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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