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T. V. Johnson (Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA), J. I. Lunine (Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA)
The composition of material condensed from a solar composition nebula is highly dependent on the abundance of oxygen, the dominant solid forming element (silicates and water ice), and carbon, roughly half as abundant. These abundances, and the partition of gaseous C between CO and CH4, determine the relative abundances of water ice and rock/metal in the solid condensates. The mean densities of satellites made from this material reflect the rock, metal and ice abundances, corrected for the effects of pressure and porosity. Recently proposed changes to the solar abundance values for carbon and oxygen have a large effect on the expected density solar composition condensates. The effects of these changes on the interpretation of new satellite density determinations from the Cassini mission include: 1. Solar composition condensates have higher mean densities regardless of the state of gas phase carbon, 2. Phoebe is significantly denser, for plausible porosities, than the regular icy satellites and is likely representative of solar nebular condensates in CO-rich conditions in the outer solar system rather than circum-Saturn planetesimals, 3. The mass-weighed average icy satellite density (~1220 kg m-3) is too low for a solar composition condensate from either the solar nebula or a circumplanetary nebula unless significant amounts of carbon were incorporated as low density solids, and 4. Large variations in mean density among the icy satellites imply post-formation fractionation processes that result in significant differences in current ice to rock/metal proportions. A portion of this work was done at the Jet Propulsion Laboratory, Caltech, under a grant from NASA.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.