Io
The body in the Solar System that generates the most heat, pound for pound, is not the Sun. It’s Jupiter’s giant moon Io.

On 8 March 1979, when Voyager 1 had streaked through the Jupiter system and was heading for its rendezvous with Saturn, the space probe’s camera was pointed back to take a parting shot of Io. Navigational engineer Linda Morabito, studying the image, could not believe her eyes. Spouting from the tiny crescent moon, silhouetted against the starry backdrop of space, was a phosphorescent plume of gas.

Io is the most geologically active body in the Solar System, with more than 400 active volcanoes. The heat driving all this violence comes from the “tidal” stretching and squeezing of the moon by the gravity of the Jupiter and its other giant moons. Squeeze a squash ball repeatedly and it gets hot. It’s the same thing on Io. The moon’s orange, yellow and brown “pizza” surface is peppered with vents, reminiscent of the geysers of Yellowstone Park.

And, actually, that is what they are: geysers rather than volcanoes. Lava from the moon’s molten interior, instead of erupting directly, super-heats liquid sulphur dioxide beneath the surface, which bursts from the vents like pressurised steam in a terrestrial geyser.

Io is pumping about 10,000 million tonnes of matter into space every year. As it falls back in the moon’s low gravity, it coats the surface with sulphur like the deposits around a Yellowstone fumarole. The colours of the pizza moon are the “phases” sulphur exhibits at different temperatures.

Marcus Chown’s latest book, co-authored with Govert Schilling, is Tweeting the Universe: Tiny Explanations of Very Big Ideas (Faber)