Space Weathering: an Ultraviolet Indicator

Introduction: We present evidence suggesting that the spectral slope of airless bodies in the UVvisible wavelength range can be used as an indicator of exposure to space weathering. While space weathering generally produces a reddening of spectra in the visible-NIR spectral regions, it tends to result in a bluing of the UV-visible portion of the spectrum, and may in some cases produce a spectral reversal. The bluing effect may be detectable with smaller amounts of weathering than are necessary to detect the longerwavelength weathering effects. Effects of Space Weathering in the Visible/IR: Space weathering, the bombardment of airless bodies by micrometeoroids and irradiation by solar wind particles, affects solar system bodies by darkening and reddening their surfaces, as well as degrading absorption features [1], as seen at visible and near-infrared wavelengths. These effects, particularly in the visible and near-IR, are well documented for the Moon (e.g., [2]), where powdered rock samples are spectrally different from spectra of the lunar soil. The precise effect of weathering may be the vapor deposition, through solar wind irradiation and micrometeorite bombardment, of submicroscopic iron (SMFe) [3]. In the asteroid belt, fewer studies of space weathering effects have been performed than for the Moon. Recently weathering has been proposed as the source of spectral differences between ordinary chondrite (OC) meteorites and their proposed parent bodies, Sclass asteroids [1][3]. The addition of nanophase Febearing coatings to the grains of an OC meteorite sample results in spectral features similar to those of an Stype asteroid [3]. Although the solar wind flux is lower in the asteroid belt than at the Moon, less nanophase Fe is required to produce weathering effects on asteroids. Furthermore, on the S-type asteroid Ida Galileo SSI camera data indicate that a number of small, relatively fresh craters are blue compared with the reddish rest of the surface, using the 0.4/0.56 μm ratio, and exhibit stronger 1 μm absorption bands [4][1], effects that are consistent with lunar-like space weathering. Effects of Space Weathering in the UV/Visible: Evidence indicates that effects of space weathering at UV-visible wavelengths include a bluing of the spectral reflectance and in some cases a spectral reversal. These effects are shown in analysis of lunar observations, laboratory spectra of lunar samples, UV data of Vesta, and laboratory space weathering experiments. The Moon and Lunar Samples. 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) [5]; the phenomenon was also seen more recently in EUVE images [6]. The lunar spectral reversal was linked to space weathering when it was found that lunar soils exhibit the spectral reversal, while powdered lunar rocks do not [7], as shown in Fig.1. The lunar spectral reversal can be explained as being due to the higher index of refraction of mare material relative to highlands material [8]. The index of refraction of many materials increases with decreasing wavelength, so that they become brighter at shorter wavelengths. This is important because at shorter wavelengths, surface scattering dominates over volume scattering so that reflectance is directly related to the index of refraction [8]. However, the correlation between visibly bright and UV-dark lunar regions is imperfect, and UV spectra may therefore contain further information than what is known from visible spectra [8]. In particular, since far-UV radiation is less penetrating than visible radiation, short wavelengths are more sensitive to thin coatings on grains that may be the result of weathering processes [5].