Progress in Theoretical Modeling of Space Weathering in Lunar and Asteroid Regoliths, B
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Lunar and Planetary Science XXXII (2001) 1132.pdf PROGRESS IN THEORETICAL MODELING OF SPACE WEATHERING IN LUNAR AND ASTEROID REGOLITHS, B. Hapke, Dept. of Geology & Planetary Science, University of Pittsburgh, Pittsburgh, PA 15260 ([email protected]). Introduction: The vapor-deposition model of The values that produced the best spectral match space weathering, first introduced by Hapke et al [1] (figure 1) are D = 10 µm and f = 0.025%. This value (see also [2, 3]), has become widely accepted [4] with of D is smaller than the lunar grain size, which is rea- the discovery [5] of ubiquitous vapor-deposited coat- sonable because micrometeorite impact velocities in ings containing submicroscopic metallic iron (SMFe) the asteroid belt are probably too small to produce sig- particles on grains of lunar soil. The SMFe-rich rims nificant melting and agglutinate formation. It should are deposits of vapor produced by both solar wind also be noted that the value of f required to change the sputtering and micrometeorite impact vaporization. meteorite spectrum into a semblance of an asteroid Because the solar wind bombards asteroid surfaces, the spectrum is only 1/20 that in the lunar regolith (f = spectra of these bodies must be affected by the same 0.5%). process. Hence, the relevant question is not whether Most importantly, it was also found that, while the space weathering occurs on asteroids, but rather, why addition of the SMFe decreases the albedo, reddens the the spectra of some bodies appear pristine, while others continuum and decreases the mineralogical band appear to be heavily altered. In order to understand depths, it does not appreciably affect the central and be able to remove the spectral effects of space wavelengths and widths of the bands. This provides weathering, it is important to be able to model them strong support for attempts to identify asteroid classes quantitatively. Earlier attempts at theoretical models as parent bodies of meteorites by band positions. In [2, 3, 6] made the simplest possible assumptions. This particular, it may be deduced that 26 Proserpina is paper reports the results of more sophisticated model- probably not the parent of Nanjemoy because the ing. bands are in slightly different positions, and the aster- To study the effects of several theoretical parame- oid bands are significantly wider than those of the me- ters, the spectrum of a typical ordinary chondrite, teorite.. Nanjemoy, was arbitrarily chosen from [7] and the References: [1] Hapke, B. et al (1975) The Moon, spectral effects of adding SMFe calculated. During the 32, 339-354. [2] Hapke, B. (2000) LPSC XXXI, abstr. modeling the parameters were varied in attempts to 1087. [3] Hapke, B. (2001), JGR-Planets, in press. match the spectrum of a similarly-chosen S asteroid, [4] Pieters, C. et al (2000) Met. Planet. Sci. 35, 1101- 26 Proserpina. The following parameters were varied: 1107. [5] Keller, L et al (2000) LPSC XXXI, abstr. asteroid regolith grain size, Fe grain size, mass fraction 1655. [6] Hapke, B. (1993) LPSC XXIV, 605-606. [7] of SMFe, regolith particle angular scattering function, Clark, B. et al (1992) Icarus 97, 288-297. and effects of adding internal scatterers to the soil grains. In addition, the earlier models assumed that the complex refractive index of the coatings could be de- scribed by a Maxwell-Garnett effective medium 1.5 model; instead, Mie theory was used to calculate the Model: D = 10 µm, f = 0.025% absorption by the SMFe. 1.4 Results: It was found that the spectral effects are 1.3 relatively insensitive to Fe grain size, particle scatter- 26 Proserpina ing anisotropy, presence of internal scatterers and 1.2 whether Mie theory or effective medium theories are 1.1 used. The only important parameters are D, the rego- 1 lith grain size, and f, the mass-fraction of SMFe. This normalized reflectance is important because it means that mathematically sim- 0.9 Nanjemoy ple analytic models can be used to rapidly process large numbers of spectra. Increasing D while holding f 0.8 0 0.5 1 1.5 2 2.5 3 constant decreases the albedo, increases the spectral wavelength ( µm) continuum slope and increases the mineralogical ab- sorption band depth. Increasing f while holding D Figure 1. Arbitrarily normalized reflectance spec- constant decreases the albedo, increases the slope and tra of an H6 ordinary chondrite, an S asteroid and the decreases band depth. best-fit model..