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F. Vilas (NASA JSC), S. M. Lederer, S. L. Gill (CSU - San Bernardino), J. E. Thomas-Osip, D. J. Osip (OCIW Las Campanas)
The faint, irregular outer planet satellites are hypothesized to be fragments of larger parent bodies disrupted by collisional impacts and captured in their present locations. Their faintness limits spectroscopic observations, so we need other methods of constraining their surface compositions. Visible reflectance spectra of many main-belt and outer-belt low-albedo asteroids show the presence of an absorption feature centered near 0.7 um attributed to a ferrous-ferric charge transfer transition in oxidized iron in phyllosilicates. This feature is highly correlated with the 3.0-um water of hydration absorption feature, suggesting that aqueous alteration operated on the surfaces of these asteroids. We searched for evidence of aqueous alteration in BVRI photometry obtained in 2001-2 of 21 irregular Jovian and Saturnian satellites by Grav et al. (Icarus 166, 2003). We converted the V-R and V-I colors to ECAS v-w and v-x colors by inverting formulae determined by E. Howell (Ph.D. Thesis, U. Arizona, 1995). We then applied Vilas’ formula (Icarus 111, 1994) for identifying the presence of this 0.7-um feature in ECAS photometry. Evidence for the presence of the 0.7-um absorption feature is found in photometry of Jovian irregular satellites in all of the inclination clusters, often mixed with satellites having photometry not showing this feature. The presence of the 0.7-um feature does not necessarily correlate with the grey/light red division of photometry offered by Grav et al. No evidence for the feature is seen in this photometry of the Saturnian irregular satellites, including Phoebe. As a proof of concept, we obtained UBVRI photometry of Ch-class asteroid 19 Fortuna using the Swope 1.0-m telescope at Las Campanas Observatory in June 2004; the absorption feature is easily identified. Analyses of results will be presented.
SML and SLG thank the NASA MUCERPI program. Everyone thanks LCO OCIW.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.