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 µm (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 = λ/∆λ ≈ 1, 500, while the Keck observations were performed with NIRSPAO near 1.5525 µm at higher resolution, R ≈ 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 CH 4 and CH 3 D correlated k distributions from the HITRAN 2012 database were used, which incorporate revised line assignments near 1.5 µm. 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 • S and 80 • N. The tropospheric methane in the Southern mid-latitudes was enhanced by a factor of ∼10-40% over the nominal profile that was measured using the GCMS on Huygens. The Northern hemisphere had ∼90% of the nominal methane abundance up to polar latitudes (80 • N). These measurements suggest that a source of saturated polar air is equilibrating with dryer conditions at lower latitudes.

Titan's atmosphere and climate

“…The decrease is a result of condensation and the temperature at the tropopause limits the methane mixing ratio in the stratosphere. Unfortunately, this profile is limited to a single location at a single point in time (10.3°S 167.7°W, 14 January 2005), and there is some evidence of tropospheric methane variations of ∼10–40% [ Tokano , ; Ádámkovics et al , ]. The average stratospheric value from GCMS is 1.48 ± 0.09%, which is consistent with initial determinations of the stratospheric methane mixing ratio from the Composite Infrared Spectrometer (CIRS) (1.6 ± 0.5% [ Flasar et al , ]) and DISR [ Bézard , ].…”

Spherical Radiative Transfer in C++ (SRTC++): A Parallel Monte Carlo Radiative Transfer Model for Titan

“…Quality one-dimensional algorithms exist and have been thoroughly tested. And 1D codes' approximations allow for fast radiative transfer calculations, thus enabling their practical use for spectral modeling (Young et al 2002;Hirtzig et al 2013) and the inversion of surface albedo (Coustenis et al 1995;Griffith et al 2012a;Solomonidou et al 2014;Maltagliati et al 2015b;Solomonidou et al 2016;Ádámkovics et al 2016).…”

Science goals and new mission concepts for future exploration of Titan’s atmosphere, geology and habitability: titan POlar scout/orbitEr and in situ lake lander and DrONe explorer (POSEIDON)

“…What is the degree of oxygen incorporation in photochemical species? • [2] What drives the dynamics of the upper atmosphere and what is its origin?…”

“…• [2] How do the polar vortices form, evolve, and dissipate? • [3] What is the complexity of the chemistry attained inside the polar vortices?…”

“…CH 4 , which plays a key role in the complex chemistry of Titan's atmosphere, comes from the surface and/or subsurface (Section 3.5). Monitoring the vertical and latitudinal distribution [2,157] and seasonal evolution of tropospheric methane humidity may allow us to identify the CH 4 main evaporation sources (e.g. polar lakes, hypothetical tropical lakes, or ground humidity).…”