The first direct measurement of the saturnian stratospheric winds
- 1Université de Bordeaux, Laboratoire Astrophysique de Bordeaux, Physics, Pessac, France (bilal.benmahi@u-bordeaux.fr)
- 2LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, Meudon, France
- 3Laboratoire de Météorologie Dynamique/Institut Pierre-Simon Laplace (LMD/IPSL), Sorbonne Université, Centre National de la Recherche Scientifique (CNRS), Ecole Polytechnique, Ecole Normale Supérieure (ENS), Campus Pierre et Marie Curie BC99, 4 place Jussi
- 4Institut Universitaire de France, Paris, France
Numerous past observations of Saturn by ground based and space telescopes have monitored the movements of clouds and derived direct measurements of tropospheric wind speeds, giving insights into the tropospheric circulation of the planet. The most remarkable feature is a broad and fast equatorial prograde jet, reaching 400-450 m/s. Saturn's stratospheric dynamics is less well known. At low latitudes, it is characterized by the thermal signature of an equatorial oscillation: the observed thermal structure implies that there is a strong oscillating vertical shear of the zonal winds throughout the stratosphere, however, wind speeds in this region cannot be measured by cloud-tracking techniques and remain unknown.
The objective of our study is to measure the stratospheric zonal winds of Saturn and unveil the circulation of this layer by observing it in the submillimeter range with the ALMA interferometer. For this, we observed the spectral lines of HCN at 354 GHz and CO at 345 GHz emitted from the limb of the planet. The pressure level at which we measure the winds is about 0.2 mbar. Thanks to the high spatial and spectral resolution of ALMA observations at 345 GHz, we measured the central frequencies of the emission lines in the whole limb, subtracted the rigid rotation of the planet, and thus derived the Doppler shift due to the atmospheric motions of the probed layer, i.e. the stratospheric winds. The method we used in this study was first developed to observe the stratospheric winds in Jupiter (Cavalié et al. 2021).
Saturn's rings have limited our wind observations to latitudes north of 20°S. The zonal winds obtained in the eastern and western limbs are consistent within error bars. We most noticeably detected a very broad eastward jet that spreads from 20°S to 20°N with an average speed of exceeding 250 m/s.
How to cite: Benmahi, B., Cavalié, T., Fouchet, T., Lellouch, E., Moreno, R., Guerlet, S., Spiga, A., and Bardet, D.: The first direct measurement of the saturnian stratospheric winds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11527, https://doi.org/10.5194/egusphere-egu22-11527, 2022.