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R.A. Baragiola, M.J. Loeffler, C.A. Dukes, W.Y. Chang, J.M. Fitz-Gerald (University of Virginia), L.A. McFadden (University of Maryland)
We study in the laboratory the effects of ion and micrometeorite bombardment on mineral surfaces of airless bodies such as asteroids, the Moon, and Mercury. We use 4 keV He and 1 keV H ions to simulate the solar wind and nanosecond laser pulses to simulate micrometeorite impacts.
Related to the question of the origin of the Na exospheres around the Moon and Mercury, we studied sputtering of Na from plagioclase feldspars and from Na deposited on olivine and on albite. Chemical characterization of the samples is made by X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS), secondary ion mass spectrometry (SIMS), and neutral mass spectrometry. We find that ion bombardment depletes Na from the surface of the feldspars but that the Na segregates back to the surface in a few hours at room temperature. Analysis of the removal of sodium adsorbed on the surface of olivine by He ion irradiation gives a sputtering cross section in line with what is expected for knock-on sputtering, and five orders of magnitude larger than for photo-stimulated desorption. We also determined that the energy distribution of the sputtered ions has a peak at a few eV and a long tail to about 50 eV.
Regarding the interpretation of NEAR data from Eros we irradiated powders of olivine with nanosecond 248 nm laser pulses and studied reflectance changes in the visible and near-infrared and chemical changes with XPS. We have collected the ejecta on a quartz-crystal microbalance to quantify the amount of ejecta per pulse. The deposit was found to be crystalline olivine. When collected on a fine-grained olivine surface, it produced visible darkening. We will also report on experiments that serve to explain sulfur depletion at Eros by the solar wind and micrometeorite bombardment.
Supported by NASA Cosmochemistry and NEAR Data Analysis Programs.
The author(s) of this abstract have provided an email address for comments about the abstract: raul@virginia.edu
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.