Among the driest parts of the Sahara Desert, this image taken through a window on the International Space Station (iss) by students operating the Sally Ride EarthKAM, shows the boundary between a major dune field and dark hills along the border between Algeria and Libya. Large dune fields are known to geologists as “ergs,” the Arabic term for these extensive regions of sand. This eastern erg (known as the Oriental) of Algeria includes hundreds of dune mounds. Many winding water courses are visible on the right half of the photo. These typically dry channels drain occasional rain water towards the edge of the vast erg. Sediment carried by such streams has accumulated over a few million years to make the dunes. The margin in the image marks the line between a zone dominated by wind as the main landforming agent and a zone dominated by water movement. The Sally Ride Earth Knowledge Acquired by Middle School Students (Sally Ride EarthKAM) program allows thousands of students to remotely ...