Phoebe: A Surface Dominated by Water
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 (KBOs), 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 46 images, producing spectral maps covering the majority of Phoebe's surface, roughly a third 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. 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 that exposed richer subsurface layers. The range of Phoebe's water–ice absorption spans the same range exhibited by dynamically excited KBOs. 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 KBOs also originated with water-poor surfaces that were enhanced through stochastic collisional modification.
Additional Information© 2018. The American Astronomical Society. Received 2018 March 12; revised 2018 April 29; accepted 2018 April 30; published 2018 June 22. The authors would like to thank Pedro Lacerda for his useful insights during discussions of this work. This research has made use of the USGS Integrated Software for Imagers and Spectrometers (ISIS). STSDAS is a product of the Space Telescope Science Institute, which is operated by AURA for NASA Raw data were generated at the Planetary Data System Cassini Archive (http://pds-atmospheres.nmsu.edu/data_and_services/atmospheres_data/Cassini/Cassini.html). Processed data products are available from the corresponding author upon request (email@example.com). WCF acknowledges support from Science and Technologies Funding Council grant ST/P0003094/1. M.E.B. and W.C.F. acknowledge support from the National Aeronautics and Space Administration Cassini Data Analysis program grant NNX10AF11G. Software: USGS Integrated Software for Imagers and Spectrometers, Image Reduction and Analysis Facility(Tody 1993), Space Telescope Science Data Analysis System.
Published - Fraser_2018_AJ_156_23.pdf
Submitted - 1803.04979