Reanalysis of the Huygens GCMS dataset

II. Trace species in Titan's lower atmosphere

¹ LATMOS-IPSL, CNRS, Sorbonne université, UVSQ Université Paris-Saclay, France
² LESIA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CNRS, Meudon, France
³ Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA
⁴ Groupe de Spectroscopie Moléculaire et Atmosphérique (GSMA), Université de Reims Champagne-Ardenne, Reims, France

^★ Corresponding author.

Received: 11 July 2024
Accepted: 28 March 2025

Abstract

The Huygens probe landed on Titan in January 2005, following the arrival of the Cassini-Huygens mission in the Kronian system. The Gas Chromatograph Mass Spectrometer instrument (GCMS) aboard the Huygens probe performed measurements in the lower stratosphere (from an altitude of 150 km) and in the troposphere to constrain the composition of Titan’s atmosphere. Earlier works on this dataset focused mainly on the major constituents of Titan atmosphere and on noble gases and were not able to quantify the trace gas composition in the atmosphere because of limitations in their mass spectrometry (MS) analysis techniques. In this paper, the GCMS dataset was analyzed with a MS deconvolution code that uses a Monte Carlo approach to quantify these trace gases. The mole fractions of four trace species retrieved from the GCMS data are significantly higher than the ones retrieved by the inversion of data returned by the Cassini-Composite InfraRed Spectrometer (CIRS) and the mixing ratio predicted for these volatiles by the Titan Planetary Climate model. As such, our results do not match with anything that we know of Titan’s lower atmosphere, which very likely points toward the fact that the GCMS instrument did not in fact measure atmospheric trace gases only. We investigated several hypotheses that could explain this discrepancy. In our opinion, the most likely one is that the trace gases measured by GCMS correspond to an outgassing from Titan atmospheric aerosols that could have been trapped inside the instrument. However, mole fractions of trace species retrieved from GCMS Leak 3 measurements matched with CIRS retrieved values and modeled data, and these are the only atmospheric trace gas measurements from Huygens GCMS. This dataset can be used further to identify other minor species present in this atmospheric region.