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New Map Reveals Secrets of Io, the Solar System's Most Volcanic Moon

Scientists can say two things with certainty about Io. First, this innermost 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 sinuously stretch 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 beneath its crust, or do they draw from some heat source welling up from far deeper, near the moon’s furious heart? Solving this mystery could 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 just published in Nature Astronomy think they have an answer: they’re placing their bets on almost “skin-deep” 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: both sculpt 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 raises tides 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 focused—and, by proxy, where the tidal heating for Europa and other oceanic moons may focus as well.

Patterns among Io’s erupting volcanoes—those whose thermal emissions can be tracked by passing spacecraft—presumably offer clues. Scientists have spent decades pursuing them by remotely charting most of Io’s volcanic hotspots, but those around its poles proved difficult to see. Thankfully, NASA’s intrepid Juno spacecraft managed to glimpse Io’s caps so that scientists could complete a global map of the moon’s volcanic hotspots.

These infrared Juno 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 that there is considerably more volcanic heat coming from Io’s lower latitudes and equatorial expanses, while its poles are comparatively lukewarm. This 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 Katherine de Kleer, a planetary scientist at the California Institute of Technology 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.” These two antipodal scenarios hold distinct implications for where Io’s volcanism ultimately emerges on the moon’s surface. Predominantly deeper tidal heating would create profuse volcanism at the poles, whereas shallower baking would kindle volcanic fires at lower latitudes.

Finding out which of these models work 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—primarily designed to investigate Jupiter’s magnetic field and polar auroras—to get a prolonged peek at Io’s poles.

A composite view of Io from instruments aboard NASA’s Juno probe.
A composite view of Io from instruments aboard NASA’s Juno probe. The spacecraft’s JunoCam captured the Jovian moon’s blotchy, colorful surface; the red, yellow and white regions are infrared hotspots recorded by Juno’s JIRAM instrument, and pinpoint sites of active volcanism. Credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM

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 positions of erupting volcanoes don’t perfectly match the expectations of any heating hypothesis. “Io’s going to be a lot more complicated than these end-member models,” Davies adds.

The poles are also volcanically active, which implies a modicum of tidal heating is occurring at depth. “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 geologic barrier below the south pole—perhaps a thicker crust or some other a heat-resistant tectonic structure—is inhibiting the flow of hot rock to the surface.

Although these results may be the closest anyone can get to an x-ray of this 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.

Another issue is that this map of Io’s volcanic hotspots is merely a snapshot in time that 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, while others have short-lived paroxysms. “That’s the delightful thing about Io,” the fact that its fiery face is ever changing, 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 of Io.”

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 may 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.

Juno’s closest-yet flyby of Io is slated for December, giving further opportunity for the spacecraft to spy the moon’s more elusive volcanic outbursts. Scientists can hardly wait to crack open that holiday gift. “This is the purest form of discovery you can imagine,” Davies says. “It’s an absolute thrill to see these things.”

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