Detection of Dynamical Instability in Titan's Thermospheric Jet

TL;DR

This study uses ALMA observations to map Titan's 3D wind field, revealing a significant slowdown and broadening of its thermospheric equatorial jet over nine months, indicating dynamical instability in Titan's upper atmosphere.

Contribution

First detailed 3D wind mapping of Titan's thermosphere showing temporal variability and instability of the equatorial jet.

Findings

47% reduction in wind speed over 9 months

Evidence of dynamical instability in Titan's thermosphere

Rapid variability suggests strong changes in atmospheric dynamics

Abstract

Similar to Earth, Saturn's largest moon, Titan, possesses a system of high-altitude zonal winds (or jets) that encircle the globe. Using the Atacama Large Millimeter/submillimeter Array (ALMA) in August 2016, Lellouch et al. (2019) discovered an equatorial jet at much higher altitudes than previously known, with a surprisingly fast speed of up to ~340 m/s, but the origin of such high velocities is not yet understood. We obtained spectrally and spatially resolved ALMA observations in May 2017 to map Titan's 3D global wind field and compare our results with a reanalysis of the August 2016 data. Doppler wind velocity maps were derived in the altitude range ~300-1000 km (from the upper stratosphere to the thermosphere). At the highest, thermospheric altitudes, a 47% reduction in the equatorial zonal wind speed was measured over the 9-month period (corresponding to L_s = 82-90 degrees on Titan). This is interpreted as due to a dramatic slowing and loss of confinement (broadening) of the recently-discovered thermospheric equatorial jet, as a result of dynamical instability. These unexpectedly-rapid changes in the upper-atmospheric dynamics are consistent with strong variability of the jet's primary driving mechanism.