SOIL TEXTURE AND GRANULOMETRY AT THE SURFACE OF MARS

The physical behavior of the Martian surface soil has been characteriz ed remotely by both photopolarimetry and radiometry. The degree of lin ear polarization defines a coefficient b which is related to the top s urface soil texture and is calibrated in terms of grain size, or as a fraction of the area exhibiting uncovered clean rocks, This coefficien t b was recorded with the instrument VPM (Visual Polarimeter Mars) on board Soviet orbiter MARS 5 in 1974. The radiometric thermal inertia c oefficient I is essentially a measurement of the soil compaction, or a n effective average particle size in the soil texture, through the few decimeters below the top surface sensed by polarimetry. The instrumen t IRTM (Infra Red Thermal Mapper) was used on board the Viking spacecr aft between 1976 and 1982. The polarimetric scans raked a strip coveri ng two contrasting regions, the dark-hued Mare Erythraeum and the ligh t-hued Thaumasia. Over these wide areas, several smaller typical terra ins were characterized by the three parameters A (albedo), b (related to top surface grain size) and I (underlaying compaction or block size ). The large dark region Erythraeum is characterized everywhere by a u niform polarization response, despite the large geomorphological diver sity of the surface. The values of A and b indicate a ubiquitous coati ng or mantling with small dark grains of albedo 12.7%, with a radius o f 10 to 20 mum. Thermal inertia coefficient I indicates that the sub-s urface is divided in pieces around 300 to 600 mum in size. A simple mo del consisting of sand-size particles completely coated with 15 mum bl ack grains is compatible with both measurements. Conversely, the brigh ter terrain Thaumasia discloses a large variety of soil properties. A typical location with albedo 16.3% has a surface covered with orange g rains, probably very dispersed in size, for which the largest grains a re 20 to 40 mum. The subsurface is divided into pieces 180-300 mum or smaller, if cemented. On the basis of terrestrial analogs of the Marti an soil (Morris et al., 1990), it is surmised that the near-surface so il on the dark areas could be tachylite sand-size grains surficially c oated by cohesive black particles of titanomagnetite. The bright orang e grains in the Thaumasia-like terrains could be made of the weathered (palagonitized) basalt glass particles of sideromelane, as found in t errestrial analogs (Singer, 1982). Thaumasia is known to be a source a rea for dust storm production. The observed soil texture provides the large grains needed for saltation to occur, causing the intermixed sma ll grains to be ejected from the surface and carried by wind.