Earth, Planets Space, Vol. 51 (No. 11), pp. 1255-1265, 1999
Maho Yamada1, Sho Sasaki1, Hiroko Nagahara1, Akira Fujiwara2, Sunao Hasegawa2, Hajime Yano2,
Takahiro Hiroi3, Hideo Ohashi4, and Hisashi Otake5
1Geological Institute, School of Science, University of Tokyo, Tokyo 113-0033, Japan
2Institute of Space and Astronautical Science, Sagamihara, Kanagawa 229-8510, Japan
3Department of Geological Sciences, Brown University, Providence, RI 02912, U.S.A.
4Laboratory for Physics, Tokyo University of Fisheries, Minato-ku, Tokyo 108-0075, Japan
5Tsukuba Space Center, National Space Development Agency of Japan, Ibaraki 305-8505, Japan
(Received December 1, 1998; Revised June 30, 1999; Accepted September 4, 1999)
For the purpose of simulating the surface alteration process called "space weathering", experiments of pulse laser irradiation, proton implantation, and laser irradiation to proton implanted samples were performed and reflectance spectra of altered materials were measured. To simulate the impact heating by micrometeorite bombardments, we made a new apparatus using a pulse laser whose pulse duration is 68 nanoseconds, comparable with a timescale of micrometeorite impacts. We find that the degree of space weathering, i.e., change of reflectance spectrum should depend on mineral composition. Laser irradiation onto olivine produces the largest reduction of albedo and the highest reddening of reflectance spectrum. In general, variation of olivine spectra is much larger than that of pyroxenes. Depths of absorption bands do not change in the scaled spectra. The olivine spectrum after the laser irradiation can match spectra of some olivine asteroids within a subtype of S-type asteroids. Comparison of Vesta spectrum with altered pyroxene spectra suggests that Vesta surface would be relatively older than olivine asteroids.
We also investigate the influence of solar wind proton and pyroxene FeO content. The proton implantation causes small changes in olivine and enstatite spectra. Implanted protons do not influence spectral change by the laser irradiation: the laser irradiation and the proton implantation do not produce multiplicative but additive changes on the reflectance spectra. FeO content of pyroxenes does not relate to the degree of reflectance change.
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