Carbon in the outer solar system

by: Simonelli, D. P.; Pollack, J. B.; Mckay, C. P.

ABSTRACT

The satellites of Uranus, with densities between 1.3 and 1.7 g cm(-3)
(from Voyager 2 observations) and the Pluto-Charon system, with a mean
density of just above 1.8 g cm(-3) (from terrestrial observations of
mutual eclipse events), are too dense to have a significant amount of
methane ice in their interiors. However, the observed densities do not
preclude contributions from such organic materials as the acid-insoluble
residue in carbonaceous chondrites and laboratory-produced tholins, which
have densities on the order of approximately 1.5 g cm(-3). These and
other considerations have led researchers to investigate the carbon mass
budget in the outer solar system, with an emphasis on understanding the
contribution of organic materials. Modeling of the interiors of Pluto and
Charon (being carried out by R. Reynolds and A. Summers of NASA Ames),
assuming rock and water ice as the only constituents, suggests a silicate
mass fraction for this system on the order of 0.65 to 0.70. The present
work includes the most recent estimates of the C H enhancements and high
z low z ratios of the giant planets (Pollack and Bodenheimer, 1987), and
involves a more careful estimation of the high z low z mass ratio expected
from solar abundances than was used in Pollack et al. (1986), including
the influence of the fraction of C in CO on the amount of condensed water
ice. These calculations indicate that for a particular fraction of C in
CO and a given fraction of C-bearing planetesimals that dissolve in the
envelope (most likely in the range 0.50 to 0.75), (1) Jupiter and Saturn
require a larger fraction of C in condensed materials than Uranus and
Neptune, but (2) the Jupiter and Saturn results are much less strongly
constrained by the error bars on the observed C H enhancements and high
z low z ratios than is the case for Uranus and Neptune. The clearest
result is that in the region of the solar nebula near Uranus and Neptune,
the minority of carbon that is not in gaseous CO (1) must include a
nonzero amount of condensed material, but (2) is most likely not condensed
material alone, i.e., there must be a third carbon-bearing component
besides condensed material and gaseous CO. Given the implied dearth of
methane ice, the condensed carbon is likely dominated by organic material,
and the third component present in addition to CO and organics is assumed
to be CH4 gas.

Carbon in the Galaxy Studies from Earth and Space p 342-343,
N90-27562 21-88, 1988.