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http://marsrovers.jpl.nasa.gov/spotlight/o...y/20050427.html

Opportunity Discovers Tiny Craters on Mars

[John M. Dollan]

Do we have any idea about the movement rate of material in this region of Mars? How long would it take for such a small pit, for instance, to be filled in by wind-blown material?

If the time frame is not that long, and the pits *are* formed by material falling into a void beneath the surface, would that imply that there is some sort of active geology going on to open these voids enough to cause the material to fall in? Or could the pits be very, very old?

Conversely, what is the liklihood of very small meteorites making it through the thin atmosphere and to the surface on a regular basis?

...John...

[dvandorn]

Do we have any idea about the movement rate of material in this region of Mars?  How long would it take for such a small pit, for instance, to be filled in by wind-blown material?

I don't have the images right at hand, but I have seen a few nearly-erased craters in the dunes, too. If I recall correctly, there is one very good example of a fresh crater and one very good example of a nearly-obliterated circular depression of similar size. Also if I'm not mistaken, they were taken from the same location, so they were located on fairly near-to-each-other dunes.

Unless we sit for months at a time at one of these highly drifted/duned locations and take a bunch of images over a fairly long baseline, I don't think we can state with any kind of certainty the time needed to fill in such a little dimple crater.

If the time frame is not that long, and the pits *are* formed by material falling into a void beneath the surface, would that imply that there is some sort of active geology going on to open these voids enough to cause the material to fall in?  Or could the pits be very, very old?

Some of the dunes near Oppy's current position show layering, so we know the dunes have built up over some span of time. And, of course, dune/drift formation is by definition a constructive process whereby new material is slowly built up onto the duneforms. So, depending on the speed of dune/drift formation (and dune/drift erosion -- it goes both ways, after all), these tiny craters could be years old or millions of years old. But since we see degradation of tiny craters, I'd have to think that really fresh-appearing tiny craters are probably not ancient.

Conversely, what is the liklihood of very small meteorites making it through the thin atmosphere and to the surface on a regular basis?

You can tell from the lower limit of the size of what appear to be primary impact craters the size of impactors that get through the air. On Mars, these *seem* to get as small as a few meters, which can be made by marble-sized to fist-sized chunks traveling at cosmic velocities. The tiny craters we're seeing in the dunes are more likely caused by something smaller than a peanut, probably less than half the size of a shelled peanut, or smaller -- if they are primary impacts.

I'm really not convinced that these are primary craters, though. They look exactly like what I would expect from a pebble or stone ejected from a nearby impact, traveling at relatively slow speed, hitting the side of a powdery-sand dune. I would say the most likely thing to have caused the really fresh one, if it *is* a primary impact, would be a small survivng piece from the cruise stage that broke off and traveled downrange a bit. But again, that would be very, very difficult to prove.

-the other Doug