Phoebe Nature Papers

Two papers have been published in the journal Nature this week with results from the VIMS instrument from the June flyby of Saturn's moon Phoebe, as well as a paper by Torrence Johnson and Jonathan Lunine on the history of Phoebe. Links and a brief synoposis of results follow.

• Clark R. N., et al. Compositional maps of Saturn's moon Phoebe from imaging spectroscopy. Nature, 435. 66 - 69 (2005).
• Phoebe is one of the most compositionally-diverse bodies in the solar system
• Compounds detected by VIMS vary in abundance and grain size at different locations
  
• Water ice
• Distributed throughout surface, highest abundance in south polar region
• Crater interiors in highest resolution data have lower water ice abundances compared to their surroundings, suggesting that ice is less abundant in the deep subsurface, though small crater do appear to excavate ice, suggesting an ice-rich layer near the surface
  
• Bound Water
• Features at 1.5 and 1.95 microns
  
• Bound CO2
• Absorbtion at 4.26 microns
• Linear structures in highest resolution data seen at this wavelength, not seen at 1 micron reflectance
  
• Fe²⁺-bearing minerals
• Absorption at 1 micron is broad (1.1 microns in width)
• More consistent with olivines and phyllosilicates than with pyroxenes
• Weak OH-stretch/metal combination features at 2.2 and 2.3 microns are consistent with an interpretation that the Fe²⁺ minerals are phyllosilicates
  
• Cyanide compounds
• Not seen elsewhere in solar system
• Absorption at 2.42 microns best fits Potassium ferrocyanide trihydrate - K₄Fe(CN)₆*3H₂O
• Mainly seen in equatorial regions
• Absorption later found on the dark side of Iapetus and Comet Borrelly
• Organics
• Weak features at 3.3 microns
• CH-stretch near 3.3 microns
• No absorptions seen between 3.4 and 3.7 microns
• 3.3 micron feature maybe due an aromatic or cycloalkane hydrocarbon (not long chain aliphatics)
• No structures associated with pyrene or anthracene
• Nitriles (CN)
  
• Absorption features at 4.50 and 2.54 microns, and near 3.3 microns
• Possibily due to alkane molecule with a nitrile group attached
• Feature at 3.62 microns maybe due to HCN
• Abundance of carbon likely results in Phoebe's low albedo, other minerals have less effect
  
• The broad Fe²⁺ absorption also seen in Saturn's rings, but not the organics and cyanides
• T.V. Johnson and J.I. Lunine. Saturn's moon Phoebe as a captured body from the outer Solar System. Nature, 435. 69 - 71 (2005).
• Density of Phoebe = 1.63 gm/cm³
• Still well above density of Mimas, Enceladus, Dione, Tethys, Rhea, and Iapetus (1.3 gm/cm³)
  
• Assuming Phoebe is made of the same material as Triton and Pluto, both with densities of 1.9 gm/cm³, Phoebe would have a porosity of 15%
• Assuming most of the Carbon in the protoplanetary disk was tied up in oxygen (but not all), a Phoebe compositional identical to Pluto and Triton emerges