Ethane abundance on Titan

Ethane (C2H6) abundance in Titanʹs stratosphere is determined from recent ground based high spectral resolution measurements of individual ethane emission line spectra. Lines near 12 μm in the ν9 band of C2H6 were measured at a resolving power of λ/Δλ ∼ 106 with infrared heterodyne spectroscopy at the NASA Infrared Telescope Facility (IRTF). Globally averaged constant-with-height C2H6 mole fractions are retrieved for various possible thermal profiles on Titan. A range of possible stratospheric temperatures is investigated with respect to the data and a corresponding range of acceptable globally averaged ethane mole fractions is retrieved. The data and physical constraints imposed by the observations limit the temperatures in Titanʹs upper stratosphere to 160–180 K. Corresponding acceptable mole fractions can range from 4 × 10−6 to 1.6 × 10−5 depending on the thermal profile used. For a currently “recommended” thermal profile a mole fraction of 9.4(−4.7, +9.4) × 10−6 is retrieved, some-what lower than previous results, but centrally placed in the acceptable temperature-abundance parameter space.