Phoebe: a surface dominated by water

Abstract

The Saturnian irregular satellite, Phoebe, can be broadly described as a water-rich rock. This object, which presumably originated from the same primordial population shared by the dynamically excited Kuiper Belt Objects, has received high resolution spectral imaging during the Cassini flyby. We present a new analysis of the Visual Infrared Mapping Spectrometer observations of Phoebe, which critically, includes a geometry correction routine that enables pixel-by-pixel mapping of visible and infrared spectral cubes directly onto the Phoebe shape model, even when an image exhibits significant trailing errors. The result of our re-analysis is a successful match of 40 images, producing spectral maps covering the majority of Phoebe's surface, roughly a 3rd of which is imaged by high resolution observations (<22 km per pixel resolution). There is no spot on Phoebe's surface that is absent of water absorption. The regions richest in water are clearly associated with the Jason and South Pole impact basins. We find Phoebe exhibits only three spectral types, and a water-ice concentration that correlates with physical depth and visible albedo. The water-rich and water-poor regions exhibit significantly different crater size frequency distributions, and different large crater morphologies. We propose that Phoebe once had a water-poor surface whose water-ice concentration was enhanced by basin forming impacts which exposed richer subsurface layers. Finally, we demonstrate that the range of Phoebe's water-ice absorption spans the same range exhibited by dynamically excited Kuiper Belt Objects. The common water-ice absorption depths and primordial origins, and the association of Phoebe's water-rich regions with its impact basins, suggests the plausible idea that Kuiper Belt Objects also originated with water-poor surfaces that were enhanced through stochastic collisional modification.