Sand Dunes in Arabia Terra
I wanted to point out some features you might have noticed in the Planet Four: Ridges images. Dark wiggly blobs as shown in the images below are actually sand dunes on Mars.
We’re showing images from orbit, but to give you a sense of what some of these might look like from the ground, here’s a selfie of Mars Curiosity at “Namib Dune”, part of the informally named Bagnold Dune Field located in Gale Crater. I learned recently that on Mars, sand dunes are black because they are made of ground up basaltic lava rocks, just like black sand beaches of Hawaii are.