Researchers have now observed water vapor rising from the surface of Ganymede. This moon of Jupiter is the largest moon in our solar system, potentially holding more liquid water than all of Earth’s oceans combined deep beneath its icy exterior.

Ganymede is usually too cold for ice to melt and evaporate. However, research published in Nature Astronomy has shown that the rain of charged particles and sunlight at high noon is enough to sublimate (change from frozen to gas) molecules such as oxygen and water vapor.

The very tenuous presence of an oxygen atmosphere has been known for over two decades, thanks to Hubble observations of this moon. The space telescope was able to detect auroral bands, long colorful ribbons of ultraviolet light emitted by electrified gas.

The most likely culprit was molecular oxygen, O₂ – but there were some emissions that didn’t match up with an atmosphere of pure molecular oxygen. A possibility raised was that atomic oxygen was present so that the occasional single atom of oxygen would be freed from the surface.

The latest analysis instead shows that it is water vapor that is the mysterious other component of Ganymede's thin atmosphere.

"So far only the molecular oxygen had been observed," lead author Lorenz Roth, from the KTH Royal Institute of Technology in Stockholm, said in a statement. "This is produced when charged particles erode the ice surface. The water vapor that we measured now originates from ice sublimation caused by the thermal escape of water vapor from warm icy regions."

The data used to make this discovery are historical observations collected between the original 1998 campaign and 2010, plus studies conducted in 2018 to help observations conducted by NASA’s Juno around Jupiter.

The Juno spacecraft itself recently delivered the closest images of the surface of the moon in decades, and visual artists were even able to create a reconstructed flight from Ganymede all the way to Jupiter.

But it’s not just Juno that Hubble is helping. The findings from this work will be crucial for the Jupiter Icy Moons Explorer, or JUICE, a mission to explore Europa, Callisto, and Ganymede, entering in orbit around the latter to uncover the secrets of these moons.    

"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.

JUICE is expected to be launched next June and it will enter orbit around Jupiter in October 2029.

 THIS WEEK IN IFLSCIENCE