Tuesday, May 21

How our vision of Europa’s habitability is changing


THE WOODLANDS, TEXAS — On stage, before a silent assembly of scientists, many of whom are experts on alien worlds, planetary scientist Paul Byrne assumed his position behind the podium. He had come to present research on Europa, a moon of Jupiter that almost certainly harbors a subsurface ocean. The moon is thought to be among the most promising places to explore for life in our solar system. But much of that promise clings to an unknown — the geologic activity of Europa’s seafloor.

“I don’t think there’s anything happening on the ocean floor,” said Byrne, of Washington University in St. Louis, to the crowd gathered at the Lunar and Planetary Science Conference on March 11.

Europa is one of three worlds in our solar system — along with Saturnian moons Enceladus and Titan — generally thought to possess the three ingredients for habitability: liquid water, energy and the chemical building blocks for life. What’s more, Europa is thought to be around 4.5 billion years old, about as old as Earth. In other words, life has had roughly the same amount of time to emerge on Europa as it has here.

As a testament to all those promising qualities, the largest spacecraft NASA has ever developed for a planetary mission, Europa Clipper, is slated for launch in October.

But as Clipper’s maiden voyage nears, it’s unclear whether the moon’s ice-covered sea can sustain life. As Byrne and other researchers question whether the seabed is dead, enigmatic quakes detected on Earth’s moon hint that mysterious mechanisms could operate within Europa, too. And even if the icy moon is uninhabitable today, it may not have always been that way.

The geologic activity of the moon’s seafloor and its ability to nurture life may form the crux of the moon’s habitability problem, says Robert Pappalardo, a planetary scientist at the Jet Propulsion Laboratory in Pasadena, Calif., who works on the Clipper mission. “It’s such a profound question,” he says. “Either way it comes out, it’s going to be important for understanding how common life is out there in general.”

All quiet on the ocean floor

Europa’s ocean is plunged in darkness. It lurks beneath a layer of ice, estimated to be at least 20 kilometers thick, that encapsulates the entire moon (SN: 5/14/18). And the ocean’s waters are unfathomably deep, somewhere around 60 to 150 kilometers. The average depth of Earth’s big blue is 4 kilometers.

Anything living within that blackness would probably be chemosynthetic in nature. While plants and phytoplankton synthesize food from light, water and carbon dioxide, chemosynthetic organisms harvest carbon-bearing molecules and energy released from chemical reactions in their environments. On Earth’s ocean floor, microbes of this nature crowd hydrothermal vents and methane seeps, chemical oases sustained by tectonic forces and volcanic activity (SN: 10/20/23; SN: 3/15/23).

For such organisms to persist in Europa’s ocean, it is thought that similar geologically-sustained environments, or at least chemical reactions between water and fresh rock surfaces, would be necessary, Byrne said. “Our basic question is: How likely is that to happen?”

He and his colleagues constructed computer simulations of Europa’s seafloor, accounting for its gravity, the weight of the overlying ocean and the pressure of water within the seafloor itself. From the simulations, the team computed the strength of the rocks about 1 kilometer below the seafloor, or the stress required to force faults in the seafloor to slide and expose fresh rock to seawater.

Compared with the stress applied to the seafloor by Jupiter’s gravity and by the convection of material in Europa’s underlying mantle, the rocks comprising Europa’s seafloor are at least 10 times as strong, Byrne said. “The take home message is that the seafloor is likely geologically inert.”

Austin Green, a planetary scientist at the Jet Propulsion Laboratory, followed Byrne in presenting. Green began by expressing his sympathies for continuing the “parade of depressing news.” He and his colleagues, including Byrne, had simulated flows of molten rock originating in Europa’s interior, to test whether magma could rise from below to reach the seafloor and bring it in contact with water.

For that to happen, the magma first needs to be sufficiently buoyant to breach the overlying rock. And second, the magma source must be voluminous enough to steadily feed molten rock to the rising flows, which would otherwise cool and solidify during the ascension.

Simulations suggest that the first condition — an adequate oomph — was unlikely. Europa’s low gravity and its inability to generate large bodies of molten rock — magma instead forms small and dispersed volumes in the mantle — limit the magma’s buoyant force, Green said.

The measly magma volumes also precluded the second requisite: a sufficient supply. Assuming buoyancy was no problem, the team simulated flows of magma rising in the mantle. They found that diffusive pockets of molten rock formed roughly 200 kilometers below the seafloor. From that depth, the highest reaching magma flows rose just 5 percent of the way to the seafloor before solidifying. “They did really, really, really, really, really bad,” Green said.

“Present day volcanism of the seafloor is highly unlikely,” he said. “If this volcanism is necessary for habitability, Europa’s ocean is uninhabitable.”

A mystery of two moons

Just before Byrne had stepped onto the stage, planetary scientist Laurent Pou of the Jet Propulsion Laboratory had his turn. He too had things to say about Europa, things that did not suggest the moon was geologically quiescent. And Pou would rope another moon into the discourse: Earth’s.

Decades ago, seismometers planted on the surface of Earth’s moon during the Apollo missions detected rumblings from deep within its rocky mantle, from 700 to 1,000 kilometers underground (SN: 5/13/19). These moonquakes were prompted by internal stresses caused by Earth’s gravity.

Both Earth’s moon and Europa are believed to possess mantles made of silicate rock. The two moons are also roughly the same size, with Jupiter’s gravity inflicting more stress on its moon than Earth’s does. Deep quakes in Earth’s moon could provide insights about the possibility of Europa-quakes, Pou and his team proposed.

They created computer simulations of the interior of Earth’s moon under the influence of Earth’s gravity, which revealed the rock properties needed for Earth’s gravity to trigger the observed moonquakes. The results suggested the moonquakes would require weaknesses within the moon, such as preexisting fractures, to occur. The team then computed the requisite conditions for quakes on Europa, simulating the moon’s interior under the influence of Jupiter’s gravitational field. A comparison of the results revealed that Europa-quakes were at least 10 times as likely to occur as temblors on our moon.

The discrepancy between Pou’s and Byrne’s results may arise out of unknown differences in the construction of the two moons. “We’re missing something,” Pou said of the seemingly conflicting findings. There may be some sort of weakness in the moon, and if that weakness doesn’t occur in Europa’s interior, then Europa-quakes may not either, he speculates. “That’s something that will be really interesting to see, with the future [Artemis II] mission to the moon.”

If quakes do shake within Europa’s rocky interior, they could revive notions of a geologically active seafloor. “It’s an important reminder,” says planetary scientist Alyssa Rhoden of the Southwest Research Institute in Boulder, Colo. “Until we can actually take data that tells us whether or not a process is occurring, it can be premature to decide whether or not it’s possible.”

And yet Byrne remains resolute. “We do not understand the physical process that makes [deep moonquakes] work,” he says. But we do know that they are deep and release relatively low amounts of energy, so if something similar occurs in Europa, those tremors probably don’t expose new rock on the seafloor.

While Pou’s work focused on deep moonquakes, which are the most common seismic event on Earth’s moon, shallow temblors also occur. Many scientists attribute these quakes to the moon’s shrinkage over time, rather than to tidal forces. Such shallow quakes are unlikely to be as common in Europa’s mantle, Byrne says, due to the presence of the overlying ocean. The weight of the sea, bearing down on the seafloor, would suppress this sort of seismicity.

An ebb and flow of habitability

Even if Europa is not habitable today, that doesn’t mean it wasn’t in the past.

“There’s a phrase that’s catching on in the planetary community … dynamic habitability,” says Pappalardo, the planetary scientist working on the Europa Clipper mission. “The habitability of a world, it could change over time.”

Europa is caught in a rhythmic dance with its sibling moon Io — for every two orbits around Jupiter that Io completes, Europa makes one (SN: 8/6/20). What follows from this orbital resonance is the periodic flexing and frictional heating of Europa’s interior, Pappalardo says.  

Though these pulses of heat follow a regular beat, the intensity of that beat fluctuates. That’s because Europa’s eccentricity, a measure of its orbit’s deviation from a perfect circle, oscillates over time.  

“It’s kind of a 100-million-year-ish cycle,” Pappalardo says. That’s consistent with the average age of Europa’s icy surface, which is roughly 60 million years old. “We may be in a phase of lesser activity now,” he says. “Maybe it was most active 100 million years ago.”

If Europa has passed through periods of more habitable conditions, a multitude of questions emerges, Pappalardo says. “Would life die off? Would there be natural selection, and microorganisms are making it through the difficult period? Can they make it through 100-million-year cycles?”

Byrne is uncertain. “Maybe if you were a particular form of alien life that evolves to be extremely adept at taking very low reaction rates and somehow living off that,” he says.

A Europa that lacks life could still assist the search for life, if researchers can prove it is or was habitable, Pappalardo says. For instance, such knowledge could inform our understanding of what fraction of habitable planets life eventually develops on. That fraction is one of the six terms in the Drake equation, a formula that estimates the number of communicable civilizations in the Milky Way (SN: 11/1/09).

An image showing people in white lab suits crouched around the assembled core of NASA's Europa Clipper spacecraft.
The partly assembled core of NASA’s Europa Clipper spacecraft sits in JPL’s Spacecraft Assembly Facility, in this picture from August 2022. JPL-Caltech/NASA

As for Clipper, Byrne says the craft probably won’t resolve the debate surrounding Europa’s seafloor activity, as it will look on from beyond the ice shell. But Pappalardo points out that the spacecraft will hopefully confirm that the ocean exists in the first place.

And if Clipper finds material from the ocean on Europa’s surface, and if it can collect enough compositional data, maybe it can reveal whether water in the ocean is reacting with rocks on the seafloor, Rhoden says. That could help address this question of whether the alien ocean contains the ingredients life needs, she says. “Also, you know, if we find a sea urchin, we know the answer.”

source: www.sciencenews.org