Hubble in another first: Water vapor spotted in atmosphere of Jupiter’s Ganymede

We speak to the scientists involved

Plumes of water vapor on Ganymede have been spotted for the first time in the atmosphere of Ganymede, Jupiter’s largest moon.

Astronomers have studied the Solar System’s largest planetary satellite for decades, though have lately only managed to detect water vapor rising from its surface, using the Hubble Space Telescope. The findings were published on Monday in Nature Astronomy; a preprint version is here.

“We’ve known for many years that Ganymede has water ice on its surface, and there’s also evidence of both ice and liquid water within its interior, but until now we hadn’t detected water vapor in the atmosphere of Ganymede or any other icy moon,” Philippa Molyneux, co-author of the study and a researcher at Southwest Research Institute (SwRI) in the US, told The Register.

“Since the presence of water is one of the essential requirements for a planet or moon to be considered potentially habitable, it’s important to understand which forms it exists in within different locations, and how this might vary over time.”

Scientists believe that 100 miles below the frozen shell of ice covering Ganymede's surface is a great body of water, possibly holding more liquid than all of the oceans on Earth combined. This subsurface water is too far below the surface of the icy crust for it to be the source of the water vapor detected in the atmosphere. Instead, the team reckons the vapor was formed when the ice at Ganymede’s surface sublimates; the solid substance directly turns into gas without melting into a liquid first.

The vapor was spotted as they analyzed ultraviolet-light emissions collected by NASA's Hubble Space Telescope from the moon. These images showed its atmosphere is made up mostly of oxygen (O₂) and atomic hydrogen (H) atoms. In some spots, however, the concentration of single oxygen (O) atoms increases.

These gases are formed when the aforementioned water vapor molecules break apart in the atmosphere from being bombarded by solar rays, Randy Gladstone, co-author of the paper and an astronomer at the SwRI, explained to The Register. When this breakdown occurs, light is emitted that we can detect from afar.

“Most of the oxygen emissions are from electrons breaking apart and exciting oxygen molecules (O₂), with a smaller portion from electrons directly exciting oxygen atoms,” Gladstone explained.

“This work shows that to get the ratios of the different oxygen atom emission colors right, a lot of the emissions have to come from electrons breaking apart and exciting the oxygen atoms in water molecules (H₂O) in the regions on Ganymede where the Sun is nearly overhead.

“It’s a subtle effect, but it allows us to confirm that the models are correct, and gives us confidence that we understand Ganymede’s atmosphere pretty well. It’s also sort of interesting that the major species in Ganymede’s atmosphere changes from H₂O at around noon to O₂ at other times of day.”

The scientists are looking forward to following up their work with the European Space Agency’s upcoming JUpiter ICy moons Explorer (JUICE) mission to Jupiter and three of its largest moons: Ganymede, Callisto, and Europa.

“We’re currently refining our plans to observe Ganymede with JUICE, and we’ll use these results to help us identify important parts of the surface and atmosphere to study in more detail,” Molyneux told us.

"The JUICE-UVS instrument is able to detect the same emissions we studied using Hubble, but since it will be in orbit around Ganymede we’ll get much better spatial resolution and will be able to watch how the atmosphere changes over time.

"One question I’d like JUICE to answer is what else is in Ganymede’s atmosphere, other than oxygen and water? Whatever we find will help us to understand the composition of Ganymede’s surface and potentially its subsurface ocean, giving us a much better understanding of how habitable it may actually be."

Other puzzling questions remain, include how deep and salty the ocean is, and how the water affects auroras on the satellite. ®