For the first time: Hubble finds water on Jupiter’s moon Ganymede

We have solid evidence of water vapour in the atmosphere of Jupiter's moon Ganymede – the solar system's biggest moon — for the first time. The frozen water on Ganymede's surface may have sublimated, moving from solid to gas without becoming liquid.

The surface of Ganymede is a mix of dark, cratered regions and bright grooved terrain that produces fascinating patterns. Researchers have long thought that Ganymede has a large amount of water — possibly more than the Earth — but because Ganymede is so far from the Sun, water could only remain liquid behind a thick covering of ice.

Ganymede is assumed to have three primary layers: a metallic iron core, a rocky mantle, and a liquid and frozen layer of water. The ice shell on the outer is extraordinarily thick (around 500 miles / 800 kilometres), and any liquid water might exist beneath it. Regardless, there is water — and where there is water, there may be life.

For the first time, researchers have discovered non-ice water on the surface.

As part of a larger observation program, Lorenz Roth of the KTH Royal Institute of Technology in Stockholm, Sweden was using Hubble to measure the amount of oxygen on Ganymede. Roth and his colleagues used data from two instruments: Hubble’s Cosmic Origins Spectrograph in 2018 and archival images from the Space Telescope Imaging Spectrograph (STIS) from 1998 to 2010.

The UV data showed the presence of atomic oxygen — at least that’s what the original interpretation from 1998 noted. But much to their surprise, Roth’s team found hardly any evidence of atomic oxygen in Ganymede’s atmosphere. If this is the case, there must be another explanation for the apparent differences in these UV aurora images.

When the researchers took a closer look at the relative distribution of the colorful ribbons of electrified gas called auroral bands in the UV images, they found another piece of evidence: Ganymede’s surface temperature varies strongly throughout the day. Around noon, the equatorial parts of Ganymede may become sufficiently warm that the ice surface releases (or sublimates) some small amounts of water molecules.

This fits excellently with the Hubble data. The presumed oxygen (which Roth now believes to be water vapor) was found exactly around the equator.

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