The saturnian satellites in the near-infrared: Absolute photometry at RingPlane Crossing

We report the first absolutely calibrated photometry of nine saturnian sate llites beyond 1.0 mum, in canonical near-infrared filters, including the fi rst such spectrum of the leading side of Enceladus. The satellites were obs erved during Ring Plane Crossing in August and September of 1995 with the N SFCAM instrument at the NASA/IRTF These observations were contemporaneous w ith those of the uranian system, acquired by K. H. Baines et al. (1998, Ica rus 132, 266-284), using the same instrument and filters. Results are repor ted for J, H, and K filters near 1.27, 1.62, and 2.20 mum a 0.1-mum-wide H' filter centered at 1.73 mum; and a 0.17-mum-wide K' filter centered at 2.2 7 mum. We find that the opposition surge, absolute brightness, and spectral band depths of Enceladus are consistent with fresh water ice, uncontaminat ed by the kinds of low albedo, spectrally neutral materials encountered in the uranian system. Specifically Enceladus displays a large near-infrared o pposition surge of 0.0817 mag deg(-1) between 3.51 degrees and 0.39 degrees solar phase and a peak brightness at 1.27 mum with a geometric albedo of 0 .904 +/- 0.063, while also having a J-H band depth of about 30%. All of the se are consistent with a highly backscattering coating of fresh water frost on the surface of the satellite. The J-H band-depth is consistent with the relatively large (30-56%) water-ice-induced 1.52-mum band depths noted for the other saturnian satellites by Clark et al. (1984, Icarus 58, 265-281). By contrast, the darker uranian satellites display a J-H band-depth of les s than 10%. From H to 1.73 mum, the full-disk albedo of Enceladus increases by 27%, similar to the uranian satellites. We find that Dione, Janus, and Epimetheus are all brighter on the leading sides than the trailing sides. H owever, Enceladus is brighter on the trailing side, consistent with 0.89 mu m measurements of B. J. Buratti et al. (1998, Icarus 136, 223-231) and with the suggestion by M. R. Showalter et al. (1991, Icarus 94, 451-473) that n egatively charged E-ring particles would preferentially impact Enceladus' t railing side. Further, there is a bluish slope to the satellite's spectrum, similar to that of the E-ring itself. These observations suggest, along wi th various other lines of evidence, that a substantial and evolving interac tion between the E-ring and Enceladus exists, with Enceladus possibly being the source land sink) of E-ring material. Finally, our results for the 2.2 7-mum albedos of some of the other saturnian satellites-including Prometheu s, Janus, Epimetheus, Mimas, Tethys, Dione, Rhea, and Hyperion, as well as Enceladus-are comparable to the visible albedos reported for these satellit es by B. J. Buratti and J. Veverka (1984, Icarus 58, 254-264) and D. Morris on et al. (1986, Satellites (Burns and Matthews, Eds.), pp. 764-801). (C) 2 000 Academic Press.