There are more than 200 moons in the solar system, but none quite like Io, the third largest of Jupiter’s 80 moons. Io is really, really volcanic. In fact, it’s peppered with so many hundreds of powerful active volcanoes that there must be something unusual beneath its crust.
That something could be a thick moonwide layer of molten rock—or a “subsurface magma ocean,” according to a new study published in the Planetary Science Journal on Nov. 16 from Yoshinori Miyazaki and David Stevenson, planetary scientists at the California Institute of Technology.
That possible super-hot sea of melted rock—which is unique in the solar system—could harbor secrets, weird mechanisms for forming moons and planets, and even recipes for exotic alien life. Only further scrutiny of the 2,200-mile-diameter moon will tell.
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Miyazaki and Stevenson aren’t the first scientists to make an educated guess at what lies beneath Io’s potentially 20-mile-thick rocky crust. It’s been the subject of heated debate for years. But their new peer-reviewed study of the moon’s mantle might be the most thorough yet.
A volcanic explosion on Io, Jupiter's third largest moon, as captured by NASA's New Horizon spacecraft.
NASA/JPL/University of ArizonaTo peer beneath Io’s surface, Miyazaki and Stevenson revisited reams of data from NASA’s Galileo probe, which orbited Jupiter for eight years starting in 1995. Initial analysis of the probe’s magnetic data led to a loose consensus that Io’s mantle—the layer under the moon’s crust—includes a 30-mile thick top layer that should be “molten or partially molten,” according to NASA.
Compare this to Earth’s own mantle, as well as the mantles of every other planetary body in the solar system, which are mostly solid and consist largely of ice or superheated rocks. Broadly speaking, planetary scientists reading the Galileo data assumed Io either has an underground magma ocean or a kind of sponge-like rocky outer mantle soaked in magma.
A fresh look at the data led Miyazaki and Stevenson concludes it’s the molten sea. They based their conclusion on estimates of the mantle’s temperature via analysis of Io’s volcanoes, which can spew magma hundreds of miles into the moon’s sulfur dioxide atmosphere. The top of the mantle might register as hot as 2,800 degrees Fahrenheit.
That’s hot. But not hot enough to sustain a spongy interior. The analysis is complicated, but it boils down to this: Like a pot of gravy on a stovetop, Io would need a lot of heat to stay consistently spongy in its upper mantle. Without enough heat, the gravy—er, the spongy rock—would separate: rock on bottom, magma on top.
Miyazaki and Stevenson crunched the numbers, calculating the heat from Io’s core as well as the effects of its weird, highly-elliptical orbit, which sloshes the mantle, spreads heat around, and keeps Io from ever permanently cooling.
They concluded that the gravy would separate. “The amount of internal heating is insufficient to maintain a high degree of melting,” they wrote. Hence what they believe could be a topmost magma ocean.
Luckily, we’ll know more soon. NASA’s Juno probe, which arrived around Jupiter in 2016, is scheduled to take readings of Io in 2023 and 2024—specifically measuring the “Love number,” a gauge of a planet’s rigidity or lack thereof. “If a large Love number is found, we can say with more certainty that a magma ocean exists beneath Io’s surface,” Miyazaki told The Daily Beast.
We already knew Io is weird. It’s possible it’s even weirder—and that weirdness could have implications across the space sciences. “I don’t think it greatly changes understanding of planetary formation, but it does change how we view the internal structure and thermal evolution of tidally heated bodies like Io,” David Grinspoon, a senior scientist with the Arizona-based Planetary Science Institute, told The Daily Beast.
Io and Europa, Jupiter's two largest moons, captured by NASA's Juno spacecraft.
NASA/JPL-Caltech/SwRI/MSSS/Roman TkachenkoLurking in the academic shadows are the astrobiologists. The experts in how and where life could evolve in the universe. If there’s extraterrestrial life out there somewhere and it looks like Earth life, we should expect to find it—or evidence of its extinction—on planets and moons that have, or had, Earthlike environments. Mars. Venus. A moon of Saturn called Enceladus.
But volcanoes with their extreme transfers of energy are widely considered key components of a living ecosystem. So planets and moons with lots of volcanoes are great places to look for E.T. In theory, that should include Io.
However, Io might have too many volcanoes. So if there’s life evolving there, it’s probably very strange life that really likes heat. “Lava tubes could be creating a condition favorable for microbes,” Miyazaki said.
The question, for astrobiologists, is whether a magma ocean would create more or fewer lava tubes than a magma sponge. “I don’t have an explicit answer,” Miyazaki said. “But it’s interesting to think about such implications.”
Dirk Schulze-Makuch, an astrobiologist at the Technical University Berlin, has long advocated a thorough search for life on Io. A magma ocean would only spoil that search if it were really close to the surface. A nice thick crust should insulate the outermost regions of the planet from scouring heat, and preserve the potential for evolution. “There seems to be quite a bit of crust,” Schulze-Makuch told The Daily Beast.
If anything, the possibility of a magma ocean on Io just underscores how interesting and exciting the moon is—and why it should be a top target for future space probes, Schulze-Makuch said. “Io is a unique kind of moon, very dynamic, and we should not dismiss it altogether.”
