New spectra of Pluto were obtained with the Gemini Near-Infrared Spectrometer (GNIRS) on the Gemini South 8-m telescope covering the region 1.9-2.5 µm. We have analyzed these data and two spectra of Triton with particular emphasis on a weak absorption feature detected at 2.405 μm. While this wavelength is coincident with a 13CO absorption band that is the isotopic variant of the 12CO band (2.35 μm) seen on both Pluto and Triton, our analysis, supported by new lab spectra of CO, shows that the strength of the 2.405-μm band is much too great to be attributed to any plausible abundance of 13CO. Instead, we identify this band as the 2.4045 μm absorption of pure ethane in solid form (Quirico & Schmitt Icarus 127, 354, 1997). Published models of the spectra of Triton (Quirico et al. Icarus 139, 159, 1999) and Pluto (Douté et al. Icarus 142, 421, 1999) show small variations from the data at 2.28 μm. The addition of absorption from the ethane band at 2.274 μm removes this small discrepancy. We do not see evidence for the 2.461 μm ethane band, although this is a somewhat noisy region of both spectra. Other investigators (Nakamura et al. P.A.S. Japan 52, 551, 2000) noted that Pluto's absorption bands at 2.28 and 2.32 μm are best fit with ethane, but their 2.405 μm region is discrepant with ethane. At longer wavelengths, Sasaki et al. (Ap.J. 618, L57, 2005) noted that models fit their Pluto data best when ethane was added, but they did not clearly identify ethane bands. Estimates of the abundances of ethane on Triton and Pluto suggest that this ice is deposited on relatively short time-scales by precipitation from the atmosphere, where it is produced by photochemistry (Krasnopolsky & Cruikshank JGR 100, 21271, 1995; JGR 104, 21979, 1999).
AAS/Division for Planetary Sciences Meeting Abstracts #38
- Pub Date:
- September 2006