I believe that the darkness seen in dust devil tracks and gully streaks is the darker "soil" layer that lies underneath lighter color dust that covers everything at the end of the dust storms. For the same reason, the rover's tracks are so distinct. According to the rover's studies, this dust is almost all pure magnetic iron. I found this very startling, as I formerly thought of Mars as the "rust planet" i.e. iron oxide. However, given the very low oxygen percentage of the atmosphere, and the even lower partial pressure, I suppose this is quite possible. This fact is also startling when one considers the difference between the dust and the darker material below.

If the dust is light colored, then why is it underlain just a few millimeters below, in every rocky hill, sandy dune or crater bottom, planet wide, by dark, sulfate rich material after billions of years of tectonically static erosion??? Should not the soil accumulations have the same general composition as the dust derived from it? Is something actively altering these soil deposits, keeping them from becoming one with the dust, or vice versa. This darker soil seems to be in some cases, a cohesive "duricrust", in others, it seems to be hollowed out or fragile underneath. In other cases, it seems to be moist. I think the best explanation for all of these observations is moisture, in the form of brines and/or acids and consequent processes such as dissolution and precipitation. The abundance of unusual micro forms, the loops, "wagon wheels," and lichen like forms that have been noted, as well as the methane and possible ammonia, are highly suggestive of current biological processes as well. These could be the source of the alterations that have led to this widespread cohesive soil.

Some would argue that the pervasive iron dust derives from the impact of iron meteorites. I would argue that this idea has huge problems. Where is all the other dust that any impact would generate from striking the basalt or sulfate minerals that cover almost all of Mars' surface? Most of the particles generated would be from the planet, not the impactor. Where did all of these go too? There would have to be active processes that eliminated other dust types (while leaving the iron alone). For this, I can only envision a selective dissolution that would involve liquids of some type. Iron, being denser, less stable chemically, and more resistant to physical abrasion than most other common minerals, is one of the least likely candidates to predominate in the top layer of Martian dust!. Granted, these physical properties could lead to its long term accumulation in "wind placer," or lag deposits, but its own movement would create fine erosional particles from the other, less dense minerals of the surface, creating other dust types. Since exposures of iron rock are rare (or unknown) at the Martian surface, the lack of other dust types is strange indeed.

Alternatively, perhaps some process generated only iron dust. Generating pure iron from other iron containing materials requires heat and reducing agents, and could not occur chemically under Martian surface conditions. Therefore, one possible explanation is biological origin for the iron dust. Some sulfate reducing life form could produce fine iron particles as waste. Bacteria such as this are found on Earth.
This possibility provides an appealing "solution" to the enigma in that it could explain both the apparently cemented darker layer and the iron dust.

The Enigma, more clearly; in order for a heavy, maleable, and uncommon mineral to be the almost exclusive type of dust in the outermost layer of Mars, then selective processes must be occurring that have either (1.) eliminated other dust types, (2.) produced only iron dust, or (3) both.

1a.) Chemical selectivity - Iron is highly reactive chemically, a strong reducing agent, and soluble in dilute solutions of all major acids. Most other minerals are more far more stable. Pure iron would thus be selectively leached out, not selectively preserved. Once in solution, it could not return to the pure form, but form salts or oxides etc.

1b.) Physical selectivity- One property that could favor iron in the dust is the electrical conductivity and magnetic permeability of 88,400 Gauss. Static charges might build up on other mineral particles and agglomerate them, while iron dust could share and neutralize charge during collisions, and be transparent and dissipating for transient magnetic fields. This might make it the first to move, and the last to settle out. This seems plausible, and it could also explain the cohesiveness of the soil layer (if bone dry and super charged with stable, oppositely charged particles), and its darkness could be due to the larger particle size from agglomeration, or the differential composition. I think this "static explanation" is worthy of some real experimentation, since the only other conceivable explanation to me is biological. This would give some new meaning to the term "Gaussian Distribution!"

The other physical properties of iron make it difficult to form dust, especially fine dust, in the first place. Density = 7.87. Tensile strength = 30,000 psi. Malleable. It could have been selectively winnowed out over eons by the above static selection process.

2.) Since the surface contains little or no pure iron outcrops, and iron is so tough to break down, a natural surface process that produced only iron particles is highly unlikely. Active volcanism is not observed. If super fine particles are being created, then almost all other types are more likely to be produced by weathering than iron. A possibility exists that ongoing biological processes are producing particles of iron that are finer than the other particles types typically produced by weathering, leading to a predominance of iron in the last settling dust.

3.) The both, or all of the above answer. Life forms could be currently active, consuming minerals and producing solutions that lead to dissolution for some minerals, followed by cementation. This consumes particles of many types, including the pure iron, and cements all types into the dark crust. The remaining modest amount of non-cemented particles, including different types of minerals (and berries), is mobilized during the peak winds, and free to form dunes. As the winds subside, static electricity agglomerates dust particles that accumulate opposite charges. The charge on iron particles would neutralize upon collision, leaving the superfine iron particles to settle out last. The fineness of these last iron particles, and the "silver-white" color of iron, makes for a reflective, light colored surface layer. The last tiny particles, all possessing the same charge polarity in slight local excess settle into the "dimples" as a virtual gas where the slightest shearing breeze causes them to form steep ripples (just showing the air movements), or mix and blow out like smoke (both of which can be seen in some pictures). These late stage dusts are all that is active at this time after the end of a major dust storm period.

The alert reader may realize that I have adequately explained the iron dust enigma without any need to invoke liquids or life forms. The alternative "bone dry and super static" explanation "does hold water," forgive the pun. Why then do I continue to klingon to my bio-solution model? The reason is the mottled distribution of the dusts and duricrusts. Both rovers have run over patches with hard crusts, patches where the rocks caved in, patches of powdery soil (apparently dry), and patches where the soil clung to the wheels (which were mostly in low spots). If the Nasa "wicking solutions precipitating salts near the surface" model is correct, then the crusts should be hardest and thickest in the topographic lows where more solutions would be drawn by gravity, and uniformly thinning toward the higher regions. The opposite is observed. The most caving in of the crusts (illustrating thick crusts capable of supporting rocks - but not Rovers) has occurred in the higher regions, like around the Endurance and Bonneville rims. The lower regions, like Eagle crater bottom and Laguna meadows, had no caving in or deep tracks, but did have soils with much cohesion shown in the trench wall's stability and Rover tracks that were suggestive of moisture. These trenches did not reveal a differential surface crust. If solutions were wicking up from farther down, then why would they fail to dry out in the low spots? For these reasons, I speculate that the soil column or surface itself is the primary source of the solutions which are brines or acids produced by lithotrophs, possibly photosynthetic. Some of these solutions may dry into surface crust, leaving the more concentrated liquids to drain into the depressions, remaining moist by the deliquescence of the remaining mineral solutions.

The highly mobile superfine dust envisioned above could provide a mechanism for continued gully formation. During still conditions (which predominate) solar heating could cause sufficient updrafts to entrain these dust particles from the hot crater slopes and floor. (Charge development from the photoelectric effect could possibly aid this by either creating all positive repellent particles, or positive and negative charged particles (or complex zonally charged multiparticle clusters??) ?) Subsidence of the air along the cooler slopes would allow the dust to accumulate there. (The shade could also stop the charging, allowing the particles to collide and neutralize, thus loosing suspension). The low gravity of Mars greatly aids the suspension of these particles, charged or not. This accumulation would lead to periodic dust avalanches, or possibly even daily flow of these tenous gas like particle clouds. The resultant morphology would be the deeply etched fan headed structures seen. I would like to point out that these dendritic heads would not be expected from the “sudden burst out” model, nor the “weeping layer” model, but are the form expected from preferential precipitation over the whole slope as seen in the post above and many, if not most of the other gully examples.