7 Apr 2004 21:34 AR AR211-EA32-18.tex AR211-EA32-18.sgm LaTeX2e(2002/01/18) P1: GCE10.1146/annurev.earth.32.101802.120453Annu. Rev. Earth Planet. Sci. 2004. 32:539–67doi: 10.1146/annurev.earth.32.101802.120453Copyrightc° 2004 by Annual Reviews. All rights reservedFirst published online as a Review in Advance on March 17, 2004SPACE WEATHERING OF ASTEROID SURFACESClark R. ChapmanSouthwest Research Institute, Suite 400, 1050 Walnut St., Boulder, Colorado 80302;email: [email protected] Words reflectance spectra, lunar surface, ordinary chondrites, Eros,S-type asteroids■ Abstract Visible and near-infrared spectra of reflected sunlight from asteroidsurfaces exhibit features that hold the promise for identifying surface mineralogy.However, the very surfaces that are observed by remote-sensing are also subject to im-pingement by micrometeoroids and solar wind particles, which are believed to play thedominant role in space weathering, which is the time-dependent modification of an as-teroid’s reflectance spectrum. Such space weathering has confused the interpretationsof telescopic spectra of asteroids, especially concerning the possible association ofcommon ordinary chondritic meteorites with so-called S-type asteroids. Recent space-craft studies of asteroids (especially of Eros by NEAR-Shoemaker) have documentedaspects of space weathering processes, but we still do not understand the physics ofspace weathering well enough to confidently assay mineralogy of diverse asteroids byremote-sensing. A review of the intellectual history of this topic reveals the complexityof interdisciplinary research on far-away astronomical bodies.INTRODUCTIONRemote sensing is an essential approach to learning about the nature of asteroids.Asteroids are very numerous (orbits of more than 220,000 have been determinedas 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 veryfewof them. Missions to date have relied on remote-sensing observations, whetherduring flyby, orbit, or even resting on the surface of the asteroid Eros (in the caseof NEAR-Shoemaker). A few missions involving in situ analysis or sample returnare in progress or being proposed. Yet the intensive studies of a few asteroidscan be extrapolated to the whole population only through reliance on Earth-basedtelescopic remote-sensing observations.Meteorites have been a prime resource for addressing early epochs of SolarSystem formation and history. However, the specific provenance of most mete-orites remains unknown. A very small percentage of meteorites come from theMoon and Mars, demonstrated by comparisons with returned lunar samples or insitu measurements on Mars, although the locations of origin on those bodies are0084-6597/04/0519-0539$14.00 539Annu. Rev. Earth Planet. Sci. 2004.32:539-567. Downloaded from arjournals.annualreviews.orgby STEWARD OBSERVATORY on 11/02/08. For personal use only.7 Apr 2004 21:34 AR AR211-EA32-18.tex AR211-EA32-18.sgm LaTeX2e(2002/01/18) P1: GCE540 CHAPMANunknown.The basaltic achondritic meteorites (HEDs = howardites, eucrites, dio-genites) are generally believed to have originated on Vesta (Keil 2002), but eventhis plausible association (based on interpretations of remote sensing) remains notproved in full. For all other meteorites, individual parent bodies remain unknown.It is possible, however, that remote sensing can help associate classes of mete-orites with certain classes or groups of asteroids; for instance, analysis of reflectedvisible and infrared (IR) sunlight from asteroid surfaces can provide clues aboutthe mineralogical composition of asteroid surfaces and permit comparisons withmeteorites. In the decades since fledgling attempts to do this in the 1940s, finalsuccess of such endeavors remains elusive.Processes generically called space weathering impede our ability to remotelyassess the mineralogy and other attributes of the surfaces of asteroids and otherairless bodies. There is the unfortunate conjunction between the immediate surfaceof a body that is remotely sensed and the surface that is exposed to space andhence is potentially subject to contamination or modification over long periodsof time. Such changes might involve accretion or erosion of particular materials,or modification of materials in situ by energetic impacts or irradiation. I definespace weathering as the observed phenomena caused by those processes (knownor unknown) operating at or near the surface of an airless Solar System bodythat modify the remotely sensed properties of the body’s surface from those ofthe unmodified, intrinsic, subsurface bulk of the body. Two primary questionsconcerning asteroids have been (a) is space weathering happening, and (b)ifso,what are the physical processes responsible and exactly how are they manifested?Space weathering processes were first recognized to be operating on the Moonin the early 1970s from comparisons of returned lunar samples with telescopicreflectance spectra (visible and near-IR) of the relevant Apollo landing sites; how-ever, a consensus about the specific physical processes responsible for lunar spaceweathering has been reached only during the past decade (Hapke 2001). It is ex-pected that similar processes must be operating on Mercury. A variety of spaceweathering processes affect the surfaces of Jupiter’s satellites, augmented by thatplanet’s strong magnetic field (Johnson et al. 2003). What has been less clear, andmuch more controversial, is the degree to which space weathering processes maybe operating on asteroid surfaces, and whether they have therefore confused ourinterpretations of asteroid compositions.The most salient issue associated with space weathering of asteroids concernstherelationshipbetweentheso-called S-type asteroidsand the ordinarychondrites.S-typeasteroids (which Idefine below)are the most common type of asteroid in theinner half of the asteroid belt, the zone from which we now expect most meteoritesare derived. Ordinary chondrites (OCs) are the most common meteorites in ourcollections. (Biases of various sorts affect the relative proportions of these kindsof asteroids and meteorites, but both are very common.) It was thus surprising thatstraightforward application of remote-sensing techniques in the 1970s found thatessentially no OCs had been measured to have the spectral reflectance character-istics of S-type asteroids (or almost any other type