Regolith

The uppermost layer of the surface, composed of the fragments produced by weathering processes, is called the regolith. The terms "regolith" and "soil" are often used interchangeably in planetary applications, although terrestrial geologists argue that biologic activity is an important component of the composition and mixing processes of "soil." Fragment/clast size is expected to generally increase with depth. The martian regolith likely contains mixtures of soil and ice, particularly at the higher latitudes. A possible cross-section of the martian regolith is shown in Figure 4.20.

The color of the martian regolith ranges from bright red dust to darker red and gray material. The differences in color are primarily due to variations in iron mineralogy, degree of alteration, and particle sizes and shapes. The darkest landing site yet

Increasing depth

Crater ejecta Volcanic lava flows Weathering products Sedimentary deposits

Crater ejecta Volcanic lava flows Weathering products Sedimentary deposits

Figure 4.20 Idealized cross-section of the martian crust. The upper level is the regolith, composed of fragmented material from ejecta, volcanic flows, sedimentary deposits, and weathering products. The middle layer is basement, fractured by geologic processes such as impact cratering. At some depth, the overlying pressure is great enough to cause self-compaction of the material. (After Clifford, 1993.)

investigated is Meridiani Planum (albedo ~0.12), which is largely covered by finegrained sand (<150 ^m) of Fe-rich basaltic composition and hematite spherules (Figure 4.21). Meridiani Planum is less dusty than the other landing sites, probably because of eolian deflation which leaves a dark cohesionless lag deposit on the surface (Soderblom et al., 2004). This dark surface layer is underlain by higher-albedo soils in many locations, as revealed by trenches excavated by the rover's wheels (Figure 4.15). The crests of small dunes and drifts encountered by Opportunity across the Meridiani plains are armored by millimeter-sized rounded granules.

In contrast, the regolith at the Spirit landing site in Gusev Crater generally consists of five components (Greeley et al., 2006). The topmost layer is a thin deposit (< 1 mm) of dust which has settled out of the atmosphere. This is underlain by a lag deposit of coarse sand and granules, under which is a layer of subangular fragments larger than a few millimeters in size. A cohesive crust ("duricrust") several millimeters thick is next, with a layer of dark soil forming the bottom of the regolith. Gusev Crater regolith displays many of the same general characteristics as regolith at the MPF (Moore et al., 1999) and Viking landing sites (Moore and Jakosky, 1989), although the VL1 and MPF sites display more fine-grained drifts than either VL2 or Gusev. Martian soil-like deposits are similar to moderately dense soils on Earth. Table 4.3 lists some of the mechanical properties of soils at the three rover sites.

All five landers have carried experiments to investigate the magnetic properties of the martian regolith. The Viking landers carried two magnets that were attached to the sampling arm so they could be directly immersed into the soil. The magnets had magnetic field strengths of 0.25 tesla (T) and 0.07 T. A third magnet with a field

Table 4.3 Soil properties at the three rover sites

MPFa Spiritb Opportunityc

Friction angle Soil bearing strength Cohesion Angle of repose Soil bulk density Grind energy density a Moore et al. (1999). b Arvidson et al. (2004a). c Arvidson et al. (2004b).

Figure 4.21 Opportunity has encountered several dune and drift fields during its traverse. This navigation camera image shows dark drifts deposited by wind inside Erebus crater. (NASA/JPL-Caltech/Cornell.)

strength of 0.25 T, attached to the lander, was passively exposed to atmospheric dust. All of the magnets attracted magnetic particles - the sampler arm magnets were essentially saturated with magnetic particles after immersion in the soil (Hargraves et al., 1977, 1979). The Viking analysis suggested that the particles, probably a

2 _i ferromagnetic oxide, had magnetizations in the range 1-7 A m (kg soil) .

MPF carried ten magnets distributed between two arrays located on the lander. The magnetic field strengths of these magnets ranged from 0.011 to 0.280 T (Madsen et al., 1999). These magnets were passive collectors of airborne dust and the results of this experiment were similar to those for the dust at the Viking landers (Section 4.3.3). The Viking and MPF results indicated that the soil and dust on Mars must contain about 2% by weight of a ferrimagnetic mineral, suggested to be either

5-200 kPa -1-15 kPa up to 65° 1200-1500kg m 11-166 Jmm-3

maghemite (gamma-Fe2O3) or magnetite (Fe3O4) with maghemite being the preferred component.

Spirit and Opportunity each carry seven magnets, four of which are integrated into the RAT, two on the front of the rover near the Pancam mast, and one on the solar panels (Bertelsen et al., 2004). Magnetic particles in the soil and rocks are analyzed using the three magnets on the RAT, which have magnetic field strengths of 0.28, 0.10, and 0.07 T. The capture (0.46 T) and filter (0.2 T) magnets near Pancam and the sweep magnet (0.42 T) on the solar panels attract magnetized dust particles. Combined magnetic and Mossbauer spectroscopic analysis suggests that magnetite is the most probable magnetic carrier rather than the maghemite which was suggested from the Viking analysis (Bertelsen et al., 2004).

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  • sophie
    Why is soil in Mars referred to as regolith?
    8 years ago

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