Photochemical enrichment of deuterium in Titan's atmosphere: new insights from Cassini-Huygens

TL;DR

This study re-examines the sources of deuterium enrichment in Titan's atmosphere using Cassini-Huygens data, concluding that photochemical processes alone cannot explain the observed D/H ratio, implying a primordial origin is also necessary.

Contribution

It provides a new analysis combining Cassini data and updated models to assess the role of photochemistry in deuterium enrichment on Titan, highlighting the need for primordial contributions.

Findings

Photochemical enrichment alone is insufficient to explain D/H levels.

A possible but limited fractionation during methane escape occurs.

Primordial deuterium enrichment likely contributes to current D/H ratio.

Abstract

Cassini-Huygens data are used to re-examine the potential sources of the D/H enhancement over solar, measured in methane, in Titan's atmosphere. Assuming that the system is closed with respect to carbon, the use of constraints from the Huygens probe for the determination of the current mass of atmospheric methane and the most up-to-date determination of D/H from Cassini/CIRS infrared spectra allow us to show that photochemical enrichment of deuterium is not sufficient to be the sole mechanism yielding the measured D/H value. A possible fractionation between CH3D and CH4 during the escape process may slightly enhance the deuterium enrichment, but is not sufficient to explain the observed D/H value over the range of escape values proposed in the literature. Hence, alternative mechanisms such as a primordial deuterium enrichment must be combined with the photochemical enrichment in Titan's atmosphere in order to explain its current D/H value.