Figure 4.24 Dust devils moving across the floor of Gusev Crater have been imaged several times by Spirit's Navigational Camera. This dust devil occurred on sol 486 of Spirit's mission and is about 1km from the rover's location. (Image PIA07253, NASA/JPL.)
Adhesion of atmospherically deposited dust on the MPF and MER passive magnets indicates that the dust contains a magnetic component similar to the soils (Section 4.3.2). The global homogeneity of dust suggests a similar composition regardless of location. MPF and MER magnetic investigations suggest that airborne dust has a saturation magnetization of 2 to 4 A m2 (kg soil)-1 (Hviid et al, 1997; Bertelsen et al., 2004). These magnetizations are too high for maghemite particles, leading the investigators to propose magnetite as the dominant magnetic phase in the dust (Bertelsen et al., 2004), a composition consistent with Mossbauer results (Morris et al., 2006).
Atmospheric transport leads to electrostatic charging of the dust through collisions, particularly in dust storms and dust devils (Ferguson et al., 1999; Zhai et al., 2006). No discharges have been observed in martian dust devils (Krauss et al., 2006), but electrostatic charging may be responsible for the accumulation of dust on rover wheels. Ferguson et al. (1999) estimate that the wheels on MPF's Sojourner rover acquired a charging voltage of ~60-80 V from dust buildup during its traverses. Abrasion of Sojourner's wheels suggests that martian dust has a hardness of 4.3 on the Mohs' scale of hardness, similar to platinum (Ferguson et al., 1999).
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