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W.M. Grundy, M.W. Buie, S.D. Kern (Lowell Observatory)
For the past several years, we have been carrying out a campaign of ground-based infrared spectral monitoring of the Pluto/Charon system, using cooled-grating infrared spectrometers mounted on telescopes on Anderson Mesa and on Kitt Peak. We hope to detect any changes in Pluto's near-infrared spectrum which may occur as Pluto moves away from perihelion and as Pluto's sub-solar latitude moves ever farther north. We seek to obtain data frequently, over a long time base, in order to distinguish between diurnal, transient, and secular spectral effects.
Under routine observing conditions, light from Pluto and Charon is combined in our near-infrared spectra. It is necessary to subtract off the Charon contribution in order to correctly evaluate Pluto's spectrum. For this purpose, we require accurate spectra of Charon, sampling its complete rotational cycle. To obtain suitable data, we carried out four HST/NICMOS visits in 1998, resulting in four separate Pluto and Charon spectra (see companion presentation by Buie et al.).
In this paper we interpret the HST/NICMOS Pluto and Charon data in greater detail, discussing what can be learned from the data directly, as well as through their use in analysis of our long-term monitoring data. Emphasis is placed on the shapes, strengths, and wavelengths of infrared absorption bands of ices such as CH4, N2, and H2O, using recent laboratory data (e.g. Schmitt et al. 1998, in Solar System Ices; Grundy and Schmitt 1998, JGR 103, 25809; and additional unpublished work). These spectral characteristics vary with temperature as well as physical and chemical state, offering valuable information about processes acting on the surfaces of Pluto and Charon.
W. Grundy is a Hubble Fellow, supported by grant # HF-01091.01-97A awarded by STScI, which is operated by AURA for NASA under contract NAS~5-26555.
Support for M.~Buie was provided by STScI through grant GO-07818.01-96A\null.
S.~Kern was supported by the National Science Foundation's REU grant to Northern Arizona University.