In March 1979, NASA’s Voyager 1 probe swept past Jupiter and, in an image taken not for science but for navigation, captured the first active volcano ever seen on a world other than Earth. The volcano was on Io, the innermost of Jupiter’s four Galilean moons. The person who found it was Linda Morabito, an optical navigation engineer at the Jet Propulsion Laboratory, who was using the picture to work out exactly where the spacecraft was.

She was not looking for a volcano. She was looking at the stars behind Io to fix the position of the probe.

The plume was in the way.

How a navigation image found a volcano

Spacecraft far from Earth can be located partly by photographing a known moon against the background stars and measuring the geometry. Voyager did this routinely. After its closest approach to Jupiter on 5 March, the probe took a set of long-exposure images of Io for exactly this purpose, the lowest-priority frames of the encounter.

On the morning of 9 March, Morabito processed one of them, taken the day before. To bring out the faint background stars she stretched the contrast of the image, and a shape appeared off the edge of Io: a large, umbrella-shaped cloud rising some 270 kilometres above the surface and catching the sunlight. Her own account describes working through the rest of the day, with colleagues, to rule out the ordinary explanations, an imaging artifact or an unknown moon behind Io. By evening the answer was clear. It was a plume from an erupting volcano, later named Pele.

When the team went back through the images Voyager had already taken, they found more. The Voyager 1 images held eight active volcanic plumes in all, on a moon no one had expected to be doing anything at all. Across the two Voyager flybys that year, the tally rose to nine erupting volcanoes.

The prediction that arrived first

Except that someone had expected it, just days earlier, and almost no one had noticed yet.

On 2 March 1979, three days before Voyager 1’s close pass of Io, the journal Science published a paper by Stan Peale, Patrick Cassen and Ray Reynolds titled “Melting of Io by tidal dissipation.” They argued that Jupiter’s gravity should flex Io enough to heat its interior and melt much of it, and they ended with a specific forecast: the consequences of a largely molten interior might be visible in the pictures Voyager was about to take.

The timing is hard to improve on. A theoretical paper predicted volcanism on Io, and within a week a navigation image confirmed it. Morabito has said she was unaware of the paper at the time, though others on the mission, including the project scientist Edward Stone, were already watching for anything that might bear it out. It is one of the cleaner examples in planetary science of a prediction and its confirmation arriving almost together.

Why Io melts

The mechanism the paper described is tidal heating. Io’s orbit is not quite circular, and it is kept that way by a steady gravitational tug-of-war with two of Jupiter’s other large moons, Europa and Ganymede, whose orbits are locked in a simple ratio with Io’s. Because the orbit is slightly eccentric, the strength of Jupiter’s pull on Io rises and falls as it goes round, and the moon is squeezed and released over and over. That constant flexing generates heat through friction inside the body, enough to keep much of Io’s interior molten.

The result is the most volcanically active body in the solar system. Io is only a little larger than Earth’s Moon, yet it carries an estimated several hundred volcanoes, with plumes that can reach hundreds of kilometres into space. Its surface is constantly remade by eruptions.

What it changed

Before Io, internal heat in a planet or moon was mostly explained by two things: leftover warmth from formation, and the slow decay of radioactive elements. Both favour large bodies, which hold their heat and have more radioactive material to begin with. A moon the size of Io should, by that reasoning, be cold and dead.

Tidal heating added a third source, one that does not depend on size but on orbit. That idea has travelled well beyond Io. It is now central to why scientists think Jupiter’s moon Europa and Saturn’s moon Enceladus may hold liquid water oceans beneath their ice, kept liquid by the same kind of flexing. The search for habitable environments in the outer solar system rests, in part, on the heat source that a navigation engineer stumbled onto in 1979.

What endures

Io is still erupting now, more than four decades on, and later missions have watched its volcanoes come and go. The first sighting still stands apart. It was not a planned observation by a dedicated instrument. It was a side effect of checking where the spacecraft was, found by someone careful enough to ask what the smudge on the edge of a moon actually was, three days after a paper had said it should be there.