Evidence for Methane Segregation at the Surface of Pluto

by: Doute, S.; Schmitt, B.; Quirico, E.; Owen, T. C.; Cruikshank,
Dale P.; deBergh, C.; Geballe, T. R.; Roush, T. L.

ABSTRACT

In May 1995, a set of spectrophotometric curves of the system Pluto-Charon
was recorded with the UKIRT telescope equipped with the spectrometer CGS4.
As for the previous observations, the spectra cover a part of the near
infrared range, between 1.4 and 2.55 micrometers, but with a higher
resolution of approximately 700. In both the 1992 and 1995 data, the
existence of solid methane is confirmed by numerous absorption bands, and
the carbon monoxide and the nitrogen ices are identified by their respec-
tive signatures at 2.35 and 2.15 um. The solid nitrogen seems to be the
principal icy component and forms a matrix in which the CH4 and CO molecules
are diluted. However a spectroscopic analysis of the 1995 observations
indicates that pure methane may coexist with its diluted phase in N2. In
order to derive the horizontal and vertical distribution of these diffe-
rent species and to obtain some quantitative information about their
characteristics, we have modeled the spectrum of May 15 that corresponds
to the maximum of Pluto's visible light curve. This was achieved by means
of a radiative transfer algorithm dealing with compact and stratified
media. Among the various representations we have tested to describe the
surface of Pluto, only a geographical mixture of three distinct units
explains all the significant structures of the analyzed spectrum. The
first unit is a thin granular layer of pure CH4 covering a compact poly-
crystalline substratum of N2-CH4-CO, which are in a molecular mixture
(concentrations of and CO of the order of 0.45, 0.1-0.2 respectively).
It covers about 70 of the observed area and corresponds to volatile
deposits that are sublimating under solar illumination. The second unit
is either (a) a single thick layer of pure granular methane or (b) a unit
similar to the first unit but with the two components inverted (i.e. with
CH4 forming a substratum and the N2-CH4-CO mixture a superficial layer
of fine grains). Covering 20 of the surface, it represents some old
surfaces that have been sublimated for a long time, and eventually
recovered later by very small amounts of fresh deposits of the molecular
mixture N2-CH4-CO. Finally, the third unit may result from the condensation
of very fine grains of nearly pure N2. It covers the remainder of the
surface (about 10). All these results allow a better understanding of
the processes of deposition, metamorphism, sublimation and transport
affecting the different ices detected on Pluto during its climatic cycles.

Ames Research Center, 1999, Document ID 20000085159