The shuttle imaging radar (SIR-A) carried on the space shuttle Columbia in November 1981 penetrated the extremely dry Selima Sand Sheet, dunes, and drift sand of the eastern Sahara, revealing previously unknown buried valleys, geologic structures, and possible Stone Age occupation sites. Radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand- and alluvium-filled valleys, some nearly as wide as the Nile Valley and perhaps as old as middle Tertiary. The now-vanished major river systems that carved these large valleys probably accomplished most of the erosional stripping of this extraordinarily flat, hyperarid region. Underfit and incised dry wadis, many superimposed on the large valleys, represent erosion by intermittent running water, probably during Quaternary pluvials. Stone Age artifacts associated with soils in the alluvium suggest that areas near the wadis may have been sites of early human occupation. The presence of old drainage networks beneath the sand sheet provides a geologic explanation for the locations of many playas and present-day oases which have been centers of episodic human habitation. Radar penetration of dry sand and soils varies with the wavelength of the incident signals (24 centimeters for the SIR-A system), incidence angle, and the electrical properties of the materials, which are largely determined by moisture content. The calculated depth of radar penetration of dry sand and granules, based on laboratory measurements of the electrical properties of samples from the Selima Sand Sheet, is at least 5 meters. Recent (September 1982) field studies in Egypt verified SIR-A signal penetration depths of at least 1 meter in the Selima Sand Sheet and in drift sand and 2 or more meters in sand dunes.

Massed crescentic ridges are the most common dunes in the north circumpolar erg and crater floor dune fields on Mars; they are similar in plan to dunes that are typical of many desert basin ergs and dune fields on Earth. This correspondence implies that the dynamics of dune formation are similar on the two planets, despite martian constraints on dune formation that include much higher velocity winds required to move 'sand' in saltation, the possible inhibition of sand movement by absorbed water vapor and the seasonal 'snow' cover in the north circumpolar erg, and a probably sparse sand supply. Scale ratios derived from measurements of martian dunes at latitude 72øN, longitude 53 ø and latitude 47.5øS, longitude 331 ø are almost identical to scale ratios for much smaller gypsum dunes at White Sands, New Mexico. Isolated, scattered barchan dunes probably show thinning of dune sediments toward the southern margin of the north circumpolar erg on Mars, indicating a diminished sand supply and/or deflation of parts of the region by high speed winds. A complex dune formed by the apparent collision of barchans on Mars suggests that dunes of different sizes there, as on Earth, migrate at different rates. The present activity of the martian dunes, however, is not known, and they may be at least seasonally inactive. Uniform dune patterns in areas of 'sand' accumulation on Mars, as on Earth, reflect the long-term adjustment of eolian bedforms to wind regimes, sand supply, and topographic variations. Boundary ridges of sand that delineate the margins of many topographically controlled dune fields and ergs on Earth also occur on Mars. Complex ak16 patterns on Mars, as on Earth, probably reflect locally complex wind regimes. Wind erosion grooves in some of the martian dunes, and the apparent superposition of some dunepatterns in the north circumpolar erg may record changes in the eolian regime there. Preservation of these features implies condsiderable cohesion of the dune sands. Large through-going longitudinal dune belts, common on Earth, are not seen on Mars. The absence of 'sand-passing' dunes of this type and the restriction of 'sand-trapping,' massive crescentic dunes to a few sites on Mars, suggest that much of the available sand has been removed from the martian plains. Mars may have a long eolian history in which much of the sand suitable for saltation has already been transported to the north polar erg and crater floor fields. On Earth the formation of 'mature' ergs is estimated to require 2 x 106 years or more. By analogy, the massive dunes on Mars probably required as much or more time to accumulate. The age of their formation is not known; it may extend to periods when climatic conditions were possibly more earthlike than at present. INTRODUCTION A concentric swirl of dunes forms a 'sand' sea (erg) of Saharan proportions around the north polar ice cap on Mars (Figure 1). Between latitude 75 ø and 80øN, the dunes occur over an area of abøUt 1 x 106 km 2, about the same as the total area of the active basin ergs of No...

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