Ground-based measurements of the methane distribution on Titan

Using spectra taken with NIRSPEC (Near Infrared Spectrometer) and adaptive optics on the Keck II telescope, we resolved the latitudinal variation of the 3 ν 2 band of CH 3 D at 1.56 μm. As CH 3 D is less abundant than CH 4 by a factor of 50 ± 10 × 10 - 5 , these CH 3 D lines do not saturate in Titan’s atmosphere, and are well characterized by laboratory measurements. Thus they do not suffer from the large uncertainties of the CH 4 lines that are weak enough to be unsaturated in Titan. Our measurements of the methane abundance are confined to the latitude range of 32°S–18°N and longitudes sampled by a 0.04″ slit centered at ∼195°W. The methane abundance below 10 km is constant to within 20% in the tropical atmosphere sampled by our observations, consistent with the low surface insolation and lack of surface methane [Griffith, C.A., McKay, C.P., Ferri, F., 2008. Astrophys. J. 687, L41–L44].