THE ORIGIN AND COMPOSITION OF PLUTO AND CHARON - CHEMICALLY UNIFORM MODELS

Observations of the Pluto-Charon system using the Hubble Space Telesco pe are soon to provide another test of the modem Laplacian theory of s olar system origin. According to this theory, Pluto, Charon and Neptun e's moon Triton are condensed remnants of the gas ring which was shed by the proto-solar cloud at Neptune's orbit, and from which Neptune fo rmed. Each body should have the same bulk chemical composition provide d that the influence of secondary events such as physical collision ca n be neglected. Here we report a model for the time-dependent, surface -catalysed production of CH4, solid carbon C(s) and CO2 within the out er layers of the proto-solar cloud. This model is used to determine th e condensate mix which best fits the Voyager 2 spacecraft measurements of Triton's mean density. We suggest that Triton is a chemically homo geneous structure consisting, by mass, of 44.2% anhydrous rock, 2.1% g raphite, 33.0% water ice and 20.7% dry ice (i.e., solid CO2 ice, of wh ich a fraction nearly 2/3 is present as the clathrate hydrate CO2.5.75 H2O). On the basis of this composition, the individual mean densities of Pluto and Charon are predicted to be 2.02 +/- 0.02 g/cm3 and 1.94 +/- 0.02 g/cm3, respectively, assuming a mean surface temperature of 3 5 K. The mean density of the combined system is predicted to be 2.01 /- 0.02 g/cm3.