Seasonal variation of the main gases in Titan's ionosphere from Cassini INMS data

¹ LATMOS-IPSL’ CNRS, UVSQ, Université Paris-Saclay, Guyancourt, France
² Max Planck Institute for Solar System Research, Goettingen, Germany
³ LESIA’ Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris-Cité, Meudon, France
⁴ Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA
⁵ NASA Headquarters, Washigton, DC, USA
⁶ NASA Goddard Space Flight Center, Greenbelt, MD, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 September 2023
Accepted: 29 January 2026

Abstract

With 13 years of observations, the Ion and Neutral Mass Spectrometer (INMS) on board the Cassini spacecraft has observed the upper atmosphere of Titan through two seasons: northern winter and spring. The complex atmosphere is mainly composed of N₂, CH₄, H₂, and Ar, but many other carbon and nitrogen bearing trace species have been observed by INMS and other instruments. Using data from the closed source neutral (CSN) mode of the INMS instrument, we studied the abundance of the main gases and their variation in Titan’s ionosphere between 1500 and 950 km of altitude with a new mass spectra deconvolution code based on a Monte Carlo approach. Our results showed a hemispheric dichotomy in gas mole fractions at a constant altitude, with an enrichment of methane mole fraction in the southern hemisphere that seems to decreases after 2011 during northern spring. At constant N₂ density, however, regardless of the hemisphere, we observe an increase in the N₂ mole fraction with time. We note a strong correlation between the gas mole fraction and the solar cycle as well. We also derived an ¹⁴N/¹⁵N isotope ratio of 197.7 ± 3.9(3σ).