The Solar System’s Most Volcanic Moon Has Been This Way For Billions Of Years

A 4-billion-year-old volcano? New research finds that Io, the solar system’s most volcanically active moon, can brag just that.

A recent study of Io’s sulfurous atmosphere suggests that Jupiter’s moon has been a volcanic hellscape for almost the Solar System’s entire 4.57 billion-year history. And it’s been recycling the same stockpile of sulfur from its boiling magma mantle through volcanic eruptions that whole time. That could have implications for two of Jupiter’s other moons, Ganymede and Europa: the Solar System’s iciest astrobiology hotspot.

Caltech planetary scientist Katherine de Kleer and her colleagues published their work in the journal Science.

Io’s Drama Has Been Going on Forever

The planetary scientists used ALMA to measure the wavelengths of light emitted and absorbed by molecules in Io’s hellish atmosphere. Sulfur, like most chemical elements, comes in several forms, or isotopes – some with more neutrons, and some with fewer. And each of those isotopes emits and absorbs light slightly differently. By measuring those subtle differences with ALMA, de Kleer and her colleagues learned that Io’s atmosphere contains a lot more of the heavier isotopes of sulfur than lighter ones. And that’s a dead giveaway that the noxious gas surrounding the moon has been recycled for billions of years.

“It’s not so surprising, our conclusion,” de Kleer tells Inverse. Since at least 2002, computer models of how the Solar System formed have predicted that as Jupiter and its system of moons came together, Jupiter’s gravity could have pulled its moons Io, Ganymede, and Europa into what’s called an orbital resonance.

A resonance means that the moons' orbits sync up very precisely. Every time Io makes two orbits around Jupiter, it aligns with Europa; every fourth orbit, Io aligns with Europa and Ganymede. The tidal pull of the nearby moons during those alignments, along with Jupiter's powerful gravity, keep Io's interior roiling and hot. And that constant heat and motion drive the moon’s constant volcanic eruptions.

The Subtext is Good News for Alien Hunters

De Kleer and her colleagues’ work confirms that the models are right; the three moons, one fiery and two icy, have been in resonance since the Solar System formed. Io has always been the violent, fiery landscape that it is today. But there’s another, more subtle point there:

“I think one kind of interesting implication of this work that's very much not in the papers, just kind of a logical development,” says de Kleer, is that if Io has been being heated for 4 billion years, then Europa has also been receiving tidal heating for 4 billion years, because it's part of that resonance.”

That could mean that Europa’s ocean, like Io’s volcanism, could be as old as the Solar System itself – which means that hypothetical alien life could have had even longer to evolve on Europa than on Earth.

Digging into the Details

On Io, the ephemeral atmosphere actually records more of the moon’s history than its rocky surface does. Fresh lava flows are constantly burying all traces of Io’s past.

“And so it's like this mystery: you only get the past 1 million years, which is like nothing on geological timescales,” says de Kleer. “So we don't know what it's been doing since it evolved, or how it got to be the way it is today.”

Sometimes, scientists’ only option is to get creative.

Io’s famous volcanoes (more than 400 of them) blast sulfur dioxide gas from deep in the moon’s interior high into its yellowish-brown sky. In Io’s upper atmosphere, fast-moving particles caught in Jupiter’s magnetic field strip gas away from Io. And the lighter molecules – the ones with lighter, smaller isotopes of sulfur – tend to get stripped away first.

The heavier molecules eventually settle back to Io’s surface, where they get buried by layers of fresh lava, mingle with molten rock in freshly-melted magma, and eventually erupt again as part of another volcanic plume. After billions of years of this cycle, Io’s sulfur supply has been distilled down to the heaviest molecules.

So de Kleer and her colleagues measured the ratio of heavier isotopes to lighter ones in Io’s atmosphere, and calculated how long it would have taken the volcanic moon to distill its atmosphere to that extent. The answer turned out to be around 4.57 billion years, about the same amount of time Jupiter and its moons have existed. Io was a hot mess from the beginning.

Did Io Ever Have an Ocean?

For de Kleer, what’s next is a whole slew of other questions about Io and its buried history. One big question – which will probably require an entirely new space mission to answer – is about whether Io ever looked like its icy sisters Ganymede and Europa.

Today, Io has no water and no carbon dioxide; both gases have long since evaporated away from the sizzling surface of the moon. But the outermost of Jupiter’s four largest moons – Europa, Ganymede, and Callisto – all have thick crusts of ice over globe-spanning oceans of churning liquid water.

“I want to know if Io doesn't have that because it formed inside of some kind of snow line, where water wasn't able to condense, or if it once had an ocean and lost it,” says de Kleer. The best way to answer that question would be by measuring the ratio of hydrogen to deuterium (a heavier isotope of hydrogen with an extra neutron). That’s how other planetary scientists first realized that Mars and Venus once had oceans of liquid water on their surfaces.

The challenge is that Io doesn’t have enough hydrogen left to measure with instruments like ALMA’s. Instead, a visiting spacecraft would need to take samples of the moon’s atmosphere.

The Future of a Troubled Moon

For Io, what’s next is probably more of the same, at least for a while. Our Solar System’s most volcanic locale is in no danger of losing its title; the orbital resonance with Ganymede and Europa will keep Io’s volcanic interior churning until the dying Sun swallows us all up in about 5 billion years. But by then, Io’s volcanoes will probably look a bit different.

Io loses about a ton of sulfur every second – mostly thanks to Jupiter’s pesky magnetosphere and its energetic particles. The result may be a very hot moon, but a less explosive one.

“It’s going to take hundreds of millions of years, but it’s going to run out of sulfur a long time before the Sun expands,” says de Kleer. “The heat will still be melting rock, but it’s the gases that are dissolved in the magma that really drive the magma to come up to the surface. So Io could conceivably look very different in the future.”