What is it about erosional, burial and exhumation of craters that you have trouble with?

Our view of Mars has gone through numerous paradigm shifts -- and it is going through another one right now, with no doubt many more shifts in understanding to come when the current crop of landers arrive and do their thing. All well and good.

The impact cratering history of Mars is inferred from the appearance of circular formations from orbit and from small chunks of the surface of Mars that have found their way to the Earth (presumably as a result of impact.)

Much of the present understanding of cratering on Mars is derived from theories of cratering on the Moon, but with little or no reference to planetary surface discoveries made since Magellan and the Voyagers.

In some respects, Mars resembles Mercury, the Moon and the Earth, but in other respects, Mars is more similar to Venus, Io, and the icy moons of the outer solar system. An impact origin for most circular formations on Mercury and the Moon is likely, on the Earth somewhat less likely (for internal processes are capable of producing mock craters on the Earth -- apparently more frequently than we find them on the Moon and Mercury.

There are only about 900 impact craters on Venus, and some of those have probably been misidentified. However, there are many thousands of circular formations on the surface of Venus, so many showed up in early radar scans Carl Sagan once claimed the Venusian surface was saturated with more impact craters than any other body in the solar system. He was wrong, but he also expressed the then current belief that if there was a circular formation on a planetary body it was a priori an impact crater. We now know that with Venus, that is not the case at all.

There are virtually no impact craters on the surface of Io, but -- again -- there are many circular or quasi-circular formations and depressions caused by internal processes only. Some of them (albeit not many) resemble impact craters -- except for the absence of secondary characteristics.

Examine Europa and Triton especially, and you will find additional circular and quasi-circular formations that resemble impact craters but either certainly or probably are not.

Now, compare and contrast the varieties of crater-like formations on all these bodies: Earth, Mars, Io, Europa, and Triton. Find the similarities and the differences.

We may find that many craters on Mars are probably not of impact origin at all, and if we do, it changes our understanding of the history of Mars, likely making the planet into a much more dynamic place than previously believed -- a step which is already being taken tentatively without re-evaluation of cratering origins.

And if Mars is more dynamic then it is potentially much more likely to host biology than not.

Among other things.

The Cold Mars hypothesis is gaining in strength. I happen to think that Mars is probably in one of its warmest eras right now and that historically it has generally been colder on the Martian surface than we see today. That means that the atmospheric pressure at the surface was generally less than now, among other things, and that weathering and erosion would usually have proceeded at a slower pace than now.

That would suggest, though it doesn't require, that ancient impact structures on the surface would generally be very well preserved -- as we find is the case with ancient craters that have reasonably identifyable secondary characteristics. There are lots of other circular formations, though, that appear to be about the same age that have few or no secondary characteristics of impact. Were they eroded differently? If so, how?

Could they have formed by some other process?

Could the supersaturated regions, often in troughs, have become cratered by means other than impact?

These are some of the questions I ask. It's the quest for knowledge that drives me, not any particular point of view about Mars or the Solar System -- or any particular worry (or lack of it) about impacts in general.

(Which probably has nothing to do with what you asked. Oh well!)

Cheers, BFelix