13 Septembre – Thesis defense - Bilal Benmahi

14 h Room Univers - Laboratory of Astrophysics of Bordeaux - Building B18N (University of Bordeaux - Bordes campus)

Composition and dynamics of the atmospheres of giant planets: preparation of the JUICE mission.

The Jupiter Icy Moon Explorer (JUICE) is the next major mission of the European Space Agency. JUICE will embark 10 instruments and will be launched in 2023 to study the Jovian system from 2031 during 4 years. Among its instruments, we are particularly interested in the Submillimeter Wave Instrument (SWI, PI P. Hartogh, MPS, Germany). SWI is a 29 cm diameter telescope that will observe Jupiter and the Galilean satellites in the submillimeter range. SWI is intended to study the composition and the dynamics of the atmospheres of these bodies as well as the surfaces of the satellites. This thesis work consists in analyzing and interpreting observations of Jupiter and Saturn, taken with ground-based and space-based telescopes, to prepare the scientific program of JUICE/SWI.
We first analyzed the temporal evolution of the H2O abundance in the Jupiter stratosphere with two decades of data collected by the Odin space telescope. This first study allows us to predict the observability of H2O in Jupiter’s stratosphere by SWI thanks to our photochemical modelling.
We also analyzed ALMA (Atacama Large Millimeter/submillimeter Array) observations of CO and HCN, two molecules initially deposited in Jupiter’s stratosphere during the impacts of comet Shoemaker-Levy-9 in 1994. Like H2O, these compounds will be observable in the SWI spectral bands. We have developed a method to directly measure the Doppler shift induced by atmospheric winds on the spectral lines, thanks to the very high spectral and spatial resolutions of ALMA. This method, which allows us to study the atmospheric dynamics of giant planets, is based on the Markov Chain Monte Carlo modeling technique.
Our results in the upper stratosphere of Jupiter indicate that jets are present in the equatorial zone. These wind measurements, coupled with near-simultaneous measurements of stratospheric temperature, have allowed us to characterize the equatorial stratospheric circulation using the thermal wind equation. We found that westward and eastward jets are vertically stacked at the equator.
We have also applied this wind measurement technique to ALMA observations of CO and HCN in Saturn's stratosphere. Our results reveal a surprising equatorial circulation. We observe at a pressure of 0.1 mbar the large equatorial jet of Saturn previously observed by Hubble and Cassini at the cloud level at pressures between 60 and 2000 mbar. Our observations demonstrate that this broad jet extends 500 km higher than the Hubble and Cassini observations.
Moreover, we have discovered that unique chemistry and dynamics occur in the polar zones of Jupiter and Saturn. Indeed, in the stratosphere of Jupiter, our Odin and ALMA observations show that H2O and HCN are destroyed more rapidly in these zones compared to the low and middle latitudes. Moreover, we have detected the presence of auroral jets that are located under the northern and southern auroral ovals of Jupiter. It seems that a similar jet is also present under the northern auroral oval of Saturn. We think that these jets could form giant vortices up to the ionosphere where similar dynamics have been detected on Jupiter. It is therefore possible that we are in the presence of gigantic vortices that would participate in the confinement of matter in the auroral zones where energetic electrons precipitate and contribute to the chemistry. This is why we have finally developed a tool to simulate the transport of magnetospheric electrons in order to combine it with an ion-neutral photochemical model in the auroral zones of Jupiter to better understand the chemistry and dynamics of these particular regions. In the long term, we plan to apply this model to Saturn also.

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