Detection of Dynamical Instability in Titan's Thermospheric Jet
  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.
