QUOTE (ngunn @ Oct 10 2008, 09:57 AM)
Ok, that 'dark stain' is at about 25 degrees N. Plenty of other dunefields at that latitude - what's special about this one?
"It just seems funky"
To my eyes, it just appears darker than most of the other other mid-latitude dark zones. It also streaks upward to the NE in the direction you'd expect the prevaing wind vector to go.
Most of the mid-latitude dune fields have a reasonably well defined border (altough deeply incised) on the Eastern side. This one seems much more diffuse.
Compare it's aspect with the dune/temperate region area in NE Shangri-La.
Then compare with the diffuse dark pattern near Omacatl Macula or (better) Elpis Macula.
To me, (and I could be way wrong) the 'dark stain at [25N,280W]' looks like dark was deposited onto bright, rather than bright deposited onto dark.
In other areas, the temperate zone bright dark boundary is distinct, but fades in contrast going away from the Equator. To me that implies that the dunes sands are less mobile than the temperate/polar deposition.
In my model, atmospheric deposition only occurs where condensation happens "somewhere" in the atmospheric column. The equatorial zones are dry, no condensation, no deposition, bright-dark contrast maintained. The temperate zones have condensation happening at some level in the atmospheric column, condensation agglomerates organic fallout, deposition occurs (maybe as dry organic flakes), and the bright/dark contrast gradient is obscured, or faded.
So while the dark brown dune sands are mobile on a reasonable geologic timescale, the organic deposition occurs faster, obscuring the dune shoreline in the temperate regions. At least this scenario fits the ISS and RADAR observations (and the crater distribution, too).
The exceptions to this are Elpis Macula, Omacatl Macula, that dark stain in S Senkyo, and (maybe) this 'dark stain at [25N,280W]'.
-Mike