Dynamics and Structure of Titan's Polar Vortices Using Cassini CIRS Observations

  • Jason Sharkey

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

In this thesis we present a comprehensive study of
Titan’s polar vortices using infrared observations
recorded by the Composite InfraRed Spectrometer
(CIRS) on-board the Cassini spacecraft. Between 2004
and 2017, Cassini toured the Saturnian system,
Entry Details
performing 127 targeted flybys of Titan and providing
almost half a Titan year of coverage. A single Titan
year lasts approximately 30 Earth years. Observations
range from Titan’s northern midwinter through to
northern summer solstice, with northern spring
equinox occurring in August of 2009. With the use of
an appropriate forward model, we are able to
combine infrared observations with the NEMESIS
radiative transfer and retrieval code to obtain
estimates of the temperature and gaseous
composition throughout Titan’s middle atmosphere.
The tilt of the stratospheric rotation axis is
investigated, placing constraints on its magnitude
and attempting to determine its orientation. Our
results are consistent with previous measurements
and suggest a complex precession of the axial tilt.
Zonal variations in the northern
hemisphere are found to remain broadly
axisymmetric, suggesting that the role of upward
propagating Rossby waves into the stratosphere is
limited. The size and evolution of both vortices is
determined by thermal winds and potential vorticity
(PV) derived from temperature retrievals. A stable
annulus of PV is found at the poles, an unexpected
result, which may be attributed to a
combination of adiabatic heating and latent heat
released from condensing gases. Agreement is found
between latitudinal gradients of gas abundance and
PV, suggesting that the vortices act as efficient mixing
barriers. Quantitative maps of stratospheric
temperature, thermal winds and PV are also
produced, which will prove useful for future General
Circulation Models (GCMs).
Finally, compositional layering is found in the
northern vortex in 2005, approximately 2-3 years after
northern winter solstice where the vortex was likely to
have developed rapidly, although the layering
disappears shortly afterwards and is not observed in
the southern vortex. Compositional layering may be
attributed to some seasonal dynamical process which
causes differential advection across the vortex
Date of Award2 Dec 2021
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorNicholas A Teanby (Supervisor)

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