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Direct measurement of Neptune’s stratospheric winds with ALMA

Presentation #409.07 in the session Uranus and Neptune Systems (Oral Presentation)

Published onOct 23, 2023
Direct measurement of Neptune’s stratospheric winds with ALMA

Voyager 2 revealed that Neptune’s tropospheric winds are among the most intense in the Solar System. The atmospheric dynamical mechanisms that produce these winds remain however uncertain. Measuring wind speeds at different pressure levels may help understand the atmospheric dynamics of the planet. In this talk we present direct measurements of Neptune’s stratospheric winds with ALMA Doppler spectroscopy. We derived the Doppler lineshift maps of Neptune at the CO(3-2) and HCN(4-3) lines at 345.8 GHz (λ~0.87 mm) and 354.5 GHz (0.85 mm), respectively. For that, we used spectra obtained with ALMA in 2016 and recorded with a spatial resolution of ~0.37" on Neptune’s 2.24" disk. After subtracting the planet solid rotation, we inferred the contribution of zonal winds to the measured Doppler lineshifts at the CO and HCN lines.

We developed an MCMC-based retrieval methodology to constrain the latitudinal distribution of wind speeds. We find that CO(3-2) and HCN(4-3) lines probe the stratosphere of Neptune at pressures of 2+12-1.8 mbar and 0.4+0.5-0.3 mbar, respectively. The zonal winds at these altitudes are less intense than the tropospheric wind based on cloud tracking from Voyager observations. We find equatorial retrograde (westward) winds of -180+70-60 m/s for CO, and -190+90-70 m/s for HCN. Wind intensity decreases towards mid-latitudes, and becomes 0 m/s at about 50ºS. The circulation reverses to a prograde jet southwards of 60ºS. The beam size of ALMA in these observations (~0.37") prevents an accurate retrieval of wind speeds near the south pole. Overall, our direct stratospheric wind measurements match previous estimates from stellar occultation profiles and expectations based on thermal wind equilibrium. These are the first direct Doppler wind measurements performed on the Icy Giants, opening a new method to study and monitor their stratospheric dynamics.

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