Exploration of Titan’s Atmosphere using Cassini Spacecraft Observations

Abstract

The endeavour to understand the planets in our solar system is a worthy field of study that
extends beyond the romance of exploring distant worlds. Planetary atmospheres serve as
invaluable laboratories to test our knowledge of atmospheric processes that may be crucial
to understanding Earth’s evolving climate. The planet in our solar system with an atmosphere
most similar to Earth’s is not a planet at all, but to a moon. Titan, the largest moon of Saturn,
hosts a dense, nitrogen-dominated atmosphere that is the site of the most complex organic
chemistry in our solar system. Titan experiences Earth-like seasons, which drive circulation in
its middle atmosphere and many processes analogous to those on Earth, including atmospheric
superrotation and winter polar vortices.
The Cassini-Huygens mission explored the Saturn system throughout 2004–2017, providing
the unique opportunity to observe Titan in unprecedented detail. One year on Titan is approximately 30 Earth years, so Cassini provided coverage of Titan for almost half of its year; during
its northern winter and spring. In this thesis, I present a study of infrared observations acquired
by the Composite Infrared Spectrometer (CIRS) instrument onboard the Cassini spacecraft. I
estimate the temperature and composition of Titan’s middle atmosphere using a forward model
coupled with the NEMESIS radiative transfer and retrieval code.
I demonstrate that the highest spatial resolution CIRS observations of Titan can be reliably
and efficiently forward modelled. Exploiting this, I present the temperature and composition
structure of Titan’s stratosphere in the highest meridional resolution to date. I investigate the
seasonal evolution of Titan’s stratospheric tilt and find that its direction would appear fixed to an
observer looking down on the entire Solar System. Additionally, I find a suggestion that the size
of the tilt is impacted by seasonal forcing. Finer details in Titan’s zonal winds are also revealed. I
present the first evidence that multiple jets may exist in Titan’s middle atmosphere in addition
to the strongest evidence yet that Titan’s polar vortex has a ring shape for part of its lifecycle.

Date of Award	9 Jul 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Nicholas A Teanby (Supervisor) & Dann M Mitchell (Supervisor)