Clark R. Chapman Southwest Research Inst. Boulder, Colorado

1. Evolving Perspectives on Space Weathering of Asteroids Clark R. Chapman Southwest Research Inst. Boulder, Colorado Solar System Remote Sensing Symposium Honoring Bruce Hapke Univ. of Pittsburgh, 21 Sept. 2002

2. The Prime Question: • Given early (post-Apollo) demonstration that the lunar surface is space weathered… • Why has it taken so long for it to become accepted that asteroid surfaces are space weathered? • Indeed, is it even yet accepted? (One of Bruce Hapke’s early accomplishments)

3. 50 Years of Early Physical Studies of Asteroids: 1929-1978 • UBV colors (reviewed by Hapke, 1971), lightcurves, and Bobrovnikoff’s spectra... • Then, spectral and radiometric studies proliferated

4. Asteroid Remote-Sensing… Trying to Deduce Mineralogy This has been the game, from the 1950s until now... It’s as if a geologist had to pick up stones in a stream…and try to associate them with colorful layers in the distant mountains An astronomer measures asteroid spectra; a cosmo-chemist studies meteorites that have fallen…they try to determine the associations How successful has this endeavor been?

5. Early progress: 1960s, 1970s • First good UBV photometry, but spectral resolution inadequate to reveal minerals • Comparison of asteroid spectrophotometry/ radiometry with lab spectra of meteorites • “Matching” of spectral traits reveals (dis)similarities • Quantitative, physics-based mineralogical assay • Apollo lunar rocks found to differ dramatically from telescopic spectral reflectance data, implicating a “space weathering” process • Early debates about what exact lunar processes acted • Early discussion of why asteroids might or might not be affected by lunar-like space weathering

6. Remote sensing sees surfaces ... Strange South American fruit: can you guess what its insides taste like by looking at its outside? • A basic philosophical point is that we see only optical surfaces… • And surfaces are most susceptible to being affected by exogenous processes (cf. Hapke’s 1960s pre-Apollo simulations of the solar wind) Given our knowledge (by 1970) of bombardment of airless bodies by micrometeorites and the solar wind, of lunar regolith processes, and of gas-rich meteorites… we should always have been skeptical, in asteroid remote sensing, that what we see represents the bulk make-up of the body.

7. Vesta: Proposed to be HED P.B. in 1970…Still the Accepted Model • The first and greatest success has endured…but is it true? • Some “Vestoids” are in strange orbits; were there other Vesta-like bodies? • We really don’t know anything about Vesta’s interior (except gross bulk density)

8. In retrospect, Vesta gave us a false sense of confidence... • Vesta is the brightest asteroid, with high albedo, and the deepest absorption bands… every other asteroid is more difficult to observe and interpret • Like the lunar highlands (compared with the maria) achondritic Vesta is depleted in mafic materials that are most subject to optical modification by space weathering In the subsequent three decades, no other asteroid has been as reliably associated with a meteorite type as was Vesta (despite some claims to the contrary)

9. Does Asteroid Mineralogy Vary with Solar Distance? (Do Meteorites come from the Belt?) • Understanding the compositional structure of the asteroid belt has been only so good as the mineralogical interpretation of the spectral data. • In a 23 March 1971 letter to Bruce Hapke, Ed Anders wrote: • “I have plotted the four Hapke-Gehrels color groups as a function of a, and was delighted to find a correlation…[f]rom your data on meteorites it appears that Group I is richer in Fe2+ than Group II, which in turn suggests that the asteroid belt gets more reduced with increasing a.” • Anders believed that the NEAs and meteorites MUST come from the main asteroid belt, even if dynamicists hadn’t figured out the exact mechanism. • George Wetherill believed that the hard physics needed to be done (and he tutored his students to work the problem)… meanwhile, he considered that meteorites probably came from comets, because comets -- at least -- cross the Earth’s orbit.

10. Structure of the Asteroid Belt:Variation of Taxonomic Types... …with Distance from the Sun …with Diameter Despite voluminous data acquisition, no bias-corrected statistical studies have been published since the 1980s… …despite the fact that compositional types show dramatic differences in their distributions. Gradie, Chapman & Tedesco (1988) Of course, taxonomic types reflect different mineralogical assemblages, so their variations with other properties are fundamental... Gradie, Chapman & Tedesco (1988) Gradie & Tedesco (1982)

11. Stony-Iron Meteorites: S-Type Analogs? (The paradigm, 1975-1990)

12. “Matching” worked OK…but it didn’t satisfy cosmochemists • Nearly every asteroid “matched” some meteorite type • But the most common meteorites in museum collections (ordinary chondrites) were represented by almost no asteroids • That didn’t bother astronomers, who claimed that the OC parent bodies were small/unobserved and that highly selective collisional/dynamical processes caused OC meteorites to be over-represented • It did bother cosmochemists, who had reasons for believing that OC material was more fundamental • Physics (of collisions, dynamics) could explain some varia-tions in representation, but not such great a disparity Passing like ships in the night, researchers in the three sub-disciplines of planetary science generally failed to communicate with each other...

13. Some mid-1970s Perspectives “Thus we are left with two possibilities. Either the S asteroids are stony irons…or they are OCs, in which case we must explain why the spectral reflectivity data tell us otherwise…I, at least, find it easier to believe that the spectral reflectivity data mislead us than to accept the alternative: that the most abundant meteorite class (OC) has no asteroidal equivalent, and the second most abundant asteroid class (stony-iron) has no xenolithic and only rare meteoritic equivalents.” -- Ed Anders (1978) Matson et al (1977), among others, doubted that lunar-like space weathering could affect asteroids. The prevailing lunar paradigm invoked agglutinization, which was known to be a minor process in the asteroid belt.

14. Azzurra: Ejecta from a Fresh Large Crater on Ida • Geissler et al. (1996) modeled ejecta distribution • The crater and ejecta have bluer, fresher un-space-weathered colors What this means: Geologically recent features and deposits are associated with different colors from the general, older terrains. SO SOME PROCESS IS CAUSING COLORS TO REDDEN WITH TIME!

15. Ida’s Evidence on the S-type/OC Conundrum & “Space Weathering” • Fresher terrains on Ida (and Dactyl) look more like ordinary chondrites • Ida and S-types in the Koronis family could be parent bodies for OC’s • Binzel’s find spectral gradation be-tween S- and Q-types among NEAs • Impetus for laboratory simulations (cf. Moroz et al, Sasaki et al) Long-standing debate: Should one take spectra of the common S-types literally, or do the colors of asteroidal surfaces exposed to solar wind and micrometeorites change with time? Opinions started to change after Ida... But we needed an orbiter for proof !

16. Eros shows very little spatial color variability, unlike Ida IDA [Enhanced color] • Even small rocks are usually the same color as the rest of Eros • Possibilities: • Coated with electro-statically levitated dust? • Maturely space-weathered while in near-Earth orbit? • Maybe Beth Clark has the answer (next talk) • In many ways, Eros resembles Ida…but not in color heterogeneity • Probable composition: OC (favoring L/LL but not yet secure) EROS [Black & white image]

17. My Answer to the “Prime Question”(Why so long to accept asteroidal space weathering?) There are two different philosophies of scientific proof: • Detailed physics/chemistry of hypothetical processes must be understood before the process is accepted. • Multiple, plausible hypotheses should be evaluated (with due weight, but not blind acceptence, being given to interdisciplinary insights) while details are research-ed... so long as the laws of physics aren’t violated. C Space weathering is alive and well… But advances are still being made (reported at this meeting) in understanding lunar space weathering. And Eros teaches us that we have much to learn about space weathering processes on asteroids.

18. Spacecraft studies of asteroid colors and spectra • Spectral variations on Ida (from Galileo SSI images) suggest space weathering • Early NEAR colorimetry of Eros shows bland colors; NIS spectra suggest ordinary chondrite composition (also implied by X-ray spectra, though calibrations uncertain)