The European Space Agency's next big mission may be particularly juicy for scientists hunting for signs of extraterrestrial life.

The Jupiter Icy Moons Explorer, or JUICE, spacecraft has been formally approved and is now slated to blast off for the gas giant planet in 2022, ESA officials announced this week.

The space probe will tour three of Jupiter's largest moons: Ganymede, Callisto, and Europa—all were discovered by Galileo Galilei in the early 1600s. (Also see "Two New Moons Found Orbiting Jupiter.")

Previous studies have hinted that the moons could harbor liquid oceans—and maybe life-forms—below their icy surfaces.

JUICE will also make constant observations of Jupiter's atmosphere and magnetic field and will track the planet's interactions with its moons.

"This will be the first time that Europe, by itself, will explore a giant planet," said ESA's science program committee chair Richard Bonneville. "It's exciting for us."

Kevin Hand, a planetary scientist with NASA's Jet Propulsion Laboratory in California, said he's "tremendously excited" about ESA's selection of the JUICE mission.

"This will be the first robotic spacecraft dedicated to exploring an ocean on another world," said Hand, who is also a National Geographic Society emerging explorer. (National Geographic News is a division of the Society.)

"We think that both Ganymede and Europa have liquid-water oceans beneath their icy shells. And if we've learned anything about life on Earth, it's that where you find liquid water, you generally find life."

Peering Into Moons' Icy Crusts

The JUICE spacecraft is to launch from ESA's spaceport in French Guiana and should reach the Jovian system in 2030.

ESA plans to pack the solar-powered spacecraft with a suite of instruments, which will collect high-resolution pictures as well as data on the moons' chemical compositions, magnetic environments, and surface features.

During its roughly three-year mission, JUICE will perform two flybys of Europa, examining that moon's icy crust in search of sites for future exploration, perhaps by a lander.

Then, after a dozen flybys of Callisto, the spacecraft will slip into orbit around Ganymede in 2032 and will study Jupiter's largest moon for nearly a year.

"The ice shells of Ganymede and Europa serve as a window to the oceans below," Hand said. That's because, as in the Arctic on Earth, the surface ice is most likely born from oceans below, and so will carry information about the liquid water's chemical composition.

And thanks to onboard radar, JUICE will be able to delve even deeper.

"Ice-penetrating radar to a planetary scientist is somewhat akin to an ultrasound procedure on a pregnant woman," Hand said. "It lets you see into—and potentially beneath—the ice shell to observe where the ocean meets the ice."

Looking for Signs of Life

It's possible JUICE could spot some sign of biological activity—assuming any of the Jupiter moons harbor life as we know it.

Onboard instruments will assess the moons' habitability by comparing carbon chemistry, temperature, pressure, and other factors with the conditions that work for organisms on Earth.

In addition, Jupiter itself will serve as the archetype for other gas giant planets being discovered around distant stars.

JUICE will deliver unprecedented information about how the Jovian system formed and evolved, helping scientists evaluate whether any alien Jupiters could harbor life.

For instance, "Earth and Venus have roughly the same size and compositions, but Earth has a magnetic field, which protects Earth from solar wind and is essential for life to appear and be protected, while Venus has none," ESA's Bonneville said.

In the Jovian system, "Ganymede has a magnetic field, and the other moons do not. Why? That's something we would like to understand." he said. (Also see "Jupiter Auroras Fed by Largest Moon's Magnetic 'Bubble.'")

"We know on Earth there is motion in the magnetic core of the planet that can generate a magnetic field. It may be the same reason on these icy moons—one [perhaps] has an active core and the other does not."

"Bittersweet" Space Mission

Along with excitement for ESA's new mission comes a bit of regret, at least among U.S. planetary scientists.

JUICE was originally half of a two-spacecraft mission that was meant to include a NASA-built Europa orbiter, but NASA pulled out of the project due to funding issues.

"Our ESA partners have carried on ... so to some extent the announcement is bittersweet," JPL's Hand said.