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Michael E. Brown (Caltech)
Near-infrared spectroscopy offers the possibility of determining the surface composition of outer solar system bodies. Such observations have shown the presence of CH4, H2O, and N2 ices on Pluto and Triton, water ice on Charon and the moons of the giant planets, and potentially organic residues on the Centaur object 5140 Pholus. The surface composition of these objects reflects the initial conditions of the solar nebula at the location where they were formed as well as any subsequent processing that has taken place since then.
The Kuiper belt objects (KBOs), residing in the relatively cool and quiescent outer solar system beyond Neptune, are thought to be the least processed objects remaining in the solar system, so their composition should most reflect the initial formation conditions. In an attempt to probe these initial conditions, much recent effort has gone into obtaining near-infrared spectra of KBOs Unfortunately, the signal-to-noise ratios of the spectra obtained have been low because of the extreme faintness of these objects. The KBO 1993 SC was seen to have features possibly due to hydrocarbon ices in a highly smoothed spectrum, while 1996 TL66 appears featureless in a very noisy spectrum.
To circumvent the problems associated with the dimness of the KBOs, we are obtaining near-infrared spectra of the brighter Centaur objects and of irregular satellites of the giant planets at the Keck Observatory. Because they have smaller heliocentric distances than KBOs, these objects tend to be brighter, and higher quality spectra can be obtained. We present and discuss features detected in near infrared spectra of the Centaurs Chiron, Pholus, 1997 CU26, and 1995 GO and the satellites Himalia (J6), Elara (J7), Pasiphae (J8), Phoebe (S9), and Nereid (N2).