By Chris Woolston
The giant planet Jupiter has nearly 100 known moons, yet none have captured the interest and imagination of astronomers and space scientists quite like Europa, an ice-shrouded world that is thought to possess a vast ocean of liquid salt water. For decades, scientists have wondered whether that ocean could harbor the right conditions for life, placing Europa near the top of the list of solar system bodies to explore.
A new study led by Paul Byrne , an associate professor of Earth, environmental, and planetary sciences, throws cold water on the idea that Europa could support life at the seafloor. Using calculations that consider the moon's size, the makeup of its rocky core, and the gravitational forces from Jupiter, Byrne and a team of scientists conclude that Europa likely lacks the tectonic motion, warm hydrothermal vents, or any other sort of underwater geologic activity that would presumably be a prerequisite for life.
"If we could explore that ocean with a remote-control submarine, we predict we wouldn't see any new fractures, active volcanoes, or plumes of hot water on the seafloor," Byrne said. "Geologically, there's not a lot happening down there. Everything would be quiet." And on an icy world like Europa, a quiet seafloor might well mean a lifeless ocean, he added.
The study was published in Nature Communications. Co-authors from the Department of Earth, Environmental, and Planetary Sciences include Professor Philip Skemer , associate chair of the department; Professor Jeffrey Catalano ; Douglas Wiens , the Robert S. Brookings Distinguished Professor; and graduate student Henry Dawson. Byrne, Skemer, Catalano, Wiens, and Dawson are also members of the McDonnell Center for the Space Sciences.
For Byrne, a planetary scientist, Europa's appeal extends well beyond the question of life. "I'm really interested to know what that seafloor looks like," he said. "For all of the talk about the ocean itself, there has been little discussion about the seafloor."
Without a submarine, Byrne and co-authors had to combine known facts about Europa with inferences drawn from the geology of Earth and other bodies, including our own Moon.
The ice shell on Europa is thought to be 15 to 25 km thick, and the ocean covers the entire moon to a depth of up to 100 km. Even though Europa is slightly smaller than our own Moon, it likely holds much more water than Earth.
Beneath that ice and water lies a rocky core analogous to Earth's. While Earth's core still burns hot, Byrne and co-authors calculated that any heat from Europa's core would have escaped billions of years ago.
The team also calculated the gravitational forces from Jupiter, a pull that can be strong enough to keep a moon geologically alive. On its innermost large moon, Io, Jupiter's gravity roils tides and heats the rocks beneath the icy surface. Io, in fact, is the most volcanically active body in the solar system. The tides on Io are especially violent because the moon has an erratic orbit that periodically takes it closer to Jupiter, but Europa's orbit is relatively stable and distant, lessening the chance for substantial tidal forces, Byrne explained.
"Europa likely has some tidal heating, which is why it's not completely frozen," Byrne said. "And it may have had a lot more heating in the distant past. But we don't see any volcanoes shooting out of the ice today like we see on Io, and our calculations suggest that the tides aren't strong enough to drive any sort of significant geologic activity at the seafloor."
Europa's quiet seafloor geology doesn't provide much support for any contemporary life beneath the ice, Byrne said. "The energy just doesn't seem to be there to support life, at least today."
Byrne is still excited about future chances to explore Europa, especially the Europa Clipper spacecraft that will fly by the moon in the spring of 2031. That mission — conceived and championed in part by Bill McKinnon, the Clark Way Harrison Distinguished Professor in Arts & Sciences and interim director of the McDonnell Center for the Space Sciences — will take close-up pictures of Europa's surface and provide more precise measurements of its ice cap and ocean. "Those measurements should answer a lot of questions and give us more certainty," Byrne said.
Even if, eventually, modern Europa is found to be lifeless, Byrne won't be disappointed. "I'm not upset if we don't find life on this particular moon," he said. "I'm confident that there is life out there somewhere, even if it's 100 light-years away. That's why we explore—to see what's out there."
