I'd like to see some MIs too, followed by a wheel trench and more MIs, but I don't think the MER team has micro-craters very high on their priority list. From the MER Website back in April, they seem to be confident with the impact theory:



And given that Steve's 10/16 update said they "really want to get things moving" after Oppy's lost sols, I don't think there'll be a stop for MIs before Mogollon Rim...

We dont know that

If they went up range, then how could they not make it this far.

I'm just suggesting it as a possibility - there's possible other reasons, obviously. I thikn they warrant some closer inspection.

Doug

One theory is that the holes on the top of rift is not closed because at the crest of the ripple there are no much sand to cover it as the ones in lower level. The ones at the lower level might be covered easier by the upper sand caused by the wind action.

Rodolfo

It would be useful to confirm whether or not the craters occur on the downwind side of the crests, which would tie it closer to an effect of collapsing and filling a loose void below the surface as more grains pack on top and collapse it.

Here's how it could work:

A rock or some other item gets successively buried deeper below the crest of the dune as the dune flows over it (and we still have no idea of how long would that take). this would allow formation of a somewhat stable void against one of the rock's overhanging edges which would fail and fill in at some point due to continued sand loading, creating the divot like an hourglass draining its sand.

Given the 'natural' shape of the craters we see, we would only be noticing the craters that form when the failure occurs at just the right distance below the sand. voids that fail at too deep a depth would only register as a slight slumping and might not be getting noticed by us at all. Voids that fail too close to the surface would likely reveal part of their associated rock, which we havent seen, except when we see the divots at the edges of the polygonal bedrock, which i think was not always due to the rover disturbing the plate of bedrock as it drove near. on the other hand, if this works, then we should see plenty of examples of this on earth, do we? i havent noticed any. but then again this isnt quite like earth sand is it?

atomoid, I completely agree with your explaination. Latest pictures clearly shows examples of all the 3 cases you described. See, for example, this one:

Dilo,

Thanks for the timely image, from looking at that image, it looks more like a crack has opened up in the more solidified dune matrix below the surface, (perhaps due to contraction as part of seasonal thermal or humidity associated expansion/contraction?) and let in some sand, forming the divoted line.

This suddenly appears more plausible than the rocks i intially supposed, since we would see a lot of larger rocks in the troughs, which we dont, just small ones which wouldnt be tall enough.

...still, but then why only near the tops of the crests? this doesnt explain that aspect, unless the sand somehow 'soaks-up' humidity, changing its volume under this influence, so that only areas thick enough to incur the associatedly larger amount of expansion/contraction would be able to produce cracks big enough to show up as such when the seasonal changes arrive and contract the dune.

to test that hypothesis, are there any craters visible on any of the smaller dunes?

I don't know if it's just because I got back from Hawaii, but this thread always seems to make me think of Don Ho.

Still, some very interesting ideas here.... the divots do remind me of the depressions in an hourglass.

Good observation, Dilo, et al. I saw that image with the crater and incipient craters and thought "hmmm...". It also dawned on me that as far as I can recall, all of the mini-craters have been on the downwind side of the dunes.

Another piece of the puzzle...

--Bill

I thought that up range was before or closer to atmospheric entry. If that is wrong, then read down range for up range. And yes, I think we should know that.

The dynmaics of something as complex as the cruise stage entering the martian atmosphere is almost un-calculatable.

And iirc, there was a distance of several tens of km's between the cruise stage and the entry shell before entry.

Doug

Very interesting observation, Bill... it seems wind is plaing an important rule, and in fact you already explained an "aeolian origin" hypothesis.
Another possibility is that "minicraters" generates everywhere, but the ones formed in the dune side exposed to wind could be subject to a very fast cancelation with fresh sand falling inside them, while on the other side this process is a lot slower and follows different modalities...
This NavCam mosaic from Sol618 (which is part of the stitch I'm using for the new updated route map) shows, in addition to the beautiful big dunes field in fron of Erebus, a circular dark feature close to the rover... these strange dark circles were already highlighted in previous weeks (the first one that I remember was in the Purgatory dune region and generated a lot of rumors about possible "oil spill" from the rover ).
Now, I strongly suspect that these patches are old "minicraters" filled with new, darker sand transported by wind... they appear also in the downwind side, maybe due to finer sand filling them (on the other side, fill is made by sand very similar to the dunbe material, so they are completely masked).
What do you think?

Dilo, it is obvious that the dark circular features are related to the craters, whether they be fresh windblown sand or material that is being "sapped" (withdrawn) from the surface. Although I call these depressions craters, they may areas where the sand is draining away to subsurface cavities.

I honestly don't know what the cause of these craters is. My first thought is that they are impact features, but there are questions about that. My second thought is that they are sand loss features, but there are also questions about that. We will need to keep looking at them as we pass by and keep collecting data on the fly. We can catalog craters of different stages of development and that will help to determine the nature of these features. Ideally, we'll be able to examine one closely and do an excavation around it, but for now, THE priority is to get to the eastern rim of Erebus.

One of the many mysteries of Meridiani.

--Bill

Tens of km... So, it was about three seconds behind it.

Summarizing all the proposed hypotheses from this thread, and my opinions (for whatever they are worth):
1. micrometeorites, perhaps from comet tails
- unlikely, these tiny meteors would burn up in the Martian atmosphere
2. micrometeorites, from larger meteors exploding in the upper atmosphere.
- possible, but I think we see too many microcraters to account for this origin
3. subsidence into holes in the substrate
- unlikely that pre-existing holes in the substrate would last 4B years without being already filled.
4. microcraters formed from secondary impacts 'tektites'
- most likely
5. lightning stikes
- very unlikely
6. dust devils
- they wouldn't form pits
7. outgassing vents
- very unlikely
8. ant lions
- overactive imaginations

I do believe that the aelian erosion with the winds and dust devils observed here is rapid enough that the microcraters are unlikely to remain more than a hundred years (and probably much less) without being refilled with dusts and sands. I would love to see some estimates of this contention!

Well let me offer yet another hypothesis for the microcraters (admitedly a stretch):
a. Let's suppose ice nodules form under the (thermally insulating) sand surface due to very slow seepage of brines from cracks in the bedrock. These ice nodules can grow to various sizes, most often perhaps the size of a fist.
b. At times, the brine seepage ceases or decreases and the nodules cease growing.
c. As the winds shift the dunes slowly about, the sands above the nodule become thin enough that the nodule (still below the sand surface) can absorb enough solar heat to cause the ice to slowly sublime. (especially in the Martian summertime) (especially on south facing dune faces)
d. As the module mass evaporates, the sands settle downwards to fill the void space. This causes a pits to form in the sand which o look like microcraters.
e. Larger nodules surface cause larger pits.

Interesting. I do think that the dark circles are likely linked to the microcraters. In your hypothesis, would the dark circles occur prior to or after the formation of the microcrater? While the circle could be a filled in crater, you may want to consider that it could be an incipient crater instead, with sublimating ice causing a change in color prior to collapse. Of course another "far out" idea would be a mold or fungus-like growth on the buried ice ball, that when mature would pop to release spores......

Scott

Yes, interesting. Reminded me of the article from last month about ice in Martian dunes:

http://news.bbc.co.uk/2/hi/science/nature/4217528.stm

Perhaps these observations could be of something related, just on a smaller scale than what is discussed in the article (ice deposits in larger dunes)?:

Earth dunes containing solid water are seen to have overhanging cornices, jagged steps, and exposed ridges which result from the ice and snow binding the sand grains together and strengthening them against erosion. That similar features are observed in the Martian dunes suggests that they, too, are strengthened by water-ice. Even stronger evidence comes from signatures of melting, according to Dr Bourke. In regions where ice warms and seeps through the sand, dunes develop cracks and fan-shaped depressions. Both features have been found in the Martian dunes. One fan-shaped basin seen on Mars spans over 400m, suggesting it has lost an extremely high fraction of water.

Re the above "melting signatures," I'm interested also in some of the other "depressions" in the drifts (as well as the "dark spots") seen in some of the recent Opportunity images such as these again from sol 617:

http://marsrovers.jpl.nasa.gov/gallery/all...00P0755L0M1.JPG

Any relation to the more circular "craters" or just other erosion? I've noticed a few more very small craters in the latest images also, since that beauty in sol 616, some looking like "partial" ones, either partly filled in or still in the process of forming.

Re the ice in dunes, does anyone know of any other links besides the BBC News article?

That's where my idea originated.

The footprint-shaped depressions in that dune (picture you posted) obviously could not come from micrometeorites. Perhaps they formed from the sublimation of oblong or linearized ice chunks, followed by subsequent collapse of the sand into the void?

(I am new to this forum. Just thought the intriguing microcraters was an interesting subject!)

3. subsidence into holes in the substrate
- unlikely that pre-existing holes in the substrate would last 4B years without being already filled.

What about an "active" substrate that shrinks and cracks creating new holes for the sand to redistribute and fall in ?

That would be a pretty active substrate to keep on developing new holes for so many years, if it kept shrinking and cracking. I would expect after it dehydrated, that's it, it finished shrinking.

Now if the substrate had flowing brine that opened up new holes in it even recently, that would be more active (and interesting!)

I rather like the idea of aolean processes being involved.

It was mentioned earlier in this thread that while the tiny craters seem to be relatively common in Meridiani, Spirit hasn't seen any in Gusev. So what makes Meridiani so special? How about blueberries?

What kind of wind speed would a dust devil have to produce to pick up not only dust, but perhaps some small blueberries, as well. Even if they weren't completely lifted into the air, wouldn't it be possible for them to be bounced along the ground, and up the windward face of a dune. As the blueberry crests the dune, the wind drops off and it "plops" into the leeward side of the ripple, creating a small crater.

Please tell me what's wrong with my idea.

Blueberries are only as big as BB's. And if wind is moving them along, at best they will only move as fast as the wind speed. They simply don't have enough energy to make platter-sized microcraters at this speed. I encourage you to experiment with this yourself, by throwing BB's, or blowgunning BB's through a BIC pen, or even firing BB's, into a bowl of plaster powder or a bowl of sand. Even rather high speeds don't make big enough microcraters.

What I find quite odd is that all the microcraters seem to be the same size and on the leeward side of the dunes.

If the microcraters were caused by tektites, micrometeors or other falling projectiles - you would see a range of sizes. Also, if sapping were involved the craters would be different sized as well accounting for the void cracks below. If these craters are from ices within the dunes subliming over time - you would see differing sized "slump" patterns along the dunes and not "point source" pits.

I really wish the MER team would inspect one of these close up with the micro-imager.

Heck with that, image it, rull over it with the wheel (compressibility test) and then trench it. We spent what, 300+ sols in one big crater, so I think a wee little crater deserves at least a day.

They are different sizes (from a few inches to a few feet in diameter), and some are non-circular (footprint shaped). I suggest you read back in this thread and look at the many pictures. This range of size and shape is exactly what you'd expect if they were 'slump' craters, perhaps formed from melting ice.

Unfortunately there doesnt seem to be a souce of water that would have any reason to percolate around: very little heat, little water vapor to condense, no tidal action to slosh magma or groundwater around. Mars seems essentially a fossil planet these days inthese parts. The only geological activity would seem likely to be in polar, permafrost and aolean type weathering.

Ice-veins forming in a permafrost outgassing phase to later sublimate away dont seem too likely. It might be more likely to get condensation accumulation whih would form the ice/snow solid matrix that could get buried by drifting sand to later sublimate away in the summer leaving the voids to form the hourglass-sand-style craters. Maybe in the polar areas this would seem a more possible scenario but here it seems too warm and dry to get that ball rolling. it rolled away a long time ago. We will have wait for Phoenix 2007 lander to ge closeup images of this buried-condensation-ice-evaporates or the permafrost-outgassses-forms-veins-then-sublimates scenario, which would be much more likely to exist under those more favorable conditions.

Here's a crazier idea: the solar radiation knocks electrons off the dust, leaving pools of negative charged dust, if the dust is insulating enough it would reach several millions volts threshold before it discharges into the moister ground below it, explosively leaving a little crater when it liberates the moisture in the low pressure/ light dust /low gravity. Or maybe no solar radiation effect is necessary, don't sand dunes build up static as the wind blows over them? isnt there 'sand-lightning' or was that just in Dune? ugh oh no its almost 5 am!!!

So whats the simplest answer?
: Nasa, please brush away the sand and look for an impactor.

What about the cobblestones laying around? They have not been explained, to my knowledge. Perhaps there is a connection...

On top of all the other comments being made, may I make a plea for s-l-o-w processes? I have no particular view with regard to the landforms we're seeing, other than a feeling that interpretations based on Terrestrial timescales may be quite misleading...

Bob Shaw

My vote is for slow, too. Given that the anatolia features are pervasive in this area, I'd suppose that these crater features are the result of a sand-sapping process. But until we get more information, I'd give a definite maybe to either impact or sapping...

--Bill

Meridiani probably sees very few changes over time, so any cracks opening up would either be very old or would reflect some new condition going on. The area should have reached an equilibrium very long ago and all potential cracks would have already opened up and have long been progressively swept over and filled in by the relatively quickly migrating sand dunes, which are still maybe thousands of years young (how can the rover's instruments be used to determine dune deposition layer age in the trench? might there be seasonal differences that can be determined like tree rings? has anyone worked on this problem?).

Sapping:
- evaporating ice nodules, their initial formation powered by either global warming (or ome unlikely sort of magma movement) releasing subsurface permafrost as vapor that condenses in winter and evaporates in summer leaving crater-forming voids.
- buried icecrusts from relatively recent surface frosting episdes similarly evaporating (however long it takes to bury the frost, id guess frost would collect in the lee-side of the dune top and get buried in relatively 'short' order).

impact:
- good ol' impact craters; and so maybe all the rocks (tectites?) were seeing in the troughs of the dune ridges are remnants of these craters left behind as the sand dunes migrated away.

NASA/JPL seem to be convinced they are impacts, but I think the court is still out on these things. I was initally convinced they were impacts too, but they just seem to be too common. We're seeing them everywhere. It may be that I am being misled, but they seem to come in a variety of shapes and sizes, and appear often in the vicinity of bedrock fractures. It's hard to ignore the similarity to the larger Anatolia features.

I've been wracking my brain trying to come up with a reasonable, uniformitarian process that operates on long time scales, but I can't help but wonder if some more exotic process may be at play, as atomoid suggests.

It's been noted too, but I'll mention it again, that Spirit's not seen anything like this at Gusev. If they were coming from space, I'd expect a fairly even distribution over the entire planet.

But it looks like at Gusev, there aren't many sand dunes to show these craters, im guessing they would hit the rocky-pebbly ground and not make much of a dent in the surface, especially since theres a lot of texture to hide them from human perception and mark they would leave would be likely smaller.

Also, it seems there is more wind at Gusev (if dust devils are any indication) and could account for faster crater burial/removal if there were any.

-speaking of this, one sure way to test the speed of dust movement was available in the heatshield wreckage near Endurance, having a half-mars year of time for the wreckage to be exposed to the dust, it looked pretty fresh, not very covered up at all, which woudl suggest these features persist for many years, whereas at Gusev we missed out on that opportunity (please be glad i spared you the pun i attempted to write), the small craters would probably last less time given the wind conditions, which woudl probably tend to smooth away such features quickly since any protective duracrust would have been breached...

come to think of it, im not finding any craters smaller than a yard or so in diameter at Gusev, perhaps any smaller ones are indistinguishable from the general terrain noise, or am i just not looking hard enough?

There are just too many microcraters...and some are not round...erosional processes in the dunes should give these microcraters too short of a lifetime to accumulate over the eons....and as I recall there were no microcraters at any of the other landing sites on Mars (Pathfinder and Vikings) {is this correct?}. All of these reasons strongly suggest that the microcraters are not impact-originated.

If the microcraters are not impact-originated, then alternate hypotheses require examination. In my earlier post in this thread I summarized all the alternative explanations. I then suggested a sapping mechanism.

However, a sapping mechanism must be renewable. Otherwise erosional processes in the dunes would erode away the microcraters rather quickly. And the sapping process must somehow cause microcraters near crests of dunes too.

A sapping process caused by ground cracks opening up seasonally is unlikely, because this should preferentially cause microcraters (especially larger microcraters) in areas where the dunes are thin - such as in the sands of the valleys between the dunes. In areas where the dunes are high, the sand would fill the ground crack without leaving a visible mark on the dune crest. However, this isn't observed (microcraters are, if anything, more common near dune crests). In places where the dune sands are especially thin, the soils from the sapped crater would be expected to not completely fill the ground crack, and the crack should remain vsisible ni the base of the microcrater. This hasn't been seen either (yet).

So the sapping process has to be renewable, and not caused by cracks in the bedrock. This is the reasoning that led to my hypotheses that subliming or melting ice nodules is the origin of the sapping process, which then causes the microcraters. Now the difficulty is explaining how the ice nodules are renewable, and how these ice nodules came to form within the dunes.

Perhaps the ice nodules originate from upwellings of brines from pores or channels the bedrock. Or perhaps, the the dunes contain fossil ices from depositions which occured when Mars has a greater axial tilt. I don't know. But I would certainly like to see a discussion of how ice might enter a dune!

These things have been so common recently at Meridiani that I would have to expect them to show up in some Gusev images if they are impacts. Ripples and drifts are less common there, but common enough that we should have noticed a few. But yes, it's possible there is more dust/sand movement in Gusev and that might cover the mini craters more quickly. I just don't have a good handle on how different aeolean processes are between the two locations. We've certainly seen some dust movement on both sides of the planet.

I'd love to find the time to go back and scan through all the Meridiani images to kind of map out just where we've seen these things. It might be simpler to scan through this thread to get a feel for the distribution. If my memory serves me correctly, they have been increasing in frequency as we've come south.

(putting on my best Ricky Ricardo voice) Ok, Lucy, 'splain this:

Here is an image of one of the drifts taken from the Four Lane, cropped from the left side of the image. There may be a better/closer/later view of this, but it isn't packratted on my local disk. It would be a good idea to map the distribution of these fellers.

As I recall, we saw the first sand/ripple crater about the time we hit Purgatory. This is another piece of our puzzle, since that was about the time that the driving surface changed from "ideal" to "potentially troublesome".

--Bill

Actually didnt we see the first crater about halfway between endurance and purgatory?, but alas, the number of them have certainly been increasing in frequency.

As for a renewable ice source, seasonal atmospheric or ground-originated water vapor condensing to frost on the lee side of dunes, building up into dusty nodules and getting buried by migrating sand seems a simple scenario, and would explain the increasing number of them as we near victoria, implying that were reaching an area of greater local ground vapor release...

if this scenario works, i'd expect many existing analogs on Earth, which shoudl be more favorable to this process. anyone aware of any similar phenomemon?

I thought I noticed one between dunes a few sols ago (but after that good one on sol 616); I'll have to go and find it again though.

If, as the pevious BBC article indicated, there is evidence for ice within Martian dunes, at least larger ones, perhaps it could happen here also in these ones? I like the ice nodules / sapping theory. I was sure initially they were from impacts also, since they looked like it and then JPL said they were, so they must be then, right?... They still could be, I'm just questioning that idea more now.

Re CosmicRocker's comment about going through previous Oppy images, I'm willing to do that as much as I can, if it helps. These micro-crater things have piqued my curiosity.

I also noted, as mentioned a while ago, that they do seem to be more common in drifts close to bedrock, at least that is the impression I get. I can't recall offhand seeing as many yet in dunes away from bedrock areas (except those two first ones back in April were I think).

On a much larger scale (say 30m across), you do get ice-heave features in permafrost which can cause what look like mini-volcanoes, often with little ponds in the centre - these features are called 'pingoes'...

Bob Shaw

See also:

http://www.unmannedspaceflight.com/index.p...240&#entry25382

Bob Shaw

Yes im very familiar with those, we got plenty, was about to say that they can be somewhat larger but you are quite right saying they are 30m on average.
These are among many things I've been on the lookout for in the rover images. So far I havnt seen any. So my conclusion have been that any possible subsurface ice are long gone and any 'pingoes' filled in by drifting sand, or alternatively its too deep down to cause any such features.

heres a giant tiny crater, that Dilo poseted in another thread,
reposting it in this thread for further investigation...

Interesting zigzag cracks in the foreground (sand-sorting along the faults due to vibration effects from the impact? if so, then we could infer something intersting about the local seismic characteristics)

sol 649!

cant seem to link the image itself so heres the link to the image

That's an interesting photo!

I see the larger crater, and two possible microcraters, the zig zag, and what looks like several minor slump on the closest dune.

It seems like that zone has an unstable underground. (cracks, zig-zag, hole).

Rodolfo

The Ultreya area in Gusev has some good dune fields which should be good substrates for showing microcraters (assuming microcraters are formed by impacts). I wonder if we will find any of the enigmatic microcraters there?

Overhead view (vertically stretched 4x, from Ultrax):
http://i16.photobucket.com/albums/b14/ustrax3/4x.jpg

Side view:
http://www.unmannedspaceflight.com/index.p...pe=post&id=1354

Hey, I'm wondering how dark (deep?) is this small, round hole viewed by MI camera on Sol 649! I this this kind of hole can create minicrates (as the wind cover it with fresh sands)... any theory on the origin?

See the other thread on Mogollon MI's - it was almost certainly caused by the MI Contact Sensor as explained in Dot.dk's post.

You *SURE* it isn't caused by baby sandworms?....
Where's the Harkonens when we need them....

Eureka! Sandtrout. That's the answer. Read this:

http://en.wikipedia.org/wiki/Sandtrout

Sandtrout are drawn to water in the open desert and together multiple sandtrout will gather to encapsulate water, creating deserts safe for the adult worms

So that's what happened to the water on Mars!

Eventually, [...] a build up of gasses that cause an eruption of the pre-spice mass, blasting out Carbon Dioxide gas

And that's where all that CO2 came from! The tiny craters are CO2 vents!!!!

The water evaporates leaving behind dried spice

Oppy's found spice deposits!

Spotter control... no sign of the carryall...
Damn it... Harkonnens.
Spice! ... pure unrefined spice!
Damn the spice! Get out of there.

D'oh!
If Oppy suddenly goes silent, and Mars Recon orbiter spots a fresh minicrater where it was last heard from.....
We'll know why!