Scientists can say two things with certainty about Io. First, this moon of Jupiter is the most volcanic object in the known universe. Its surface is festooned with so many lava-spewing calderas that it resembles an oven-baked cheese pizza; its glowing rivers of molten rock stretch sinuously from horizon to horizon; and its endless eruptions spray towering arcs of matter into the vacuum of space.
Second, no one really knows the depth of this flashy orb’s fiery plumbing. Are Io’s volcanoes fed from reservoirs just underneath its crust, does the heat well up from some far deeper source, near the moon’s metallic core? Solving this mystery could also help reveal how Io’s lunar sibling Europa and other icy moons manage to harbor vast, potentially habitable liquid-water oceans despite the outer solar system’s sunlight-starved chill. Now the authors of a new study in Nature Astronomy think they have an answer: they’re placing their bets on “heat engines” buried not too far below Io’s surreal surface.
“Research like this provides invaluable insights into the diversity of volcanic activity and the interior heating of other worlds,” says Anna Gülcher, a planetary scientist at the California Institute of Technology, who was not part of the new study. While the paper’s conclusions are not unequivocal, they are helping researchers winnow down their models of where and how heat arises within otherwise frozen alien moons.
In a way, Io’s internal heat can be traced to the presence of Europa and its other nearest neighboring moon, Ganymede: the two bend Io’s orbit around Jupiter into a distinctly noncircular oval that brings the hypervolcanic moon swooping closer to and then farther from the gas giant and its wrenching gravitational grip. This movement raises tidal forces within Io that squeeze the moon’s geological guts, generating enormous amounts of magma-making frictional heat. The question is where within Io that heating is concentrated—and, by proxy, where Europa’s and other oceanic moons’ tidal heating may originate as well.
Glimpses from NASA’s Juno spacecraft have helped fill in views of Io’s volcanic hotspots, letting scientists gather clues. These infrared images “are showing things nobody has ever seen before,” says Ashley Davies, a volcanologist and planetary scientist at NASA’s Jet Propulsion Laboratory and one of the study’s authors. In particular, they reveal considerably more volcanic heat coming from Io’s lower latitudes and equatorial expanses than from its comparatively lukewarm poles. This distribution suggests Io’s tidal heating is concentrated not at great depths but higher up, closer to the crust.
“We have been wanting to have this data set for decades, and it’s finally here,” says Caltech planetary scientist Katherine de Kleer, who was not part of the new study. “The models [have differed] as to where the melting is mostly occurring—whether it’s down at the core-mantle boundary or whether it’s close to the surface.”
Finding out which of these models works best demanded a global map of Io’s erupting volcanoes. Because no spacecraft has been exclusively dedicated to interrogating Io, however, maps of its volcanic hotspots—especially those in its polar regions—remained incomplete. Prior spacecraft with infrared cameras mostly conducted flybys with equatorial views of Io.
Juno came to the rescue in 2016 when it entered a polar orbit of Jupiter. Taking advantage of this novel perspective, scientists used the spacecraft’s Jovian Infrared Auroral Mapper (JIRAM) instrument—designed primarily to investigate Jupiter’s magnetic field and polar auroras—to get a prolonged peek at Io’s poles.
In the new study, the authors surveyed 266 volcanic hotspots across the moon. This map showed that Io’s lower latitudes were emitting 60 percent more volcanic heat per unit area than the poles. The best explanation of this dichotomy is that Io’s tidal heating is mostly happening at shallow depths, either within a puttylike upper mantle or within a partly or fully molten ocean of rock just below the crust.
“I’m kind of leaning toward a magma ocean,” Davies says. But evidence is not clear-cut: the erupting volcanoes’ positions don’t perfectly match any heating hypothesis. “Io’s going to be a lot more complicated,” he adds.
The poles are also volcanically active, which implies a modicum of tidal heating is occurring at greater depths. “There’s probably some degree of melting happening everywhere,” de Kleer says. Weirdly, the north pole is emitting more than twice the volcanic heat per unit area of Io’s southernmost reaches. It’s unclear why; Davies posits that a geological barrier below the south pole—perhaps a thicker crust or some other a heat-resistant tectonic structure— inhibits the flow of hot rock to the surface.
Although these results may be the closest anyone can get to an x-ray of the ultravolcanic orb, they still contain huge uncertainties. Researchers (including the study’s authors) cannot even be sure that the pattern of Io’s volcanic thermal emissions is a reliable proxy for the moon’s heat flow. “Magma will come to the surface where it can, even if that isn’t directly over the melting source,” says Tracy Gregg, a planetary volcanologist at the University at Buffalo, who was not part of the study. Those circuitous migrations make pinning down the primary location of Io’s tidal heating more troublesome.
And this map of Io’s volcanic hotspots cannot be set in stone (molten or otherwise). Io’s volcanoes share something in common with Earth’s: some stay active for a long time, whereas others have short-lived paroxysms. Its ever-changing fiery face is “the delightful thing about Io,” says Jani Radebaugh, a planetary geologist at Brigham Young University, who was not part of the study. “There is no way we can ever be done mapping all the volcanism.”
This paper’s global portrait of the moon’s eruptions may be the first of its kind. But it won’t be the last. Future snapshots of Io’s volcanic hotspots might look much different from this one, potentially supporting a different conclusion. For now, however, this thermal snapshot broadly aligns with past research that used the distribution of the moon’s erupting or quiescent volcanoes to ascertain the location of Io’s heat engine—and it sure seems like that engine is shallow, not deep.
And as Juno continues its daring flybys, we will spy even more volcanic outbursts. “This is the purest form of discovery you can imagine,” Davies says. “It’s an absolute thrill to see these things.”
A version of this article entitled “Erupting Moon” was adapted for inclusion in the February 2024 issue of Scientific American. This text reflects that version, with the addition of some material that was abridged for print.