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D. P. Cruikshank, E. Wegryn, C. M. Dalle Ore (NASA Ames), K. H. Baines, B. J. Buratti, D. L. Matson, R. M. Nelson (JPL), G. Bellucci, F. Capaccioni, P. Cerroni, A. Coradini, V. Formisano, V. Mennella (Ist. Fisica Spaz. Interplanet., CNR), J.-P. Bibring, Y. Langevin (Univ. Paris Sud-Orsay), R. H. Brown (LPL U. Ariz.), R. N. Clark (USGS), M. Combes (Obs. Paris), P. Drossart, B. Sicardy (Obs. Paris-Meudon), R. Jaumann (Inst. Planet. Explor. DLR), T. B. McCord (HIGP SOEST U. Hawaii), P. D. Nicholson (Cornell U.), C. Sotin (Lab. Planet. Geo. U. Nantes)
Spectral analysis of data taken with the Visible-Infrared Mapping Spectrometer (VIMS) during the Cassini spacecraft's closest approaches to Phoebe (11 June 2004) and Iapetus (31 Dec. 2004) shows an absorption band between 3.23 and 3.35 micrometers that matches the =C-H stretching mode band produced by aromatic hydrocarbons, a common constituent of the organic fractions of comets, IDPs, carbonaceous meteorites, and interstellar dust. This paper expands on the discovery of this spectral feature on Phoebe by R. N. Clark et al. (Nature 435, 66, 2005). Centered at 3.3 micrometers, this band is located on the steeply rising long-wavelength slope of the strong band of H2O ice and bound OH. It has a depth of as much as 10 percent below the ``continuum" bracketing the wavelength range 3.23-3.35 micrometers, and spans 8 spectral channels in the VIMS instrument. While this band is found to be strongest in VIMS pixels scattered across the surfaces of both satellites, it is most prominently concentrated in patches or regions on each. The band is detected in these same geographic locations in several cubes taken at different times during each encounter. The C-H band in aromatic hydrocarbons is intrinsically weak, and its detection indicates the presence of a significant weight fraction of the one or more molecules causing it. Additional study of the band profile may narrow the range of aromatic hydrocarbons responsible. The discovery of these complex hydrocarbon molecules is significant both for determining the chemical composition of the dark material on the surfaces of these two satellites and for further exploration of the possible common origins of the material through dynamical exchange between Phoebe and Iapetus.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.