An intense thermospheric jet on Titan.

Winds in Titan's lower and middle atmosphere have been determined by a variety of techniques, including directmeasurements from the Huygens Probe over 0-150 km, Doppler shifts of molecular spectral lines in the optical, thermal infrared and mm ranges, probing altogether the ~100-450 km altitude range, and inferences from thermal field over 10 mbar - 10 -3 mbar (i.e. ~100-500 km) and from central flashes in stellar occultation curves. These measurements predominantly indicated strong progradewinds, reaching maximum speeds of ~150-200 m/s 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 mesosphereand thermosphere(500- 1200 km) has remained unconstrained so far. Here we report direct wind measurements based on Doppler shifts of six molecular species observed with ALMA. We show that unlike expectations, strong progradewinds extend up to the thermosphere, with the circulation progressively turning into an equatorial jet regime as altitude increases, reaching ~340 m/s at 1000 km. We suggest that these winds may represent the dynamical response of forcing by waves launched at upper stratospheric/ mesosphericlevels and/or magnetospheric-ionospheric interaction. We also demonstrate that the HNC distribution is restricted to Titan's thermosphereabove ~870 km altitude.