S-Type Asteroids, Ordinary Chondrites, and Space Weathering: The Evidence from Galileo's Fly-bys of Gaspra and Ida

Details S-Type Asteroids, Ordinary Chondrites, and Space Weathering: The Evidence from Galileo's Fly-bys of Gaspra and Ida S-Type Asteroids, Ordinary Chondrites, and Space Weathering: The Evidence from Galileo's Fly-bys of Gaspra and Ida

Oct 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996m%26ps...31..699c&link_type=abstract

Meteoritics, vol. 31, pages 699-725.

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109

Scientific paper

New observations of the S-type asteroids Gaspra and, especially, Ida by the Galileo spacecraft demonstrate that a "space weathering" process operates, which modifies the reflectance spectra of fresh material to be redder, straighter, and have shallower absorption bands. The weathering process appears similar to, but less potent than, that observed on the Moon. It operates in the sense that would tend to convert spectra of ordinary chondrites to have the spectral traits of S-type asteroids. These results appear to resolve the major obstacles of the long standing "S-type conundrum" about the provenance of ordinary chondrite meteorites. A wide body of recent research, reviewed here, builds on previous meteoritical evidence to support a new, developing consensus that the larger S-type asteroids are a diverse assortment of silicaceous assemblages, which includes the OC parent bodies. Recent fairly realistic laboratory simulations of space weathering processes have changed OC spectra to resemble S-type spectra. J. F. Bell's highly regarded earlier paradigm that OC parent bodies would be found only among sub-10 km main-belt asteroids has been tested by an extensive observational survey by R. Binzel and his proteges; the result is that no small, main-belt analog for the OC's has been found, not even the somewhat OC-like object Boznemcova. This article reviews the history of the S-type conundrum, which set the stage for Galileo's historic fly-bys. I then review the findings about Gaspra and Ida, including results about their geology, their geophysical structure and probable origins, and about Ida's small moonlet, Dactyl. Density constraints on Ida set by Dactyl's orbit rule out (for Ida, at least) the classic view of S-types as metallic, stripped cores of differentiated precursor asteroids. New analysis of the Galileo spectral images of Ida is presented that provides strong evidence that space weathering occurs on Ida and that Ida, and the Koronis family asteroids in general, are plausibly OC-like in composition. After reviewing 1990's developments on the S-type conundrum, I advocate a new perspective that the ordinary chondrite parent bodies are among the S-type asteroids, a diverse grouping that also contains other silicaceous and silicate/metal assemblages, presumably including various stony-irons and primitive achondrites represented in meteorite collections; Gaspra may be such a metal-rich assemblage.

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