Rabe Crater on Mars, imaged by Mars Express. Photo by ESA/DLR/FU Berlin

May 30, 2014 Martian crater draped in dunes

Sen—Last week, I wrote about a crater called Tsiolkovskiy on the Moon’s far side that had an unusual topography. Its flat floor is composed of dark basalt, solidified lava that bubbled up after the impact that formed it.

A few dozen million kilometers farther out from the Sun lies Mars. Like the Moon, it too has countless craters on its surface, and a lot of them are funny-looking as well. One of the odder ones is the 100-kilometer-wide crater Rabe, located in the Red Planet’s southern hemisphere. Much like Tsiolkovskiy (and many other impact features), it’s round, has a raised rim, and a dark, flattish floor.

But it’s also rippled.

What you’re seeing in the image above, taken by the European Space Agency’s Mars Express space probe, are sand dunes in the crater! How did they get there?

Rabe formed in an instant when a large rock slammed into the surface of Mars long ago. It may have started as a normal, if large, impact crater, but things change. The cause of the flat floor isn’t well known, but likely due to volcanic deposits filling it, plus material blown in over time.

Actually, that last bit is very important. Mars has an atmosphere, though a thin one, pushing down with less than 1% of the pressure on the surface as Earth’s air does here. But it’s enough. The ceaseless Martian wind, acting over millions or even billions of years, is a plodding source of erosion. Wind-swept material blows over the surface, moving things around.

In the case of Rabe, it formed the dunes there, and also shapes them. There’s a pit in the middle of the crater, dropping roughly a kilometer from the floor. That may be an erosive feature, both from air and long-gone water. It acts as a trap for material blown in by the wind.

The sand dunes are obviously very dark, and are almost certainly made of basaltic material, much like the floor of Tsiolkovsky. However, unlike in the lunar crater, where the gray substance flowed up from below as a liquid, in Rabe crater it most likely fell down from above… or, more accurately, from above and to the sides.

In higher resolution images, fine layers of materials can be seen in the crater walls, including basalt. Erosion from the wind would eat away at the walls, tearing apart the rock into coarse grains of sand that would then settle into the lowest place they could find: the crater pit.

As it happens, the winds in this region tend to blow somewhat steadily from the southeast, so the dunes take on a fairly regular shape, creating those ripples across the crater floor. The tallest dunes are over 200 meters above the crater floor.

The dune field is pretty big, about 35 x 50 kilometers in area. Still, it’s not big enough to fill the pit. On Earth, sand dunes migrate slowly as particles are blown around, and the same is true on Mars. In some places the motion is rapid, and in others the dunes are much more sluggish. The dunes of Rabe crater are certainly shifting around, and over time their appearance will change, but their eventual fate is uncertain. Will they ever blow away? Maybe, in millions of years or more.

However, I suspect long before then, humans will be calling Mars home. If we terraform it, making it look more like Earth, the air will get thicker, wetter, and the landscape of Mars will be changed on far faster timescales than those to which it was previously accustomed. I’m of two minds on this; I’d hate to see any of the natural beauty of this stark planet erased, but I also very much want to see human footprints in the dust there. I have no easy answer to this, but at the very least, I’m glad we have missions orbiting Mars and roving its surface now, so that we can learn as much about as we can while we can.