A new map reveals the secrets of Io, the most volcanic moon in the solar system

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 decorated with numerous calderas spewing lava so that it resembles an oven-baked cheese pizza. Its glowing rivers of molten rock stretch sinuously from horizon to horizon; Its endless eruptions spray towering arcs of matter into the vacuum of space.

Second, no one really knows the depth of this orb’s fiery plumbing. Are Io’s volcanoes fed by reservoirs just below its crust, or do they draw from a heat source flowing from much deeper, near the moon’s raging core? Solving this mystery could help reveal how Io’s lunar sister Europa and other icy moons manage to harbor vast oceans of liquid water, potentially habitable, despite the cold, sun-starved outer solar system. Now the authors of a new study just published in Nature astronomy I think they have an answer: They’re placing their bets on “very deep” heat engines buried not too far from Io’s surreal surface.

“Research like this provides invaluable insights into the diversity of volcanic activity and internal heating of other worlds,” says Anna Goelcher, a planetary scientist at Caltech, who was not part of the new study. While the research’s conclusions are not unambiguous, they help researchers sift through their models of where and how heat appears inside the frozen alien moons.

In a way, Io’s internal heat can be traced back to the presence of Europa and its other nearby moon, Ganymede: both sculpt Io’s orbit around Jupiter into a pronounced non-circular ellipse that has the volcanic moon swooping closer to and then away from the gas giant and its agonizing gravitational grip. This triggers tides within Io that press on the moon’s geological bowels, generating massive amounts of frictional heat that produces magma. The question is where the heating is concentrated within Io, and alternatively, where tidal heating might be concentrated in Europa and other oceanic moons as well.

The patterns of erupting volcanoes on Io — ones whose thermal emissions can be tracked by passing spacecraft — should provide clues. Scientists have spent decades pursuing them by mapping most of the volcanic hotspots on Io’s surface from a distance, but the regions around its poles have proven difficult to see. Fortunately, NASA’s intrepid Juno spacecraft was able to catch a glimpse of Io’s hats so scientists can complete a global map of the moon’s volcanic hotspots.

These infrared images of Juno “show things no one has ever seen before,” says Ashley Davis, a volcanologist and planetary scientist at NASA’s Jet Propulsion Laboratory and one of the study’s authors. In particular, they revealed that there is much more volcanic heat coming from Io’s lower latitudes and equatorial expanses, while its poles are relatively tepid. This suggests that Io’s tidal heat is not concentrated at great depths, but rather higher up, closer to the crust.

“We’ve been wanting this data set for decades, and it’s finally here,” says Catherine De Clare, a planetary scientist at Caltech, who was not part of the new study. “Models have differed as to where melting occurs most often, whether it is at the core-mantle boundary or whether it is close to the surface.” These two opposing scenarios have clear implications for where Io’s volcanoes eventually appear on the moon’s surface. Deeper tidal heating would often create abundant volcanic activity at the poles, while shallow baking would spark volcanic fires at lower latitudes.

Figuring out which of these models works best requires a global map of Io’s erupting volcanoes. Because no spacecraft has been dedicated exclusively to interrogating Io, maps of volcanic hotspots — especially those in its polar regions — have remained incomplete. Previous spacecraft equipped with infrared cameras have mostly performed flybys with equatorial views of Io.

Juno came to the rescue in 2016 when it entered Jupiter’s polar orbit. To take advantage of this new perspective, scientists used the spacecraft’s Jovian Infrared Auroral Mapper (JIRAM) instrument, which was primarily designed to study Jupiter’s magnetic field and aurora, to take a long peek at Io’s poles.

In the new study, the researchers surveyed 266 volcanic hotspots across the moon. This map showed that Io’s lower latitudes were emitting 60% more volcanic heat per unit area than the poles. The best explanation for this dichotomy is that Io’s heating occurs mostly at shallow depths, either within the putty-like upper mantle or within an ocean of partially or completely molten rock just below the crust.

“I kind of lean toward a magma ocean,” says Davis. But the evidence is not clear-cut: the locations of volcanic eruptions do not exactly match the predictions of any warming hypothesis. “IO will be much more complex than these end-member models,” adds Davies.

The poles are also volcanically active, meaning little tidal heating occurs at depth. “There’s probably some degree of melting going on everywhere,” says De Clare. Curiously, the North Pole emits more than twice as much volcanic heat per unit area as the southernmost point of Io. It is not clear why. Davies hypothesizes that a geological barrier beneath Antarctica—perhaps a thicker crust or other heat-resistant tectonic structure—prevents the flow of hot rock to the surface.

Although these results may be the closest anyone can get to X-rays of this supervolcanic body, they still contain a large amount of uncertainty. Researchers (including the study’s authors) cannot be certain that Io’s volcanic thermal emission pattern is a reliable indicator of heat flow on the Moon. “Magma will reach the surface where it can, even if it’s not directly above the melting source,” says Tracy Gregg, a planetary volcanologist at the University at Buffalo, who was not part of the study. These indirect migrations make identifying the primary location of tidal heating on Io more difficult.

Another problem is that this map of Io’s volcanic hotspots is just a snapshot in time that can’t be set in stone (molten or otherwise). Io’s volcanoes have something in common with Earth’s volcanoes: some remain active for a long time, while others experience short-lived outbursts. “That’s the exhilarating thing about Io,” says Gianni Radibo, a planetary geologist at Brigham Young University, who was not part of the study, the fact that its fiery face is constantly changing. “There is no way we can finish mapping the volcanoes on Io.”

The global picture of lunar eruptions provided by this paper may be the first of its kind. But it won’t be the last. Future shots of Io’s volcanic hotspots may look much different from this image, which could support a different conclusion. However, for now, this thermal snapshot is generally consistent with previous research that has used the distribution of explosive or quiescent lunar volcanoes to ascertain the location of Io’s heat engine, and it certainly appears that this engine is shallow rather than deep.

Juno’s closest flyby yet is scheduled to take place by Io in December, providing an additional opportunity for the spacecraft to spy the moon’s most elusive volcanic eruptions. Scientists can’t wait to open this holiday gift. “This is the purest form of discovery you can imagine,” says Davies. “It’s very exciting to see these things.”