A Diagnostic Spectral Indicator of the Exposure Age of an Asteroidal Surface

Extract

The energy of the SiH vibrational fundamental has been shown be extremely sensitive to the oxidation state of the silicon to which the hydrogen is bound, ranging from 4.4 microns in highly oxidized silicate grains to 4.74 microns in silicon carbide (Moore, Tanabé and Nuth, Ap. J. (Lett.) 373, L31-L34, 1991). Yin, Ghose and Adler (Appl. Spectrosc. 26, 355-7, 1972) have shown that the process of ion-sputtering a metal oxide results in chemical reduction, due to the high sputtering yield and volatility of oxygen relative to the metal. These authors have shown that solar wind ion-sputtering of lunar soils could provide an explanation for the observed solar wind darkening of the lunar surface. We hypothesize that a similar ion-reduction process could occur on asteroidal surfaces exposed to the solar wind. A second consequence of solar wind exposure would be the implantation of hydrogen ions into the asteroidal surface. Stein (J. Elec. Mat. 4, 159-174, 1975) has shown that hydrogen-ions implanted into silicon exhibit infrared absorptions between 4.5 and 5.5 microns: similar features should result from the ion-implantation of hydrogen ions into partially reduced silicate minerals and glasses. The position of such features would be indicative of the local chemical environment of the silicon atom to which the hydrogen was bound.