NASA’s Juno spacecraft made the second of two successful close flybys of Io, the solar system’s most volcanic moon, on February 3, giving scientists the best view of this moon of Jupiter in more than two decades.
The data collected from these flybys could solve a mystery about the origin of Io’s volcanoes and indicate whether a magma ocean lurks beneath its surface.
Juno launched in 2011 and arrived at Jupiter in 2016. The spacecraft’s original mission involved exploring the solar system’s most massive planet and its iconic striped atmosphere. But once Juno completed that mandate in 2021, its operators outlined a new plan for the spacecraft to examine three of Jupiter’s moons, as well as the giant’s delicate rings.
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After zipping past massive Ganymede in 2021 and 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 the second on February 3. Each time, the spacecraft flew about 930 miles (1,500 kilometers) from the surface. Much of the data awaits scientific analysis and is not yet publicly available. But the stunning photographs of the surface of this moon that have been published offer a preview of the vision of this extraordinary world that the mission offers.
“We really saw Io in a new light,” says Scott Bolton, a planetary scientist at the Southwest Research Institute and principal investigator for the Juno mission.
Juno is not the first spacecraft to glimpse Io: both Voyager probes flew over the natural satellite in 1979 and discovered a volcanic nightmare where scientists expected a gray and calm world like our own moon. And the Galileo, Cassini and New Horizons missions observed Io between the mid-1990s and mid-2000s.
Interestingly, the Jovian moon itself has appeared strikingly similar throughout all of these missions. “Io has some volcanoes that stay active for a long time,” says Rosaly Lopes, a planetary scientist at NASA’s Jet Propulsion Laboratory, who is not a member of the Juno team but works with the mission staff. “They were going at the time of Voyager and they’re still going.”
Take a look at images of Io taken by Juno in December 2023 and you might think you know precisely which features are volcanoes: the pointed, angular elevations that are on display particularly near the termination line, where day turns to night. . “Along the terminator, we were able to take pictures so you can see the shadows, and it’s amazing how tall and clear the mountains are,” Bolton says. “I’ve never seen anything so spectacular before.”
But as impressive as these craggy peaks look, they are not the famous volcanoes of Io. “We’ve looked and looked and looked at mountains, and there’s no heat source 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. “Io’s mountains appear to be a purely tectonic phenomenon.”
Instead of developing upward like many eruptions on Earth, Io’s volcanoes look more like lava lakes, Lopes says. In the new images, these volcanoes are particularly vivid and look like dark pits. Scientists monitor its hotspots from Earth, but Juno’s close approach data remains a source of excitement. Rathbun says she is particularly looking forward to an image of the region of the surface where she says a colleague using ground-based observations identified a new hotspot about a decade ago, long after the most recent close flyby by a spacecraft, performed by Galileo in 2001.
The Juno spacecraft is equipped with nine scientific instruments to collect data, including a visible light camera, which generates the earliest and most aesthetically pleasing images available. Additionally, the probe’s ultraviolet instrument, which did not participate in last December’s flyby but was scheduled to operate during this month’s maneuver, could reveal more about the composition of Io’s surface, as well as how the abundant volcanoes They affect the atmosphere of the moon.
Juno’s powerful microwave instrument, designed especially for the mission, has been vital for peering deep into Jupiter’s atmosphere. Scientists aren’t yet sure what their observations on Io will teach us about the Moon, although Bolton says ongoing experiments on Earth are working to interpret what their measurements could mean.
A key and expected scientific finding from the flybys is based 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. These data could clarify whether the moon hides a global magma ocean or if its volcanoes are fed by simple pockets of molten rock scattered throughout the interior, a key mystery about Io.
Although Juno cannot complete any additional close flybys of the active moon, it will continue to collect observations at greater distances, particularly with an infrared camera that is 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 the eruptions develop over time.
“We’ll have a very good storyboard of how volcanoes vary on Io, when new ones erupt, and how big they are,” Bolton says.
All of Juno’s observations on Io should help scientists better understand how this volcano-riddled world affects the orbits of Jupiter and its other large moons, and vice versa. “It’s a very influential beast within the Jupiter system, even though it’s very small,” Bolton says.
And all of those observations come courtesy of an existing NASA mission that is agile enough to exceed its useful life and expand its portfolio of observations. Nothing on Juno is designed to study a supervolcanic moon, but scientists say its observations will still be valuable in unlocking Io’s mysteries.
“The Juno spacecraft was designed to study Jupiter and its atmosphere,” says Rathbun. “The spacecraft, the orbit, the instruments, none of that was designed to study Io. So this is really amazing extra science.”