Jupiter’s massive size, coupled with Io’s close orbital distance to the planet, expose the moon to powerful gravitational forces. These forces constantly deform the moon, generating the heat that powers its volcanism. Similar, but less powerful, ‘tidal’ forces are thought to power geysers of water vapour erupting from Saturn’s icy moon Enceladus (Illustration of Jupiter and Io: V Lainey/IMCCE-Paris Observatory) Hot lava (left) glows on one of Io’s active volcanoes in this image taken by the Galileo spacecraft in 2000 (Image: Galileo Project/JPL/NASA)

The most volcanically active body in the solar system has just received a death sentence. Jupiter’s moon Io, whose surface erupts with active volcanoes, will one day become dormant, a new study analysing more than 100 years of observations suggests.

Io, which is about the size of Earth’s moon and is Jupiter’s closest large satellite, is covered with lava flows and dozens of active volcanoes (see image).


The heat for this activity comes from the fact that the moon travels on an elongated path around Jupiter, and therefore feels the giant planet’s gravity at different strengths along its orbit. This varying pull causes its body to deform, producing bulges that move its surface up and down by an estimated 10 metres per orbit. This generates heat that powers the moon’s volcanism.

But it will not always be so, according to a new study led by Valéry Lainey of the Paris Observatory in France.

Out of resonance

If Io were Jupiter’s only satellite, the planet’s intense gravity would eventually pull the nearby moon into a circular orbit.

The reason it travels on an elliptical path instead is because of special gravitational interactions with its nearest large sister moons, Europa and Ganymede. For every orbit that Ganymede makes, Europa makes two and Io four – a type of gravitational relationship called a Laplace resonance.

But Lainey and colleagues have found that the moons are, in fact, moving out of their resonance – Europa and Ganymede are gradually drifting away from Jupiter, while Io is moving towards the planet.

The team came to these conclusions after carrying out numerical calculations of Io’s orbital motion and plugging in observations of Io, Europa and Ganymede taken between 1891 and 2007.

Speed trade-off

Though different gravitational forces act on Io, with some pulling it towards Jupiter and others pushing it away, the new study suggests the inward forces win out.

Io’s spin gradually increases at the expense of its orbital speed. When it is closest to Jupiter, gravitational pulls on Io’s near side act to make the moon spin faster. “Io loses orbital energy, its orbital period decreases, and it moves inward towards Jupiter,” explains Gerald Schubert of the University of California, Los Angeles, in a commentary accompanying the study.

“Others have attempted the same calculation in the past, but with poorly constrained – and often contradictory – results, probably owing to approximations made in their orbital dynamical models,” Schubert writes.

Fading to black

It is not clear exactly when the moons will break free from their resonance. “If this occurs on a short timescale, say [100 million] years or less, then we have been lucky to see Io in its volcanic glory, because dormancy will be the fate of Io when the resonance is broken,” Schubert writes.

Other moons in the solar system may have already gone through a similar process. Neptune’s largest moon, Triton, boasts small geysers – “like somebody just punched a hole in the surface with a meteorite and released some gases that are trapped there”, Schubert told New Scientist.

But its volcanism may have been more dramatic if its orbit ever caused it to experience more tidal heating, he says: “It’s possible the moon was more active in the past.”

Journal reference: Nature (vol 459, p 957)