Astronomers have uncovered the first evidence of water vapour in the atmosphere of Jupiter's moon Ganymede by using new and archival datasets from NASA's Hubble Space Telescope.
According to the study published in the journal Nature Astronomy on Monday, the water vapour forms when ice from the moon's surface turns from solid to gas.
Previous studies have offered circumstantial evidence that Ganymede, the largest moon in the solar system, contains more water than all of Earth's oceans, NASA said.
However, temperatures there are so cold that water on the surface is frozen solid, according to the US space agency.
Ganymede's ocean would reside roughly 160 kilometres below the crust, therefore, the water vapour would not represent the evaporation of this ocean.
Astronomers re-examined Hubble observations from the last two decades to find this evidence of water vapour.
In 1998, Hubble's Space Telescope Imaging Spectrograph took the first ultraviolet (UV) images of Ganymede, which revealed colourful ribbons of electrified gas called auroral bands, and provided further evidence that Ganymede has a weak magnetic field.
The similarities in these UV observations were explained by the presence of molecular oxygen (O2).
However, some observed features did not match the expected emissions from a pure O2 atmosphere.
At the same time, scientists concluded this discrepancy was likely related to higher concentrations of atomic oxygen (O).
Lorenz Roth of the KTH Royal Institute of Technology in Stockholm, Sweden led the team to measure the amount of atomic oxygen with Hubble.
The team's analysis combined the data from two instruments: Hubble's Cosmic Origins Spectrograph in 2018 and archival images from the Space Telescope Imaging Spectrograph (STIS) from 1998 to 2010.
Contrary to the original interpretations of the data from 1998, they discovered there was hardly any atomic oxygen in Ganymede's atmosphere.
Roth and his team then took a closer look at the relative distribution of the aurora in the UV images.
Ganymede's surface temperature varies strongly throughout the day, and around noon near the equator it may become sufficiently warm that the ice surface releases some small amounts of water molecules, the researchers said.
The perceived differences in the UV images are directly correlated with where water would be expected in the moon's atmosphere, they said.
"So far only the molecular oxygen had been observed," explained Roth.
"This is produced when charged particles erode the ice surface. The water vapour that we measured now originates from ice sublimation caused by the thermal escape of water vapour from warm icy regions," he said.
The finding adds anticipation to European Space Agency (ESA)'s upcoming mission JUpiter ICy moons Explorer (JUICE).
Planned for launch in 2022 and arrival at Jupiter in 2029, JUICE will spend at least three years making detailed observations of Jupiter and three of its largest moons, with particular emphasis on Ganymede as a planetary body and potential habitat.
"Our results can provide the JUICE instrument teams with valuable information that may be used to refine their observation plans to optimise the use of the spacecraft," Roth added.
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