Perhaps this is just my mind finding patterns where there are non… But every time I see a new picture of crater walls or valleys the phenomenon jumps out at me.

Lee S: It is a a mistake to assume only one dark layer throughout the planet. Nor should you assume the dark streaks are from it.

There are many dark and light layers in the innumerable bedding planes that rover and orbital images have discovered all across the planet. The different shades likely relate to the nature of material coming from volcanic eruptions that placed layers on the surface.

Until we go there we will not be able to properly map out this eruption history. Once there however we should be able to create a reasonably detailed timeline for almost all of Martian history. Unlike Earth, the geology on Mars is not covered with life, and the atmosphere doesn’t change things as quickly or as drastically. And there are no oceans.

I have a question for you which I cannot recall if you have an opinion on.. in very many deep surface features on Mars ( valleys, craters, and such ) there is a dark layer in the strata, not so far down and visible almost everywhere on the planet. It seems to me that almost all the “dark streaks” originate from this layer. I know the current best guess is CO2 lumps sliding down the slope while sublimating, but why the dark layer? And why would it be dry ice rich?

You probably have more experience looking at random interesting images of Mars than anyone alive, I am very interested in your opinion, and if you ( or anyone else ) have any theories… I have given it a lot of thought and only drawn blanks.