Hubble telescope detects water vapour on Jupiter’s moon Ganymede

27 Jul 2021

Image: © ianm35/

The finding comes ahead of the European Space Agency’s planned mission to Jupiter, scheduled to launch next year, which aims to determine if Ganymede is habitable.

Astronomers have found evidence of water vapour in the atmosphere of Jupiter’s largest moon, Ganymede, for the first time.

They used data from NASA’s Hubble space telescope, which was facing technical difficulties last month but is now back online.

Ganymede, which is also the largest moon in the solar system, is believed to have more water than Earth. However, because of its sub-zero temperatures, all the surface water is frozen.

NASA said that because Ganymede’s ocean would flow around 100 miles below its crust, the water vapour observed is not a result of evaporation. Instead, it is a result of sublimation – with ice turning from a solid into a gas.

Scientists examined Hubble observations from the past two decades to discover the evidence in this new research, published in the journal Nature Astronomy.

In 1998, Hubble’s first UV images of Ganymede suggested that the moon may have a weak magnetic field. The telescope revealed colourful ribbons of electrified gas called auroral bands, which were explained at the time by the presence of molecular oxygen.

20 years later, a team led by astronomer Lorenz Roth from the KTH Royal Institute of Technology in Stockholm used data from NASA’s Juno mission to Jupiter to discover there was hardly any atomic oxygen in Ganymede’s atmosphere.

Roth and his team then found that the observations can be explained by the presence of water vapour as a result of sublimation.

NASA said that Ganymede’s surface temperature varies strongly throughout the day and around noon, near the equator, it may become warm enough that the ice surface sublimates small amounts of water molecules.

“So far only the molecular oxygen had been observed,” Roth added. “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.”

A potential habitat

The findings come ahead of the planned European Space Agency (ESA) mission to Jupiter, known as Jupiter icy moons explorer, or Juice. It is the first large-class mission in the ESA’s 2015 to 2025 space programme.

Juice is scheduled for launch in 2022 but will only reach Jupiter in 2029. It is expected to spend three years observing Jupiter and its three largest moons, with a focus on determining if Ganymede could be a potential habitat.

NASA said that Ganymede was identified for detailed investigation because it provides a “natural laboratory for analysis of the nature, evolution and potential habitability of icy worlds”, and also has “unique magnetic and plasma interactions” with Jupiter and its environment.

“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,” added Roth.

Vish Gain is a journalist with Silicon Republic