Jupiter's moon Io has been a volcanic inferno for billions of years

Jupiter’s moon Io has been continuously remodelled by volcanic eruptions for billions of years, possibly since it first formed.

Io is the most volcanically active body in the solar system, spewing plumes of sulphurous material from its many volcanoes, which can be seen from Earth. Astronomers know that this is currently driven by so-called tidal heating as the gravity of Jupiter and nearby moons deforms Io, but it was unclear if that was always the case or whether there had been a calmer past.

Now, Katherine de Kleer at the California Institute of Technology and her colleagues have found that Io has probably been blasting out lava for almost its entire history. They did this by measuring the ratio of two isotopes of sulphur in its atmosphere.

Sulphur’s most common stable form contains 16 protons and 16 neutrons in each atom, but a heavier stable form called sulphur-34 has two extra neutrons. On Io, volcanoes are constantly spewing both isotopes into its atmosphere and onto its surface. The very top layer of its atmosphere, which contains more of the lighter sulphur atoms, is lost to space as the moon travels around Jupiter, which changes the ratio of these isotopes.

De Kleer and her colleagues used observations from the Atacama Large Millimeter/submillimeter Array (ALMA), a set of radio telescopes in Chile, to measure the ratio in Io’s atmosphere. Then, by modelling how much sulphur Io might be losing each year, the team could work back to find out when Io’s sulphur ratio looked like the rest of the solar system. Although they can’t say exactly how long it has been volcanically active, it appears to have been erupting for between 2.5 and 4 billion years.

Because Io’s volcanism is down to tidal heating by Jupiter and its other moons, like Europa and Ganymede, the results can also be used to infer the arrangement of the Jovian system billions of years ago. “Io’s longevity of volcanism directly reflects how long this orbital configuration has been present,” says de Kleer.

If Io has been consistently volcanic for billions of years, then this also means it will have recycled its deeper geological layers many times over, says Lionel Wilson at Lancaster University in the UK.

This presents a rare opportunity to find out about the chemical makeup of Io’s deeper layers, such as the mantle that lies below its outer crust, by sampling the material that is blasting out, he says. “If these volcanoes have been erupting for the whole solar system history, essentially, then it’s safe to look at the composition of what’s coming out and know that that’s really a snapshot of the entire mantle of Io,” says Wilson.