Giant crayons on Mars

A piece of Mars: There are vast plains on Mars that display criss-crossing streaks like this. These are ~5 m (~16 feet) across, give or take. Did an alien drive a dune buggy all over, leaving behind tracks? Nope. These are the distinctive trails made by the passage of dust devils, which act like huge

Dunes in the spring

A piece of Mars: Dunes near the north and south poles get cold in the winter, just like they do on Earth. Except on Mars instead of H2O ice, it’s a mix of CO2 and H2O ice (mostly CO2). In the spring the white ice slowly disappears, revealing the dark dunes underneath. (HiRISE PSP_002033_1325, NASA/JPL/University

Nili Patera

A piece of Mars: Nili Patera on Mars is an ancient volcano. Some of the old volcanic material has been blown into rather striking sand dunes. It is the first place where dunes were conclusively identified as actively moving. Here’s a closeup of one of them — the steep slip face on the downwind side

Spines

A piece of Mars: Here are some old dunes that look a little like vertebrae of fossils (if you think they look like dragon spines poking out of the ground then you’re playing too many video games). The white areas are stabilized and possibly lithified. The blue areas are where the dunes are being actively

Almost a dune

A piece of Mars: Not all piles of windblown sand are able to form proper dunes, with a fully developed avalanche on the downwind side. Here, bluish sand tries to make its way through hilly terrain, which both traps the sand and makes it difficult for slip faces (avalanches) to form. Smaller, now stabilized dunes

Windswept landscape

A piece of Mars: As the wind blows sand over terrain, the grains deepen grooves in weak materials, enhancing the topography in the direction of the strongest wind. Here, over eons, sand marching from right to left has formed dunes (oriented perpendicular to the wind) and elongated grooves downwind of low hills. (HiRISE ESP_030582_1850, NASA/JPL/University