For the first time, we have conclusive proof that the atmosphere of Jupiter’s largest moon, Ganymede, contains water vapor. It’s possible that the ice on Ganymede’s surface sublimated, going straight from a solid to a gas without melting.

The surface of Ganymede is a mixture of bright grooved terrain and dark, cratered areas that create intriguing patterns. Long believed to hold a vast amount of water, maybe more than the Earth, Ganymede is so far from the Sun that water could only remain liquid behind a thick layer of ice.

Three major layers are thought to exist on Ganymede: a metallic iron core, a rocky mantle, and a water layer that is both liquid and frozen. The outside ice shell is incredibly thick (around 500 miles/800 kilometers), and any liquid water may be present underneath it. Whatever the case, water exists, and where there is water, there might also be life.

Researchers have found non-ice water on the surface for the first time.

Hubble was being used by Lorenz Roth of the KTH Royal Institute of Technology in Stockholm, Sweden, to gauge the oxygen content on Ganymede as part of a broader observation program. Hubble’s Cosmic Origins Spectrograph data from 2018 and older Space Telescope Imaging Spectrograph photos were used by Roth and his colleagues (STIS) from 1998 to 2010.

The first ultraviolet (UV) photos of Ganymede were captured by Hubble’s Space Telescope Imaging Spectrograph (STIS) in 1998, and they showed a distinct pattern in the measured emissions from the moon’s atmosphere. Similar to aurora ovals seen on Earth and other planets with magnetic fields, the moon has auroral bands. This served as concrete proof that Ganymede possesses a magnetic field that is permanent. The existence of molecular oxygen was used to account for the parallels in the UV measurements (O2). At the time, the variations were attributed to the existence of atomic oxygen (O), which generates a signal that favors one UV color over the other. Credit: Lorenz Roth, N.A.S.A, and ESA (KTH)

The UV measurements indicated the presence of atomic oxygen, at least according to the initial 1998 interpretation. Roth’s crew was astonished to discover that there was hardly any atomic oxygen in Ganymede’s atmosphere. The apparent disparities in these UV aurora photographs must have another explanation if this is the truth.

The surface temperature of Ganymede varies significantly throughout the day, according to the researchers’ analysis of the UV images’ relative distribution of the auroral bands, which are bright ribbons of electrified plasma. Around noon, the equatorial regions of Ganymede might get warm enough for some modest amounts of water molecules to escape (or sublimate) from the ice surface.

The Hubble data and this come together well. The equator was precisely where Roth discovered the purported oxygen, which he now considers to be water vapor.

Roth stated that just the molecular oxygen has been seen thus far. This results from the ice surface being eroded by charged particles. The water vapor that we have recently measured comes from the thermal escape of water vapor from warm icy places, which sublimates ice.

The discovery makes Ganymede considerably more intriguing, especially in light of the planned mission from the European Space Agency. Launched in 2022, JUICE (Jupiter ICy moons Explorer) is scheduled to reach Jupiter in 2029. Three years will be devoted to the mission’s careful examination of Jupiter and its largest moons, including Ganymede.

“Our results can provide the JUICE instrument teams with valuable information that may be used to refine their observation plans to optimize the use of the spacecraft,” added Roth.

Astronomers are increasingly looking at frozen moons around Jupiter and Saturn as places where life could emerge. They were once discarded as barren, frozen wastelands, but the more we look at them, the more the potential habitability of these moons seems increasingly likely. Of course, just because there could be life on Ganymede doesn’t mean there is — that’s up for future research to discover.

The study was published in Nature Astronomy.

Soucre: news.sci-nature.com