Scientists can say with certainty two things about Io. First, this innermost moon of Jupiter is the most volcanic object in the known universe. Its surface is adorned with so many lava-spewing calderas that it looks like a baked cheese pizza; its shimmering rivers of molten rock stretch sinuously from horizon to horizon; and its endless eruptions throw imposing arcs of matter into the void of space.
Secondly, no one really knows the depth of the fiery pipes of this striking orb. Are Io’s volcanoes fed by deposits just beneath its crust, or are they fed by some heat source bubbling up from much deeper, near the moon’s raging heart? Solving this mystery could help reveal how Io’s lunar sister Europa and other icy moons manage to host vast, potentially habitable oceans of liquid water despite the cold of the outer solar system, which lacks sunlight. Now the authors of a new study just published in Nature Astronomy They think they have an answer: they are placing their bets almost “superficial” heat engines buried not far beneath Io’s surreal surface.
“Research like this provides invaluable insights into the diversity of volcanic activity and interior warming on other worlds,” he says. Anna Gulcher, 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 narrow down their models of where and how heat arises inside otherwise frozen extraterrestrial moons.
In some ways, Io’s internal heat can be attributed to the presence of Europa and its other closest neighbor moon, Ganymede: both sculpt Io’s orbit around Jupiter into a distinctly non-circular oval that brings the hypervolcanic moon closer and then the away from the gas giant. and its heartbreaking gravitational grip. This causes tides within Io that squeeze the geological bowels of the moon, generating enormous amounts of frictional heat that produces magma. The question is where that warming is concentrated within Io, and therefore where tidal warming on Europa and other oceanic moons may also be concentrated.
Patterns among Io’s erupting volcanoes (those whose thermal emissions can be tracked by passing spacecraft) presumably offer clues. Scientists have spent decades chasing them by remotely mapping most of Io’s volcanic hot spots, but those around its poles have proven difficult to see. Fortunately, NASA’s intrepid Juno spacecraft managed to glimpse the layers of Io so scientists could complete a global map of the moon’s volcanic hot spots.
These infrared images of Juno “show things no one has seen before,” says ashley davies, a volcanologist and planetary scientist at NASA’s Jet Propulsion Laboratory and one of the authors of the study. In particular, they reveal that there is considerably more volcanic heat coming from Io’s lower latitudes and equatorial reaches, while its poles are comparatively warm. This suggests that Io’s tidal heating is not concentrated at great depths but higher up, closer to the crust.
“We’ve been wanting this data set for decades and it’s finally here,” he says. Katherine de Kleer, a planetary scientist at the California Institute of Technology who was not part of the new study. “Models (have differed) as to where the melting primarily occurs, whether it’s at the core-mantle boundary or near the surface.” These two antipodal scenarios have different implications for where Io volcanism ultimately emerges on the moon’s surface. Predominantly deeper tidal heating would create profuse volcanism at the poles, while shallower cooking would cause volcanic fires at lower latitudes.
Finding out which of these models works best required a global map of Io’s erupting volcanoes. Because no spacecraft has been exclusively dedicated However, when Io was interrogated, maps of its volcanic hot spots, especially those in its polar regions, remained incomplete. Previous spacecraft with infrared cameras primarily performed flybys with equatorial views of Io.
Juno came to the rescue in 2016 when it entered Jupiter’s polar orbit. Taking advantage of this novel perspective, scientists used the spacecraft’s Jovian Infrared Auroral Mapper (JIRAM) instrument, primarily designed investigate Jupiter’s magnetic field and polar auroras, to take a long look at Io’s poles.
In the new study, the authors examined 266 volcanic sites across the Moon. This map showed that Io’s lower latitudes gave off 60 percent more volcanic heat per unit area than the poles. The best explanation for this dichotomy is that Io’s tidal heating occurs primarily at shallow depths, either within a putty-like upper mantle or within an ocean of partially or fully melted rock just beneath the crust.
“I’m leaning towards a magma ocean,” Davies says. But the evidence is not clear: the positions of the erupting volcanoes do not perfectly match the expectations of any warming hypothesis. “It’s going to be a lot more complicated than these end-member models,” adds Davies.
The poles are also volcanically active, implying that minimal tidal heating is occurring at depth. “Some degree of melting is probably occurring everywhere,” de Kleer says. Interestingly, the north pole emits more than twice as much volcanic heat per unit area as the southernmost reaches of Io. It is not clear why; Davies postulates that a geological barrier beneath the south pole (perhaps a thicker crust or some other heat-resistant tectonic structure) is inhibiting the flow of hot rock toward the surface.
Although these results may be the closest thing to an x-ray of this ultravolcanic orb, they still contain enormous uncertainties. Researchers (including the study authors) can’t even be sure that the pattern of Io’s volcanic thermal emissions is a reliable indicator of the moon’s heat flow. “The magma will come to the surface wherever it can, even if it’s not directly above the melting source,” he says. Tracy Gregg, a planetary volcanologist at the University at Buffalo, who was not part of the study. Those tortuous migrations make it more problematic to pin down the primary location of Io’s tidal warming.
Another problem is that this map of Io’s volcanic hot spots is simply a snapshot in time that cannot be etched in stone (molten or not). Io’s volcanoes have something in common with those on Earth: some remain active for a long time, while others have short-lived paroxysms. “That’s the lovely thing about Io,” the fact that her fiery face is constantly changing, he says. Jani Radebaugh, a planetary geologist at Brigham Young University, who was not part of the study. “There’s no way we can finish mapping all of Io’s volcanism.”
The global portrait of lunar flares presented in this article may be the first of its kind. But it will not be the last. Future snapshots of Io’s volcanic hot spots may look very different from this one, which could support a different conclusion. For now, however, this thermal snapshot broadly aligns with past investigation which used the distribution of the moon’s erupting or dormant volcanoes to determine the location of Io’s heat engine, and it sure looks like that engine is shallow, not deep.
Juno’s closest flyby of Io is scheduled for December, giving the spacecraft more opportunities to spy on the moon’s most elusive volcanic eruptions. Scientists can’t wait to open that Christmas gift. “This is the purest form of discovery you can imagine,” Davies says. “It’s absolutely exciting to see these things.”