Surface composition and radius of Hyperion

Abstract

New spectrophotometric data for Hyperion in the region 1.5-2.6 microns obtained in 1981 confirm the presence of water ice bands reported by Cruikshank (1980). The bands are now shown with sufficient clarity to permit improved comparisons with other ice-bearing satellites of Jupiter and Saturn and with laboratory samples. Comparisons with Ganymede and Rhea are shown, and Hyperion is found to differ from both satellites in terms of depth and width of the water ice bands. The sense of the difference is the same as noted earlier from broadband infrared photometry, but the physical cause is not fully understood. The effective radius of Hyperion (considered circular in cross section) derived from a 20-micron flux measurement and a revised value of V(1,0) = 4.62 is r = 140 + or - 19 km. This result is in better accord with both preliminary and refined values of the radius derived from Voyager images; the Voyager result supersedes that deduced from infrared observations.