QUESTION:
The photographs returned from Mars have shown two features which seem
contradictory:

1) A layer of dust on top of Flat Top (and other rocks).
2) Staining on top of the soil, apparently from evaporated water.

Assuming liquid water has not been present for many years, why has the dust
layer not covered the staining?

ANSWER from Mike Mellon on August 25, 1997:
Yes, that is an excellent question and quite a puzzle. I have heard the
speculation that that the "staining" on the soil looks like an evaporite
deposit (salts of some type, left behind by evaporating water). But the
surface of Mars, as we understand it, has been unable to support liquid
water for some time, perhaps as long as a billion years. And in that time
physical weathering and other processes have eroded the surface and
deposited new material (like the dust on the rocks) - generally reworking
the surface. It's possible that some sort of differential erosion has
exposed this soil layer, or that we are not actually looking at an evaporite
deposit, or that physical weathering processes are much slower than we
thought, or even that the salts were deposited by some process not yet
discovered.

Unfortunately, I don't have an answer for you at this time. As the data
becomes available to the general scientific community, I'm sure this
apparent contradiction will receive a good deal of attention and provide us
with new insights into the past martian climate. But this, like many
questions, will take us some time to figure out.