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T.W. Momary, K.H. Baines, P.A. Yanamandra-Fisher, B.J. Buratti (JPL/CalTech), L.A. Lebofsky (U. of Arizona)
Absolutely-calibrated multispectral photometry of nine Saturnian satellites, including the leading side of Enceladus, are presented for canonical near-infrared filters. The satellites were observed during Ring Plane Crossing in August/September 1995 with the NSFCAM instrument at the NASA/IRTF. Observations were contemporaneous with those of the Uranian system, acquired by Baines et al. (Icarus 132, 266-284, 1998), using the same instrument and filters. Results are reported for J, H, and K filters near 1.27, 1.62, and 2.20 microns, and two filters centerd at 1.73 and 2.27 microns. We find that the spectrum and absolute brightness of Enceladus is consistent with fresh water ice, relatively uncontaminated by the kinds of low albedo, spectrally neutral materials found in the Uranian system. The satellite's peak brightness is at 1.27 microns with a geometric albedo of 0.904 ± 0.063, in contrast to the Uranian satellites Miranda, Ariel, and Titania, which are one third as bright at this wavelength. The J-H band depth of Enceladus is about 30% contrasting with the darker Uranian satellites' J-H band depth of less than 10%. Enceladus shows a large near-infrared opposition surge of 0.0817 mag /degree between 3.51° and 0.39° solar phase, perhaps consistent with a surface coated with highly backscattering fresh water-ice frost. Dione, Janus, and Epimetheus have brighter leading sides than trailing. The opposite is true for Enceladus, consistent with 0.89 micron measurements of Buratti et al. (Icarus, 136, 223-231, 1998) and with the suggestion by Showalter et al. (Icarus, 94, 451-473, 1991) that negatively charged E-ring particles would preferentially impact Enceladus’ trailing side. Finally, our 2.27 micron albedos for Prometheus, Janus, Epimetheus, Mimas, Tethys, Dione, Rhea, and Hyperion, as well as Enceladus, are comparable to the visible albedos reported by Buratti and Veverka (Icarus, 58, 254-264, 1984).