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.
Full Version: Why Is Martian Soil Dark Underneath?
It's more likely un-stuck-together dust "fines" at the surface, together with a microscopically smoother surface compared with recently mechanically disturbed soil. Note that disturbed lunar "soil" is darker than undisturbed regolith, even with relatively high sun and low phase angles so shadowing effects are small. There's a classic post-EVA apollo 11 pic taken out of the window of the LM showing the astronauts tracks all over, all darker than undisturbed soil next to the tracks.
That sounds quite reasonable; thanks, Ed! 
IIRC, even some of the Venera images showed darker soil under a rock or two that had been displaced by landing...remarkable how literally universal this phenomenon apparently is, despite radically different environmental conditions and soil compositions. It's a pity that no pebbles had been displaced within Huygen's FOV for another data point.
IIRC, even some of the Venera images showed darker soil under a rock or two that had been displaced by landing...remarkable how literally universal this phenomenon apparently is, despite radically different environmental conditions and soil compositions. It's a pity that no pebbles had been displaced within Huygen's FOV for another data point.
QUOTE (edstrick @ Jan 12 2007, 11:14 AM) 
It's more likely un-stuck-together dust "fines" at the surface, together with a microscopically smoother surface compared with recently mechanically disturbed soil. Note that disturbed lunar "soil" is darker than undisturbed regolith, even with relatively high sun and low phase angles so shadowing effects are small. There's a classic post-EVA apollo 11 pic taken out of the window of the LM showing the astronauts tracks all over, all darker than undisturbed soil next to the tracks.
It may be more complicated than that - Phil Stooke pointed out a while back that close to the landers the dust layer had been blown away by the DPS exhaust, which gives us a rather misleading impression. Having said that, there is the Apollo 17 post-EVA III panorama which certainly seems to show tracks out as far as you can follow them...
Bob Shaw
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
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
I'm not offering this as a cut and dried explanation, but there may be several reasons why the Martian regolith is darker beneath the surface.
1. It is covered with a layer of airborne dust, which contains red hematite, and other alteration products. The dark coloration underneath could be indicative of the higher prevalence of basalt sands, particularly unaltered pyroxene, which is dark in colour. Pyroxene tends to degrade to lighter minerals such as Fe3O4 when exposed to weathering processes.
2. The colour change may be associated with surface reactions of the soluble salt component, such as reduction/oxidation or loss/gain of water of crystallisation.
It's interesting that 'Cyanotype', one of the early photographic processes, relied on colour changes to Ferric salts on exposure to the sun.
1. It is covered with a layer of airborne dust, which contains red hematite, and other alteration products. The dark coloration underneath could be indicative of the higher prevalence of basalt sands, particularly unaltered pyroxene, which is dark in colour. Pyroxene tends to degrade to lighter minerals such as Fe3O4 when exposed to weathering processes.
2. The colour change may be associated with surface reactions of the soluble salt component, such as reduction/oxidation or loss/gain of water of crystallisation.
It's interesting that 'Cyanotype', one of the early photographic processes, relied on colour changes to Ferric salts on exposure to the sun.
An integrated view of the chemistry and mineralogy of Martian soils. by Yen et al. would be a good read for a comparison of the bright dust (which is homogeneous) and dark soil (which is also strikingly similar at the two MER sites).
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