BOX 3.1 Definitions of Martian Regolith, Soil, and Dust
For the purposes of this report, the committee developed operational definitions of the solid particulate materials on or near the Martian surface. The Martian regolith is the complex outer layer of loose rock and soil on the surface of Mars. Owing to repeated meteorite impacts and surface weathering processes, rock fragments of all sizes are mixed with weathered soils in various proportions to produce a regolith of unknown thickness. The composite physical properties of the regolith are likely to be different from the physical properties measured in Martian soils.
The term “soil” describes deposits of fine-grained, largely unconsolidated materials on the planet's surface. The planetary, or non-Earth, usage of the word “soil” differs from terrestrial usage, which specifies that soil must have an organic component. Martian soils may be mixtures of very small particles resulting from deposits of airborne dust and coarser, sand-size particles. The chemical composition of the soil has been measured at three spacecraft landing sites separated by thousands of kilometers, and the soil analysis results are similar. 1 The mineralogy of the Martian soil is not well understood at this time, but it is commonly inferred to resemble that of palagonite, a mixture of amorphous and poorly crystalline clays, iron oxides, and other products formed from the weathering of volcanic rocks. 2
The terms “dust” and “airborne dust” are used to identify fine particles suspended in the Martian atmosphere. The average grain diameter of airborne dust, as determined from multispectral imaging, is 3.4 microns. For the purposes of this report, dust is characterized as being less than 10 microns in diameter.3
The amount of dust in the atmosphere increases during seasonal dust storms, but even in quiet times there is enough suspended dust to impart a salmon color to the sky. There is a consensus that Martian dust has been globally homogenized by the wind, so that its composition is the same everywhere.4 Although its mineralogy is unknown, magnetic experiments have demonstrated that some dust particles contain magnetic iron oxides. Its red color indicates a high oxidation state.5
1Clark et al. (1982); Rieder et al. (1997).
2Bell et al. (2000).
3Tomasko et al. (1999).
4McSween and Keil (2000).
5Madsen at al. (1999).
a vehicle with very large inflatable tires capable of easily rolling over large rocks might be the appropriate vehicle in which to transport humans. Simply stated, understanding the shape and form of the terrain at the landing site on Mars is a critical requirement.
Finally, knowledge of the distribution of larger rocks is needed to plan traverse routes. The rock distribution will determine if rovers have to surmount objects, that is, roll over rocks that are small enough, or maneuver around them.
To ensure safe landing and operations on the surface of Mars, it is necessary for NASA to fully characterize the landing site and the topography of the anticipated surface operation zone with high-resolution stereoscopic imaging. The operation zone is the area around the landing site defined by the anticipated range of operations of EVAs, including the use of human transport and/or science rovers.
The level of resolution required of this imaging will be determined by the capabilities of the equipment to be used on the surface. Presentations to the committee illustrated human transport rover designs using wheels 1 meter in diameter. Vehicles using standard wheels can typically roll over objects one-third the diameter of the wheel being used. This suggests that if human transport and scientific rovers will use 1-meter wheels, the mission planners will need to know the distribution of rocks one-third of a meter and larger in the landing and operation zone. Imaging rocks this size requires a pixel resolution of 10 cm. The committee anticipates that the three-dimensional mapping would be conducted from Martian orbit.
Recommendation: NASA should map the three-dimensional terrain morphology of landing operation zones for human missions to characterize their features at sufficient resolution to assure safe landing and human and rover locomotion.