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C. R. Chapman (SwRI, Boulder)
After decades of debate, consensus now exists that asteroid reflectance spectra are affected by space weathering. Based on a recent consensus [1] about the physical processes that space weather the Moon, we can realistically hypothesize about the causes of asteroidal space weathering and about how the processes might be manifested on different types of asteroids. But issues and questions remain. While vapor deposition on grains dominates lunar space weathering, for asteroids the relative vapor production by solar wind sputtering versus micrometeorite impact (or even other processes masked on the Moon) remains uncertain. Particularly troublesome are the different styles of space weathering on Ida and Eros; these otherwise similarly sized SIV asteroids differ mainly in being either in the belt or in near-Earth space, but how does that affect space weathering so dramatically?
A vital issue is the timescale for space weathering to be manifested, or to mature, under various conditions (e.g. in or out of the belt, for different taxonomic types, or for different sizes). An extraordinary discovery may finally help provide an answer. Nesvorny et al. [2,3] have discovered how to date the creation of specific asteroids (i.e. in a catastrophic collisional break-up) with a robust precision of several percent, competitive with meteoritic age-dating techniques except that we know which specific asteroids the dates pertain to. The Karin cluster, within the S-type Koronis family, formed 5.8 ±0.2 Myr ago. This age is of the order of estimates of space-weathering/regolith-evolution timescales. Spectral studies of Karin cluster members, and members of other dated families, may determine the absolute rates of space weathering processes. That in turn can elucidate the physical processes and permit better extrapolation to other asteroids.
References: [1] B. Hapke (2001) J. Geophys. Res. 106, 10039-10073; [2] D. Nesvorny et al. (2002) Nature 417, 720-722; [3] D. Nesvorny et al. (2003) Astrophys. J. 591, 486-497.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.