There is water vapor in the atmosphere of Ganymede, the largest moon of Jupiter and of the entire solar system: the first direct evidence of its presence was discovered in the archive data of the Hubble Space Telescope of NASA and the European Space Agency (Esa ). The study, published in the journal Nature Astronomy, indicates that the vapor arises from the sublimation of the frozen surface of Ganymede and not from the evaporation of the ocean that hides about 160 kilometers deep under the crust: an important element in the search for possible forms. of life that will be conducted by the future Juice mission that the European Space Agency (ESA) plans to launch in 2022 with an important contribution from the Italian Space Agency (ASI).
Ganymede has been under the scrutiny of researchers for years, because the data collected so far have provided indirect evidence of the presence of a huge amount of water, greater than all that contained in the terrestrial oceans.
Extremely cold temperatures, however, cause the surface water to be frozen.
The first images of the satellite made in the ultraviolet by Hubble in 1998 thanks to the Stis spectrograph had revealed particular emissions from the atmosphere that seemed to indicate the presence of a weak magnetic field: moreover, some differences visible in two images in particular had been attributed to the possible presence in the atmosphere of oxygen in the form of a single atom (O) as well as molecular (O2).
The ultraviolet images of Ganymede taken by Hubble in 1998 (source: NASA / ESA / L.Roth)
In 2018, a research group from the Royal Institute of Technology in Stockholm (KTH) led by Lorenz Roth used the Hubble Cos spectrograph to measure oxygen in atomic form: combining data with archival data collected between 1998 and 2010 by the Stis instrument, it was discovered that in reality oxygen in atomic form is almost completely absent. Roth and colleagues tried to re-examine the UV images, discovering that the different emissions of the atmosphere could be interpreted in another way. The surface temperature of Ganymede, in fact, varies a lot during the day and in particular in the afternoon, near the equator, it could become hot enough to induce the ice surface to release water molecules. In fact,the differences observed in the Uv images are directly related to the areas where water vapor in the atmosphere would be expected.
