sol 645 view of (get out your anaglyhs)... now is it a crater or a divot, hard to say sometimes, but i tend to think the same process is behind both, this one appears to be expecially closely associated with the bedrock features that extend under the dune. other similar though less-formed features behind it might hint at something.

I've always figured Herbert was a cocaine addict or something similar.. I know, let's write a book about this one planet that has this magical 'spice' that everyone just MUST HAVE. SWEET, SWEET, SPICE

It can't be a divot this close to the green. If someone is whacking divots here, they are going to get kicked out of the club.

Another couple mini-craters, in the same drift, from sol 731:

http://marsrovers.jpl.nasa.gov/gallery/all...NTP0600L0M1.JPG

Another nice one, from sol 765:

http://marsrovers.jpl.nasa.gov/gallery/all...YQP0695R0M1.JPG

Just saw this other one from yesterday, a somewhat larger mini-crater or slump?:

http://qt.exploratorium.edu/mars/opportuni...DTP1795L0M1.JPG

Interesting how it is so nicely tangent to the top crest of the drift, yet the sharp edge of the crest itself looks to be completely undisturbed. That would seem to make impact less likely?

And also:

http://nasa.exploratorium.edu/mars/opportu...DTP1795L0M1.JPG

and:

http://nasa.exploratorium.edu/mars/opportu...DTP1795L0M1.JPG

if you look at the Navcams you can see hints of more depressions.

Although impact craters make sense as an explanation, I see many instances of the sand being sapped from around the edges of the paving stones and my intuition says that these "craters" and this phenomenon are related.

I dunno.

--Bill

I agree with the idea that they are like the cones in an hour glass caused by the dry sand falling in to a void below the dune.

There are many mysteries about these small craters. It seems to me that there are a disproportionate number of them observed near the crest of ripples. Sapping has it's problems, too. How do the very small mini-micro craters form? The sand has a characteristic angle of repose/angle of draw and I can see thatthis could be easily exceeded by a small crater on thick sand. And where is all the sapped sand going? Once an old fracture is filled, it's filled. Is there an active process creating new fractures or transporting sapped sand away?

I think that the truth is going to be stranger than fiction here at Meridiani.

--Bill

I have no real faith in the impact hypothesis, but so far we've not seen a wholly convincing alternative. My gut feeling is that we need to look to the subsurface rather than the sky. Ever since Anatolia it's been clear that sinks *can* occur, but the question in my mind is *why*?

One point regarding the location at the top of the drifts - that's where they're thickest, and highest off the ground, so maybe some volatile has been trapped in a layer which has then popped... ...think of natural gas in an oil-bearing environment, and the way it can be trapped in folded strata, salt domes and the like, well above the oil itself, but swap scales and try the word 'duricrust'. And no, that's *not* a vote for oil on Mars!

Bob Shaw

We have hydrated minerals and a tenuous atmosphere with very low water content. I can't comment for minerals (geologists, help , but simple hydrated salts will lose water under reduced pressure over time. And on Mars we have a lot of time. As they dehydrate, they will likely shrink creating a void, which eventually becomes big enough that the mass of particles above slumps downwards.

Or am I missing something ?

Interesting how Jim Bell referred to them as an "enigma" in the latest Rover Audio Update from a few days ago. Sounds like the MER team perhaps isn't quite as sure now if they are just impact craters after all, as they had first said they were when the first few were seen.

That would make sense in more stable environs, but these sand dunes cant be really that old, given the wind characteristics weve seen as well as all the sand-blasting going on, in the geologic time scale as the supposed dehydration would occur, the sand dunes would surely be sloshing about like waves on the ocean...

maybe i am missing something too?

Well, I think a crater formed at the top of a ridge would tend to be harder to "fill" vs one further down the side of the dune. I'm starting to have doubts on whether or not these craters being so far up the dunes would tend to go against sapping. Here's a simple, $50 experiment:

Get some plywood, something to set it on to create a table with space underneath it, and some sandbox sand. Create some ripples going the length of the plywood, maybe 4-5 across the width. Drill various sized holes into the plywood from underneath, at different locations relative to the trough/crest of the dunes. Let it drain a few seconds, stop it, take a picture, let it drain a little further, etc. Maybe even throw a few rocks or pebbles of differing sizes and at different parts of the dunes to see what kinds of morphology develop.

I really hope this isn't to stupid but are these dune depressions being formed by wind vortexing off of some feature upstream of them and scouring out a pothole in a location where the vortexes are sent by local topography? I think these dunes are really old. I see rather large rocks sitting on a few of them.

I too think the dunes are very old, although in some ways that probably makes the impact hypothesis more likely!

Apart from the odd collapse along a canyon wall, have any orbital images *ever* been taken which show dunes actually moving? Obviously, I don't mean 'moving' so much as 'have moved'!

Bob Shaw

I know that things move very slowly on Mars. I can easily visualize tens or hundreds of years as a timeframe for these simple erosional/depositional processes, but thousands or millions of years is a bit of a stretch for me. We've seen dust in disturbed wheeltracks blow away within a matter of hours, so decades seems to me to be a proper timeframe for dune/ripple movement.

The mini-craters are indeed an enigma. If it weren't for the anatolia features and sand sifting into fractures and the non-presense of mini-craters at Gusev I'd be 99.9% inclined to accept an impact origin.

--Bill

Bill:

Yes, *disturbed* dust does move. But, does *undisturbed* dust move, or is it bound, however lightly, in place?

Bob Shaw

The mini-craters are so high up on the drifts that its hard to imagine a connection between them and the cracked pavement that the drifts rest on. Is it possible for a cavity to form in the base of the drift as material sifts into a fracture in the pavement and to move up through the drift as the roof of the cavity caves in repeatedly until the cavity appears on the surface of the drift as a mini-crater? I find it hard to imagine that process forming the steep-sided, conical mini-craters we see.

My impression is that all the mini-craters seen by Opportunity were formed by a single recent event. This particular type of event simply did not occur in Spirit's area recently (according to my humble theory).

>Yes, *disturbed* dust does move. But, does *undisturbed* dust move, or is it bound, however lightly, in place?

Yes and no. Disturbed dust moves within hours so it's not too much of a stretch to imagine that the dust under the "desert pavement" can move in years or decades. My thought is that it is faster than centuries or millennia.

>My impression is that all the mini-craters seen by Opportunity were formed by a single recent event...

They may be, it could be that we are looking at ejecta impacts from a unique event. But I'll play devil's advocate trying to fit this puzzle-piece into the Anatolia side of the board. We may not come up with reasonable evident for either side of the board without being able to to a proper excavation of one of these critters. What we need to do so keep looking and we'll find one example that makes everything click into place.

Slightly OT question: what type of dunes are we looking at here? Transverse, longitudinal or something transitional?

--Bill

This is an excerpt from:
Sullivan et al., 2005: Aeolian processes at the Mars Exploration Rover Meridiani Planum landing site. NATURE 436 (7047): 58-61.

"Unlike the
loose basaltic sand grains observed within Eagle and Endurance
craters, much of the 50–125-micro-m sand out on the plains has evidently
remained inactive long enough for cohesion to form between grains
by some unknown process. This implies similar inactivity for smaller
and larger particles there (which are moved by wind less easily) and
the plains ripples they compose.We conclude that plains ripples have
not been active as recently as other bedforms described above.
[...]
However, excellent sorting of the 1–2-mm concretion fragment
population currently armouring plains ripples indicates that very
strong winds in the more distant past were capable of saltating these
particles."

It seems to me that the ripples and small dunes are quite undisturbed and therefore rather old formations (whatever "more distant past" means, certainly millions of years?) which have been inactive for quite a long time. So it would probably not be surprising to find large numbers of mini-craters.

Michael

Sure enough, and this recent navcam image shows probably one of the best examples yet of mini-craters in apparent association with the fissures/fractures. http://qt.exploratorium.edu/mars/opportuni...00P0705L0M1.JPG
I have trouble imagining how a sapping process still appears to be active, when one would imagine the fractures would be long filled up by now, but it sure does look like sapping. And then there are other things like the "wrinkled rug" features, as you called them. I wonder if we might see features like these as a result of a shaking event, such as a nearby impact or eathquake.
Some of those we've seen do suggest the possibility of wind erosion playing a significant role. I haven't yet ruled that out in my own mind.

The recent one I posted in post #155 makes me even more doubtful about impacts. It is so nicely tangent to the sharp edge of the crest; the crest even appears to almost "cut through" a bit of the edge of the mini-crater. I don't see how an impact could have done that, especially if very old, with the edge of the crest still so perfectly intact? This one is a bit larger also, but even more reason to think the crest would have been disrupted at the point of impact. Perhaps sapping or even wind could do this, I don't know.

The thread has drifted away from lightning/static arguments, but it is worthwhile to throw in a couple of tidbits:

1) When lightning - powerful lightning - strikes sand, it creates twisted, tree-root like glass structures that are excavated and sold as works of art - I think there is an artist in Boca Raton who works with these. One of the things archeologist do, when they find potential fire pits, is look for these glassy structures, so that they can eliminate lightning. On Earth, lightnin strikes look more like fire pits than impact craters - at least until the next storm.

2) Grit Blasting - on a commercial scale, with a big firehose full of sand sprayed at what-haveyou, creates incredible static charges. This is most obvious at night, when an operation will be covered with 'blue lightning' and everything seems to glow - lots of elevated hair. The power of the discharges is inversely proportional to the grains of moisture in the air: Not a lot of zap in Atlanta in the Summer, but if you tried to grit blast on a cold dry night in Denver, the discharges would be very loud and very uncomfortable!

Which brings us back to Mars: I think it is remarkable that the MER's have not recorded any significant static events, in spite of brushes with cleansing whirlwinds. No glow, no static 'microcraters'.

1) Yup they're called fulgurites

To save Doug's sanity i will point out so he doesn't have to: there is no evidence that the rovers have been hit by a dust devil. The cleaning events were caused by gusts of wind. One of Spirits cleaning events happened overnight and that definitely wasn't caused by a dust devil.

Your point about static/electrical activity is well taken. However, without full access to the MER engineering data (damn that pesky ITAR) i think it is possibly unrealistic to say they haven't recorded any significant satic events.

Thankyou

Doug

Oh and my $0.02 regarding the micro craters: I am firmly in the sapping camp. For the following (mostly armwaving reasons).


1) The ripples and sand at Meridiani overlay a fissured pavement into which sand can easily drain. Whilst i understand the problem of where does the sand go once it has drained, i have no problem with the drainage happening then the surface reconsolidating and forming the duricrust in the morphology of the micro-crater, thus lending them some morphological and temporal stability.

2) I have heard alternative explanations for voids forming within dunes, i'm not talking about volatiles leaving voids, but deflation, fracturing of weakly consolidated dune interiors. In the same way that we saw rotton rocks at Gusev, i imagine a process in someways analogoues to rotton dunes. with sand draining ino these fissures (hence why we see the craters mainly along the top of the dunes).

3) Where are the microcraters at Gusev? The observation of microcraters at Meridiani and not Gusev, suggests that it is something about Meridiani. I realise of course there is a far cleaner canvas on which to spot them at Meridiani (and it is higher) however, i dont believe this is enough to explain the difference.

4) It is my understanding that objects leave craters approx 10 times the size of the impactor. Anything small enough to excavate those craters would have slowed to terminal velocity in the atmosphere before impact and would not have enough energy to be leaving a crater that size, without being in them. If they are secondaries, where are the objects that cause them?

5) Morphology of the craters - to me it looks like most of the craters we have seen are at the angle of repose for the sand, analagous to sand draining in an hour glass. I simply do not believe that an impactor would leave a depression with the morphology we see.

6) I also don't think it is static/lightning. I see no evidence of vitrification of the sand and we haven't seen evidence for sufficiently energetic electrical events at Meridiani.

I propose the following investigations:

UMSF could: collect all images of the microcraters at Meridiani and compare morpholgy, size, and location on dune.

The MER team could: MI then drive over the next one we encounter.

Good post, Paxdan. Although I am an avowed sapper, I admit to sitting on the fence. If more impact puzzle-pieces started showing up I could swing that way without great angst.

Collecting and cataloging mini-crater specimens is one of my "round-tuit" (getting around to it) projects. Unless someone is now working on this I'm going to get started on it.

Hopefully, Oppy will look at a mini-crater. If we had boots on the ground, I'd already be on my hands and knees looking and poking at one with a pencil. I hope that Oppy will not be in such a headlong rush to get to Point G and rush past points E and F.

Keep your eyes open; it will be interesting to see what changes take place in the characteristics of these features as we approach Victoria. We are already seeing changes in the sand/ripples and in the bedrock exposures.

--Bill

My two cents on your two cents.


Why is the base of the micro-crater separated (well above) any fissue in the pavement that it might drained into?
Why are there only single microcraters and not chains that might follow a fissure?




here you seem to acknowledge the problem in your first proposed solution and offer an alternate solution.



That "something" could simply be that Opportunity has stumbled upon the results of a (geologically) recent single event which created all the micro-craters instantly.




These may be your strongest arguments against imapactor-caused mini-craters. I know nothing of crater science.




Right, an exotic explaination with no evidence to back it up.

...but is this a crater remnant??
?
dont foget the anaglyphs: looking left and looking right
me thinks if it were, then where is all the displaced sand?
strewn about and so evenly distributed that we cant see it?
or thats one heck of a dessication/sublimation/sapping gap...
or is this merely a clever trick of aolean processes?

Atomoid:

That's a good point. Where's the statistically predictable range of crater sizes if they're the result of some sort of secondary event? OK, assume that there's a predisposition, for whatever reason, towards clustering of sizes - but just one size?

Oh, I hope we finally catch a sandworm hatching...

(joke)

Bob Shaw

This is a three dimensional fence I'm perched upon: I see crater, I see sand-sapping and here I see wind deflation. At any rate, we now see a cross-section of one of these banded ripples.

Question: are we looking at transverse, longitudinal or something in transition? I ca't quite pin the dune type down.

--Bill

Here's a list of interesting features we should be cataloging (Bill, maybe you can create an excel spreadsheet?)

HTML link to the raw file
Diameter
Depth
Dune height (If it's sapping, I would guess we only see them on relatively shallow dunes)
Location on dune (crest, distance from crest if possible, I would do both meters and ratio of crater diameter. Maybe negative means on the windward side, positive means on the leeward side?)
Location found (Lat/long would be ideal, maybe we can find some kind of relation between location and frequency)
Notes

Any other distinguishing features of interest?

Thats the clincher, we tend to see them more near the top third of good-sized dunes (from what ive seen and other have noticed, anyway, then maybe we dont really have a good statistical sampling, hence your call for cataloging them).
As far as that goes, if these dunes really are loosely cemented not just at the top crust due to atmospheric moisture interactions, but all the way through the inside (ground moisture interactions?), then they might form more of a fault-like rupture to allow the sand near the top to fail and funnel in more than the sand below it, explaining why its possible to see them at the top (unless it can be explained otherwise, just a thought...) ...bu that still leaves out why we dont see many on smaller dunes...

I would call it a macro-minicrater!

This one looks like a larger version of the one I posted in reply #155 (crater, slump or whatever). Note how while the original drift crest is now broken by this large cavity, the upper left and lower right edges of the cavity now have the appearance themselves of new, sharp-edged crests. See what I mean? A sort of "new" continuation of the old crest, forming around the cavity. Whatever the aeolian (?) or other processes that do this, I find it quite interesting and just aesthetically pleasing to look at.

The smaller one in reply #155, where the mini-crater is perfectly tangent to the edge of the top crest of the drift, looks like maybe a smaller version of this, less developed? Do these mini-craters get larger over time?

I just noticed also, there's another possible mini-crater on top of the next adjacent drift to the left of this one (and to the right of the rover tracks).

The more I look at this, the more it doesn't look like a simple wind deflation feature. I don't recall seeing other macro-mini craters.

Good list of cataloging criteria.

--Bill

Hendric:

These numerical values might be good testbeds for the MER range-finding application described in another thread...

Bob Shaw

Does anyone know where one can get current (or even past) position data for the rovers -- either lat/long or the kind of local metric grid that Li and his group at OSU have been generating? If available, those data seem well hidden.

There is no mini-ejecta blanket. If this was from an impact, where did the displaced material from the crater go?

If the argument is that the ejecta blanket was eroded away or covered up via aelioan processes, then why does the crater still look rather pristine? If the ejecta was covered up, I think it should have left some bulges on the sides of the crater rim.

In addition to sapping mechanisms, I proposed earlier in this thread that perhaps there were occasional ice nodules in the dunes; sublimation of the nodule could then cause a crater-like feature as the sands slump to fill the volume previously occupied by the ice nodule.

I don't want to drain anyone's patience, but I'm still voting for dehydration of sulfate minerals as a likely explanation for the minicraters. There is spectroscopic evidence for abundant epsomite (MgSO4.7H2O), which is unstable under Martian surface conditions. I know it sounds kind of sappy, but dehydration to kieserite (the monohydrate) would yield a 60% reduction in volume, which ought to be enough. http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2290.pdf

Hopefully Doug won't think there's a hole in my head and void my access, but Vaniman et. al. (http://www.ifa.hawaii.edu/~meech/NAIJC/pap...niman_2004.pdf), observed epsomite to lose more than 50% of it’s water content over 4 months. That's plenty of time however old you think the dunes are. Granted, they used more stringent conditions and it didn't form kieserite anyway, but these are messy details that cloud my argument so I'm going to ignore them

The often spotted mini-craters on the Meridiani Planum is common case comparing to the Gusev plains which have not still showed up except to ones alikes as "Hollows". Due to the shape, it looks more alike to a sappying phenomenon. Something under the sand is losing its initial volume. One of them might be of hydratation of sulfated mineral as Stevo is speculating. But, anyway, it is due to the lost of volume under the sand land. Not of wind deflaction and not of any impact mini-asteroides. Finally, about its age, it must not be of many hundreds years, and I think they might be the most recent cases for Mar's long timescale.

Rodolfo

I share the doubts about an impact origin for most of these features. I would expect to see some traces of the impactor in the bottom, if they were impact craters. At these sizes the energy levels would not be high enough to vaporize the impactor. Especially if they were secondaries. I think primary impactors in this size range would burn up first in the atmosphere.
As to ages, I'd be too cautious to speculate about "hundreds" of years. I'm waiting for the papers that show changes at these latitudes over a million.

I didnt quite absorb all of the 'sappy' pdf Stevo linked (im too brain-dead tonight) but for those with knowledge of such details, is the timescale of such sulfate de-hydration anywhere near fast enough in any speculated Martian setting to keep up with the shifting sands?

I'm getting closer to considering these dunes just might actually be hundred-million-year-old fossils with a thin veneer of particles blowing around but still cant quite picture these timescales matching up to create the clean-edged crater remaining as such given a relatively active atmosphere unless it were orders of magnitude younger than the apparently loosely-cemented (and relatively fickle) sands it rests in.

Where are the impactors for all the secondaries at Gusev?

Doug

secondaries, ...was that (or impactor fragments themselves) a source all of the Meridiani cobbles? i didnt quite read the end of that story (if indeed it has been settled).

Nice "macromicrocrator" stitch, Dilo!

Two SOLs later that micromacrocrater is but a small blur on the dune behind the rover tracks looking back... makes you think there must be a heck of a lot of similar 'craters' to these we never happened to stumble upon!

For some reason the sappy stuff seemed much funnier when I was writing it

While I don't claim to actual knowledge about these things, the second paper I linked to (http://www.ifa.hawaii.edu/~meech/NAIJC/pap...niman_2004.pdf - should be accessible to all), reports partial dehydration in four months at 1 torr, 0.5% RH and, presumably, 298K. Now while this is both hotter and drier than the surface of Mars typically, there is an awful lot of room between four months and a hundred million years.
The same authors, mostly, have a more recent paper (http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1486.pdf - should also be accessible) that contains a lot of interesting detail (if you're interested in this sort of thing) but also suggests that the hexahydrate can dehydrate to an "n=3" form within 10 hours at roughly martian noonday conditions.
Cheers, Steve

This picture from a few days ago has some interesting sapping and a "stripe", possibly from an underlying fault (It was posted on the SDC SST forum a few days ago and discussed):
http://qt.exploratorium.edu/mars/opportuni...HGP1946R0M1.JPG

I recall that JonClarke commented in the thread in that forum that the presumed sapping from an underlying fault may imply that the dunes are quite old and relatively unchanging (since the fault had to appear after the dunes formed). I hope I didn't misquote you Jon!

If the dunes are quite old and relatively unchanging in the winds, I would think that this helps support that the microcraters are also old, and if so, I think then the hypothesis that the microcraters are formed by (secondary) impact is more possible.
(Sorry I can't link that thread since SDC is blocked by the firewall here)

Good question.
When they were just formed, I expect there would have been pieces of them everywhere, some near the impact point, others scattered more widely. By definition they would be Mars rocks, and so not readily distinguished from other float (including that which they excavated), especially if from local impacts.

Secondary impacts would be produced at velocities more widely ranging than primaries. Some primary ejecta would not leave the atmosphere and would be slowed in velocity before it hit. In this case it might shatter on hard ground but should not be entirely obliterated. We could have driven over hundreds of chunks of this stuff without noticing it.

But, as with craters like Erebus, we really don't know if we are seeing anything of the original surface features, including impactor fragments, or just the fossilized deep residue left from vast periods of erosion. That said, I can remember noticing a number of craters around the east slope of Husband and the adjacent ridge whose name eludes me, in the size range of 10 to 30 meters, that had one or two prominent rocks not far from their centers. I always expected the drivers to go over for a closer look, but they always seemed to head for other targets. I still wonder if they might have been impactor fragments.

Doug, I think I have found a document which Shaka was telling details about the secondary craters: GEOLOGY OF THE GUSEV CRATERED PLAINS FROM THE SPIRIT ROVER TRAVERSE.

The document is of 1.6 MB and I am not able to attach that document on that forum. Maybe, it is due to a new police. If you are interested of it, let me know in order to send it to you.

Rodolfo

I followed you exactly through your first two paragraphs, and lost you on the third.

Here (in the linked image) we see a number of these tiny dimple craters, all roughly alined with a linear feature and arc-shaped features which all seem obviously related to sapping.

If these features are pretty old, then the sapping could all have happened a long time ago and the surface dimples remain intact -- or perhaps are excavated as ancient dune faces are deflated.

At least, that lends more credence to the sapping, if we don't have to postulate a lot of recent activity resulting in subsurface voids. It could all have happened millions of years ago.

But, the image just screams at me that all of the small depressional features are products of sapping...

-the other Doug