Proton-irradiation darkening of rock powders: Contamination and temperature effects, and applications to solar-wind darkening of the Moon

Abstract

Darkening of silicate rock powders by bombardment with 2‐ to 16‐kev protons was found to be slight unless proton flux or total proton incident‐power density is sufficient to produce sample surface temperature in excess of about 150°C. Darkening then increases with incident power density for a given proton dose; i.e., the darkening is rate dependent as well as dose dependent. The darkening is due to contamination by (1) carbon from irradiation decomposition of hydrocarbon vacuum contaminants and/or (2) metal atoms deposited on the sample surface from sputtering of ion source components. The photometric properties of the contamination‐darkened samples are similar to those reported for samples darkened with hydrogen ions by Hapke and by Wehner et al.; this result suggests that contamination darkening may have caused the photometric modifications that they report; a review and comparison of experimental apparatus and procedures is presented. The experimental conditions under which silicate powders darken by proton or hydrogen‐ion bombardment are shown to be unrealistic for simulation of solar‐wind darkening of the moon's surface. It is suggested, however, that deposition of solar carbon on the moon may occur in sufficient amounts to produce darkening.

Surface modification of olivine by H+ and He+ bombardment

“…Surface darkening. Whether visible surface darkening, caused by ion bombardment on lunar fines, mineral powders, and glass beads, is a real or anomalous phenomena has long been debated [Rosenberg and Wehner, 1964;Hapke, 1966Hapke, , 1968Hapke, , 1973KenKnight et al, 1967;Nash, 1967;Lord, 1968;Dybwad, 1971;Yin et al, 1972;Hapke et al, 1975;Hapke and Cassidy, 1978;Pillinger and Jull, 1978;Benedix, 1993]. Experimenters who derive a null result have suggested that the darkening is not necessarily due to the irradiation but to poor vacuum, hydrocarbon contamination, or vitrification.…”

Laboratory simulations of redeposition of impact ejecta on mineral surfaces

“…Several low-energy ion irradiation experiments on minerals have been performed to simulate the solar wind impact on surfaces (Nash, 1967;Hapke, 1973;Dukes et al, 1999). Hapke first pointed out that different effects should occur for powder-like regoliths than for compact rock surfaces.…”

Space weathering of silicates simulated by successive laser irradiation: In situ reflectance measurements of Fo90, Fo99+, and SiO2

“…Laboratory studies have been generally successful in reproducing the space weathering characteristics seen in lunar and S‐type asteroid spectra by focusing on the changes caused by energetic ions and micrometeorite hypervelocity impacts. Using keV protons as a solar wind analog, early studies (Nash ; Hapke ) showed that irradiation caused the reflectance spectrum of the analog and returned lunar samples to evolve toward the in situ lunar soil spectra, becoming darker, redder, and having attenuated absorptions. Through more detailed studies (see Hapke for a review), Hapke was able to show that to create spectral changes consistent with space weathering, the sample needed to contain Fe.…”