Splatter
in a word, yes. The Lander leg would have been completely frigid from the long Interplanetary cruise, then you have the rockets pushing dirt and a little ice, maybe some melting. All the legs have some areas of splattering, mostly in the more-protected pockets. The more exposed ones have been blasted clean.
I guess one day some Ph.D. student will have to do a study of the gas-flow patterns under the lander to understand more about the physical properties of the soil on Mars.