Lava and frost may form the mysterious lumps on Jupiter’s moon Io

On Jupiter’s moon Io, lava creeping beneath frost may give rise to fields of towering dunes.

That finding, described April 19 in Nature Communications, suggests that dunes may be more common on other worlds than previously thought, though the lumps may form in odd ways.  

“In some sense, these [other worlds] are looking more familiar,” says George McDonald, a planetary scientist at Rutgers University in Piscataway, N.J. “But the more you think about it, they feel more and more exotic.”

Io is a world crowded with erupting volcanoes, created when the gravitational forces of Jupiter and some of its other moons pull on Io and generate heat (SN: 8/6/14). Around 20 years ago, scientists reported another type of feature on Io’s dynamic surface — hummocky ridges. The features resemble dunes, but that couldn’t be the case, scientists reasoned, because Io’s atmosphere is too thin for winds to whip up a dunescape.

But in recent years, dunelike features have been discovered on comet 67P (SN: 9/21/20) and Pluto (SN: 8/24/21), planetary bodies that also lack thick atmospheres. Inspired by those alien dunescapes, McDonald and his colleagues revisited the matter of Io’s mysterious lumps. All they needed was some type of airborne force to sculpt the moon’s dunes.

On Earth, powerful explosions of steam occur when flows of molten rock encounter bodies of water. While water isn’t found on Io, sulfur dioxide frost is pervasive. So the scientists hypothesize that when lava slowly flows into and just under a frost layer, jets of sulfur dioxide gas could burst from the frost. Those jets could send grains of rock and other material flying and forming dunes.

The researchers calculate that an advancing lava flow, buried under at least 10 centimeters of frost, could sublimate some of the frost into pockets of hot vapor. When enough vapor accumulates and the pressure becomes high enough to match or overcome the weight of the overlying frost, the vapor could burst out at velocities over 70 kilometers per hour. These bursts could propel grains with diameters from 20 micrometers to 1 centimeter in size, the team estimates.  

Analyzing images of Io’s surface, collected by NASA’s now-defunct Galileo probe, revealed highly reflective streaks of material radiating outward over dunes in front of lava flows — possibly material newly deposited at the time by vapor jets.

This image, taken by NASA’s now-defunct Galileo spacecraft, shows dunelike lumps on Jupiter’s third-largest moon Io. The dark area (lower left) is a lava flow, and the bright streaks that radiate outward may be evidence of material strewn by jets of vapor that burst from frost heated by the lava.JPL-Caltech/NASA, Rutgers Univ.

What’s more, using the images to measure the hummocky features showed that their dimensions align with those of dunes on other planetary bodies. Some of the Ionian dunes are over 30 meters high, the team estimates.

“I think a lot of [scientists] looked at those and thought, hey, these really could be dunes,” says Jani Radebaugh, a planetary scientist at Brigham Young University in Provo, Utah, who was not involved in the study. “But what’s exciting about it is that they’ve come up with a good physical mechanism to explain how it’s possible.”

Io is typically thought of as a world of volcanoes. The possibility of dunes suggests that there might be more going on there than scientists thought, McDonald says. “It certainly adds a layer of complexity.”