On 5 March 1979, Voyager 1 made its closest approach to Jupiter. What it discovered astounded navigators at the Jet Propulsion O Laboratory in California. Not least of all astronomer Linda Morabito, who had been analysing an image taken by the spacecraft and saw a puzzling feature that turned out to be a volcanic plume off the limb of Io. It was 270 kilometres (170 miles) tall, spewing sulphur into the airless sky with great ferocity. This volcano came to be known as Pele, after the Hawaiian fire goddess, and its discovery was hugely significant: it was the first time that an erupting volcano had been found anywhere other than Earth.
It wasn’t the first time that alien volcanoes had caught the imagination. Missions to the Moon had uncovered basalt samples some 3.3 billion years old, and Apollo 15 landed close to Hadley Rille, an immense groove on the Moon 1.5 kilometres (0.9 miles) wide and 300 metres (984 feet) deep. This groove likely originated as a lava tube whose roof collapsed. The unmanned Mariner 9 highlighted a varied Martian terrain in 1977 which had huge volcanoes, including the mammoth Olympic Mons. Yet these discoveries were all completely extinct.
Io proved to be a swirl of colours thanks to a thin atmosphere laden with sulphur, and was showing signs of being the most geologically active body in the Solar System: more than 150 active volcanoes – of more than 400 in total – have been discovered there. Moons Enceladus and Triton also have active volcanoes. Venus, too,
