NOTE: Detection of Gaseous Methane on Pluto

0.5% CO, 1.5% CH4 (by mass), and N2 in an intimate mixture (Owen et al. 1993). Assuming the N2–CO–CH4 frost acts like an ideal mixture, We obtained Pluto’s spectrum using the CSHELL echelle one finds that XCH4 is only 0.0001% to 0.001%. However, Lellouch (1994) spectrograph at NASA’s IRTF on Mauna Kea, on 25–26 May points out that Triton’s XCH4 is several hundred times greater than that 1992, with a spectral resolution of 13,300. The spectral range predicted by the ideal mixture that Cruikshank et al. (1993) used to (5998–6018 cm, or 1661.8–1666.9 nm) includes the R(0) and interpret similar spectra of Triton. This can be explained if the N2–CO– the Q(1)–Q(9) lines of the 2n3 band of methane. The resulting CH4 frost is nonideal. Lellouch therefore suggests that Pluto’s XCH4 is spectrum shows the first detection of gaseous methane on Pluto, inflated by a similar amount, so that XCH4 P 0.1%. Finally, there is recent with a column height of 1.20 20.87 cm-A (3.22 22.34 3 10 molecule spectral evidence of pure CH4 on Pluto’s surface (Schmitt et al., 1994). Pure CH4 frost would supply more gaseous CH4 than N2–CO–CH4 frost, cm).  1997 Academic Press raising XCH4 above the ideal-mixture value. The thermal structure of Pluto’s atmosphere is further evidence that Introduction. This paper presents the first positive detection of gasXCH4 is larger than the ideal mixture predicts. The temperature of the eous CH4 in Pluto’s atmosphere. Solid CH4 has been previously identified N2–CO–CH4 frost is 40 6 2 K, from the shape of the N2 absorption in Pluto’s visible and near-IR spectra at resolutions of a few hundred up feature at 2148 nm (Tryka et al., 1994), and the brightness temperature to 1200 (e.g., Fink et al. 1980; Spencer et al. 1990; Owen et al. 1993). of Pluto (which is a global average, including frost-covered and frostUnfortunately, these observations cannot be used to find the CH4 mixing free regions) ranges from 55 K at 60 em (Sykes et al. 1987; Tedesco et ratio in the atmosphere directly, since the spectra of solid and gaseous al. 1987) to 35 K at 1300 em (Stern et al. 1993; Jewitt 1994). In contrast CH4 are essentially indistinguishable at these resolutions. Furthermore, with the cold surface, the atmosphere is 100 K at ebar levels (Elliot and the absorption features due to gaseous CH4 are hundreds of times weaker Young 1992), implying a source of heating in the atmosphere. CH4 can than those due to solid CH4, and have not been detected in these spectra. provide the required heating, if XCH4 . 0.1% and there is no CO cooling Because Pluto’s atmosphere is expected to be in vapor-pressure equilib(Yelle and Lunine 1989). For models that include CO cooling, large rium with the surface, the surface temperature and composition can be enough amounts of gaseous CH4 can still heat the atmosphere (Lellouch used indirectly to estimate the atmospheric CH4 mixing ratio (XCH4). 1994; Strobel et al. 1996). Near-IR spectra indicate that Pluto’s surface is dominated by frost with