Solar System Remote Sensing Symposium (2002) 4048.pdf

EVOLVING PERSPECTIVES ON SPACE WEATHERING OF ASTEROIDS. Clark R. Chapman, Southwest Research Institute, Suite 400, 1050 Walnut St., Boulder CO 80302.

Efforts to measure the colors of asteroids sensing techniques or interpretations. But aster- date back at least as far as the 1920s, when Bo- oid specialists (including myself) reached a brovnikoff’s spectroscopy actually yielded nearly unanimous consensus from the late 70s Vesta’s rotation period based on cyclical through the early 90s that it was not “scientific” changes in its reflectance spectrum. There were to adopt unknown space weathering processes to several efforts in the 1940s and 1950s to com- resolve the discrepancy. They turned instead to pare asteroid colors with laboratory measure- adopting equally unknown, very selective dy- ments of colors. They proved incon- namical processes (which had little basis in the clusive for several reasons, including (a) the published literature of asteroid physics concern- blandness of mineral spectra in the short- ing dynamical or collisional processes) to derive wavelength bands commonly used in astronomi- OCs from small, rare, generally unseen asteroids cal photometry and (b) poor preparation of often while excluding meteorite delivery from the terrestrially weathered . abundant S- type asteroids, which even domi- In the early 1970s, both laboratory measure- nated the Near- Asteroid (NEA) popula- ments of properly prepared meteorite samples tion. This attitude of asteroid specialists was and astronomical observations of visible and very frustrating for cosmochemists since the de- near-IR asteroid spectra had advanced and were gree of space weathering required to resolve the sufficient to address the fundamental questions conundrum was in fact modest compared with concerning asteroid mineralogy and relationship the extraordinary (if not specifically understood) to meteorites. Two basic approaches were em- space weathering effects that clearly were re- ployed: (a) attempts to assess asteroid mineral- sponsible for the dramatic differences between ogy from first principles, based on absorption telescopic reflectance spectra of the lunar sur- band centers for instance, and (b) spectral face compared with laboratory measurements of matching, in which it was simply assumed that if lunar samples. an asteroid spectrum resembled the spectrum of Once again, despite the in-your-face evidence a particular class of meteorite it could be in- for lunar space weathering, most specialists re- ferred that the two had similar mineralogy, even fused to accept any extrapolation to the asteroids if the specific spectral features were not under- until the actual physical processes responsible stood or were not unique. for lunar space weathering were identified. In Application of both approaches yielded a few the mid-90s, several types of asteroid observa- successes, particularly in associating Vesta with tions (e.g., Galileo studies of Gaspra and Ida and basaltic achondritic meteorites, but they gener- spectral studies of small NEAs) demonstrated ally revealed an unexpected disparity: despite that space weathering processes (of some un- sharing some spectral similarities, the most known sort) must be occurring on asteroids. Si- common asteroid type (not corrected for obser- multaneously, laboratory studies finally identi- vational selection) could not be reconciled with fied the predominant causes for lunar space the most common meteorite type (again, not cor- weathering as well as showed that micrometeor- rected for atmospheric filtering processes). Thus ite “zapping” of asteroidal minerals can modify emerged the conundrum in which S-type aster- their spectra in the sense that S-types differ from oids apparently yielded few meteorites while OCs. Finally, in the last few years, advances in ordinary chondrites (OCs) had essentially no asteroid dynamics (primarily due to enhanced observable parent bodies in the . capabilities of numerical simulations) have Several prominent cosmochemists assumed demonstrated that the combination of minor that the S-types must be associated with OCs resonances and the Yarkovsky Effect necessarily and that the disparities must be due to unknown to general, representative sampling (rather technical issues associated with the remote- than highly selective sampling) from most of the Remote Sensing Symposium (2002) 4048.pdf

EVOLVING PERSPECTIVES ON SPACE WEATHERING OF ASTEROIDS. C. R. Chapman

inner half of the main asteroid belt. So there can cized cosmochemist Ed Anders for not taking no longer be any doubt about the association of dynamics seriously when Anders argued that the OCs with many S-type asteroids and the reality meteorites must somehow come from the aster- of space weathering processes (probably associ- oid belt even if the dynamicists had not yet fig- ated with the solar wind and micrometeorite ured out how. bombardment) as the dominant cause of the dis- In both cases, the respective specialists in- crepancies. sisted that it wasn’t “scientific” to rely on un- In retrospect, one wonders why it took more known processes until the specific processes than a quarter of a century for the early advo- were proven to operate. Even as recently as last cacy of space weathering (by Bruce Hapke, year I heard loud objections to the term “space among others) to gain wide acceptance. Some- weathering” because of the speaker’s belief that how, the demand for explicit physical “proof” of we do not yet know for sure, from first princi- specific space weathering processes (even as ples, exactly how space weathering works. Yet, NASA refused to fund the required research on in both cases, history has demonstrated that the space weathering) delayed for three decades the non-specialists who basically argued that any- acceptance of what we now realize is correct. thing was possible in the spheres of other fields Such attitudes can now be seen to have been so long as it wasn’t specifically disproven turned intellectually unsupportable: the asteroid spec- out to be right. I would not extrapolate these troscopists generally refused to take seriously cases to every scientific dispute and suggest that the evidence from fields outside their discipline, hand-waving generalists or non-specialists who specifically physicists, whose evolving under- ignore conservative specialists outside their standing of collisional and dynamical processes fields of expertise will always prove to be right. never supported the extreme selectivities on But I think there are lessons to be learned from which the asteroid remote-sensing community these case studies in asteroid science. Sometimes relied. intuitively plausible hypotheses may prove to be Asteroid science was similarly led astray for true — provided they don’t violate fundamental a few years (in the late 60s and early 70s), by physical laws — even if we haven’t yet figured analogous attitudes on the part of dynamicists, out specifically how they could be true. We still such as George Wetherill, who refused to be- operate in considerable ignorance and have lieve that any meteorites could be derived from much to learn, as I think Beth Clark’s following the asteroid belt because — in that pre-chaos talk will exemplify. I expect that studies of as- epoch — no specific dynamical processes had teroid space weathering will still hold some sur- yet been identified that could bring asteroidal prises for us. samples into Earth-crossing orbits. They criti-