Jupiter's banded circulation through the eyes of VLT/ESPRESSO

The thousands of exoplanets that have already been discovered launched an unprecedent drive towards the exploration of these new worlds, particularly their atmospheres. Many of these new planets fit the profile of a gas giant, although with a wide range of characteristics. To study these far away objects, we often use the Solar System as a starting point, thus a good knowledge of the atmospheres of these objects is paramount to understand these other worlds. In this regard, Jupiter often serves as model gas giant, due to its size and mass combination, among other parameters. However, despite numerous interplanetary and orbiting spacecraft combined with a long record of Earth-based observations, some fundamental questions regarding dynamical processes in Jupiter's atmosphere remain [Fletcher et al. (2020)]. The vertical structure of the colourful clouds we see with a small-sized telescope and their circulation mechanisms are still elusive [Sanchez-Lavega (2011)]. Also, to study the atmosphere dynamics of solar system planets, particularly its behaviour and evolution with time, continuous observations are required [Hueso et al. (2020)].

Multiple records of detailed observations span across more than 30 years, from first analysis of the zonal winds [Limaye et al. (1986), Vasavada et al. (1998)] using Pioneer and Galileo data, to more detailed views from the Cassini flyby [Porco et al. (2003), Salyk et al. (2006), Garcia-Melendo et al. (2011), Galperin et al. (2014)]. The most recent efforts in this regard are attributed to the Juno mission, which contributes with very high spatial resolution images [Hansen et al. (2017)], supported by observation campaigns from the Hubble Space Telescope [Garcia-Melendo et al. (2001), Tollefson et al. (2017), Hueso et al. (2017), Johnson et al. (2018)]. Although an impressive volume of data, winds at tropospheric levels have mostly been obtained with cloud-tracking techniques, which follow large patterns moving in the observable atmosphere of Jupiter. Recent efforts in studying the dynamics of the tropospheric region of Jupiter with other techniques such as high-resolution spectroscopy are gaining momentum, with the improvement of facilities which enable increased spectral resolution [Gaulme et al. (2018), Goncalves et al. (2019)].

Different techniques, such as high resolution spectroscopy applied to planetary atmospheres of the Solar System to study dynamics, can be complementary to the usually employed cloud-tracking method, by targeting slightly different levels of the atmosphere [Machado et al. (2021)], with the possibility therein to study the vertical wind shear. The technological capabilities of modern facilities that were designed to discover exoplanets can also be taken advantage of to observe Solar System atmospheres, achieving unprecedent levels of precision from the ground [Gonçalves et al. (2020)]. One such facility is ESPRESSO, assembled on the Very Large Telescope, at ESO. ESPRESSO is able to get two simultaneous spectra in a wavelength range between 378.2 and 788.7 nm with a resolving power that ranges from 70,000 in the Medium Resolution mode (MR) to more than 190,000 in the Ultra High Resolution mode (UHR) [Pepe et al. (2021)]. ESPRESSO was originally designed for exoplanet hunting and atmospheric characterisation, however, just as was demonstrated in [Gonçalves et al. (2020)] for HARPS-N, using these very high resolution spectrographs on solar system atmospheric characterisation can open new horizons on what is possible to achieve with ground-based instruments to study large objects in our cosmic vicinity.

We present an optimised Doppler velocimetry method, originally used to retrieve winds on Venus' cloud top region in the visible part of the spectrum [Widemann et al. (2008), Machado et al. (2012), Machado et al. (2014), Machado et al. (2017)]. With its successful application to Venus, this work presents an exploration of other targets within the solar system with our method. It is also an opportunity to investigate the effectiveness of ESPRESSO in the study of Solar System atmospheres, since it was used for this purpose for the first time. We show zonal wind speeds at equatorial latitudes using all the lines in the visible spectrum, from solar radiation backscattered on Jupiter’s atmosphere. These results are compared with the plethora of wind velocity data already retrieved in Jupiter’s troposphere for validation, finding consistency between both methods, despite our limited spatial and temporal coverage.

This work promotes another step in the exploration of other Solar System targets with ground-based observations, to fill the gap left by the limited availability of interplanetary space missions, ensuring continuous monitoring of the evolution of the atmospheric circulation on those planets at high spectral resolutions.

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