Plumes of water vapor over Ganymede were first spotted in the atmosphere of Ganymede, Jupiter’s largest moon.

Astronomers have studied the largest planetary satellite in the solar system for decades, but recently only managed to detect water vapor rising from its surface, using the Hubble Space Telescope. The results were published Monday in Nature Astronomy; a pre-printed version is here.

“We have known for many years that Ganymede has water ice on its surface, and there is also evidence of ice and liquid water inside, but so far we had not detected any water vapor in the atmosphere of Ganymede or any other moon, ”said Philippa Molyneux, study co-author and researcher at the Southwest Research Institute (SwRI) in the United States. The register.

“Since the presence of water is one of the essential conditions for a planet or moon to be considered potentially habitable, it is important to understand what forms it exists in different places and how this can vary over time. time.”

Scientists believe that 100 miles below the frozen ice shell covering Ganymede’s surface is a large body of water, possibly containing more liquid than all of Earth’s oceans combined. This groundwater 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 believes that vapor formed when the ice on Ganymede’s surface sublimates; the solid substance turns directly into gas without first merging into liquid.

The steam was spotted as they analyzed the ultraviolet light emissions collected by NASA’s Hubble Space Telescope from the moon. These images showed that its atmosphere is composed mainly of oxygen (O₂) and atomic hydrogen atoms (H). In some places, however, the concentration of single oxygen (O) atoms increases.

These gases form when the aforementioned water vapor molecules break up in the atmosphere after being bombarded by sunlight, explained Randy Gladstone, co-author of the article and astronomer at SwRI. The register. When this failure occurs, a light is emitted that we can detect from a distance.

“Most oxygen emissions are due to the separation of electrons and the excitation of oxygen molecules (O₂), with a smaller portion of the electrons directly exciting the oxygen atoms, ”Gladstone explained.

“This work shows that in order to obtain the ratios of the different emission colors of oxygen atoms, a large part of the emissions must come from electrons which separate and excite the oxygen atoms in the water molecules (H₂O) in the regions of Ganymede where the Sun is almost above.

“It’s a subtle effect, but it allows us to confirm that the models are correct, and gives us the assurance that we understand the atmosphere of Ganymede quite well. It is also interesting to note that the main species of the atmosphere of Ganymede change from H₂O around noon to O₂ at other times of the day.

Scientists are eager to continue 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 are currently refining our plans to observe Ganymede with JUICE, and we will 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 that we studied using Hubble, but since it will be orbiting Ganymede, we will get a much better spatial resolution and be able to observe how the atmosphere changes. over time.

“A question I would like JUICE to answer is what else is in Ganymede’s atmosphere besides oxygen and water? Anything we find will help us understand the makeup of the surface of Ganymede and potentially its subterranean ocean, giving us a much better understanding of just how habitable it can be. “

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