SPACE WEATHERING ON ASTEROIDS: NEW RESULTS FROM THE ULTRAVIOLET. F. Vilas and A

Introduction: We present evidence that space weathering manifests itself at near-ultraviolet wavelengths as a bluing of the spectrum, in contrast with the spectral reddening that has been seen at visible-near-IR wavelengths. This UV bluing often results in a spectral reversal, where surfaces that are bright at visible-near-IR wavelengths are relatively dark at UV wavelengths, and vice-versa. Furthermore, the effects of space weathering at UV wavelengths tend to appear with less weathering than do the longer-wavelength effects, suggesting that the UV wavelength range is a more sensitive indicator of weathering, and thus age. We report result from analysis of existing near-ultraviolet measurements of S-type asteroids from IUE and HST to support this hypothesis, and compare with laboratory measurements of meteorites. Space weathering is also an important process in the asteroid belt, and is the hypothesized cause of spectral differences between ordinary chondrite (OC) meteorites and their proposed parent bodies, S-class asteroids [1,3]. The visible/near IR spectral characteristics of weathering of asteroids could be similar to those seen on the Moon (i.e., darkening, red spectral slope, decreased strength of near-IR absorption features). The addition of SMFe-bearing coatings to the grains of an OC meteorite sample results in spectral reflectance features similar to those of an S-class asteroid [3]. Space Weathering: Space weathering, the bombardment of airless bodies by micrometeoroids and irradiation by solar wind particles, affects spectra of solar system bodies by darkening and 'reddening' (where the spectral reflectance increases with wavelength) their surface materials, as well as degrading absorption features (1) at visible/near IR wavelengths. These effects are well documented for the Moon (e.g., 2), where they are apparent in spectra of natural lunar soils, but not seen in spectra of powdered lunar rock samples. The cause of these weathering effects is likely vapor deposition of submicroscopic iron (SMFe) (3), through solar wind irradiation and micrometeorite bombardment of the bodies' surfaces. Effects of Weathering in the UV: Apollo 17 UVS measurements first displayed the lunar spectral reversal, where it was noted that the visibly dark lunar maria are 5-10% brighter than the highlands at far-UV wavelengths (147 nm) [4]; the phenomenon