Aromatic and aliphatic organic materials on Iapetus: Analysis of Cassini VIMS data

We present a quantitative analysis of the hydrocarbon and other organic molecular inventory as a component of the low-albedo material of Saturn's satellite Iapetus, based on a revision of the calibration of the Cassini VIMS instrument. Our study uses hyperspectral data from a mosaic of Iapetus' surface (Pinilla-Alonso, N., Roush, T.L., Marzo, G.A., Cruikshank, D.P., Dalle Ore, C.M. [2011]. Icarus 215, 75-82) constructed from VIMS data on a close fly-by of the satellite. We extracted 2235 individual spectra of the low-albedo regions, and with a clustering analysis tool (Dalle Ore, C.M., Cruikshank, D.P., Clark, R.N. [2012]. Icarus 221, 735-743), separated them into two spectrally distinct groups, one concentrated on the leading hemisphere of Iapetus, and the other group on the trailing. This distribution is broadly consistent with that found from Cassini ISS data analyzed by Denk et al. (Denk, T. et al. [2010]. Science 327, 435-439). We modeled the average spectra of the two geographic regions using the materials and techniques described by Clark et al. (Clark, R.N., Cruikshank, D.P., Jaumann, R., Brown, R.H., Stephan, K., Dalle Ore, C.M., Livio, K.E., Pearson, N., Curchin, J.M., Hoefen, T.M., Buratti, B.J., Filacchione, G., Baines, K.H., Nicholson, P.D. [2012]. Icarus 218, 831-860), and after dividing the Iapetus spectrum by the model for each case, we extracted the resulting spectra in the interval 2.7-4.0 μm for analysis of the organic molecular bands. The spectra reveal the CH stretching modes of aromatic hydrocarbons at ∼3.28 μm (∼3050 cm−1), plus four blended bands of aliphatic CH2 and CH3 in the range ∼3.36-3.52 μm (∼2980-2840 cm−1). In these data, the aromatic band, probably indicating the presence of polycyclic aromatic hydrocarbons (PAH), is unusually strong in comparison to the aliphatic bands, as was found for Hyperion (Dalton, J.B., Cruikshank, D.P., Clark, R.N. [2012]. Icarus 220, 752-776; Dalle Ore, C.M., Cruikshank, D.P., Clark, R.N. [2012], op. cit.) and Phoebe (Dalle Ore, C.M., Cruikshank, D.P., Clark, R.N. [2012], op. cit.). Our Gaussian decomposition of the organic band region suggests the presence of molecular bands in addition to those noted above, specifically bands attributable to cycloalkanes, olefinic compounds, CH3OH, and N-substituted PAHs, as well as possible Hn-PAHs (PAHs with excess peripheral H atoms). In a minimalist interpretation of the Gaussian band fitting, we find the ratio of aromatic CH to aliphatic CH2 + CH3 functional groups for both the leading and trailing hemispheres of Iapetus is ∼10, with no clear difference between them. In the aliphatic component of the surface material, the ratio CH2/CH3 is 4.0 on the leading hemisphere and 3.0 on the trailing; both values are higher than those found in interstellar dust and other Solar System materials and the difference between the two hemispheres may be statistically significant. The superficial layer of low-albedo material on Iapetus originated in the interior of Phoebe and is being transported to and deposited on Iapetus (and Hyperion) in the current epoch via the Phoebe dust ring (Tosi, F., Turrini, D., Coradini, A., Filacchione, G., and the VIMS Team [2010]. Mon. Not. R. Astron. Soc. 403, 1113-1130; Tamayo, D., Burns, J.A., Hamilton, D.P., Hedman, M.M. [2011]. Icarus 215, 260-278). The PAHs on Iapetus exist in a H2O-rich environment, and consequently are subject to UV destruction by hydrogenation on short time-scales. The occurrence of this material is therefore consistent with the assertion that the deposition of the PAH-bearing dust is occurring at the present time. If the organic inventory we observe represents the interior composition of Phoebe, we may be sampling the original material from a region of the solar nebula beyond Neptune where Phoebe formed prior to its capture by Saturn (Johnson, T.V., Lunine, J.I. [2005]. Nature 435, 69-71).