Today, NASA announced evidence that Jupiter's largest moon, Ganymede, contains a liquid water ocean floating beneath its crust. The discovery is a masterpiece of indirect detection, using Hubble Space Telescopes observations of the aurora that lights up above the moon, then using that data to work out its magnetic properties and in turn using those measurements to work out its interior structure.
Ganymede is a very large moon, bigger than the planet Mercury. It has a magnetic field, which indicates that the moon hosts a magnetic core. Like the other large moons of Jupiter, it is subjected to tidal forces from the giant planet and its other moons; these forces heat the interior of the moon sufficiently to melt enough of its iron core to generate a magnetic field.
There have been indications that the heating has also generated liquid water. Ganymede's icy surface has areas that are extensively cratered, indicating great age. But there are other areas with relatively few craters, suggesting that the ice there arrived on the surface relatively recently. Possible explanations include floods of liquid water, ice-based volcanic activity (termed cryovolcanism), or even some form of tectonic system. Modeling suggested that this could be explained by a subsurface ocean, but one deeper than that of Europa. Due to the relatively slow pace of change on the moon's surface, however, there has been little direct evidence for this ocean.
Hubble's observations changed that situatiokn. As mentioned above, Ganymede has a magnetic field, and it's bathed in charged particles, mainly from the volcanoes on the moon Io. As on Earth, this field creates aurora; Joachim Saur of the University of Cologne said that they are intense enough that you'd be able to see them if you were standing on the moon's surface. They're also visible to Hubble as two large belts—one around each hemisphere of the moon.
These belts aren't only influenced by the moon's own magnetic field, though. The whole system is embedded in the magnetic field of Jupiter, which changes as the planet undergoes its 10-hour rotation. This should cause the auroral belts to shift by about six degrees with every cycle. But by having Hubble stare at the moon for five hours, Saur said the researchers found that the "aurora barely moves." Something inside the moon is dampening the influence of Jupiter's magnetic field.
And that something is almost certainly a salty ocean of liquid water. The charged particles in this ocean could respond to Jupiter's magnetic field and produce an opposing force, limiting its effect on the aurora. While models of the moon suggest that the oceans can't be that close to the surface, the new results indicate the ocean has to be less than 330 km deep. The thickness of the ocean isn't clear; it could either be broad and not very salty or thinner with levels of dissolved minerals.
Combined with the recent results from Enceladus, these latest findings show that, in the words of Heidi Hammel of the Association of Universities for Research in Astronomy, "we live in a wet solar system." Earth and Mars clearly have water, as do at least two of Jupiter's moons and two of Saturn's. Voyager also found evidence of cryovolcanism on Neptune's moon Triton.
Jennifer Wiseman, a Hubble senior project scientist, said that the work was only possible because of the servicing missions we have flown to Hubble over the years. We have made a number of observations of Ganymede using the observatory, but it was only the last mission that provided the hardware with the ability to image the right wavelengths at the resolution needed to track the aurora.
Lest this appear like a creative bit of science that's only relevant to Ganymede, Hammel suggested that it could work for exoplanets as well. A planet with a magnetic field orbiting near a magnetically active star should experience the same effects as Ganymede; it's essentially the same system scaled up. If we can get to the point where we can image these planets with any resolution, we might be able to catch changes in their aurora and make some inferences about their composition as well.
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