37th DPS Meeting, 4-9 September 2005
Session 55 Pluto and Charon
Poster, Thursday, September 8, 2005, 6:00-7:15pm, Music Recital Room

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[55.02] Evidence of Tholins on Pluto’s Surface from Near-IR and IR Spectroscopy

C. B. Olkin, E. F. Young, L. A. Young (SwRI), W. Grudy (Lowell Obs.), B. Schmitt (Lab. de Plantologie de Grenoble, France)

We obtained spectroscopic observations of Pluto and Charon recorded on August 12, 2001 with NIRSPEC (Keck I) spanning 2.8 to 3.7 microns with a sub-Earth longitude of 198 degrees. These data have a spectral resolution of ~350 and a signal-to-noise ratio of 5 to 25 per spectral element. In the past year, we have added the ability to model spectra with Hapke’s formalism for 2-layer surfaces. We find that a geographic mixture of 20 percent pure methane and 80 percent methane diluted in nitrogen fits the Keck data from 2.8 to 3.6 microns. While Doute et al. (1999, Icarus 142, 421) found an areal component of 12 percent fine-grained nitrogen ice was consistent with Pluto’s spectrum from 1.4 to 2.5 micron, we find a three-sigma upper limit of 6 percent fine-grained nitrogen ice based on the dark albedo of Pluto’s 3.3-micron methane feature.

We expanded our wavelength coverage by including shorter wavelength spectroscopy recorded with SpeX on the IRTF at a similar sub-Earth longitude (July 16-17, 2002). The 3.3-micron methane feature is considerably darker than the methane bands around 2.35 microns. It is not possible to simultaneously fit both these features using only combinations of pure methane, methane diluted in nitrogen and pure nitrogen, but we find a much improved fit by including a 20 percent areal fraction of tholin (Khare et al. 1984, Icarus 60, 127).

This may not be a unique solution; other species (which we have not yet modeled) may also improve the fit. Future work will include modeling other laboratory tholins and water ice.

This work was supported by NASA's Planetary Astronomy Program NAG5-12516 and the NSF Life in Extreme Enviroments Program AST-0085614.

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