The distribution of methane in Titan's atmosphere during northern spring from Cassini/CIRS observations

We present a study of the methane abundance in Titan's lower stratosphere. We analyzed spectra of Titan's atmosphere in the mid- and far-infrared region recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft with nadir geometry to determine the methane mixing ratio between 1 and 20 mbar range and its variations with seasons and latitudes.

Lellouch et al. (2014) analyzed CIRS observations recorded over the first part of the Cassini-Huygens mission, from August 2005 to June 2010, during Titan's northern winter and early spring. They showed that the methane mole fraction in Titan's atmosphere varies with latitudes from about 1.0% to 1.5%, which was unexpected as, due to its long chemical lifetime, CH₄ is supposed to be homogenized by the atmospheric circulation.

The goal of this study is to analyze Cassini/CIRS data taken over the last part of the mission during northern spring and early summer (June 2010 to September 2017) in order to retrieve CH₄ mixing ratio, vertical profiles of temperature and aerosols opacity at the 19 latitudes that we have selected. We analyzed spectra acquired by two focal planes of CIRS (FP1 and FP4) covering the spectral range from 10 to 600 cm^-1 and from 1050 to 1500 cm^-1, respectively. FP1 spectra include emission from CH₄ pure rotational lines and FP4 spectra include the CH₄ ν₄ band centered at 1305 cm^-1. We use an iterative process to determine the temperature profile from the tropopause (using the FP1) to the low stratosphere (using the FP4) by fitting the continuum of the FP1 spectra in the 70-150 cm^-1 wavenumber range and the CH₄ ν₄ band in the 1200-1350 cm^-1 range. The obtained thermal profile is used to retrieve the methane mole fraction by fitting their rotational lines in the 75-150 cm^-1 range. We use the obtained value as a priori of a new iteration (retrieving the temperature profile and the CH₄ mole fraction subsequently). Convergence is obtained after a few iterations.

We will present the derived CH₄ mixing ratios during the northern spring and compare them with the results of Lellouch et al. (2014), which mostly focused on the northern winter. This will allow us to derive potential seasonal variations that could occurred after the global circulation overturning during the spring.

 

Reference :

- Lellouch et al. (2014). The distribution of methane in Titan’s stratosphere from Cassini/CIRS observations. Icarus 231, 323-337.