It's been thought that Ganymede has water layers for a while, however Ganymede's water is not nearly as habitable as the all-but-confirmed oceans underneath the surfaces of Europa and
Enceladus. Whatever water is in Ganymede must be in between thick layers of ice. Just looking at the surface shows geologic activity, but that activity is pretty sedate in comparison to Europa.
The latest evidence from Hubble is consistent with the idea that Ganymede has a single, huge ocean layer. A recent model suggests it is possible that Ganymede has ocean layers
sandwiched in between ice layers. What's special about the Hubble observations is that they're the best evidence for an internal ocean.
It is important to note that Hubble cannot directly detect Ganymede's magnetic field; what was observed in changes in the moon's auroras. Only Galileo has provided magnetic field readings of Ganymede. Presumably, the magnetometer data from Galileo was used to attempt confirmation of a subsurface ocean in the same way Europa's was confirmed. The moons experience a time varying magnetic field as they orbit Jupiter. An induced magnetic field was detected from Europa as it travels around Jupiter, showing that there must be conductive material near the surface - a saltwater ocean. This let scientists put constraints on the size, salinity, and depth of Europa's ocean.
As Ganymede's ocean
is much deeper, evidence of a single ocean layer is much more difficult to confirm. An ocean layer has been suggested using magnetic field measurements, but the data must be much harder to interpret for Ganymede than Europa.
According to the paper, some possible oceans that are consistent with the results include, but are not limited to:
- A liquid layer from 150 to 250 kilometers depth with relatively low salinity
- A liquid layer from 190 to 210 kilometers depth with relatively high salinity
- A liquid layer below 330 kilometers depth with relatively high salinity
