The near-surface methane humidity on Titan

TL;DR

This study analyzes Titan's lower troposphere methane humidity variations using ground-based spectra, revealing undersaturation in the south and increased humidity near the north pole, indicating surface liquid presence.

Contribution

It introduces a method to retrieve methane humidity profiles without supersaturation assumptions and compares results with circulation models to understand surface liquids.

Findings

Lower troposphere is undersaturated south of 60N.

Increased humidity toward the north pole suggests surface liquids.

Uncertainty remains due to haze and methane degeneracy.

Abstract

We retrieve vertical and meridional variations of methane mole fraction in Titan's lower troposphere by re-analyzing near-infrared ground-based observations from 17 July 2014 UT (Adamkovics et al., 2016). We generate synthetic spectra using atmospheric methane profiles that do not contain supersaturation or discontinuities to fit the observations, and thereby retrieve minimum saturation altitudes and corresponding specific humidities in the boundary layer. We relate these in turn to surface-level relative humidities using independent surface temperature measurements. We also compare our results with general circulation model simulations to interpret and constrain the relationship between humidities and surface liquids. The results show that Titan's lower troposphere is undersaturated at latitudes south of 60N, consistent with a dry surface there, but increases in humidity toward the north pole indicate appreciable surface liquid coverage. While our observations are consistent with considerably more liquid methane existing at the north pole than is present in observed lakes, a degeneracy between low-level methane and haze leads to substantial uncertainty in determining the extent of the source region.