But if Io doesn't have a magma ocean, what might that mean for Europa?

 

And, scientists now wonder,

how does tidal heating even work...?

 

 

 

Melting Magma

 

Heat drives geology, the rocky foundation upon which everything else, from volcanic activity and atmospheric chemistry to biology, is built.

 

Heat often comes from a planet's formation and the decay of its radioactive elements.

 

But smaller celestial objects like moons have only tiny reserves of such elements and of residual heat, and when those reserves run dry, their geological activity flatlines.

 

Or, at least, it should - but something appears to grant geologic life to small orbs throughout the solar system long after they should have geologically perished.

 

Io is the most flamboyant member of this puzzling club - a burnt-orange, crimson and tawny Jackson Pollock painting.

 

The discovery of its over-spilling cauldrons of lava is one of the most famous tales in planetary science, as they were predicted to exist before they were discovered.

On March 2, 1979, a paper in Science ruminated on Io's strange orbit.

 

Because of the positions and orbits of neighboring moons, Io's orbit is elliptical rather than circular. And when Io is closer to Jupiter, it experiences a stronger gravitational pull from the gas giant than when it is farther away.

 

The study authors figured that Jupiter's gravity must therefore be constantly kneading Io, pulling its surface up and down by up to 100 meters, and, per their calculations, generating a lot of frictional heat within it - a mechanism they described as "tidal heating."

 

They conjectured that Io may be the most intensely heated rocky body in the solar system.

"One might speculate that widespread and recurrent surface volcanism would occur," they wrote.

Just three days later, Voyager 1 flew by.

 

An image taken on March 8 documented two gigantic plumes arching above its surface. After ruling out all other causes, NASA scientists concluded that Voyager had seen an alien world's volcanic eruptions.

 

They reported their discovery in Science that June, just three months after the prediction.

 

The planetary science community quickly coalesced around the idea that tidal heating within Io is responsible for the never-ending volcanism on the surface.

"The unknown part that's been an open question of decades is what that means for the interior structure," said Mike Sori, a planetary geophysicist at Purdue University.

Where is that tidal heating focused within Io, and just how much heat and melting is it generating?

NASA's Galileo spacecraft studied Jupiter and several of its moons around the turn of the millennium.

 

One of its instruments was a magnetometer, and it picked up a peculiar magnetic field emanating from Io. The signal appeared to be coming from an electrically conductive fluid - a lot of fluid, in fact.

 

After years of study, scientists concluded in 2011 that Galileo had detected a global magma ocean just below Io's crust.

 

Whereas Earth's mantle is mostly solid and plasticky, Io's subsurface was thought to be filled with an ocean of liquid rock 50 kilometers thick, or almost five times thicker than the Pacific Ocean at its deepest point.

 

A similar magnetic field was coming from Europa, too - in this case, apparently generated by a vast ocean of salty water.

 

The implications were profound:

With a lot of rocky material, tidal heating can make oceans of magma.

 

With plenty of ice, it can create oceans of potentially habitable liquid water. 

 

 

 

Volcanic Vanishing Act

 

By the time the Juno spacecraft started swinging around Jupiter in 2016, the belief that Io had a magma ocean was widespread.

 

But Bolton and his colleagues wanted to double-check.

 

During flybys in December 2023 and February 2024, Juno came within 1,500 kilometers of Io's scorched surface.

 

Although the remarkable images of active volcanoes drew everyone's attention, the goal of these flybys was to find out if a magma ocean truly lay beneath the moon's rocky skin.

 

To investigate, the team used an unlikely tool:

Juno's radio transponder, which communicates with Earth, sending and receiving signals.

Because of Io's unevenly distributed mass, its gravitational field isn't perfectly symmetrical. That uneven gravitational field subtly alters the motion of Juno as it flies by, causing it to accelerate or decelerate a little.

 

That means Juno's radio transmissions will experience the Doppler effect, where the wavelength shifts slightly in response to Io's uneven gravitational field.

 

By looking at the incredibly small shifts in the transmissions, Bolton's team was able to create a high-fidelity picture of Io's gravitational field and use that to determine its internal structure.

"If there were indeed a global magma ocean, you'd see a lot more distortion as Io orbited around Jupiter and as the tidal forces flexed it and changed its shape," said Ashley Davies, a volcanologist at NASA's Jet Propulsion Laboratory who wasn't involved with the new study.

But Bolton's team did not find this level of distortion.

 

Their conclusion was clear.

"There cannot be a shallow magma ocean fueling the volcanoes," said study co-author Ryan Park, a Juno co-investigator at the Jet Propulsion Laboratory.

So what else might be powering Io's volcanoes?

 

On Earth, discrete reservoirs of magma of different types - from the tarlike viscous matter that powers explosive eruptions to the runnier, honey-esque stuff that gushes out of some volcanoes - are located within the crust at various depths, all created by the interactions of tectonic plates, the moving jigsaw pieces that make up Earth's surface.

 

Io lacks plate tectonics and (perhaps) a diversity of magma types, but its crust may nevertheless be peppered with magma reservoirs. This was one of the original lines of thought until Galileo's data convinced many of the magma ocean theory.

 

The new study doesn't rule out a far deeper magma ocean.

 

But that abyssal cache would have to be filled with magma so iron-rich and dense (because of its great depth) that it would struggle to migrate to the surface and power Io's volcanism.

"And at some depth, it becomes tricky to distinguish between what we would call a deep magma ocean versus a liquid core," Park said.

For some, this raises an irreconcilable problem.

 

Galileo's magnetometer detected signs of a shallow magma ocean, but Juno gravity data has emphatically ruled that out.

"People are not really disputing the magnetometer results, so you have to make that fit with everything else," said Jani Radebaugh, a planetary geologist at Brigham Young University.

Researchers disagree on the best interpretation of the Galileo data.

 

The magnetic signals,

"were taken as probably the best evidence for a magma ocean, but really they weren't that strong," said Francis Nimmo, a planetary scientist at the University of California, Santa Cruz and a co-author of the new study. 

The induction data couldn't distinguish between a partly molten (but still solid) interior and a fully molten magma ocean, he said.

 

 

 

 

Heavy Water

 

Perhaps the main reason scientists study Io is because it teaches us about the fundamentals of tidal heating. Io's tidal heating engine remains impressive - there's clearly a lot of volcano-feeding magma being generated.

 

But if it's not producing a subsurface magma ocean, does that mean tidal heating doesn't generate water oceans, either?

Scientists remain confident that it does.

Nobody doubts that Saturn's moon Enceladus, which is also tidally heated, contains an underground saltwater ocean:

the Cassini spacecraft not only detected signs of its existence but directly sampled some of it erupting out of the moon's South Pole.

And although there is some light skepticism about whether Europa has an ocean, most scientists think it does.