Kudos to Twiffer for referring me to another NASA site, Visible Earth, which features images of earth made by satellites or by astronauts on shuttles. This image is of a portion of northern Sudan, taken with a radar instrument carried aboard the shuttle Endeavour. The radar exposes hidden sub-surface features, such as, in this instance, rock formations covered by desert sand. The curving green band on the left side of the image is the Nile and its flood plain. To the right of the Nile is a nearly straight line separating reddish from yellowish portions of the image. This is a fault line that delineates a collision between two ancient continents which happened 600 million years ago. Further details can be found here.