Tiny Craters Options

[Bill Harris]

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

[hendric]

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?

[atomoid]

...Dune height (If it's sapping, I would guess we only see them on relatively shallow dunes)...

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...

[dilo]

...but is this a crater remnant??

I would call it a macro-minicrater!

Attached thumbnail(s)

 

[Guest_paulanderson_*]

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).

[Bill Harris]

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

[Bob Shaw]

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?

Hendric:

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

Bob Shaw

[SteveM]

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.

[silylene]

I would call it a macro-minicrater!

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.

[stevo]

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

[RNeuhaus]

I would call it a macro-minicrater!

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

[Shaka]

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.

[atomoid]

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.

[djellison]

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

Where are the impactors for all the secondaries at Gusev?

Doug

[atomoid]

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!