Astronomers have found the first evidence of water vapour in the atmosphere of Jupiter’s moon Ganymede, the largest moon in the Solar System. They believe Ganymede may hold more water than all of Earth’s oceans. But finding water in liquid form there is difficult. The temperatures are so cold that all the water on the surface is frozen solid and the ocean lies roughly 100 miles (160km) below the crust, said the European Space Agency. Still, scientists believe finding water is a crucial first step in knowing whether life could exist on a celestial body or not. The astronomers analysed archival datasets of the Hubble Telescope over the past two decades to come to this conclusion.

The ESA said identifying liquid water on other planets is crucial to understand whether they are habitable.

The research is based on datasets going back to 1998, when Hubble took the first ultraviolet (UV) pictures of Ganymede. These images revealed a particular pattern in the observed emissions from the moon’s atmosphere which was somewhat similar to those observed on Earth and other planets with magnetic fields.

Scientists later found that Ganymede’s surface temperature varies extremely throughout the day. Around noon, it may become warm enough that the icy surface releases some small amounts of water molecules. Since the oceans lay miles below the crust, it is unlikely that the water vapour could be from them.

“Initially only the O2 (molecular oxygen) had been observed,” said lead researcher Lorenz Roth, adding that this is produced when charged particles erode the ice surface.

He said the water vapour his team has found originates from ice sublimation.

NASA, too, has released a video that describes the new finding. Watch it here:

This development has led to curiosity ahead of ESA’s planned Jupiter Icy Moons Explorer (JUICE) mission in 2022. The mission is expected to reach Jupiter in 2029 and will spend the next three years studying Jupiter and three of its largest moons, including Ganymede.