artful) explanation, very much appreciated. Sometimes it is too easy to say some feature is ever so old or ever so young or whatever, and simply leave it at that with no real supporting information at all.

No doubt there is much to learn about what we see at Meridiani (and elsewhere on Mars) and what we know will evolve over time. It may well be that the Meridiani formations are very old, and the fact that they are not altogether gone may speak volumes about the almost impossibly slow erosion rates on Mars.

My analogy between Antarctica and Meridiani may work (or not) if we recognize that the sulfate deposits that are eroding away in Meridiani are in many respects not "rock" at all, but more like salts (in fact, they are salts, aren't they?) and their erosion rates may be very fast compared to erosion of competent rock on Mars. If this erosion is fast, it would be hard (not impossible) to claim that these deposits stopped occurring 3.5 billion years ago -- or any billion years ago. The implication of a fast erosion rate is that these deposits are still occurring.

Much as ice was still being deposited until recently (global warming, don't you know) in Antarctica.

Also, could this meteorite have actually fallen on an icy surface? If the upper surface of Meridiani was once ice, wouldn't that, too, explain the paucity of small craters?

In falling onto an icy surface, the meteorite may have formed a small crater in the ice (or not, depending); the ice may over time have sublimed away, leaving little or no present evidence, but the meteorite would still be there, settled comfortably into the present surface, looking very out of place but oddly cozy. If ice was ever involved, we might find some under the meteorite.

Wouldn't that be fun?

Cheers, BR