Millions of miles from our planet, extraterrestrial life could reside in salty seas encased by hard spheres of ice. Lacking close proximity to the sun, as well as the proper evolutionary advantages to glean energy from sunlight, life on these so-called “icy ocean moons” might harness chemical energy instead. Either bacteria or archaea—bacteria’s primitive counterparts—would be the most likely candidates, said Peter Girguis, a professor of organismic and evolutionary biology at Harvard University. “That said, until we have the appropriate technology to send really sophisticated instruments directly to these planets, we’re simply left to make inferences,” he said.
Today, scientists can roughly estimate the thickness of the ice shells on distant ocean moons (like Jupiter’s Europa) using orbiting spacecraft or detect salinity via changes in planetary magnetic fields. But what’s transpiring at the seafloor remains anyone’s guess.
Researchers centered at Caltech’s Jet Propulsion Laboratory (JPL) advocate using a familiar method to assess habitability—the same technology employed to understand what lies beneath our feet here on Earth. In a recent article submitted to Astrobiology, the team proposed landing seismology experiments on the ice shells to determine their moon’s internal structure and potential for life. However, the mission’s first challenge begins on Earth—convincing NASA that bringing seismology instruments to icy ocean moons is a worthwhile endeavor.
Most of what we know about the inside of our own planet stems from data collected by seismometers—sensitive devices that record the waves of energy released by subterranean tremors. As they spread underground, these seismic waves bounce off formations inside the planet (which, in the case of an ocean moon, include a rocky core engulfed by an icy sea). Just as we map the interior of the human body by tracking X-rays, recording seismic waves as they reach the surface of a planet reflects internal structure.
This could reveal alternative sources of life-sustaining energy (like ice volcanoes or hydrothermal vents), and indicate potential nutrient cycling through the ice shell as well as the presence of liquid water near the surface. So far, seismometers have been positioned on the Moon and Mars, but not on any icy moons.
“Interest in these icy moons is at an all-time high,” said Jennifer Wiseman, the Hubble Space Telescope senior project scientist. “Seismic activity will certainly contribute to our understanding of whether habitability is likely in these places.”
Based on signs of surface geological activity, like fractures, scientists are relatively certain the icy moons are seismically-active. However, seismometers could verify this immediately upon landing. According Steve Vance, first author of the paper, seismology should be used in tandem with preexisting techniques that examine planetary interiors.
The JPL paper is the first to offer a holistic approach to assessing internal structure of five such icy moons in the context of potential life. Girguis, who was not affiliated with the paper, said it “comprehensively reviews” the advantages and limitations to applying this technology to questions of habitability.
Ralph Lorenz, an author of the paper, hopes the piece will spur researchers to explore these moons in new ways. “Many seismologists have not considered the potential for making measurements on worlds like these,” he said. “Perhaps the prospect of implementing one of these investigations is close at hand.”
There are two main icy worlds that could be well-suited to test the technology—Jupiter’s moon Europa and Saturn’s moon Enceladus. “The activity at the surface of these moons suggests they are really dynamic places,” said Alyssa Rhoden, another author of the paper. If recent observations on Europa are confirmed, these will also be the only two icy moons known to harbor geyser-like water vapor “plumes.” These bursts signify seismic activity and would permit sampling the oceans without drilling through miles of ice.
Delivering a seismometer to Europa or Enceladus would require a lander—a spacecraft that makes direct contact with the ground. However, NASA has only initiated flyby missions thus far: The Voyager mission launched in 1977, Galileo in 1989, and New Horizons in 2006. Cassini also collected images of Europa on its way to Saturn, where it has remained since 2004, snapping photos of Enceladus and Saturn’s other moons. According to Rhoden, there has yet to be a mission solely devoted to either Enceladus or Europa.
That may change as NASA is currently developing the Europa Multiple-Flyby Mission, nicknamed the Europa Clipper. The orbiter is slated for lift off in the early 2020s, performing approximately 42 passes at 16 to 1700 miles above Europa’s surface. It will capture high-resolution images, as well as samples from the erupting plumes. Analyzing the chemical make-up of these plumes, which might spew from the frothing sea below, could provide insight into the potential for life.
NASA is also considering the possibility of a Europa lander as a future mission post-Clipper, which would be the first of its kind to touch down on an icy ocean moon. However, these plans are far from concrete.
“To put it in perspective, we spent about fifteen years working on the Europa Clipper mission before NASA gave us the go-ahead,” said Curt Niebur, lead program scientist of the mission. “We still have to come up with a design for the lander that’s possible and scientifically worthwhile.”
The NASA team began their preliminary studies a mere six months ago. “We are considering including instruments that could help detect life, as well as instruments like a seismometer that could tell us about Europa’s interior structure,” Niebur added.
Mark Panning, a collaborating author on the JPL paper, is optimistic. “There’s a very good chance that the Europa lander could include some sort of seismic instrument,” he said. “We want [NASA] to look at this paper and start thinking about what kinds of science they could do with a proposed seismic instrument.” He said the group’s follow-up studies, which will incorporate more precise numbers and models, could be even more informative.
Neibur emphasized that they would need to ensure the seismometer could be securely coupled to Europa’s surface. “Unless you can do that, you’ve just spent a lot of money accomplishing nothing,” he said.
Jay Melosh, professor of earth, atmospheric, and planetary sciences at Purdue University, agreed. He also said landing a spacecraft on Europa would be extremely challenging due to the moon’s powerful gravitational field. A lander would require extra fuel to have enough power to reach the surface. And, unlike Mars or the Moon, Europa lacks an atmosphere—making a “soft” parachute landing to avoid damage to the delicate seismometer quite difficult. Plus, without this layer of insulation, intense radiation from Jupiter constantly bombards Europa’s surface. Coupled with the frigid temperatures, this makes for an extremely hostile environment.
“I would actually think that a seismometer to Titan, Saturn’s largest moon, would fly earlier,” Melosh said. “Titan has a very substantial atmosphere, and it doesn’t suffer from the horrendous radiation problem that Europa does.”
Consensus here, too, is split. NASA planetary scientist Terry Hurford noted that an atmosphere means wind can disrupt the sensitive seismology instruments. (This was the case with one of the 1976 Viking probes to Mars.)
But both Melosh and Hurford agreed that, in order for an icy moon seismology experiment to succeed, there must be planetary quakes large enough to detect.
Vance recognized there is much to learn before seismometers can probe our solar system’s icy ocean moons. “This paper is a jumping off point for more detailed studies,” he said.
“The fervor is certainly there,” Wiseman said.
