography of the underneath of the spld and the surroundings. Makes the layers even thicker at our current point of interest as it seems to sit atop a depression.
http://m.sciencemag.org/content/316/5821/92/F2.large.jpgThe paper this comes from http://m.sciencemag.org/content/316/5821/92.full also indicates that the co2 ice cap on top of the spld is only 10's of meters thick.
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Kitharode
moderator
in response to wassock's comment.
Excellent finds wassock. Will enjoy reading up on this. My understanding is that martian topography is mapped by the MOLA thingy which does not cover the polar area to latitude -87-ish.
I'll get back when I can ... 😉
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wassock
moderator
Yes looks like the orbit misses going straight over the pole, think the little circles on the rhs image show the tracks of thd various measurement orbits
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wassock
moderator
Here's an 'ancient cratered surface' from else where on mars http://www.uahirise.org/AEB_000002_0000
And the HiRise greyscale,map oriented image
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wassock
moderator
Amongst the clear fresh craters there are lots of older more worn and infilled ones which (ses me) look similar to my fossil ones
And there's also this one (look familiar?) casual scan shows there are quite a lot of these around.
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wassock
moderator
Heres a HiRise target showing the edge of the Chasma Australe, http://www.uahirise.org/ESP_014360_0985
The bottom of the Chasma is at the top and is covered with what to me are clearly old craters. As I understand it the Chasma is an erosion feature (although there's some debate as to how it formed) meaning that the layers (SPLD) evident in the side of the Chasma once extended across the floor which I suggest is the original surface of mars upon which the layered deposits formed.
The image below (from the linked image above) shows a crater at the bottom of the slope which is clearly part covered by the layered deposits apart from at the very bottom there is no sigh that the slope is collapsing into the crater (all the strata lines hold their shape and thus th layers are on top of this ancient 'fossil' crater
It looks more crater like when you zoom out a bit
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Kitharode
moderator
in response to wassock's comment.
I see the link in the first line of your post above showing the edge of the Chasma Australe, but I don't see "the image below" showing a crater at bottom of slope ... ?
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wassock
moderator
Kitharode, links are fixed
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p.titchin
Great old craters Wassock, fascinating to find such a clear one cut by the SPLD's. I note there is another shot of this from 2011. Superb find!~Pete
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wassock
moderator
Here's another, from http://www.uahirise.org/ESP_032530_0975
Double whammy this time as it's a double impact as well
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wassock
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And this one's the best yet, from http://www.uahirise.org/PSP_006238_0950
It's very obvious on the grey scale .jpg image
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wassock
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BTW while you are looking at the last image on HiRise take a look at the spiders on the top of the SPLD, the image size on this one is around 450 meters
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Kitharode
moderator
Outstanding work once again wassock. I'm certainly convinced now that you've uncovered some fine examples of fossil craters. Well done indeed. Help yourself to a frozen yardang and go to the top of the planet. 😃
My doubts at the beginning of the discussion centred around the location of your initial finds. We were unsure whether they were inside the Chasma Australe, or just off to one side on top of the spld's. Had they been on top I think my objections would have been valid, but it seems clear now that they're in the vacinity of your later images and I'm happy to admit defeat.
The double-whammies are mounting up too. More very interesting finds and very mysterious. Love the spider. That's definately got my juices flowing again - not that they'd dried up mind you. Congratulations on a great thread. Encore.
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p.titchin
in response to wassock's comment.
Hi Wassock, I just found this image while classifying. APF00025wn . There appears to be an icey double crater in the top left corner. I found it ⅝ the way down the HiRISE colour strip, and in context, it still looks like a double crater. What think you?
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wassock
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Could be will have a look at the jp2 version and see 7f I can findit un iced
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