Meridional variation in tropospheric methane on Titan observed with AO spectroscopy at Keck and VLT

Abstract

The spatial distribution of the tropospheric methane on Titan was measured using near-infrared spectroscopy. Ground-based observations at 1.5 mu m (H-band) were performed during the same night using instruments with adaptive optics at both the W.M. Keck Observatory and at the Paranal Observatory on 17 July 2014 UT. The integral field observations with SINFONI on the VLT covered the entire H-band at moderate resolving power, R = lambda/Delta lambda approximate to 1500, while the Keck observations were performed with NIRSPAO near 1.5525 mu m at higher resolution, R approximate to 25, 000. The moderate resolution observations are used for flux calibration and for the determination of model parameters that can be degenerate in the interpretation of high resolution spectra. Line-by-line calculations of CH4 and CH3D correlated k distributions from the HITRAN 2012 database were used, which incorporate revised line assignments near 1.5 mu m. We fit the surface albedo and aerosol distributions in the VLT SINFONI observations that cover the entire H-band window and used these quantities to constrain the models of the high-resolution Keck NIRSPAO spectra when retrieving the methane abundances. Cassini VIMS images of the polar regions, acquired on 20 July 2014 UT, are used to validate the assumption that the opacity of tropospheric aerosol is relatively uniform below 10 km. We retrieved methane abundances at latitudes between 42 degrees S and 80 degrees N. The tropospheric methane in the Southern mid-latitudes was enhanced by a factor of similar to 10-40% over the nominal profile that was measured using the GCMS on Huygens. The northern hemisphere had similar to 90% of the nominal methane abundance up to polar latitudes (80 degrees N). These measurements suggest that a source of saturated polar air is equilibrating with dryer conditions at lower latitudes.