The Surfaces of Pluto and Charon

by: Cruikshank, D. P.; Roush, T. L.; Moore, J. M.; Sykes, M. V.; Owen,
T. C.; Bartholomew, M. J.; Brown, R. H.; Tryka, K. A.

ABSTRACT

Much of the surface of Pluto consists of high-albedo regions covered to
an unknown depth by Beta-N2, contaminated with CH4, CO, and other molecules
. A portion of the exposed surface appears to consist of solid H2O. The
remainder is covered by lower albedo material of unknown composition. The
N2 ice may occur as polar caps of large extent, leaving ices and other
solids of lower volatility in the equatorial regions. The low-albedo mate-
rial found primarily in the equatorial regions may consist in part of solid
hydrocarbons and nitriles produced from N2 and CH4 in the atmosphere or in
the surface ices. Alternatively, it may arise from deposition from impac-
ting bodies and or the chemistry of the impact process itself. Charon's
surface is probably more compositionally uniform than that of Pluto, and
is covered by H2O ice with possible contaminants or exposures of other
materials that are as yet unidentified. The molecular ices discovered on
Pluto and Charon have been identified from near-infrared spectra obtained
with Earth-based telescopes. The quantitative interpretation of those data
has been achieved through the computation of synthetic spectra using the
Hapke scattering theory and the optical constants of various ices observed
in the laboratory. Despite limitations imposed by the availability of
laboratory data on ices in various mixtures, certain specific results
have been obtained. It appears that CH4 and CO are trace constituents,
and that some fraction of the CH4 (and probably the CO) on Pluto is dis-
solved in the matrix of solid N2. Pure CH4 probably also occurs on Pluto's
surface, allowing direct access to the atmosphere. Study of the nitrogen
absorption band at 2.148 micrometers shows that the temperature of the
N2 in the present epoch is 40 -2 K. The global temperature regime of
Pluto can be modeled from observations of the thermal flux at far-infrared
and millimeter wavelengths. The low-albedo equatorial regions must be
significantly warmer than the polar regions covered by N2 (at T = 40 K)
to account for the total thermal flux measured. At the present season,
the diurnal skin depth of the insolation-driven thermal wave is small,
and the observed mm-wave fluxes may arise from a greater depth. Alterna-
tively, the mm-wave flux may arise from the cool, sublimation source
region. The surface microstructure in the regions covered by N2 ice is
likely governed by the sintering properties of this highly volatile ma-
terial. The observed nitrogen infrared band strength requires that expan-
ses of the surface be covered with cm-sized crystals of N2. Grains of
H2O ice on Charon, in contrast, are probably of order 50 micrometers in
size, and do not metamorphose into larger grains at a significant rate.
Because of the similarities in size, density, atmosphere and surface
composition between Pluto and Neptune's satellite Triton, the surface
structures observed by Voyager on Triton serve as a plausible paradigm
for what might be expected on Pluto. Such crater forms, tectonic structures,
aeolian features, cryovolcanic structures, and sublimation-degraded topo-
graphy as are eventually observed on Pluto and Charon by spacecraft will
give information on their interior compositions and structures, as well
as on the temperature and wind regimes over the planet's extreme seasonal
cycle.

Ames Research Center; Jet Propulsion Laboratory, CASI, 1996, Document ID
19970019549, Report Number NAS 1.15112310; NASA-TM-112310