Europa, one of the four large moons of Jupiter first seen by Galileo 414 years ago, may have a deep, salty, global ocean hidden beneath a thick crust of ice. Where there is water, there might be life. In an ambitious $5 billion mission decades in the making, NASA is poised to send a jumbo robotic probe called the Europa Clipper to see if the icy moon has the key characteristics of a habitable world.

“This is a huge deal,” said Robert Pappalardo, the project scientist for the Europa Clipper at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

NASA officials had hoped to launch the spacecraft Thursday on a SpaceX Falcon Heavy rocket from Florida’s Kennedy Space Center. But Hurricane Milton — the eye of which passed directly over Cape Canaveral — put everything on hold. Friday night, NASA said the launch window will open Monday. It extends to Nov. 6.

Life beyond Earth is among the greatest unknowns in science. Finding the first confirmed example of alien life has been a goal of NASA for decades. The scientific community has narrowed its focus to a few enticing targets, and at or very near the top of the list is this strange moon that looks like nothing else in the solar system.

The Europa Clipper, weighing 13,000 pounds fully fueled at launch, is the largest space probe NASA has ever built. It will deploy a solar array more than 100 feet across, a requirement for a mission that will fly so far from the sun.

A flyby look at the moon Europa

The Europa Clipper space probe carries nine science instruments and will use its telecommunications system to perform additional gravity experiments.

Diagram of the Europa Clipper

Cameras,

thermal

imaging

PIMS (Magnetic

field sensor)

High-gain

antenna

Solar

panels

ECM

(Europa Clipper

Magnetometer)

PIMS (Magnetic

field sensor)

REASON

(Ice-penetrating radar)

Solar

panels

Cameras,

thermal

imaging

PIMS (Magnetic

field sensor)

High-gain

antenna

ECM

(Europa Clipper

Magnetometer)

PIMS (Magnetic

field sensor)

REASON

(Ice-penetrating radar)

Solar

panels

Cameras,

thermal

imaging

PIMS (Magnetic

field sensor)

High-gain

antenna

REASON

(Ice-penetrating radar)

ECM

(Europa Clipper

Magnetometer)

PIMS (Magnetic

field sensor)

Solar

panels

Cameras,

thermal

imaging

PIMS (Magnetic

field sensor)

High-gain

antenna

REASON

(Ice-penetrating radar)

ECM

(Europa Clipper

Magnetometer)

PIMS (Magnetic

field sensor)

This is not, as NASA is quick to emphasize, a life-detection mission. Instead the agency has set a more modest goal of discerning whether Europa has the right environmental conditions for life — whether it’s habitable. That doesn’t mean inhabited by any alien creatures, only that it looks like the kind of place life might find a toehold.

If the habitability answer is a resounding yes, scientists across the planet would likely lobby for a more ambitious mission to orbit Europa or land a spacecraft on the moon’s surface.

Scientists have been mulling the possibility of a subsurface ocean on Europa since the Voyager probes sent back images in the 1970s revealing that the moon’s icy surface is covered in cracks. Subsequent probes produced images of features that resemble icebergs.

Europa is also locked into a stable orbit of Jupiter that leads scientists to suspect that a Europan ocean could be billions of years old — intriguing both scientists and sci-fi writers. Arthur C. Clarke fictionalized an ocean on Europa in his 1982 novel “2010: Odyssey Two” and two sequels. He described a bizarre alien life form — “like huge strands of wet seaweed” — emerging through an ice crack and destroying a spaceship.

“One of the big objectives we’d like to accomplish with this mission is to absolutely prove that the ocean exists today,” said Geoff Collins, a geologist at Wheaton College in Massachusetts who is on the camera team for the Europa Clipper.

The Clipper’s journey to Europa

The Europa Clipper isn’t taking a direct route to Jupiter. It will use the gravity of Mars and later, Earth, to accelerate to speeds that will enable it to reach the Jupiter system. The trip will take about five and a half years.

The Galilean moons

Galilean moons diagram

JUPITER DIAMETERS

JUPITER DIAMETERS

JUPITER DIAMETERS

JUPITER DIAMETERS

JUPITER DIAMETERS

Life as we know it needs several things, including liquid water, energy and organic molecules, ideally in a stable environment. Europa — one of Jupiter’s 95 known moons, and the smallest of the four large moons Galileo spotted in 1610 with his early telescope — may satisfy all those requirements.

Europa whole

Europa interior

Surface ice

Subsurface

water

Surface ice

Subsurface

water

Surface ice

Subsurface

water

Surface ice

Subsurface

water

Despite its icy cracks, Europa’s surface is also remarkably smooth, with hardly any craters — certainly nothing like the pocked surface of our own moon. That’s a compelling sign that the surface is young, and refreshed by the convection of warmer ice from below.

Several space probes have obtained intriguing images of the surface, but what lies beneath remains a mystery. The discovery by NASA’s Voyager 1 space probe in 1979 of active volcanoes on the nearby moon Io indicated that Europa could have its own hot interior, including the kind of hydrothermal vents seen in the depths of Earth’s oceans.

Robotic probes, including NASA’s Voyager and Galileo spacecraft, have detected features on Europa suggestive of the existence of a deep and salty ocean beneath a crust of ice. Although scientists cannot be certain of the moon’s interior structure, they hypothesize that the crust could be 10 miles thick and the ocean 50 miles deep. That is more water than all of Earth’s oceans combined.

Scientists theorize that radiation from the sun and Jupiter would free hydrogen from the frozen water on Europa’s surface, leaving oxygen molecules that could be transported through convection to the hypothesized ocean below, potentially making the ocean more hospitable to life as we understand it.

The spacecraft’s journey to Europa has been long and arduous already. NASA engineers discovered in May that transistors on the spacecraft might fail under the bombardment of radiation from Jupiter’s powerful magnetic field during the flybys of Europa. Only after months of anxious troubleshooting did engineers conclude that the spacecraft’s elliptical orbit would enable it to get far enough away from the giant planet for the transistors to recover from the radiation assault during the flybys.

The Europa Clipper mission owes its existence in significant part to one passionate supporter, former congressman John Abney Culberson (R-Tex.), a space buff. He began advocating for a Europa mission, ideally a lander, in the early 2000s when he served on the House appropriations subcommittee in charge of NASA. Early efforts to earmark funding for the mission were stymied by resistance at NASA headquarters, he said.

But then came a bulletin from space: In 2013, astronomers published a paper saying the Hubble Space Telescope had seen evidence of 120-mile-long plumes of water vapor jetting from Europa. The observation remains ambiguous, but it gave the mission the push it needed, and Culberson wrote legislation that mandated NASA spend money on a mission to Europa.

Tight budgets and engineering challenges ruled out a lander or a Europa orbiter. Instead, the spacecraft will go into a highly elliptical orbit of Jupiter that will enable 49 flybys of Europa, coming as close as 16 miles from the surface.

Culberson said he thinks the space program could regain the public support it had during the glory days of the Apollo program if NASA could find signs of life on Europa.

Such a discovery would be a “civilization-changing” event, he said.

“There might be frozen krill in that snow,” he said. “But we don’t know. We’re not sure if there’s life there but it’s the most likely place to find it.”

‘What does it mean for something to be alive?’

“Follow the water” has long been the mantra of NASA’s search for life beyond Earth. Tidal forces generated by Jupiter’s massive gravitational field could provide the energy to keep the subsurface water liquid, scientists believe.

For life to exist in the darkness of a subsurface Europan sea, it would presumably rely on chemosynthesis — energy derived from chemical interactions — as do some organisms at the bottom of Earth’s oceans. The question yet to be answered is whether Europa has the molecular ingredients for life as we know it. Life on Earth relies heavily on a short list of elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.

Europa Clipper has nine instruments to study the moon. Among them is a mass spectrometer that, during close passes, could characterize particles ejected from the surface by the constant bombardment of tiny, dust-sized meteoroids, or by hypothesized plumes of water vapor. Detecting amino acids, which are building blocks of life, would be a significant discovery, particularly if scientists see a pattern in the amino acids suggestive of a biological origin.

NASA’s incremental approach — shooting for an understanding of Europa but not a go-for-broke attempt to detect life — has been shaped by a painful experience. In 1976, NASA landed two Viking spacecraft on Mars and famously conducted experiments to detect living organisms. The results were confusing. Initial euphoria over a seemingly positive result gave way to dismay as evidence suggested the Martian surface was inhospitable and probably sterile.

With Viking, said Pappalardo, “We went for the brass ring, but then said, ‘Oh, we didn’t understand quite how this place worked.’”

Since then, NASA has taken a more deliberate approach to Mars and other targets of astrobiological interest.

Scientists are still wrestling with one of the most basic questions in biology, according to Manasvi Lingam, a planetary scientist at the Florida Institute of Technology.

“In order to know if something is alive or not, a closely related question is, what is life?” he said. “What does it mean for something to be alive?”

Pappalardo points out that life on Earth is tenacious and versatile. Unclear, though, is how life begins.

“The big mystery is, how does it get going? How does life start out? And how specific are those conditions?”

If the scrutiny of Europa shows that it looks habitable but has no sign of life, that would still have important astrobiological implications, Pallapardo noted. It might mean that it’s hard to get life rolling even in seemingly congenial places.

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