Optical Effects of Regolith Processes on S-Asteroids as Simulated by Laser Shots on Ordinary Chondrite and Other Mafic Materials

Abstract

up ȁ80% of observed meteorite falls, but so far very few asteroid spectral analogs have been identified for these meLaboratory results from a simulation of possible optical effects of impact melting and repeated crystallization on asteroi-teorites. dal surfaces are presented. Quick melting and crystallizationThe S-asteroids are widespread in the inner belt and of surface materials were simulated by impulse laser treatment appear to consist mostly of olivine, orthopyroxene, and of powdered materials in vacuum. The mafic materials used metal. They resemble OCs spectrally but are characterized in this study were: ordinary chondrite (OC) Elenovka L5, car-by steep red continua unlike those of the OCs, and their bonaceous chondrite Allende CV3, terrestrial olivine, clinopy-spectrally derived mineralogies are far outside the OC roxene, and olivine-clinopyroxene mixture 1 : 1. The reflectance range (Gaffey et al. 1993a). Moreover, the character of the spectra of powdered samples before and after laser treatment rotational spectral variations of surfaces of two large were recorded in the range 0.3-25 m. The laboratory laser S-asteroids, 8 Flora and 15 Eunomia, appear to be inconsisalteration produced a reduction in overall reflectance and spectent with the primitive undifferentiated OC composition tral contrast, a noticeable increase in spectral slope, the shifts (Gaffey 1984, Gaffey andOstro 1987). Consequently, the in the wavelength positions of absorption band centers, and a majority of investigators are inclined to interpret S-type decrease in band area ratio. The continuum slope of the ordinary chondrite Elenovka after laser treatment is comparable asteroids as composed of various differentiated assemto that of S-type asteroids. Laser impulse alteration raises the blages which are the products of melting in the deep interispectrally derived olivine/orthopyroxene ratio of Elenovka ors of asteroid parent bodies. These materials are similar from 2.8 to 5.4. This value is far outside the OC range, but to stony-iron meteorites and some achondrites (Gaffey falls within the olivine/orthopyroxene range for S-asteroids. 1993a, Hiroi et al. 1993a).The results suggest that quick melting and subsequent crystalliThus, the source of ordinary chondrites remains unclear. zation can enhance the spectral similarity between S-asteroids On the basis of some spectral resemblance between the and ordinary chondrites.

Evidence for the effects of space weathering spectral signatures on low albedo asteroids

“…In particular, a study by Moroz et al (1996) showed that weak irradiation of a carbonaceous chondrite led to a blueing of the spectrum. With higher irradiation doses, they found a reddening.…”

“…In fact, low dose laser irradiation of the carbonaceous meteorite Allende initially results in blueing. However, after an increased dose of laser irradiation, Moroz et al (1996) found that the spectrum reddens. Studying color variations between young and old asteroids families, Nesvorny et al (2005) found that aging surfaces of S-type become increasingly redder and that aging surfaces of C-type asteroids become increasingly bluer.…”

“…The effects of space weathering on S-type asteroids have been widely analyzed (Moroz et al 1996;Clark et al 2001;Brunetto et al 2005;Ishiguro et al 2007). Because S-types have high albedos and strong absorption band features, space weathering effects are easier to detect.…”

Surface Composition of Vesta: Issues and Integrated Approach

“…Meteorite regolith breccias, as proxies for asteroidal regolith, were searched for similar nanophase particles, with the howardite Kapoeta being one of the few minor, but successful detections (Noble et al 2010). Laboratory experiments have various degrees of success in simulating these particular space weathering products along with their optical effects (e.g., Cassidy and Hapke 1975;Moroz et al 1996;Yamada et al 1999;. A model describing the optical effects of nanophase metallic iron was successfully developed by Hapke (2001).…”

Space weathering trends among carbonaceous asteroids

“…These spectral changes are attributed to the vapor deposition of sub-micron metallic iron (SMFe) particles onto grains during micrometeorite impacts and solar wind irradiation (Yamada et al 1999;Hapke 2001;Sasaki et al 2001;Brunetto & Strazzulla 2005). The intrinsically dark (p v ∼0.06; Mainzer et al 2011) nature of carbonaceous material and the lack of prominent absorption features at visible and near-IR wavelengths implied space weathering trends would be difficult to identify for C-complex asteroids (Hapke 2001;Moroz et al 1996). However, two recent studies indicate significant spectral slope variation among this class of asteroids as a function of age (Nesvorný et al 2005;Lazzarin et al 2006).…”