“Space Weathering” Defined:

1. This poster is based on an invited review article for Annual Reviews of Earth & Planetary Science. It is currently undergoing review, so criticisms and suggestions are still welcome! I have some preprints with me, but you can download the draft from: www.boulder.swri.edu/clark/annrevsf.doc Figs. 1-8: …/clark/ARFIGX.JPG (where X=1…8) Take-away Conclusions: We now know that space weathering of asteroids MUST occur; indeed it is observed to occur, on timescales of millions of years. But puzzling differences between Eros and Ida remind us that we have a lot to learn about the specific physical processes of space weathering and how they interact with regolith processes. “Space Weathering” Defined:  the observed phenomena caused by those processes (known and 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, sub-surface bulk of the body. The space weathering debates did not occur in an intellectual vacuum confined to remote-sensing specialists alone. There was similar progress, and occasional stasis, in understanding other issues -- especially small-body orbital dynamics and collisional processes -- which also affect how meteorites (particularly ordinary chondrites, OCs) might be derived from main-belt asteroids (particularly S-types). The question to keep in mind is this: how much should specialists in one field rely on the current paradigms of another field? As Alfvén remarked in his reply to Anders (1971), "It is obviously unreasonable to write one evolutionary history [of the solar system] for physicists and another, completely different, for chemists." But accepted hypotheses in one specialty are, of course, incompletely developed, and may turn out to be actually wrong. From the history of the space weathering debates, we can learn about the dangers of either blindly accepting, or blindly rejecting, the insights from different research specialties that bear on our problem. And we must turn a self-critical eye to the uncertainties in our own specialty, since our assertions can similarly affect researchers in other disciplines who wish to take into account our scientific results. CONCLUSIONS… Space weathering of asteroids is a concept that has evolved over the decades in ways that may say more about the sociology of science than about physics. As I show in my review, the study of asteroid compositions has taken place within the context of many other relevant developments in fields as distinct as lunar sample studies and solar system orbital dynamics. Sometimes the conclusions of investigators in different specialties have been inappropriately ignored; occasionally their results should have been ignored, in retrospect, since they turned out to be wrong. Relevant aspects of the physics of impacts (by micrometeoroids and solar wind) into mineral grains were surely being studied in the 1960s and 1970s and, together with laboratory studies of then-recently-returned lunar samples, should have been recognized as potentially relevant to the rather modest spectral differences between OC meteorites and S-type asteroids (modest at least compared with the huge differences between Moon rocks and lunar regolith soils). But several iconic "facts" (e.g. vitrification/agglutinization as the cause of lunar space weathering or the apparent spectral purity of Vesta) stood in the way, and even as late as the mid-90s, strongly negative attitudes toward asteroidal space weathering inhibited the funding (at least in the United States) of laboratory simulations and other research concerning plausible space weathering processes. When such studies were finally undertaken, primarily in Russia and Japan, they demonstrated that the optical properties of asteroidal minerals are necessarily changed by the space environment impinging on asteroid surfaces. Indeed it wasn't until the 1990s when much improved telescopic techniques plus close-up studies of two asteroids by the Galileo spacecraft essentially proved that space weathering was occurring on some asteroids. Recent studies of the colors of a very youthful subset of asteroids (Karin cluster) within the Koronis family (Nesvorný et al. 2003) reinforce these conclusions. Yet even after the dedicated, close-up investigation by NEAR-Shoemaker of Eros, an archetype of a potentially space weathered OC, serious questions remain about the suite of processes that comprise space weathering (and the processes against which space weathering competes, like regolith turnover), so that we cannot fully explain important differences between Eros and Ida. Thus we remain chastened in our expectations of the robustness of remote sensing techniques applied to unreachable, or rarely reachable, objects in space. Remote sensing applied to terrestrial problems can frequently be checked against ground truth. But when we rely on theory, imperfectly relevant laboratory simulations, and indirect inference to determine the compositions of solid-surfaced solar system bodies, we must be wary that we could well be led astray. For even though with hindsight we can see that some prescient hypotheses were offered decades ago that are relevant to asteroidal space weathering, the stop-and-start interdisciplinary research that has evolved since then has only very slowly directed us closer to the truth about these processes. And we have much more to learn about them. EROS AND IDA… The very different photometric and colorimetric styles of these two bodies [both S(IV)s] is puzzling. The consensus of NEAR-Shoemaker investigators (from MSI, NIS, GRS & XRS) is that Eros is an ordinary chondrite (OC); the relative absence of sulfur is, in fact, attributed to space weathering of this volatile substance. It is plausible that Koronis family members, like Ida, are OC’s too. Yet on Ida, darkening apparently precedes reddening (since the relatively un-weathered units are only marginally brighter than the rest of Ida), but for Eros, the darkening happens after the reddening (since both the dark and bright patches within Psyche, above, show reddened, typically S-type colors). The differences in the space environments of the two bodies may be the explanation. Three things have differed for as long as Eros has been removed from the belt (tens of Myr): (a) solar wind flux, much reduced in the belt; (b) micrometeoroid impact rates (surely velocities are higher for Eros, but relative rates are poorly known); and (c) rates for large impacts that churn and cover regolith (2 orders of magnitude higher in the belt, with a stronger carbonaceous component). There are two other clues concerning Eros: First, even the smallest, freshest blocks are reddened, perhaps implicating electrostatic redistribution of dust. Second, the down-slope movement within Psyche must be triggered by something other than the infrequent jostlings by rare impacts. • Post-Apollo discovery of lunar space weathering (s-w); McCord et al.: Vesta = HED pb • Anders expresses belief that remote-sensing will identify asteroidal parent bodies (pb) for meteroites, as Wetherill says dynamics requires that meteorites come from comets • C,S,M… taxonomy of asteroid colors; meteorite spectral matching with asteroids • Hapke et al. identify vaporization deposits of nanophase metal as cause of lunar s-w, but idea is overlooked as funding for lunar research declines (vitrification holds sway) • Arguments (in context of Vesta and vitrification) are that asteroid s-w cannot occur: spectra must be interpreted literally, so S-types cannot be ordinary chondrites (OCs) • Wisdom shows that chaos near resonances delivers meteorites; 1st Q-type (OC) found • Keller & McKay demonstrate that nanophase metal in vapor deposits causes lunar s-w • Galileo data on Gaspra and especially Ida demonstrate s-w is occurring on those S-types; Binzel et al. show spectral trends for NEAs consistent with s-w of Q- to S-types • Laboratory laser simulation of asteroidal s-w by micrometeoroid impact (OC  S-type) • Dynamics of Yarkovsky Effect show that meteorites come from throughout inner belt • Hapke reviews modern consensus on s-w, as NEAR-Shoemaker’s studies of Eros demonstrate that s-w is occurring, but in ways different from what was seen on Ida Space Weathering of Asteroids: Issues Remain Poster Presentation 34.06, Friday afternoon, 5 September 2003, 35th DPS Meeting, Monterey, California USA Clark R. Chapman Southwest Research Inst., 1050 Walnut St., Boulder CO 80302 cchapman@boulder.swri.edu Spectral reflectance of lunar soil and regolith breccia vs. lunar lithic fragments. The mature soil shows only the weakest hints of the deep absorption features in the spectrum of the fragmental breccia (from B.E. Clark et al., “Asteroids III). Ida shows large color variations (enhanced in false-color image above) but minimal albedo differences. Eros (as in the crater Psyche, right) shows prominent albedo variations on steep slopes, but shows only the most minimal color differences. Nanophase iron deposits on olilvine grains from a laser simulation of micrometeoroid impact (from Kurahashi et al., 2002). Similar laser experiments have changed the spectra of asteroidal silicates to look like S-types. Asteroidal Space Weathering: A Brief Intellectual History Timeline 1970 1971 1973 1975 1977 1985 1993 1995 1999 2000 2001 Rates and Time Scales for Space Weathering Hapke (2001) calculates that solar wind should theoretically space-weather asteroids in just 50,000 years, ignoring regolith processes. The tightest observational constraint is from Nesvorny et al. (2003) who show, from analysis of Sloan DSS colors of members of the Karin cluster (part of the Koronis family) that those asteroids are only partly space weathered (like the less space-weathered portions of Ida, shown in figure to left). Since the Karin cluster formed precisely 5.8 Myr ago, that shows that space weathering time scales in the main asteroid belt are millions of years. Space weathering trends on Ida: youthful terrains are intermediate between OCs (bottom 2 curves) and typical S (top curve); similar spread shown in inset. A continuum of spectra range from S- types down to Q-types, among NEAs studied by Binzel et al. (More asteroids were observed at shorter wavelengths.)