re: A few new pictures
"That stuff still looks a lot like mud, although I know it's too cold."
No it's not. It only has to be above freezing at *some* point of the day for mud to form. Since daytime maximum temperatures near the equator, as is Gusev, can reach 80 degrees F, it is likely the daytime temperature can exceed freezing at this time in late southern Summer in Gusev.
Bob Clark