The distribution of methane in Titan's stratosphere from Cassini/CIRS observations

Cassini/CIRS spectra in the far- and mid-infrared region are used to determine the abundance of methane in Titan's lower stratosphere and investigate its distribution with latitude. The CIRS spectra include emission from both the CH4 ν4 band at 7.7 μm and pure rotational lines longwards of 50 μm, which show differential sensitivities to thermal profile and methane mole fraction. We analyze nadir and limb data taken over the first part of the Cassini mission (August 2005 to June 2010), including a selection of 12 latitudes that provides a reasonably complete and regular sampling of both hemispheres. Unexpectedly, but in a consistent manner for limb and nadir geometries, large variations of the methane mole fraction near 15 mbar (∼85 km) are found, with values ranging from ∼1.0% (at low latitudes and near ±50-55°) to ∼1.5% (at ±30-35° and polar latitudes). Error bars on the retrieved methane mole fraction are 0.07-0.12% at low latitudes in the Southern hemisphere and 0.14-0.21% northward of 40°N. A 1.0% methane mole fraction at low latitudes permits us to reconcile the HASI-measured temperatures below 147 km altitude (2.7 mbar) with inferences from CIRS. The roughly hemispherically-symmetric distribution of methane gas is reminiscent of that observed or predicted for the tropospheric methane clouds, which on a yearly-averaged basis, show preferential occurrences at tropical and polar latitudes. We speculate that convective events at these latitudes result into local stratospheric methane enrichment, which may persist year-round due to dynamical mixing times in the lower stratosphere only moderately shorter than a Titan year.