New observations of Saturn's coorbital satellites

Abstract

We present observations of the Saturnian coorbital satellites, Janus and Epimetheus, obtained with the Caltech Cassegrain infrared camera at the Palomar Hale telescope in July and August 1990. Exploiting the strong planetary methane and hydrogen absorption at λ2.0–2.4 μm, we observed both satellites as they passed over Saturn's north pole at superior conjuction. At that time, the bright rings were blocked or eclipsed by the planet, permitting observations of the much fainter satellites. The larger coorbital, Janus, was detected in a single image on 1 July and in seven images on 4 July. Epimetheus was first detected in four images on 4 July and subsequently in five images on 11–13 July and in seventeen images on 14 August 1990. Exposure times varied from 60 to 240 sec, and both narrowband (CVF) and broadband K filters were used. Astrometric reductions relative to the brighter satellites, chiefly Mimas and Enceladus, yield planetocentric positions of the coorbitals with accuracies of 0.15–0.35″, or 0.4°–0.9° in longitude. These measurements are combined with Voyager and previous Earth-based observations made in 1980–1981 and the discovery observations in 1966, in a revised solution for the dynamical parameters of the system. The orbital model is that of Yoder et al. (1989, Astron. J. 98, 1875–1889). Our new observations confirm their results, in particular the low densities of both satellites, but provide a much stronger solution which is essentially independent of lingering uncertainties in the interpretation of the 1966 data for Epimetheus. We obtain masses of (1.98 ± 0.12) × 10 21 g for Janus, and (0.55 ± 0.03) × 10 21 g for Epimetheus which, together with the volume estimates by Thomas (1989, Icarus 77, 248–274) and Yoder et al. (1989) based on Voyager images, yield mean densities of 0.65 ± 0.08 and 0.63 ± 0.11 g cm −3, respectively. Similar densities for these bodies have recently been derived by Rosen et al. (1991, Icarus 93, 25–44) from an analysis of density waves driven in Saturn's rings. The most natural interpretation of these densities is that these objects (whose mean radii are 90 and 59 km) are composed of relatively pure water ice, but with porosities of ∼30%. A similar porosity has been inferred by Dermott and Thomas (1988, Icarus 73, 25–65) and Eluszkiewicz (1990, Icarus 84, 215–225) for the outer 50–70 km of the mantle of Mimas.