USE OF ORBITAL AND GROUND-PENETRATING RADAR TO UNDERSTAND CLIMATE CHANGE ON EARTH AND MARS

In the nearly two decades since the discovery of subsurface channels beneath the sand cover of southern Egypt, many questions remain about the response of geologic materials to orbital radar, as well as the origin of patterns detected in the subsurface for the evolution of the northeast Sahara. In the Bir Kiseiba region of Egypt, 100 km west of Abu Simbel, SIR-C data revealed a tributary network of channels that originates as a widely dispersed pattern in the depression, with an apparent paleoflow direction to the west. The network then becomes a single channel between the bounding scarp to the west and a relict, elevated surface to the east. Following geometric correction of the image data using ground control points, topographic surveys, pebble counts of surface lag, and numerous trenches to determine near surface stratigraphy, it is apparent that a combination of surface and subsurface layers are responsible for the drainage patterns seen in orbital data. Radar dark areas are a function of the near total absorption of the signal, while bright zones are combinations of surficial carbonate lenses, fluvial pebble lenses (point bars) in the near subsurface, and local caliche and iron-rich horizons in the near surface sediments. Using a ground penetrating radar at 400 and 900 Mhz, the dry sand allowed penetration commonly to 2-3 m to local caliche and iron rich horizons, and in the single channel north of Bir Kiseiba, up to 12 m of sediment was transparent to the 400 Mhz radar, allowing us to map the broad base of the channel in three cross-sections. This use of GPR has allowed us to uniquely specify the strength of returns seen in SIR-C images, and to predict subsurface stratigraphy in areas that have not yet been studied in detail. The combination of these two radar techniques has shown that the now hyperarid climate of this desert is not responsible for the present-day landforms, which are inherited from prior pluvial periods. On Mars, where the evidence for climate change is evident even at visual wavelengths, the use of SAR and GPR will help to constrain some of the critical issues of timing, sources and sinks of water.