Such imagery allows scientists to determine which areas warrant an expensive and difficult visit. The concentric shapes on the left are sandstone hills, rising above white salt flats. White rivers snake through the rocks—the salt remains after being leached from the soil by the rare rainstorms that pass through the Sahara Desert. Prevailing wind usually shapes sand dunes into curves or lines, but the yellow cluster of dunes, known as Erg Mehedjibat, grows upward instead of horizontally, suggesting that this region may lack consistent directional winds. And that thin line in the upper right, squiggling above the Erg? That's the only road for miles.