Unlike Earth, Mars is covered in sand and dust. The sand is made up of dark gray basalt, and is heavier than the much finer-grained red dust which covers it. So when a dust devil sweeps over the ground on Mars, it lifts up the red dust and blows it away, revealing the gray sand underneath. And when dozens of them do it in one region, you get this incredibly beautiful Martian calligraphy.
Related post: Martian Swirly
Credit: NASA/JPL/University of Arizona