NASA’s Juno spacecraft successfully made the second of two close flybys of Io, the most volcanic moon in the solar system, on February 3, giving scientists their best look at this satellite of Jupiter in more than two decades.
The data gathered from these flybys could resolve a mystery about the source of Io’s volcanoes and indicate whether a magma ocean is hiding beneath its surface.
Juno launched in 2011 and arrived at Jupiter in 2016. The spacecraft’s original quest involved exploring the most massive planet in the solar system and its iconic striped atmosphere. But once Juno completed that mandate in 2021, its operators sketched out a new plan for the craft to examine three of Jupiter’s moons, as well as the behemoth’s dainty rings.
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After zipping past the massive Ganymede in 2021 and the icy Europa in 2022, Juno executed its first close flyby of Jupiter’s volcanic moon Io, the innermost of the planet’s four large satellites, on December 30, 2023, followed by its second one on February 3. Each time, the spacecraft flew within about 930 miles (1,500 kilometers) of the surface. Many of the data await scientific analysis and aren’t yet publicly available. But the stunning photographs of this moon’s surface that have been released offer a teaser of the mission’s view of the remarkable world.
“We really saw Io in a new light,” says Scott Bolton, a planetary scientist at the Southwest Research Institute and principal investigator on the Juno mission.
Juno is not the first spacecraft to glimpse Io: both Voyager probes flew by the natural satellite in 1979 and discovered a volcanic nightmare where scientists had expected a quiet gray world like our own moon. And the Galileo, Cassini and New Horizons missions observed Io between the mid-1990s and mid-2000s.
Intriguingly, the Jovian moon itself has appeared surprisingly similar throughout all these missions. “Io has some volcanoes that stay active for a very long time,” says Rosaly Lopes, a planetary scientist at NASA’s Jet Propulsion Laboratory, who is not a Juno team member but works with mission personnel. “They were going at the time of Voyager, and they’re still going.”
Glance at Juno’s December 2023 images of Io, and you may think you know precisely which features are volcanoes: the spiky, angular rises that are particularly on display near the terminator line, where day becomes night. “Along the terminator, we were able to get pictures so you can see shadows, and it’s amazing how tall and sharp the mountains are,” Bolton says. “I had never seen anything quite that spectacular before.”
But as stunning as these craggy peaks appear, they aren’t Io’s notorious volcanoes. “We’ve looked and looked and looked at mountains, and there’s no source of heat associated with the mountains,” says Julie Rathbun, a planetary scientist at Cornell University, who studies Io’s volcanoes but is not involved in the Juno mission. “The mountains on Io seem to be a purely tectonic phenomenon.”
Instead of building upward like many eruptions on Earth, Io’s volcanoes appear to be more like lava lakes, Lopes says. In the new images, these volcanoes are particularly vivid, looking like dark pits. Scientists monitor their hotspots from Earth, but Juno’s close-approach data are still a cause for excitement. Rathbun says she’s particularly hoping for an image of the surface region where she says a colleague using ground observations identified a new hotspot about a decade ago, long after the most recent close spacecraft flyby, which was performed by Galileo in 2001.
The Juno spacecraft is equipped with nine science instruments to collect data, including a visible-light camera, which generates the earliest-available and most aesthetically pleasing images. In addition, the probe’s ultraviolet instrument, which sat out last December’s flyby but was set to operate during this month’s maneuver, could reveal more about the composition of Io’s surface, as well as how the plentiful volcanoes affect the moon’s atmosphere.
Juno’s powerful microwave instrument, designed especially for the mission, has been vital in peering deep into Jupiter’s atmosphere. Scientists aren’t sure yet what its observations at Io will teach us about the moon, although Bolton says ongoing experiments on Earth are working to interpret what its measurements could mean.
A key hoped-for science finding from the flybys relies on Juno’s gravity science instrument. The Juno team was careful to coordinate two flybys of Io to increase the chances of seeing changes between the encounters, Bolton says. Those data could clarify whether the moon hides a global magma ocean or whether its volcanoes are feeding on mere pockets of molten rock peppered through the interior, a key mystery about Io.
Although Juno can’t complete any additional close flybys of the active moon, it will continue to gather observations at greater distances, particularly with an infrared camera that’s valuable for mapping the moon’s volcanoes. A combination of data from previous distant flybys, the recent pair of close flybys and future distant observations should reveal how eruptions play out over time.
“We’ll get a very good storyboard of how the volcanoes are varying on Io, when are new ones erupting, how big are they,” Bolton says.
All of Juno’s Io observations should help scientists better understand how the volcano-ridden world affects the orbits of Jupiter and its other large moons, and vice versa. “It’s a very influential beast inside of Jupiter’s system, although it’s very tiny,” Bolton says.
And all of those observations come courtesy of an existing NASA mission being agile enough to exceed its lifetime and expand its portfolio of observations. Nothing about Juno is tailored to studying a supervolcanic moon, but scientists say its observations will still be valuable in cracking Io’s mysteries.
“The Juno spacecraft was designed to study Jupiter and Jupiter’s atmosphere,” Rathbun says. “The spacecraft, the orbit, the instruments—none of it was designed to study Io. So this is some really awesome bonus science.”