An intense thermospheric jet on Titan

Abstract

The presence of winds in Titan’s lower and middle atmosphere has been determined by a variety of techniques, including direct measurements from the Huygens Probe1 over 0–150 km; Doppler shifts of molecular spectral lines in the optical, thermal infrared and millimetre ranges2–4, which together have probed the ~100–450 km altitude range; inferences from the thermal field over 10–0.001 mbar (that is, ~100–500 km)5,6; and inferences from central flashes in stellar occultation curves7–9. These measurements predominantly indicated strong prograde winds, reaching maximum speeds of ~150–200 m s−1 in the upper stratosphere, with important latitudinal and seasonal variations. However, these observations provided incomplete atmospheric sounding; in particular, the wind regime in Titan’s upper mesosphere and thermosphere (500–1,200 km) has remained unconstrained so far. Here we report direct wind measurements based on Doppler shifts of six molecular species observed with the Atacama Large Millimeter/submillimeter Array (ALMA). We show that contrary to expectations, strong prograde winds extend up to the thermosphere, with the circulation progressively turning into an equatorial jet regime as the altitude increases, reaching ~340 m s−1 at 1,000 km. We suggest that these winds may represent the dynamical response of forcing by waves launched at upper stratospheric/mesospheric levels and/or of magnetospheric–ionospheric interaction. We also demonstrate that the distribution of the hydrogen isocyanide (HNC) molecule is restricted to Titan’s thermosphere above ~870 km altitude. Molecular emission lines originating in Titan’s relatively unexplored upper mesosphere and thermosphere reveal a strong prograde jet that reaches speeds of 340 m s–1 at 1,000 km altitude.