The Saturnian Satellites in the Near-Infrared: Absolute Photometry at Ring Plane Crossing

We report the first absolutely calibrated photometry of nine saturnian satellites beyond 1.0 μm, in canonical near-infrared filters, including the first such spectrum of the leading side of Enceladus. The satellites were observed during Ring Plane Crossing in August and September of 1995 with the NSFCAM instrument at the NASA/IRTF. These observations were contemporaneous with those of the uranian system, acquired by K. H. Baines et al. (1998, Icarus132, 266–284), using the same instrument and filters. Results are reported for J, H, and K filters near 1.27, 1.62, and 2.20 μm; a 0.1-μm-wide H′ filter centered at 1.73 μm; and a 0.17-μm-wide K′ filter centered at 2.27 μm. We find that the opposition surge, absolute brightness, and spectral band depths of Enceladus are consistent with fresh water ice, uncontaminated by the kinds of low albedo, spectrally neutral materials encountered in the uranian system. Specifically, Enceladus displays a large near-infrared opposition surge of 0.0817 mag deg−1 between 3.51° and 0.39° solar phase and a peak brightness at 1.27 μm with a geometric albedo of 0.904±0.063, while also having a J–H band depth of about 30%. All of these 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-μm band depths noted for the other saturnian satellites by Clark et al. (1984, Icarus58, 265–281). By contrast, the darker uranian satellites display a J–H band-depth of less than 10%. From H to 1.73 μm, 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. However, Enceladus is brighter on the trailing side, consistent with 0.89 μm measurements of B. J. Buratti et al. (1998, Icarus136, 223–231) and with the suggestion by M. R. Showalter et al. (1991, Icarus94, 451–473) that negatively charged E-ring particles would preferentially impact Enceladus' trailing side. Further, there is a bluish slope to the satellite's spectrum, similar to that of the E-ring itself. These observations suggest, along with various other lines of evidence, that a substantial and evolving interaction between the E-ring and Enceladus exists, with Enceladus possibly being the source (and sink) of E-ring material. Finally, our results for the 2.27-μm albedos of some of the other saturnian satellites—including Prometheus, Janus, Epimetheus, Mimas, Tethys, Dione, Rhea, and Hyperion, as well as Enceladus—are comparable to the visible albedos reported for these satellites by B. J. Buratti and J. Veverka (1984, Icarus58, 254–264) and D. Morrison et al. (1986, Satellites (Burns and Matthews, Eds.), pp. 764–801).