Observations in the Saturn system during approach and orbital insertion, with Cassini's visual and infrared mapping spectrometer

Abstract

The Visual and Infrared Mapping Spectrometer observed Phoebe, Iapetus, Titan and Saturn's rings during Cassini's approach and orbital insertion. Phoebe's surface contains water ice, CO₂, and ferrous iron. Iapetus contains CO₂ and organic materials. Titan's atmosphere shows methane fluorescence, and night-side atmospheric emission that may be CO₂ and CH₃D. As determined from cloud motions, the winds at altitude 25-30 km in the south polar region of Titan appear to be moving in a prograde direction at velocity ∼1 m s^-1. Circular albedo features on Titan's surface, seen at 2.02 µm, may be palimpsests remaining from the rheological adjustment of ancient impact craters. As such, their long-term persistence is of special interest in view of the expected precipitation of liquids and solids from the atmosphere. Saturn's rings have changed little in their radial structure since the Voyager flybys in the early 1980s. Spectral absorption bands tentatively attributed to Fe²⁺ suggest that iron-bearing silicates are a source of contamination of the C ring and the Cassini Division.