re: no follow-up?)
Not to mention that the Martian atmosphere ought to filter out objects small enough to form a crater that size -- we did not expect to see *any* craters that small.
Given that the number of objects goes up exponentialy as size decreases, if foot-wide craters can be formed by meteorites, we should have seen lots of them at all the landing sites, given how old those surfaces are (much lower resurfacing rates than Meridiani).
Pretty much has to be a low energy object, not a primary impact. Maybe a rock lobbed out as secondary from a nearby young crater.