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C. M. Pieters (Brown University)
If it were not for the return of lunar soil, we would still not understand why the spectrum of lunar soil is so unusual. We observe the intricacies of particles in the laboratory, but have never been able to duplicate the effects of weathering processes accumulated over time. We are in a directly parallel situation with our current understanding of Martian soil from spectroscopic techniques. We know it contains a ferric component and we know something of its elemental composition; we know it is very fine grained; we know it is the cumulative weathering product of Martian lithologies, some of which are known from meteorites. A summary of several relevant lessons from studying lunar soils include: 1) Physical and chemical processes fractionate the soils with respect to local rocks. 2) Meteoritic contamination (largely from the constant rain of micrometeorites) cumulates in the soils. Lunar estimates are about 2-3 percent. 3) The fine fraction dominate observed optical properties, regardless of the presence of larger particles. Individual particles may accumulate coatings and rinds. 4) The spectral characteristics of the weathering products of iron dominate soil spectra. On the Moon it is highly reduced iron (typically 10's of nm in scale); on Mars it is highly oxidized (nano-phase?) iron. Modeling this 'gunk' or 'crud' is illusive. 5) Although weathering products dominate most spectra, signatures of the mineralogy of regional terrain can nevertheless be detected as subtle superimposed features. 6) Small-scale outcrops where soil has not been able to form or accumulate are critical markers of local lithology diversity. In planning exploration strategies using remote detectors, this latter lesson is particularly important and underlines the need for high resolution.