Jupiter's Moon Ganymede Has Water Vapour, Confirms NASA Hubble Space Telescope

For the first time, astronomers have found evidence of water vapour in the atmosphere of Jupiter's moon, Ganymede. This exciting discovery, published in the journal Nature Astronomy, was made using new and archival datasets from NASA's Hubble Space Telescope.

NASA

Previous research provides evidence that Ganymede, the largest moon in the solar system, contains more water than all of Earth¡¯s oceans combined.

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But temperatures on Jupiter¡¯s moon are so cold that water on the surface remains frozen solid. However, scientists have found evidence suggesting that the moon¡¯s ocean is located in liquid form some 160 km beneath the surface. This means that the water vapour observed does not come from that body of water.

The first UV images of Ganymede offered hope

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.

Observations deduced from the UV images suggested the presence of molecular oxygen (O2) on the moon. But, the images did not match the expected emissions from a pure O2 atmosphere. Scientists had then concluded that this was likely related to higher concentrations of atomic oxygen (O).

NASA

But Roth and his team re-examined Hubble observations from the last two decades to find this evidence of water vapour. They compared Hubble¡¯s photos from 2018 to those from Space Telescope Imaging Spectrograph from 1998 to 2010.

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Contrary to the original interpretations, the team discovered there was hardly any atomic oxygen in Ganymede's atmosphere, which meant there was another explanation for the apparent differences in these UV aurora images.

A sublimated water atmosphere on Ganymede

This water vapour discovered by the researchers forms when ice from Ganymede¡¯s surface sublimates -- transforms directly from solid to gas (without changing to liquid). This is unlike the phenomenon witnessed on Earth, where water vapour is formed after evaporation, i.e. after heating a liquid.

Ganymede's surface temperature varies strongly throughout the day. And around noon near the equator, the moon's icy surface becomes warm enough to release small amounts of water molecules. This was the water vapour the Hubble observed.

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"So far only the molecular oxygen had been observed," Lorenz Roth of the KTH Royal Institute of Technology in Sweden told NASA. "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."

JUICE mission to Jupiter

This finding adds anticipation to European Space Agency's (ESA) upcoming JUpiter ICy moons Explorer (JUICE) mission. Planned for launch in 2022 and arrival at Jupiter in 2029, it 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.

Ganymede has interested scientists as it offers a natural laboratory for analysis of the nature, evolution and potential habitability of icy worlds in general, the role it plays within the system of Galilean satellites, and its unique magnetic and plasma interactions with Jupiter and its environment, the US space agency said.