An international group working with archival Hubble Space Telescope observations identified the presence of water vapor in the tenuous atmosphere of the Jovian moon Ganymede.
It is the largest moon in the Solar System, slightly larger than Saturn’s Titan and larger than the planet Mercury (though less dense). But speaking of atmosphere in this case deserves a qualification: it would be better to say exosphere, a very tenuous presence of gases, which cannot be compared to the denser envelopes even in rarefied atmospheres, such as that of Mars (which in turn has a hundredth the density of terrestrial).
In 1998, a Hubble spectrograph took the first ultraviolet images of Ganymede and revealed a pattern of emissions that indicated the presence of the exosphere and a magnetic field, with patterns similar to those of auroras. Molecular oxygen, O2, was detected, and it was thought that there was also atomic oxygen, to explain the distribution of emission bands over the moon.
New observations were collected in 2010 and 2018 (on this last occasion with a different instrument) and then combined with those made previously. The processing did not reveal the presence of atomic oxygen, but water vapor, H2O. The result is not altogether unexpected, given the moon’s composition. It is covered by a thick crust of ice, under which a global ocean of liquid water (kept in this state by the powerful tidal effect provided by Jupiter) hides.
Lorenz Roth’s team, from the Royal KTH Institute of Technology in Sweden, found that there are places near the equator where the temperature can rise to the point of sublimating some of the ice, converting it to gas, and feeding the exosphere. In these regions, there is a predominance of water vapor, in contrast to cooler areas, where molecular oxygen predominates.
The work was published in Nature Astronomy and already serves as a warm-up for future Ganymede missions, such as the European Juice, which will be launched in 2022 and should reach the Jupiter system in 2029. the spacecraft being built by the Airbus company will also visit Calisto and Europa, before entering definitive orbit around Ganymede in 2034.
Juice’s main objective will be to investigate Ganymede’s potential to harbor life. In this sense, the best bet is still Europe. Both have subsurface oceans, but Europa’s is in direct contact with a bedrock and hydrothermal vents (a possible analogue of where life first appeared on Earth). The ocean of Ganymede is sandwiched between two layers of ice, above and below. Luckily, there’s it for everyone, as NASA is also preparing a new Jovian mission, Europa Clipper. It should leave in 2024 and get there a little after Juice, in 2030.
This column is published on Mondays in Folha Corrida.
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