M. Deschamps

Soil texture and granulometry at the surface of Mars

The physical behavior of the Martian surface soil has been characterized remotely by both photopolarimetry and radiometry. The degree of linear polarization defines a coefficient b which is related to the top surface soil texture and is calibrated in terms of grain size, or as a fraction of the area exhibiting uncovered clean rocks. This coefficient b was recorded with the instrument VPM (Visual Polarimeter Mars) on board Soviet orbiter MARS 5 in 1974. The radiometric thermal inertia coefficient I is essentially a measurement of the soil compaction, or an effective average particle size in the soil texture, through the few decimeters below the top surface sensed by polarimetry. The instrument IRTM (Infra Red Thermal Mapper) was used on board the Viking spacecraft between 1976 and 1982. The polarimetric scans raked a strip covering two contrasting regions, the dark-hued Mare Erythraeum and the light-hued Thaumasia. Over these wide areas, several smaller typical terrains 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 uniform polarization response, despite the large geomorphological diversity of the surface. The values of A and b indicate a ubiquitous coating or mantling with small dark grains of albedo 12.7%, with a radius of 10 to 20 μm. Thermal inertia coefficient I indicates that the sub-surface is divided in pieces around 300 to 600 μm in size. A simple model consisting of sand-size particles completely coated with 15 μm black grains is compatible with both measurements. Conversely, the brighter terrain Thaumasia discloses a large variety of soil properties. A typical location with albedo 16.3% has a surface covered with orange grains, probably very dispersed in size, for which the largest grains are 20 to 40 μm. The subsurface is divided into pieces 180–300 μm or smaller, if cemented. On the basis of terrestrial analogs of the Martian soil (Morris et al., 1990), it is surmised that the near-surface soil on the dark areas could be tachylite sand-size grains surficially coated by cohesive black particles of titanomagnetite. The bright orange grains in the Thaumasia-like terrains could be made of the weathered (palagonitized) basalt glass particles of sideromelane, as found in terrestrial analogs (Singer, 1982), Thaumasia is known to be a source area for dust storm production. The observed soil texture provides the large grains needed for saltation to occur, causing the intermixed small grains to be ejected from the surface and carried by wind.