It seems, these white blotches are reflective enough that they caused oversaturation of a spot of image taken from a satellite at the L1 lagrangian point (click to open at max resolution, the purple spot is squat in the middle of the picture).
Clear quartz crystals has been found nearby ( link ), but could it be this? Can quartz so clear and reflective appear in such abundance? Or is it some other cause?
(I thought it might be frost, which shows up on the desert in the mornings, but the photo taken from L1 places the point at peak of the summer with the Sun precisely in zenith above it; I doubt any frozen water could withstand that. Also, Wikimapia has some photos taken nearby with what looks like frost, but the daytime is obviously different).