Thank you, but I already have it

Doug

That is exactly what I think and tried to express in this post. Sullivan et al. report that the particles the ripples/dunes are armored with cannot be moved by today's winds. That does not apply to the small drifts found in Endurance and some other locations which consist of much finer grains.

If one consideres that these dunes have been inactive for a very long time ("more distant past") the accumulation of secondaries or even small primaries is quite likely, isn't it?

Michael

If the dunes are old and unchanged for a long time, then why does that larger mini-crater(?) just posted from a few days ago look the way it does? As in, as I noted previously, how the edges of the cavity have partially become so sharp-edged and well-defined, looking like a newer continuation of the former dune crest which had been broken and disrupted as such by the cavity? It looks like wind or some other process has re-shaped the sand along these edges so they now more closely resemble the other, normal (and continuous) dune crests. Something must have done this.

And I still think that the other smaller cavity I noted before that doesn't fit the impact scenario, since that dune crest, tangent to the cavity, looks completely undisturbed, unless some other re-shaping has gone on there also since the initial formation of the cavity?

Just guesswork...

MichaelT, thanks for explaining better what I was attemtping to communicate. You also have answered dvandorn's earlier question to me.

PaulAnderson, I know what you are saying. I am not entirely comfortable with the "inactive dunes" model either, for exactly the reasons you state.

I just wanted to show all the observational data, including that which supports the "inactive dunes hypothesis". We need to be discussing with careful consideration all possible explanations and angles to the intriguing question of the origin of the micro-craters. JonClarke I believe supports the inactive dunes hypothesis, and that at least some of the microcraters have an impact origin.

My own opinion is that some of the microcraters are formed from sapping, others from impact events. I am becoming more favorable to the idea that the dunes are ancient and inactive.

I can buy all of that, but I'm really curious about this notion of an "armor" of cemented sand covering the dunes. Has anyone actually seen this armor in any of our MIs?

Well, if you assume that the large mini-crater was formed when the dunes were still active the explanation is quite simple. Unfortunately, we don't know how old the mini-craters are. Some may be as old as the dunes, some much younger. Who knows? But, I don't see a contradiction there.

Michael

more sapping cracks lateley,
but why havent we seen more of them (or any of them at all) up near the top of the dunes like we were seeing way back when...?
if the sapping explanation were true, as if clearly appears to be for these cracks, then does it still explain the mini-craters near the tops of the other dunes. in other words if they are so plentiful here, then why are the ones at the top so rare now, it begs a different explanation for the ones near the tops of dunes...

Good questions - and these really do look like sapping...

Bob Shaw

I had to revive this topic before we leave this location. While we've been here I've had plenty of time to look around for various reasons and couldn't help but notice that there seems to be an anomalous concentration of these mini-crater-like objects in this vicinity.

Our discussions of this subject have ranged far and wide, and I have come to believe that there are several phenomena involved in the formation of the things we have been observing and discussing. Some seem to be related to bedrock fractures, others appear to be aeolian, and a few look very much like tiny impacts. I think we have also seen some that appear to be hybrids of the above.

In order to re-establish my baseline, I went back to the original "tiny craters." JPL published a special feature about them at the time. http://marsrovers.jpl.nasa.gov/spotlight/o...y/20050427.html

We haven't seen very many of these things that look that fresh. However, it seemed curious to me that there were a large number of dimples on the drifts around here, in the vicinity of an apparent impact feature that has scattered debris all around. Could it be that scattered debris created these things, and that they were later modified by the winds and other forces?

I've not tried to highlight all of them, and I'm afraid that some of my arrows obscured some of the features that I was attempting to highlight. There are other panoramas showing them all around here. Just look around at the navcams and pancams from the sols where opportunity was near that pile of debris off to the right/west, and post your comments.

We've seen these *&#^% minicraters scattered about since Purgatory I or earlier and they still tend to be enigmatic. Are they impact features or sapping features? Rather like Lunar crater chains, IMO. One of my Round Tuit (americanism for "getting around to it") projects is collecting images of and cataloging minicraters seen during this mission. I'd better hurry up before things start getting _real_ interesting for Oppy...

--Bill

Bill, small craters were seen from nearby since Endurance.

What I see s that they all have about the same diametre (as far as we can judge) but some are fresh looking, while others seem blunt.

I also noted a round dark stain, just after leaving Purgatory.

I don't know how they were made, and I agree that we've been seeing them for a long time. But there are _way_ more per square meter around here than we've seen anywhere else.

As a commited 'Sapper' (well, maybe) it hurts to say it, but: Corner Crater is nice and fresh, and very close. Secondaries can have secondaries too...

There, said it.

Bob Shaw

Picture from Joffan's post in the other thread shows several microcraters (I count possibly 5) in the distance on the dunes an example looking roughly west).

If the microcrater count can be shown to increase as we approach the corner crater, perhaps this can confirm that the origin of at least some of these microcraters are secondaries (or tertiaries).

It's been noted by others here that the number of small "craters" seems to be on the increase as we approach VC. This image here, taken on Sol 846(?) shows a couple of examples on the dune midway between the bottom of the picture and the horizon:

http://qt.exploratorium.edu/mars/opportuni...78P0705L0M1.JPG

These particular examples have small pebbles associated with them and I think this could be a key part of the process. If there are pebbles near the surface, they will act as windflow modifiers and they could cause erosion of the dune on the downwind side. There would be deposition of sand on the upwind side of the pebble, which may eventually bury it. This is perhaps best imagined by thinking about the shape of a river's surface as it flows over a boulder.

I'd be very surprised if small impact craters could survive in an active aeolian system for very long. Some sort of process of windflow over pebbles seems more likely to me.

Castor

True...But craters would also tend to crack and fracture the ground as well!

Has a method been proposed for how the secondaries manage to disappear but leave the craters?

Why don't we see any chain of tiny craters with a rock at the end?

If the rocks managed to bury in the dunes, shouldn't we see half-buried rock+crater near the edges of the dunes?

I recall seeing a rock on top of the dunes, but there wasn't of a depression, if any.

Ive been racking my brain about that 'stain', and many others like it. could it be that once a microcrater (regardless of whether its an impact or a 'sapping' feature, and im now 85% convinced its sapping since you can see at shallower sand depths, outlines of cracks below elongated divots and the way the sand sinks in suggests that the sand above the bedrock cracks as well, as if its very loosely cemented up to a certain point near the surface where it then collapses into a divot or microcrater, and especially if we see an increase in craters and divots, then its gotta be sapping or something that mimics its effects).. ..so once the microcrater gets 'filled in' by blowing sand, it is leaving evidence that only the darker character sand is getting blown around to fill in the voids? Which fits nicely with the dark streaks blowing out of Victoria and all the dark filled areas such as Erubus and other 'sinks' visible in the overhead view.

The problem is the timescales. If the dunes really predate the rubble (how else could rocks be on top of the sand?), then the craters would have to have filled up before the rubble was deposited, which would make them very old indeed. Curiously, I haven't seen any windtails around the rubble that is in top of the dunes. ? ...it makes the rubble looks relatively young, or else everything else is extremely old... I'd expected the microcratering to be an ongoing perhaps even contemporary process, since we see various stages of erosion. but this makes the microcraters look older than the rubble, the rubble being secondaries as old as Victoria, or at least CC. So could it be the sand just isnt blowing around much anymore? except for maybe a few loose grains here and there, but not enough to form drifts any longer...? this might explain why so many of the bedrock polygon plate boundary cracks arent filled in, increasing the potential age of those cracks.

Atomoid:

You put your finger on it: these are *very* old surfaces. Our usual expectations don't - quite - apply.

Bob Shaw

In the "FWIW" category, I think that the stains-- both circular and linear-- are loose evidence of sapping. Note that the ripples here tend to be layered-- lighter/darker, more/less erosion resistant-- and by removing the sand in a circular cone at the angle of repose/angle of draw the light or dark layers are displaced, much like the rocks in a graben. You end up with a dark-toned stain where the sand has been withdrawn.

Of course, this does not explain why the stains are only dark and not light, but it's the germ of a hypothesis, take it and run...

--Bill

Should it be noted that, every time Oppy looked at a dune, the sand grains were relatively large, into a submillimetre range. But wind tails we see everywhere (white in Endurance, dard around it, white at Purgatory) seem to be made of much thinner dust grains. So this would left us with the impression that the dunes are modeled by a contemporary process, when today wind would be in reality completelly unable to move their large sand grains. If this is true, we must admit that dunes formed at least millions years ago, when a large volcanic eruption of flood created a temporary atmosphere able to move the grains and form the dunes.

Also the surface of the dunes has an unusual texture, as if it rained on it (little blocks and holes much larger than sand grains). I don't know what process created these, but it is clear that there was no wind since (large enough to change the dunes shape). Perhaps it really rained, at a time whith a temporary atmosphere of steam. (I discuss this hypothesis in another thread, I don't remember where).

About the dark stain besides Purgatory, it was really a stain, not changing the overal surface "rainy" texture. It was not a hole filled with something. It looked as if it was an oil stain from Oppy. Usually the team don't comment such incidents, because they think they are of no interest. But for us who don't know, we should not be left wondering for nothing.

It's indeed a mystery. Were I there, I'd be inclined to flop down on my belly alongside one of these mini-craters and stains, whip out the trowel, butter knife, whisk broom, air-bulb, etc, and do a fine-scale paleontological cross-section/dissection of one of these features. Ditto across one of the layered ripples. Ditto with several other features we've seen. Understanding the erosional-depositional processes here is a key element in the overall picture.

At my day job, I've spent a lot of time looking at fine structures and at the chemistry of the Pottsville rocks to piece togther the depositional environment back in the Pennsylvanian times. Understanding _then_ helps us to understand _now_.

--Bill

About the dark stain besides Purgatory, it was really a stain, not changing the overal surface "rainy" texture. It was not a hole filled with something. It looked as if it was an oil stain from Oppy. Usually the team don't comment such incidents, because they think they are of no interest. But for us who don't know, we should not be left wondering for nothing.
[/quote]

Oil stain from oppy - why I don't believe oppy was even near any of these features before they were seen. As far as secondaries - the microcraters seem to be too similar is size to be secondary crater features - you would expect to see a wide range of diameters - from many meters to centimeters in size.

Unless these enigmatic features are studied closely by the MI, which I can't believe that the MER team has not deemed them to be worthy of further investigation (hint, hint) we can all keep speculating.

idealy, a cross section would be fine, on a clearly shaped one. It would at least confirm/dismiss the presence of a stone or something.

About these things being secondaries, there are plenty of cobbles around here, some on the top of dunes. But none seems to be related with the tiny craters.

By the way theres is around a very recent crater, which looks as being more recent than the dunes. A look in would be interesting, as at least features are not buried into sand.

From my observations, there might have been a moderate increase in the number of tiny craters and similarly appearing phenomena between endurance and the present location near Victoria, but in this vicinity the number seems to have spiked to an all time high. There also appears to be an anomalous number of the "oil" stains around here, too.

I am making a distinction between the circular to elliptical depressions and the "sapped" cracks and elongated divots, which have always been apparent where bedrock has a relatively thin veneer of sand/dust. I still think there are multiple processes that are modifying/have modified these old aeolian deposits. Remember that the original tiny craters that were featured on the rover's JPL home page were nicely conical depressions.

The fact that the number of circular to elliptical depressions greatly increased as we passed the rubble pile that apparently provided many of the cobbles scattered around it draws me to the conclusion that there is some causative connection between the two. I'm still sticking with my earlier hypothesis that the rubble pile is a secondary (or tertiary) that rained down fragment in this area.

Here's a thought:

You have an impact, at quite a slow speed - probably a secondary. It blasts out debris, as they do. The debris lands, and makes whatever disturbance it makes. A zillion years pass, and the drifts migrate s-l-o-w-l-y around the debris, which tends to sink.

Everybody with me so far?

Here's what gradually happens: the fluffy dune particles are all surface area, and no volume (you know what I mean) and thus have no thermal inertia. The debris has a more normal surface area/volume ratio. It retains heat/stays cold as appropriate. The cobbles act as cold traps/heat sinks and cause what is in effect convection within the dunes, but only where there is *just* the right depth of fluff above the debris relative to the size of the cobble, so that all the heat transfers add up - hence the effects being seen at the peak of the dunes. What appear to be cobbles which have migrated into the inter-dune troughs are actually just representatives of a more-or-less continuous scattering of cobbles across the plains.

So, it's both sapping (in the sense of a sub-surface effect) and impacts!

Bob Shaw

The flip side of the water argument...

At any point, were depositional processes affected by frozen CO2 that might have frozen at one point and then sublimated away? Perhaps not here but closer to the poles in today's Martian climate, yes... but -

If there was frozen CO2 acting as a temporary agent on the "soil" surface, fixing it as it were, could that influence the transportation of the larger blocks without significant transportation "evidence"? Could the larger rocks be simply rolled into place without disturbing the dunes because they were under some thin layer of frozen CO2 (I hate calling it dry ice). I'm thinking frozen CO2 because I would imagine (frozen water) might leave more traces on the surface, but this also presupposes a history of significantly more CO2 in the atmosphere history, which probably isn't in evidence. This also presupposes a "warming cycle" to sublimate away the CO2, dropping the rocks in place, and then the thinner atmosphere of today.

I'll agree that mechanism most likely didn't happen this far toward the current Martian equator, but at what point could we see frozen CO2 affect depositional morphology, and how will we know?

Nice concept. It works for me but as you say the location is critical. I also don't think it could be applied to these locations right now but who can tell what situation applied 250MY ago - if Mars obliiquity has extreme variability over the 250MY timeframe (or less) then the current equatorial locations could easily have been subject to the conditions you describe. If this conjecture is true then our standard analyses of what we are seeing needs to be quite careful in the assumptions that are made. Your proposal is entirely plausible over a long time scale (IMO at any rate) even if the current conditions are very different.

One problem with the otherwise rather interesting 'wandering poles' hypothesis discussed above is that Meridiani would need to be in high latitudes (you know what I mean) and there's no particular way to tell where and when it might have been in such circumstances.

Or... ...is there?

What about a global search for old gullies, seeking a statistical distribution which might indicate which areas enjoyed the combination of latitude and insolation which appears to be a requirement for their formation but which are not presently within such zones. If a model can be identified which embraces the sort of polar wandering which may well take place over a period of millions of years, then it might be used to predict areas where CO2-mediated material movement may have taken place. This sounds like the sort of activity which MRO could help to carry out, with it's unique synoptic observation capability making image interpretation somewhat easier.

Two other points: might polygonal ground patterns, previously attributed to the presence of water ice, be the products of sorting processes mediated by CO2?

And, what might Phoenix see - particularly on the way down?

Bob Shaw

Thanks for reminding me about pole procession, that reshuffles the deck, i was unconciously assuming a geologically stable climate, but if Mars were covered with some thickness of frost or snow at some recent point then this would change quite a bit. Remeber, some folks think Mars, like Earth, is exiting from a relatively recent ice age so it doesnt seem out of the question, imho, that there could be a source of volatiles locked below the surface that is now under some temperature and low atmospheric pressure gradient to be released in an ongoing fashion in geologically recent times.


Here's another (wacky) thought: if there were some source of volatiles (now not so implausible assuming the above), and the sand grains are packed tightly beow the surface and effectively air-locked with dust, could those volatile gasses such as CO2 'bubble-up' in small events, displacing some sand forming the divots?

This _has_ to be another way in which we need to reset our Earth-based perceptions. On Earth, if I go out into the field and observe a 250 million year old river delta and swamp I can go to the Mississippi delta and see similar processes. The flora and fauna are markedly different, but the geological processes are virtually identical.

Not so on Mars. What we see today is not necessarily what was. And on Earth, we have the hydrologic cycle which drives amny of the geologic processes; on Mars, we have that and also a "CO2 Cycle" which cannot happen on Earth.

An alien world, indeed.

--Bill

To admit that the tiny craters were made by the disappearance of some underground frozenice or CO2 arises a difficulty:

why should be this frozen material just in a given place, and nearby alway the same size?Why not larger items (as wit hte impact terory) and even, why alway round shapes, not any shape? We need that the frozen material would be alway in forms of small blocks of the same size.

And, remember, they don't seem all the same age, while the dunes seem to be all of the same age.

Bill:

Couldn't agree more. On Mars, walking like a duck and quacking may not imply a duck. We have to challenge our Earthly preconceptions at every turn - Mars ain't Earth.

Bob Shaw

I would suspect all the CO2 ro H2O ice to be pretty much distributed below the surface, at least initially, due to snowpack overhead dissolving/seeping into the subsurface minerals/pores, any 'chunks' would be solid aggregations formed by settling and seepage over time during such a 'wet' period, which would create thicker pocket remnants in voids or faults after most else has evaporated, so there could be localized deposits beneath the crust, or even in sand dunes if weather changes provided enough sand movement to cover deposits and insulate them for posterity, however, even if not, we still have the faulting and cracks that serve as escape routes for volatiles so that would, hand in hand with sapping, create voids for which the divots would be created over time as the sapping/evaporation continues, hence the relative ages of the divots (microcraters).

There is an interesting theory regarding ice locked up in Martian dunes.

We have seen various shapes and sizes of divots, they tend to be more linear in form where the sand is somewhat shallow (although this would depend highly on the 'cementedness' of the sand) and more conical where it is deepest.

There are larger ones too, see upper left of this image. The largest one seen is about a meter wide. but none seem to be much smaller than a poker chip.

The youngest most pristine ones seem to have been the original microcraters that we saw last year. but why would these ones here near Victoria be older? maybe its just that more sand is blowing around here than there?

Is it just me, or are many of these... "things" preferentially distributed along the crest of the parent dune? The crest very often appears to divide into equally sized semi-circular shapes, pass around the rim of the divots, and rejoin on the other side of the dimple. The 'larger one' you've pointed out looks pretty typical of many others in that respect.

That is the frustrating thing about these microcraters: several theories, all equally valid with the data we have, fit these features. This is the classic "blind men and the elephant" paradox.

What is needed is to flop down on our bellies beside one of these features, whip out the trowel, butter knife, whisk broom, etc, and do a fine-scale paleontological cross-section/dissection.

Argghhh.

--Bill

I'm hoping they could have Oppy do just that, use the wheel as a butter knife to relatively easily and safely excavate a little bit at a time by spinning a wheel just downslope of a good representative divot, taking pictures all tehy way, and seeing what the sands reveal as the divot features fall away. if its an impactor we should almost surely see it fall out as well. If not, then do we see any relatively stable crack at the root of the divot, which would be some evidence of sapping. It might take couple of days to go through all tho motions, but time well spent i think..

It's worse, at least the blind men were only looking at one elephant. Here different features may have different causes. The original microcrater Atomoid refers to looks to have raised edges, consistent with an impact origin (to this naif), while many of the others look much more like sapping features.

Steve

I agree, stevo. The wide range of shapes and sizes of these depressions at least suggests the possibility of multiple origins. I think some have been actual impact craters, many are created by some kind of sapping/subsidence process, and some may even be aeolian in origin.

Regarding the rubble pile we saw several sols ago, I recently noticed that the June 15th update for Opportunity mentioned taking pancams of a small crater before the drive on sol 848. It said, "Before the drive, the panoramic camera took an image of a small crater, nicknamed "Sjaelland" for the biggest island of Denmark and the site of the capital." I wanted to verify that the rubble pile was the crater they mentioned, but those pancams haven't come down yet. However, the thumbnails are down. I manually stitched the two R2 thumbnails and am attaching it to this post. The resolution is obviously quite poor, but there is no doubt that Sjaelland is the feature we have been calling the rubble pile.

That has convinced me that the many small depressions we saw as Opportunity passed that area are very likely of impact origin.

Regarding sapping/subsidence, we saw an example of a long, linear, and pretty wide depression that crossed two ripple crests in a navcam from sol 850. http://marsrovers.jpl.nasa.gov/gallery/all...1P0705L0M1.HTML It leaves little doubt that sapping/subsidence related to fractures has occurred after the ripples were formed. (or, at the same time the ripples were formed.) I am beginning to think that subsidence caused by the slow compaction of sand/dust filling deep fractures would be a simple and plausible process.

Here we have a good looking one:

http://qt.exploratorium.edu/mars/opportuni...WXP1605R0M1.JPG

I don't understand a thing about this, can someone tell me if this is a fresh one?

Ustrax,

Assuming you mean the feature on the crest of the ridge in the foregound it seems unlikely to be an impact crater to me. I know it's possible for an impact to occur exactly at the crest of the dune, but it seems too coincidental to me. The crest of the dune is probably the most active part and a small impact is unlikely to be preserved for long.

I think this is more likely to be an interference effect between two dune crests as they migrate westwards. You'll notice that the rim of the crater-like feature extends for a short-way anti-clockwise from the crests on each side. It doesn't make a full circle.

I'll try and dig out some Earth examples, but I think as two dune crests approach each other they will cause local wind interference leading to the pattern we sometimes observe. I've seen this on other Martian dune crests as we've travelled along.

Castor

Thanks for the answer Castor, I see what you mean...
I had a second look and make another question, couldn't it be that the dune crest may be suffering an alteration in it's course due to something in it's course, let's say, a tiny bit of evaporite? Because it looks to me that there is something in the center of the crater like feature:

http://i16.photobucket.com/albums/b14/ustrax3/tc1.jpg

I just "know" that I have seen such features while wandering around in the Great Sand Dunes in Colorado, but I can't find a picture.

Scott

Here is an L257 image as well as a stereo pair of a minicrater seen on Sol 812. This site is about 150m north of the "Sinkhole" feature.

Still mysterious...

--Bill

I'm a sapper to the core, but I *did* have a thought regarding baby impacts. What sort of projectile might make a crater then utterly vanish, leaving no more than a stain?

An ice one...

Think of an impact into one of the polar ice caps. Miles and miles of ice - there must be secondaries created by flying icebergs on Mars!

Bob Shaw

Tiny craters? You said : tiny craters?

http://origin.mars5.jpl.nasa.gov/gallery/a...CNP0705L0M1.JPG

There is a beautiful example of it.

Surely the process of being ejected, flying at very high speed through the atmosphere to cover a few thousand miles...that's going to melt anything of that sort isn't it?
Doug

I'm not sure what the initial shock from the primary impact would do to ice at martian polar temperatures, but assuming a big enough block of ice could leave the impact site intact I think it could survive its flights through the atmosphere. The frictionally heated surface would ablate quickly resulting in very little heat being conducted to the interior of the block.

Ah, that old "murder-by-icicle-knife", huh?

...But I think this idea is intriguing. They wouldn't need to be secondaries from impacts at the poles, either...icey bodies must be breaking up in the Martian atmosphere every day. On Earth, we'd hardly notice the addition of a splash of water, and a small crater would be gone in no time. Here - with erosion on Dead Slow - we need to think about a very alien environment.

'Ice idea, Bob.

Andy G

I'm a sapper, too, but also an on-the-fence-rocker on these critters. The thing about the icy-projectile idea is that we ought to also be seeing rocky-projectile impacts since the ejecta won't be pure ice.

I'm not sure that not having seen the projectiles is a problem here. We haven't had a super-close look at any minicrater so we can't say it ain't there.

Has anyone calculated a probable size of the projectile needed to produce the minicraters we've seen? I'm not aware of any such guesstimates in the literature.

--Bill

Doug:

I don't think so, no - or at least, not always. Impacts can result is some quite non-intuitive effects (for example, it's been suggested that on Earth it's possible that tektites 'surf' shock waves and end up with very non-linear distributions). And meteorites are often cold in their interiors - one with a nice ablative coating of ice should survive really well...

Bob Shaw