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.
Image credit: NASA/JPL-Caltech/MSSS
Planet Four Blog 
I had tagged a similar image as :beaded-terrain, but had begun to suspect :image-artifact. I went back and reviewed the original image and still couldn’t come down on the bad data conclusion. Glad to see my instinct isn’t entirely bad.
Dunes may also be found on a handful of select bodies in our Solar System!
Apart from Mars: Venus, our own Earth and Titan also hosts these features. All of course in conditions which are different and unique to the body in question…