One of the Gigapan assembled Phoenix site panoramas has 'sink' spots which can be seen as the apparent lowest local spots in the scene, and it appears in enhanced image out-takes (from the larger image) the spot is ringed by depositional and fluid transport margins, with inflow from from the polygon margins around that spot.
Dark drainage from local cones within a quarter mile of Phoenix, and again a larger dark cone at about a mile or a little further show possible sources for the liquids, or, for chemistry or heat which could melt ice or percolate water or other liquids. Those cones drain toward the Phoenix site in apparent sloping.
A real intrigue, but they are others images, and showing altered sections should accompany permissions. Perhaps I should approach the source author, Mr. Malin, or others to possibly bring those here.
The polar dunes are like these drainage or 'hoarfrost' semi-liquid rolling movement streaking, at varied temperatures and climate slightly, so, perhaps the action is not all liquid which would tend to draw down and dissipate I would expect. Would a dune of permafrost or ice/particulates be hydrophobic and force a liquid to 'ride the slopes'?
What other than water would be dark in the near IR/IR images from HiRISE/MRO?
Very cold, but movement for a great distance, and in some images, the dark material moves along horizontal shallow troughs or dune bases towards secondary steeper descents tens to hundreds of feet in distance. Not a solids or a semi-liquid path of expected paths.
Can supersaturated atmospheric dense fogs become dark in IR satellite imagery?
I've never seen that, any experienced witnesses with a story?
It seems to me that the local weather may be percolating liquids upward to the heat source peak energy spots at the crests, allowing muddy hoarfrost descents, or even sandy liquifaction /liquid trails. If true, can it be the cause of more temperate slope dark streaking similarly?
Many of the streaking dunes have bright margins or fan extensions which are nearly white, as either ice or salts in transience. I would expect salts would be a stable presence, so ice conversion of liquid water is my preferred view currently.