SMASSIR Measurements of Vesta Chips: Evidence for Weathering?

T. H. Burbine, R. P. Binzel (MIT)

"Space weathering" has been proposed as a mechanism for causing the spectra of asteroids with ordinary chondrite compositions to become disguised by redder spectral slopes and suppressed spectral features as compared to laboratory measurements of ordinary chondrite meteorites. To gain insight into the question of weathering on basaltic asteroids, we have observed four Binzel and Xu (1993, Science 260, 186) Vesta-like objects in the near-infrared and compared their spectra to laboratory measurements of diogenitic material subjected to an experimental weathering process (Wasson et al., 1997, LPSC XXVIII, 1505). Our new telescopic data were obtained using the new "Asteroid Grism" system and NSFCAM detector developed by one of us (RPB) for use at the IRTF at Mauna Kea. These data are part of a near-infrared extension (SMASSIR) of the MIT Small Main-Belt Asteroid Spectroscopic survey (SMASS). The combined data typically cover 0.4 to 1.7 microns. Our telescopic measurements show all four Vesta-like objects have distinctive basaltic spectra, but with significantly redder slopes than Vesta. Partially altered diogenite samples are also redder than Vesta, but are not as red as these Vesta-like objects. Interestingly, the laboratory spectrum of a more thoroughly altered diogenite is a very good match for Vesta's spectrum out to 1.5 microns and resembles a eucritic spectrum in this wavelength region. Past 1.5 microns, the altered diogenite spectrum is much redder than Vesta. This combination of observational and experimental data is consistent with a scenario where Vesta, instead of having a young surface age as discussed by Pieters and Binzel (1994, LPSC XXV, 1083), has a diogenitic surface that has been weathered severely to appear eucritic. The younger Vesta chips appear to have been affected by some process to appear redder than Vesta and the HEDs. This research is supported by NSF grant AST- 9530282.