36th DPS Meeting, 8-12 November 2004
Session 11 Pluto, Triton, and TNO Surfaces
Oral, Tuesday, November 9, 2004, 10:30am-12:00noon, Clark

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[11.03] Interpreting Pluto's Spectra Using A New Radiative Transfer Model

J. Eluszkiewicz (AER, Inc), W. Grundy (Lowell Observatory), L. Young (Southwest Research Institute), M. Buie (Lowell Observatory)

The spectra of Pluto and Triton are characterized by long absorption path lengths in the overtone band of solid nitrogen at 2.15 micron. In the past, these long path lengths have been interpreted as large grain sizes, on the order of centimeters and up to a meter. However, as such large pebbles or boulders are unlikely to form on any planetary surface, an alternative radiative transfer (RT) model for the spectra has been developed. In the model, nitrogen frost is represented as a porous slab containing spherical voids and ``dust" particles (where the "dust" on Pluto refers to any minor ice, e.g., methane). The single-scattering properties (asymmetry parameter and single-scaterring albedo) of the voids and dust particles are calculated using a generalized Mie code in which the complex refractive indices of refraction for the particle and the surrounding medium can assume arbitrary values. The single-scattering properties are used in a multiple scattering code to compute albedo and emissivity of the frost.

The new model computes reflectivity as a function of porosity and pore size and naturally relates long absorption path lengths to low porosity (rather than to large grain sizes). Moreover, the model has some predictive capability, as changes in reflectivity can be related to changes in porosity caused by frost metamorphism. In principle, porosity and pore size inferred from the spectra (with the new RT model serving as the forward model in the inversion) have wider applications, e.g., they could be used to constrain surface energy balance models. The new RT model is applicable to any planetary surface covered by volatile ices (e.g., Pluto, Triton, Io, and the martian CO2 caps).

In this presentation, we will describe an initial application of the model aimed at interpreting Pluto's spectra measured between 2001 and 2003 at the Infrared Telescope Facility using the SpeX spectrometer.

Acknowledgements: The work of JE has been supported by the NASA Planetary Geology and Geophysics Program (contract NNH04CC40C). WG, LY, and MB are Visiting Astronomers at the Infrared Telescope Facility which is operated by the University of Hawaii under Cooperative Agreement no. NCC 5-538 with the NASA Planetary Astronomy Program.

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