[X] My Assistant

[nprev]

This occurred to me over at the Pathfinder site thread: Given the extreme dessication of Mars, why does the immediate substrate of Martian soil invariably appear darker than the surface layer regardless of location and disregarding the salt deposits exposed by Spirit?

This is normal and expected for most terrestrial soils in temperate climates, and the apparent reason for the difference is evaporation. Now, I am not suggesting widespread subsurface moisture on Mars, and understand that wind-borne dust distribution may at least partially explain this observation. However, I have to wonder if the lighter appearance of surface layers might be in part due to the presence of the widely-hypothesized surface oxidants.

[dvandorn]

On the Moon, the soil only looked darker where the atronauts' feet had disturbed it in the area right around the LM. As Phil has pointed out, this is because the area right around the LM has been "swept" of loose dust and appears lighter than the surrounding terrain. This is obvious in all of the images of landed LMs that were taken during the J missions.

From some angles, lunar rover tracks in areas away from the LM could look lighter than the surrounding terrain, and from other angles they looked darker. Same with astronaut bootprints -- though for the most part, away from the LMs, the bootprints were usually the same color as the surrounding soils.

On Mars, there are more active chemical reactions happening (or potential) in the soil than on the Moon, and Mars features a so-called "duricrust" over much of its soils which protects a possibly different chemical environment just below. I can well imagine that buried soils are darker than soils exposed to the very low-pressure atmosphere and the nearly unfiltered UV radiation to which the surface is exposed.

I think it's interesting that this dark quality seems to fade over time with at least some exposure to surface conditions.

-the other Doug