An international group working with archived observations from the Hubble Space Telescope identified the presence of water vapor in the thin atmosphere of Jupiter’s moon Ganymede.
It is the largest moon in the solar system, slightly larger than Saturn’s Titan and larger than the planet Mercury (albeit less dense). But to speak of atmosphere in this case deserves a qualification: it would be better to say exosphere, a very faint presence of gases, even in dilute atmospheres like that of Mars (which in turn is a hundredth of the density of the earth).
In 1998, a Hubble spectrograph took the first ultraviolet images of Ganymede and showed an emission pattern indicating the presence of the exosphere and a magnetic field, with patterns similar to those of auroras. Molecular oxygen, O2, has been detected and it has been assumed that there is also atomic oxygen to explain the distribution of emission bands over the moon.
In 2010 and 2018 new observations were collected (most recently with a different instrument) and then combined with those previously made. The processing did not reveal the presence of atomic oxygen, but of water vapor, H2O. The result is not entirely unexpected, given the composition of the moon. It is covered by a thick ice crust under which a global ocean of liquid water is hidden (kept in this state by the strong tidal effect of Jupiter).
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 enough that some of the ice is sublimated, converted into gas and fed to the exosphere . In these regions, water vapor predominates, in contrast to cooler areas where molecular oxygen predominates.
The work has been published in Nature Astronomy and is already serving as a warm-up for future Ganymede missions, such as the European Juice, which will be launched in 2022 and set to reach the Jupiter system in 2029. Also visit Calisto and Europe before heading to the 2034 enter final orbit around Ganymede.
Juice’s main goal will be to explore Ganymede’s potential to harbor life. In this sense, Europe is still the best choice. Both have subterranean oceans, but Europe is in direct contact with a bedrock and hydrothermal springs (a possible analogy to where life first appeared on earth). The Ganymede Ocean is enclosed between two layers of ice above and below. Fortunately, there is something for everyone because NASA is also preparing a new Jupiter mission, Europa Clipper. It was supposed to leave in 2024 and arrive there shortly after Juice in 2030.
This column is published in Folha Corrida on Mondays.
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