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SPACE WEATHERING in the ASTEROID BELT. B. Hapke, Dept

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SPACE WEATHERING in the ASTEROID BELT. B. Hapke, Dept Lunar and Planetary Science XXXI 1087.pdf HOW TO TURN OC's INTO S's: SPACE WEATHERING IN THE ASTEROID BELT. B. Hapke, Dept. Of Geology and Planetary Science, Univ. of Pittsburgh, Pittsburgh, PA 15260, [email protected]. This paper demonstrates that the same belt impact velocities probably are too low for process that darkens and reddens lunar soil appreciable vapor production; however, solar also operates on asteroid regoliths and can wind sputtering is still effective, although at a cause the spectra of powdered ordinary rate less by an order of magnitude from the chondrites to closely resemble those of S moon because of the greater distance from the asteroids. sun. Space weathering on the moon is now well understood. The remarkable recent discovery of ubiquitous vapor-deposited coatings on lunar regolith particles by Keller and his colleagues [1], [2] has amply confirmed the space weathering model first suggested by the author and his colleagues 25 years ago [3]. (See also [4].) According to this model, the lunar optical and magnetic ESR properties result from submicroscopic metallic iron (SMFe) particles 5-10 nm in diameter in coatings produced by the deposition of ferrous silicate vapor. The SMFe is made by a physical reduction process, the selective partial loss of oxygen that occurs during deposition of the coatings, and does Fig. 1 demonstrates the spectral effects not require heating, vitrification, or a reducing of SMFe-bearing coatings in the lunar environment. This space weathering regolith. The figure shows the spectra of mechanism has been demonstrated by pulverized Apollo 11 rock 10017 and extensive laboratory experiments [3], [5]. On powdered glass made by vitrifying this rock in the moon the vapor is generated by two vacuum. Note that the glass is not dark nor processes: solar wind sputtering and red and has deep broad absorption bands, micrometeorite impacts, both contributing contrary to widespread erroneous belief based roughly equally. It occurs only in a porous on melting experiments that were carried out medium, such as a regolith, where the vapor is under inappropriately oxidizing conditions injected downward into the medium, rather [8]. The equations developed in [9] were used than escaping [6], but is not effective on a to calculate the result of covering the grains in solid surface. The SMFe-rich vapor deposits a mixture of rock and glass powder with darken, redden and decrease the depths of silicate coatings containing SMFe. The absorption bands of the regolith particles they complex refractive indices of Fe in [10] were coat. used. In Fig. 1 the model spectrum is On Mercury the magnetic field compared with lunar soil 10084. The bulk prevents the solar wind from reaching the mass fraction of SMFe in the model is 0.5%, a surface most of the time, so that impact value typical of lunar regolith. vaporization dominates [7]. In the asteroid Lunar and Planetary Science XXXI 1087.pdf HOW TO TURN OC's INTO S's: B. W. Hapke Although the overall spectrum of the coated OC is similar to to the S, the match is not perfect. Complete agreement is not expected, because 26 Proserpina probably is not the parent of Nanjemoy. In addition, however, asteroid absorption bands generally appear to be wider than those of meteorites. This can be understood if the asteroid regoliths contain abundant glass (compare the rock and glass spectra in Fig. 1) in addition to crystalline grains. This glass could be made by impact vitrification. If so, it would imply that at least part of the micrometeorite population impacting asteroid surfaces have higher velocities than is commonly believed. The result of a similar calculation on In conclusion, there appears to be no the spectrum [11] of a pulverized sample of good reason to believe that S asteroids are not Nanjemoy, a type H ordinary chondrite, is the parent bodies of the ordinary chondrites. shown in Fig. 2. In this figure the effect of References: [1] Keller, L. and adding 0.05% SMFe-bearing coatings to the McKay, D. (1993) Science, 261, 1305-1307. grains of Nanjemoy is compared with 26 [2] Wentworth, S. et al (1999) Met., Planet. Proserpina [11], an S asteroid. Again, note Sci., 34, 593-604. [3] Hapke, B. et al (1975) how the coatings darken, redden and subdue Moon, 13, 339-353. [4] Hapke, B. (1986) the absorption bands, so that the spectrum of Icarus, 66, 270-279. [5] Hapke, B. (1973) the meteorite resembles the asteroid's. Moon, 7, 342-355. [6] Hapke, B. and Laboratory experiments indicate that on the Cassidy, W. (1978) Geophys. Res. Let., 5, moon a time of the order of 100,000 years is 297-300. [7] Hapke, B. (1977) Phys. Earth, required to darken maifc rock powder by solar Planet. Interiors, 15, 264-274. [8] Adams, J. wind sputtering [5]. The ion flux is less by a and McCord, T. (1971) Proc. Lunar Sci. Conf. factor of 10 in the asteroid belt, but only 1/10 2nd, 2183-2196. [9] Hapke, B., (1993) as much SMFe is required to match the Theory of Reflectance and Emittance asteroid spectrum, so that the exposure time Spectroscopy, Cambridge Univ. Press, New required to alter an asteroid spectrum is about York. [10] Johnson, P. and Christy, R. 100,000 years also. (1974), Phys. Rev. B, 9 5056-5070. [11] Clark, B. et al (1992), Icarus, 97, 288-297..
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