In another forum it was noted that Endurance crater seems to have a dearth of ejecta around it. Could this be due to the way the crater was formed, age of the crater or perhaps the composition of the bedrock?

I'm thinking that the lack of ejecta around *all* of the craters we're seeing at Meridiani (with the exception of Fram) has two explanations:

1) The craters were made a *long* time ago, possibly when there was still water overlying the site (though I don't at all insist on that). The upper layers of the whitish rock unit (the "evaporation layers") have all obviously weathered down significantly and have been covered with the basaltic sand that, along with the hematitic blueberries, makes up the dark regolith at the site. Any significant amount of ejecta made up of the evaporation layers has probably long since been weathered away. Hence the lack of visible ejecta.

2) I think a good number of the craters we've seen (including possibly Eagle, although I'm not as sure about that) are sinkholes and not impact craters. So the lack of ejecta around the sinkholes doesn't surprise me.

Now, obviously, Endurance is an impact crater, the crater rim morphology makes that clear. But I think it's quite possible that Endurance is simply old enough that the ejecta has all been weathered away.

I bet we find a lot of that on Mars.

Doug
dvandorn@NOSPAM.mn.rr.com

Another explanation is that the Meridiani bedrock is made up of fairly soft, easily erodible materials (sulfate salts, eolian sands/sandstone) and the ejecta has been worn away completely by wind erosion (leaving berries, small chunks of tightly cemented bedrock and other hard things lying about) since its deposition after the impacts. The erosion patterns in the craters also suggest layers of easily and differentially erodible materials. There seem to be "flaps" of the whitish sulfate-rich beds that have been undercut and have slumped down along the crater walls in fairly coherent sheets. The big craters have been eroded into aerodynamically stable forms with fairly smooth walls with some trapped, more recent eolian-reworked material in the bottoms. I wonder if this material may be of local bedrock origin (the darker, cross-bedded material in the crater wall) and may be a safer place to analyze the composition rather than scaling the crater wall.