7 Apr 2004 21 34 AR AR211 EA32 18 tex AR211 EA32 18 sgm LaTeX2e 2002 01 18 P1 GCE 10 1146 annurev earth 32 101802 120453 Annu Rev Earth Planet Sci 2004 32 539 67 doi 10 1146 annurev earth 32 101802 120453 c 2004 by Annual Reviews All rights reserved Copyright First published online as a Review in Advance on March 17 2004 SPACE WEATHERING OF ASTEROID SURFACES Clark R Chapman Annu Rev Earth Planet Sci 2004 32 539 567 Downloaded from arjournals annualreviews org by STEWARD OBSERVATORY on 11 02 08 For personal use only Southwest Research Institute Suite 400 1050 Walnut St Boulder Colorado 80302 email cchapman boulder swri edu Key Words reflectance spectra lunar surface ordinary chondrites Eros S type asteroids Abstract Visible and near infrared spectra of reflected sunlight from asteroid surfaces exhibit features that hold the promise for identifying surface mineralogy However the very surfaces that are observed by remote sensing are also subject to impingement by micrometeoroids and solar wind particles which are believed to play the dominant role in space weathering which is the time dependent modification of an asteroid s reflectance spectrum Such space weathering has confused the interpretations of telescopic spectra of asteroids especially concerning the possible association of common ordinary chondritic meteorites with so called S type asteroids Recent spacecraft studies of asteroids especially of Eros by NEAR Shoemaker have documented aspects of space weathering processes but we still do not understand the physics of space weathering well enough to confidently assay mineralogy of diverse asteroids by remote sensing A review of the intellectual history of this topic reveals the complexity of interdisciplinary research on far away astronomical bodies INTRODUCTION Remote sensing is an essential approach to learning about the nature of asteroids Asteroids are very numerous orbits of more than 220 000 have been determined as of 2003 and they are distributed throughout an enormous volume of space mainly between the orbits of Mars and Jupiter Spacecraft can visit only a very few of them Missions to date have relied on remote sensing observations whether during flyby orbit or even resting on the surface of the asteroid Eros in the case of NEAR Shoemaker A few missions involving in situ analysis or sample return are in progress or being proposed Yet the intensive studies of a few asteroids can be extrapolated to the whole population only through reliance on Earth based telescopic remote sensing observations Meteorites have been a prime resource for addressing early epochs of Solar System formation and history However the specific provenance of most meteorites remains unknown A very small percentage of meteorites come from the Moon and Mars demonstrated by comparisons with returned lunar samples or in situ measurements on Mars although the locations of origin on those bodies are 0084 6597 04 0519 0539 14 00 539 7 Apr 2004 21 34 Annu Rev Earth Planet Sci 2004 32 539 567 Downloaded from arjournals annualreviews org by STEWARD OBSERVATORY on 11 02 08 For personal use only 540 AR AR211 EA32 18 tex AR211 EA32 18 sgm LaTeX2e 2002 01 18 P1 GCE CHAPMAN unknown The basaltic achondritic meteorites HEDs howardites eucrites diogenites are generally believed to have originated on Vesta Keil 2002 but even this plausible association based on interpretations of remote sensing remains not proved in full For all other meteorites individual parent bodies remain unknown It is possible however that remote sensing can help associate classes of meteorites with certain classes or groups of asteroids for instance analysis of reflected visible and infrared IR sunlight from asteroid surfaces can provide clues about the mineralogical composition of asteroid surfaces and permit comparisons with meteorites In the decades since fledgling attempts to do this in the 1940s final success of such endeavors remains elusive Processes generically called space weathering impede our ability to remotely assess the mineralogy and other attributes of the surfaces of asteroids and other airless bodies There is the unfortunate conjunction between the immediate surface of a body that is remotely sensed and the surface that is exposed to space and hence is potentially subject to contamination or modification over long periods of time Such changes might involve accretion or erosion of particular materials or modification of materials in situ by energetic impacts or irradiation I define space weathering as the observed phenomena caused by those processes known or unknown operating at or near the surface of an airless Solar System body that modify the remotely sensed properties of the body s surface from those of the unmodified intrinsic subsurface bulk of the body Two primary questions concerning asteroids have been a is space weathering happening and b if so what are the physical processes responsible and exactly how are they manifested Space weathering processes were first recognized to be operating on the Moon in the early 1970s from comparisons of returned lunar samples with telescopic reflectance spectra visible and near IR of the relevant Apollo landing sites however a consensus about the specific physical processes responsible for lunar space weathering has been reached only during the past decade Hapke 2001 It is expected that similar processes must be operating on Mercury A variety of space weathering processes affect the surfaces of Jupiter s satellites augmented by that planet s strong magnetic field Johnson et al 2003 What has been less clear and much more controversial is the degree to which space weathering processes may be operating on asteroid surfaces and whether they have therefore confused our interpretations of asteroid compositions The most salient issue associated with space weathering of asteroids concerns the relationship between the so called S type asteroids and the ordinary chondrites S type asteroids which I define below are the most common type of asteroid in the inner half of the asteroid belt the zone from which we now expect most meteorites are derived Ordinary chondrites OCs are the most common meteorites in our collections Biases of various sorts affect the relative proportions of these kinds of asteroids and meteorites but both are very common It was thus surprising that straightforward application of remote sensing techniques in the 1970s found that essentially no OCs had been measured to have the