Crowded dark ripples, like folded sand, dominate this image. At the edges, the folds break down into trickling streams, like paint drips on a Jackson Pollock canvas. Lighter-colored areas beneath the darker layer emerge at right, and below: a complicated surface of stacked, paper-thin, seemingly random shapes. Aeolian geology is the study of landforms formed by wind (Aeolus is the wind god in Greek mythology). This image shows aeolian activity on Mars; it’s the most dynamic, and visible geologic process on the planet. Mars orbiters have observed other formations whose names form a poetic vocabulary of wind: ripples, yardangs, wind tails, and dust devil tracks.