[Illustration: The northern Mojave Desert. The Landsat Thematic Mapper (TM) acquires data in seven bands of the electromagnetic spectrum. On this image, white and yellow colors indicate rocks rich in clay minerals and limonite in rocks of red and yellow hues. The large concentrations of limonite and clays may indicate mineral deposits exposed at the surface or buried up to several thousand feet below it (photograph courtesy of Melvin Podwysocki).] Remote Sensing of Arid Lands The world’s deserts are generally remote, inaccessible, and inhospitable. Hidden among them, however, are hydrocarbon reservoirs, evaporites, and other mineral deposits, as well as human artifacts preserved for centuries by the arid climate. In these harsh environments, the information and perspective required to increase our understanding of arid-land geology and resources often depends on remote-sensing methods. Remote sensing is the collection of information about an object without being in direct physical contact with it. Remote-sensing instruments in Earth-orbit satellites measure radar, visible light, and infrared radiation. Radar imaging systems provide their own source of electromagnetic energy, so they can operate at any time of day or night. Additionally, clouds and all but the most severe storms are transparent to radar. The first Shuttle Imaging Radar System (SIR-A), flown on the U.S. space shuttle _Columbia_ in 1981, recorded images that show buried fluvial topography, faults, and intrusive bodies otherwise concealed beneath sand sheets and dunes of the Western Desert in Egypt and the Sudan. Most of these features are not visible from the ground. The radar signal penetrated loose dry sands and returned images of buried river channels not visible at the surface. These images helped find new archeologic sites and sources of potable water in the desert. These “radar rivers” are the remnants of a now vanished major river system that flowed across Africa some 20 million years before the development of the Nile River system. Radar imagery also is a powerful tool for exploring for placer mineral deposits in arid lands. In 1972, the United States launched the first of a group of unmanned satellites collectively known as Landsat. Landsat satellites carry sensors that record “light,” or portions of the electromagnetic spectrum, as it reflects off the Earth. Landsat acquires digital data that are converted into an image.