IO

The cold inferno

Mass: 8.93 x 1022kg (1.97 x 1023lbs) Diameter: 3,643km (2,264 miles) Parent planet: Jupiter Discovered: 1610, Galileo Galilei

Io is the innermost of the four giant Galilean moons that orbit the Solar System’s largest planet, Jupiter. But while the outer three are – at least outwardly – placid, frozen worlds of rock and ice, Io’s landscape is a virulent mix of yellows, reds and browns, full of bizarre and ever-changing mineral formations created by sulphur that spills onto its surface in many forms. Io is the most volcanic world in the Solar System. Io’s strange surface was first observed during the Pioneer space probe flybys of the early 1970s, but its volcanic nature was only predicted weeks before the arrival of the Voyager 1 mission in 1979.

The moon is caught in a gravitational tug of war between its outer neighbours and Jupiter itself, and this prevents its orbit from settling into a perfect circle. Small changes in Io’s distance from Jupiter – less than 0.5 per cent variation in its orbit – create huge tidal forces that pummel the moon’s interior in all directions. Rocks grinding past one another heat up due to friction, keeping the moon’s core molten and creating huge subsurface reservoirs of magma.

While the majority of Io’s rocks are silicates similar to those on Earth, these have relatively high melting points, and so are mostly molten in a hot magma ocean that lies tens of kilometres below the surface – most of Io’s surface activity, in contrast, involves sulphur-rich rocks that can remain molten at lower temperatures.

Together these two forms of volcanism have long since driven away any icy material that Io originally had, leaving a world that is arid and iceless despite an average surface temperature of -160 degrees Celsius (-256 degrees Fahrenheit).