Europa is an icy ocean world—and NASA is finally going to explore it
For the first time, NASA is journeying to this mysterious moon to determine if it might be able to host alien life.
For more than a quarter century, scientists have wanted to send a robot to explore Jupiter’s moon Europa. Now, it’s finally happening: A spacecraft bound for the icy world blasted off from Kennedy Space Center on a SpaceX rocket on October 14, after delay due to Hurricane Milton. NASA’s Europa Clipper mission will spend five and a half years cruising to the Jovian system and then conduct nearly 50 flybys of the enigmatic moon to investigate its many mysteries.
Among the top puzzles scientists are hoping to tackle is whether Europa might be habitable—meaning it has the water, energy, and chemical building blocks required to host life as we know it. The frozen world is roughly the same size as our moon but hides a vast ocean beneath its icy exterior, potentially containing twice the amount of water as all of Earth’s oceans combined. This makes Europa an important target for astrobiologists hoping to learn whether living organisms exist beyond our planet.
During its initial three-year stint in orbit, Europa Clipper will provide unprecedented views of the moon’s surface, glean information on the dynamics of its ice shell, and determine if spectacular geysers spout from its surface, much like Saturn’s moon Enceladus. The probe will give investigators insight into the inner workings of icy ocean worlds, which may exist in enormous numbers throughout the cosmos.
“We've never sent a mission dedicated to an icy ocean world before,” says Curt Niebur, program scientist for the mission at NASA. “There are so many discoveries lying in wait for us, it's going to be fantastic.”
What do we know about Europa?
Italian astronomer Galileo Galilei discovered Europa and three of its sibling moons in 1610 when he pointed his telescope at Jupiter and saw four bright dots surrounding the giant planet. Our first close-up views came in 1979 when NASA’s Voyager probes sped past Jupiter and photographed its many satellites. Voyager 2 took high-resolution images of Europa’s exterior, showing it was chock full of long, irregular cracks and relatively devoid of craters—indicating the surface was young and likely geologically active.
In 1996, NASA’s Galileo mission flew by Europa and beamed back some extremely curious data: Apparently, this tiny frozen moon possessed a magnetic field, something normally only found on larger bodies like the Earth. The space physicist Margaret Kivelson and her colleagues showed that Jupiter’s magnetic field could generate one on Europa if the moon had a gigantic global ocean of liquid saltwater beneath its crust.
“Everybody was suitably skeptical,” says Niebur. “Margie sat down and ran through her assumptions and her math, her logic, and this wonderfully brilliant woman convinced everybody that she was correct.”
For the first time, scientists had evidence for an icy ocean world, a category that is now known to include Europa’s siblings Calisto and Ganymede, as well as Saturn’s Enceladus and Titan, and Neptune’s Triton. The Galileo probe also showed that enormous patches of reddish carbon-containing organic material covered Europa’s surface, while later observations found table salt in the same patches—all tantalizing hints that a livable environment might lie beneath its frigid exterior.
Yet large gaps remain in our understanding of Europa: Is its frozen carapace is a few kilometers or tens of kilometers thick? What kind of chemistry does the moon’s subsurface ocean have and how deep is it? Is there geothermal activity on the seafloor that could provide energy for living organisms? Could the organic material on Europa’s outer shell provide food for undersea creatures? Such questions are the driving forces behind the mission.
What is Europa Clipper?
The Clipper spacecraft is the largest planetary explorer NASA has ever built, with solar panels extending out to the size of a basketball court. The robot carries a suite of nine state-of-the-art instruments, each of which will record data as the probe sweeps past Europa.
Clipper will not be orbiting Europa itself because of Jupiter’s formidable magnetic field—roughly 20,000 times stronger than our own planet’s. The field’s radiation would fry any electronics that remain too long inside its influence (and may give the moon a blue or green glow). But the spacecraft will swoop in and out of the radiation to snap high-resolution photos of 95 percent of the moon’s exterior, sometimes coming in as low as 25 kilometers (16 miles) from the surface. These images will give us the most-detailed map of Europa yet.
(To prepare for Europa Clipper, scientists tested techniques and instruments here on Earth.)
How will Clipper study Europa?
Our current best images of the moon contain limited detail, the equivalent of somebody flying over the Earth and being able to tell that Manhattan is an island situated between two bodies of water, says planetary scientist Catherine Walker of the Woods Hole Oceanographic Institution in Massachusetts. After Clipper “we'll be able to see Central Park, individual streets, things like that,” she says, as well as measure the heights of many features.
Such resolution will allow researchers to investigate how the ice shell moves and breaks apart, whether it contains sections that are sliding underneath one another like tectonic plates on our planet, and, perhaps most crucially, whether there are passageways or conduits connecting Europa’s exterior to its interior ocean.
Clipper will use ice-penetrating radar to peer inside the frozen ice shell all the way to its very bottom, searching for hidden pockets of liquid water akin to the buried Lake Vostok in Antarctica. Such spots are potentially habitable abodes that energy-rich organic molecules on the surface might have an even easier time reaching than the subglacial ocean.
“We know life loves interfaces and places that materials can mix,” says astrobiologist Kate Craft of Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “And so if we see places like that within the ice shell, that would be a really compelling place we could go back and try to sample in the future.”
Using a device called a spectrometer, Clipper will investigate what’s in the reddish-orange organic material on the moon’s surface. Such material has three potential origins: It might be bubbling up from the interior ocean via cracks, the result of simple chemicals interacting with Jupiter’s powerful magnetic field, or debris blown out into space by the nearby volcanic moon Io before settling on Europa.
Clipper will also be joined at the Jovian system by the European Space Agency’s Jupiter Icy Moons Explorer (JUICE). That mission will mainly be investigating Ganymede and Calisto but should also pass by Europa, providing opportunities for the two spacecraft to work in tandem to solve mysteries about these worlds.
Will Clipper drill beneath Europa’s crust?
Sadly, the spacecraft can’t land and take samples, but many scientists are hoping to spot geysers spouting from Europa’s surface. Hubble images from around a decade ago hinted that such plumes could be erupting. But “all the detections have been at the detection limits,” says planetary scientist Francis Nimmo of the University of California, Santa Cruz. “If [the plumes] exist, then they're pretty intermittent, and they may not actually be there at all.”
Clipper should hopefully clear up this conundrum, either by taking direct photos of a geyser or finding evidence of a recent deposit on the surface. If the plumes exist, they would be a major boon because mission directors might be able to fly Clipper through a spout, tasting and analyzing material from within or below the ice sheet.
(Explore Europa’s icy ocean through this interactive graphic.)
What’s going on in Europa’s depths?
No one is quite sure how geologically active the rocky surface at the bottom of Europa’s ocean is. Some suspect that hot, smoking hydrothermal vents could be present there, the same as those that exist in the depths of Earth’s oceans, providing warm and energetic habitats to microbes, crabs, and worms. But recent models splashed cold water on this idea: Based on the strength of the rocky crust and low tectonic activity of our own similarly-sized moon, some researchers questioned whether Europa’s seafloor might actually just be super low energy.
“If we're right, it's not going to be the kinds of hydrothermal systems that we might be familiar with on Earth, with those big worms and those really hot black and white smokers,” says Paul Byrne, a planetary scientist at Washington University in St. Louis who helped devise the new models.
He’s careful to add that this doesn’t totally place the kibosh on life, especially considering that many terrestrial microbes thrive just fine in lower-energy places where water and rock interact likes large cracks in the undersea crust. The fact that researchers are having such debates is good, says Craft, since it bolsters the need for a mission like Clipper to clear them up. During its flybys, the probe will measure the moon’s magnetic and gravity fields, providing information about Europa’s interior composition to help resolve such debates.
What might this mean for life beyond our planet?
While Clipper is not a life-detection mission, it will be the first to assess whether this alien world could sustain alien life. Understanding this frozen moon will provide insight others like it such as Enceladus and Triton and, most likely, icy ocean worlds in distant star systems. Niebur points out that there is only one terrestrial ocean world in our solar system—the Earth—but at least six celestial bodies similar to Europa.
“If Europa Clipper shows that icy ocean worlds are habitable,” he says, “then the implications for how common habitable environments are in the universe as a whole are absolutely staggering.”
