Jupiter's volcanically active moon Io casts its shadow

Three exciting alien moons to get first look up close in nearly 20 years

NASA's Juno probe will soon fly close to a trio of Jupiter's intriguing moons, including one that scientists think is among our best bets for finding life beyond Earth.

Jupiter's moon Io—a volcano world orbiting the planet once every 42 hours—casts its shadow on the Jovian clouds in this image from NASA's Juno spacecraft.

Photograph by NASA/JPL-Caltech/SwRI/MSSS, Kevin M. Gill

After five years orbiting Jupiter, NASA’s Juno spacecraft is spiraling into the heart of the Jovian system.

The four-year, 42-orbit journey will plunge the craft into a treacherous, brutal environment—while offering new, dramatic views of three alien worlds. But Juno, sheathed in titanium armor that protects it from harsh radiation, is ready for its mission. And its team back on Earth is eager to follow it from afar during this daring voyage.

“I’m really excited, just thrilled that we were able to come up with this plan,” says Juno principal investigator Scott Bolton of the Southwest Research Institute. ”It was an out-of-the-box idea, to be able to literally make a transition from a Jupiter orbiter that studied primarily only Jupiter, to something that is going to be a full-system explorer.”

As Juno continues its surveillance of the system, its loops around Jupiter will tighten, bringing the spacecraft closer and closer to our solar system’s largest planet and within range of three of Jupiter’s most intriguing moons.

This June, the solar-powered spacecraft will brush by Ganymede, the largest moon in the solar system. Next, in 2022, it will visit Europa—an ice-encrusted ocean world that could harbor life. And finally, as Juno sweeps daringly close to Jupiter, it will encounter Io, a violently volcanic moon dusted with sulfuric frost. Seeing those moons up close for the first time in nearly two decades—since the Galileo spacecraft’s mission ended in 2003—will thrill scientists who study the elusive icy worlds of the outer solar system.

“I keep trying to get this hashtag trending: #PlanetsAreOverrated,” says Julie Rathbun of the Planetary Science Institute, who studies Io. “Moons and satellites are way more interesting.”

In addition to studying those three alien moons, Juno will also scrutinize Jupiter’s rings, making several passes through the bangles themselves. Dark and diffuse, the rings are not nearly as spectacular as Saturn’s, and scientists know very little about them.

“It’s so rare to get a mission to the outer solar system, and it’s just amazing that we can take advantage of this spacecraft that’s there,” says Cynthia Phillips of the Jet Propulsion Laboratory, who studies the outer solar system’s icy moons. “It’s basically a whole new mission.”

Half a decade at Jupiter

Launched in 2011, Juno pulled into orbit around Jupiter on July 4, 2016. The spacecraft’s primary goals have included studying the gas giant’s gravity, magnetic fields, atmosphere, and interior.

But Juno, named after the god Jupiter’s wife, is perhaps best known for its exquisite images of the solar system’s largest world. For centuries, we’ve watched from afar as massive maelstroms crossed the planet’s face. But up close, through JunoCam’s lens, Jupiter is a watercolored world dappled by numerous cyclones that swirl and cluster near its previously unseen poles.

In addition to its thin ring system, the planet is orbited by some 79 moons—including 12 that were accidentally discovered just three years ago. The four largest moons, known as the Galilean satellites, were first spotted in the early 1600s by the Italian astronomer Galileo Galilei, who initially mistook them for stars. Now, Ganymede, Callisto, Europa, and Io are among the most intriguing places in the solar system—although few spacecraft have gotten close enough to study them in detail. The two Voyager spacecraft flew by in 1979, and the Galileo spacecraft surveyed the Jupiter system for eight years starting in 1995.

“The thing that still shocks me is how little data we have on all of these Galilean moons,” Rathbun says.

The first target on Juno’s traipse inward is Ganymede. A hulking moon that stretches more than 3,700 miles across and loops around Jupiter once every 172 hours, Ganymede is larger than the planet Mercury. On June 7, Juno will make the first of two close passes by the cratered world which, like all the Galilean satellites, is named after one of Jupiter’s mythological lovers.

“It’s hard to grasp the immense size of the Jupiter system,” Phillips says. “Just because you're in orbit around Jupiter doesn't mean you're anywhere near Io, Europa, or Ganymede.”

Ganymede’s chief claim to weirdness is that it’s the only moon in the solar system that generates its own magnetic field—a phenomenon that scientists think is the work of a churning, liquid core like that of Earth.

“That’s the assumption, but we don’t know for sure,” Phillips says. It’s not clear how Ganymede has maintained its intrinsic magnetism over the age of the solar system while other bodies, such as the moon and Mars, lost their magnetic fields billions of years ago.

Scientists also suspect that a global ocean lies beneath Ganymede’s pockmarked, icy shell—and observations from Juno should help map that shell and probe for thin spots. Eventually, Juno’s gravitational encounters with Ganymede will shave 10 days off the spacecraft’s orbit around Jupiter, sending it on a path toward Europa and Io.

In September 2022, Juno will fly by Europa, a moon that’s considered one of the most likely places in the solar system to find alien life. The smallest of the Galilean satellites, Europa is a true water world: Beneath its crisscrossed, icy shell lies a salty global ocean that contains more water than all of Earth’s seas.

Scientists suspect that this alien ocean contains all the ingredients necessary for life as we know it to evolve and thrive: water, an energy source, and essential chemical elements. But we won’t know if Europa is inhabited until we can get a spacecraft near enough to search for signs of life. NASA is planning to do just that in the next 15 or so years with a mission called Europa Clipper that is getting ready to sail to the outer solar system.

Next year, Juno will fly by Europa three times. As it does, it will look for any signs of water vapor plumes or geysers, a phenomenon that astronomers have claimed is occurring—at least sporadically—at Europa.

“There’s a lot of debate,” Phillips says. “So having another spacecraft there, having one just take a look—does it see anything suspicious? That’ll be really important.”

Juno will also be able to scan the ice shell for any particularly thin spots, or for places where there might be pockets of liquid water tucked into the crust. Such locations would make it easier for a future robotic explorer to collect samples from the buried ocean. And as it scans the surface, Juno will make a map of Europa’s never-before-seen poles.

“By imaging these unseen parts of Europa, we can tie together a global map,” Phillips says. “It’s not like with Mars, where we’ve sent a zillion mission.”

As Juno swings by Europa, it will lose another five days from its orbit. That will shrink the time it takes to loop around Jupiter to just 38 days and send the spacecraft on to the infernal moon Io.

With more than 400 active volcanoes, Io is the most volcanic place in the solar system. When NASA’s New Horizons spacecraft zoomed by Jupiter in 2007, it spotted an enormous fountain of fire stretching 200 miles above the moon’s surface.

Snuggled up next to Jupiter, Io is among the trickier and more dangerous destinations to reach. Its proximity to Jupiter, orbiting the planet once every 42 hours, puts it in an extremely harsh environment. The moon itself is heated, squeezed, and stretched by Jupiter and Europa’s gravity, which explains its hyperactivity. In 2023, Juno is set to make 11 close flybys of the world, during which Rathbun hopes to see some dramatic activity, like the outburst from the volcanic region Tvashtar that New Horizons captured.

“Tvashtar always seems to be active when there’s something nearby,” Rathbun jokes. And Io is basically always erupting. The material it produces is captured by Jupiter’s gravity and goes on to feed the planet’s mammoth auroras—swirling, vivid, ultraviolet light shows at the poles, similar to the Northern and Southern Lights on Earth.

As with Europa and Ganymede, there aren’t many observations of Io from up close. Most of the information we’ve gathered about the moon since the Galileo mission ended has come from Earth-based telescopes.

“Every time you look, something new is going on, and with Io in particular, you have to look as often as you can,” Rathbun says.

Unlike Earth’s volcanoes, Io’s volcanoes are randomly scattered across the moon’s surface—a pattern that raises questions about the moon’s magma reservoirs and internal heating. Rathbun says that higher-latitude volcanoes appear to behave differently than those nearer the equator, erupting less frequently but more powerfully.

Rathbun says she was “jumping-up-and-down excited” when she heard about the plans to zoom by Io: “A good look at the volcanoes at the poles? Finally!”

Bolton says Juno could help scientists figure out whether Io’s volcanoes are powered by a global magma ocean, or if smaller, more discrete pockets are fueling local eruptions. And he isn’t worried about sending Juno so close to Jupiter, or about the spacecraft’s traverses through the tenuous ring system. If there’s any spacecraft that’s capable of surviving Jupiter’s harsh radiation, or a collision with a ring particle, it’s Juno.

“We built it like an armored tank,” he says. “And the armor seems to be holding up very well.”

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