Some windblown dunes on Mars are just beyond words. These are a peculiar type of bedform called a transverse aeolian ridge, or TAR for short. Nobody really understands these things: are they dunes or ripples or something totally different? They’re common on Mars but unusual (maybe even nonexistent) on Earth. They’re also pretty dark-toned here, whereas on most of Mars they tend to be lighter than the surrounding rocks.