One theory of Ganymede's interior structure, with a huge ocean beneath the ice. This creates ‘magnetic friction’ that suppresses the rocking of Ganymede’s auroras. However, they don’t appear to rock as much as they should, and so scientists have postulated that it is the presence of a salty ocean that creates a second magnetic field that counteracts Jupiter’s. Ganymede’s auroras are ‘rocked’ by changes in Jupiter’s magnetic field. Using the Hubble Telescope, scientists studying how those auroras changed with shifts in Jupiter’s magnetic field surmised that Ganymede has an ocean of salty water beneath its icy surface. The magnetic field causes auroras, which are affected by Jupiter’s magnetic field. Ganymede is pretty special-it’s the only moon in our solar system to have its very own magnetic field, probably caused by convection in its iron-rich liquid core. Other than a very thin atomic oxygen atmosphere, there’s no breathable air. Ganymede is Jupiter’s largest moon, made of roughly equal amounts of rock and ice. Future missions, such as the planned Europa Mission, may be able to take samples from these plumes of water and test them for the presence of microbial life. The Hubble Space Telescope has also taken snaps of something that could be a geyser of water, erupting around 200 kilometres above the surface of Europa-and it looks like Galileo even got sprayed by such a geyser during a 1997 flyby. It was also the Galileo mission that noticed how Europa seems to alter Jupiter’s magnetic field as it moves through it, which is the best evidence we have yet of a salty ocean under the moon’s icy crust. This sort of activity is a plus for life. NASA’s Galileo mission returned lots of images of Europa’s surface that clearly show some extensional features and ridges on the icy crust, suggesting that convection is occurring beneath it. It would be not much below freezing temperature, compared with the chilly –160 ☌ on the surface. All life on Earth requires liquid water, and so what better place to look than an ocean? Furthermore, Europa’s ocean would be protected from Jupiter’s radiation, and rather warm. Underneath a spiky, icy crust, Europa is thought to have a huge, planet-wide liquid water ocean. Io (top right) erupting with three volcanic plumes and Europa (bottom left) as they move past each other Image adapted from: NASA/Johns Hopkins University APL/Southwest Research Institute (used with permission) Europa However, scientists believe that water may have been present on Io early in its formation, and there is a possibility that some form of life may exist underground. Io is also bathed in huge amounts of radiation from Jupiter and there’s no evidence of any water. And although Io’s atmosphere is one of the thickest of all the moons in the solar system, it’s still relatively thin-Earth’s atmosphere is around 200 million times denser. However, for all its tidal heating, the surface of Io is around –143 ☌. Io is the most volcanically active moon in our solar system, with plumes of material reaching up to 300 kilometres from the surface, spewing out masses of what is possibly either silicate rock or sulfur-rich material into space. The uneven gravitational pull causes the moon to bulge, then bounce back, causing friction inside Io’s interior, driving its intense volcanic activity. As Io orbits around Jupiter, its oval-shaped orbit means that Jupiter’s extremely strong gravitational pull is stronger at some times during the orbital path, and weaker at others. Gravitational pulls from Jupiter’s next two moons, Europa and Ganymede, have tugged Io’s orbit into an oval shape. Io’s vigorous dynamic activity comes from something called ‘tidal flexing’. Io is the closest moon to Jupiter, and a veritable hotspot of volcanic activity. Image adapted from: NASA (used with permission) Io An artist’s impression of the Galilean moons of Jupiter. Nevertheless, they are all tantalising prospects for finding life beyond Earth. All four moons are extremely cold, and all have thin atmospheres. Although some have speculated that life may be possible within the atmosphere of Jupiter itself, more likely candidates are the four icy Galilean moons around it.
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