Hubble Space Telescope observations show traces of water in Ganymede's atmosphere
An international scientific team - including researchers from the Laboratory for Planetary and Atmospheric Physics (LPAP) at the University of Liège - just reported the first detection of water vapour in the atmosphere of Ganymede, one of the four largest moons of Jupiter. This discovery - based on new observations with the Hubble Space Telescope - is published in Nature Astronomy.
Virtually all the planets in the solar system have an atmosphere, the physical and chemical properties of which vary greatly from planet to planet. Even our Moon has an extremely thin atmosphere. In this case, we speak of an exosphere. The satellites of the planets can therefore also have some kind of atmosphere. As far as Jupiter's satellites (Io, Europa, Ganymede and Callisto) are concerned, these moons have sufficient mass to retain the heaviest compounds that form their exosphere," explains Prof. Denis Grodent, Director of the STAR Research Unit (Faculty of Science) and head of the Laboratory for Planetary and Atmospheric Physics (LPAP) at the University of Liège and co-author of the study published in Nature Astronomy. For Ganymede, we can even say that this atmosphere contains mainly oxygen (O2) and water vapour (H2O). These constituents, which are certainly present in the Earth's atmosphere, come from the surface of this satellite, which is mostly composed of water ice.
There are at least three mechanisms for supplying Ganymede's atmosphere from this surface ice: sputtering, radiolysis (the decomposition of matter by ionising radiation) and sublimation (the change of state of a body from solid to gas). In addition, there are numerous physico-chemical reactions. The presence of O2 and H2O in Ganymede's atmosphere was already predicted by physical models," explains Dr Bertrand Bonfond, astrophysicist and FNRS Research Associate at LPAP and co-author of the study, "but until now only oxygen had been detected, in particular thanks to the observation by the Hubble Space Telescope (HST) of the ultraviolet aurorae that surround the poles of Ganymede. We are talking about tiny quantities of these molecules, as the pressure of the atmosphere at the surface of Ganymede is a hundred billion times lower than on Earth. It therefore took new and more accurate observations of this moon of Jupiter with the Hubble telescope to finally detect water vapour. "This detection is very indirect," says Denis Grodent, "because it is based on the ratio between two auroral emissions of atomic oxygen at two different wavelengths, which is called a 'colour ratio'.
This study clearly shows that the concentration of water vapour is highest where Ganymede is most exposed to sunlight (at the subsolar point, the point closest to the Sun, where the solar rays reach the moon perpendicularly) and disappears on the night side, which demonstrates the major role of the process of sublimation of the surface water ice. "Actually", continues Bertrand Bonfond, "at the subsolar point, the atmosphere is almost entirely made up of water vapour, whereas on the night side, there is only oxygen (O2). This asymmetry between the day and night sides must certainly lead to the transport of atmospheric constituents in the form of winds". This is clearly one of the properties of Ganymede's atmosphere that the ESA-JUICE mission, in which LPAP is actively involved, will have to study in detail when it will orbit Ganymede... in 2033.
Scientific reference
Lorenz Roth, Nickolay Ivchenko, G. Randall Gladstone, Joachim Saur, Denis Grodent, Bertrand Bonfond, Philippa Molyneux, Kurt Retherford. Evidence for a sublimated water atmosphere on Ganymede from Hubble Space Telescope observations. Nature Astronomy, 2021
