Beneath the surface of Jupiter’s icy moon Europa, there’s an ocean up to 100 km (62 mi) deep that has two to three times the volume of every ocean on Earth combined. Even more exciting is how this ocean is subject to hydrothermal activity, which means it may have all the necessary ingredients for life. Because of this, Europa is considered one of the most likely places for extraterrestrial life (beyond Mars). Hence, mission planners and astrobiologists are eager to send a mission there to study it closer.
Unfortunately, Europa’s icy surface makes the possibility of sampling this ocean rather difficult. According to the two predominant models for Europa’s structure, the ice sheet could be a few hundred meters to several dozen kilometers thick. Luckily, new research by a team from Stanford University has shown that Europa’s icy shell may have an abundance of water pockets inside, as indicated by features on the surface that look remarkably like icy ridges here on Earth.
The study team was led by Riley Culberg, a Ph.D. candidate and geophysicist at Standford’s School of Earth, Energy & Environmental Sciences (Stanford Earth). He was joined by Dustin Schroeder, an associate professor of geophysics at Stanford Earth; and Gregor Steinbrügge, a former postdoctoral fellow at Stanford Earth, now a planetary scientist at the NASA Jet Propulsion Laboratory. The paper that describes their research and findings recently appeared in the journal Nature Communications.
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As they explain in their study, the research was motivated by a similarity the team noticed during a presentation at Standford. While discussing Europa’s double-ridges, Culberg noticed how similar these landforms were to features they had studied extensively in northern Greenland. Between 2015 and 2017, NASA collected ground-penetrating radar data of the region as part of Operation IceBridge, an aerial observation campaign that conducts geophysical studies of the growth and retreat of ice sheets.
This investigation confirmed the existence of a double ridge in northwestern Greenland and provided details of how it evolved. Geophysicists and glacial experts have determined that these features form when water from nearby surface lakes drains into an impermeable layer within the ice sheet. It then refreezes and fractures the ice above, causing it to be forced upward and outwards to create the characteristic double ridge feature on the surface.
The similarities came as a surprise to the team because of how different Earth’s land-based subsurface is compared to Europa’s subsurface ocean of liquid water. “We were working on something totally different related to climate change and its impact on the surface of Greenland when we saw these tiny double ridges – and we were able to see the ridges go from ‘not formed’ to ‘formed,’?” Schroeder said in a recent Stanford News release.
Upon further examination, they found that the M-shaped feature in Greenland could be a miniature version of Europa’s most prominent surface feature. On Europa, double ridges appear as gashes that cut across the surface, with crests reaching nearly 300 m (1,000 ft) tall, separated by valleys about 800 meters (2,625 ft) wide. Scientists have known these features since the Galileo spacecraft took images of the Galilean Moons in the 1990s, leading to the first detailed surface maps.
Since then, however, scientists have not been able to come up with a definitive explanation of how these features formed. By conducting a comparative analysis between the radar data collected by Operation IceBridge and the geophysical data they had for Europa, the team could conceive a possible answer. As Culberg explained:
“In Greenland, this double ridge formed in a place where water from surface lakes and streams frequently drains into the near-surface and refreezes. One way that similar shallow water pockets could form on Europa might be through water from the subsurface ocean being forced up into the ice shell through fractures – and that would suggest there could be a reasonable amount of exchange happening inside of the ice shell.”
These findings suggest that Europa’s ice shell may be much more dynamic than previously thought, undergoing various geological and hydrological processes. This is supported by other recent findings, such as Hubble’s discovery of plume activity on the surface in 2012, which were later confirmed in 2018 based on a new analysis of Galileo data. A dynamic ice shell model is consistent with the exchange of subsurface water and nutrients from neighboring celestial bodies on the moon’s surface.
Said Steinbrügge, who started working on the project as part of his postdoctoral research at Stanford:
“People have been studying these double ridges for over 20 years now, but this is the first time we were actually able to watch something similar on Earth and see nature work out its magic. We are making a much bigger step into the direction of understanding what processes actually dominate the physics and the dynamics of Europa’s ice shell.”
The existence of these pockets is especially good news for the Europa Clipper mission, in which both Schroeder and Steinbrügge will be participants. This robotic orbiter will launch in October 2024, reach the Jovian system by April 2030, and spend the next four years (barring extensions) examining the surface of Europa through a series of flybys. In addition to analyzing Europa’s surface ice and plume activity, it will select landing sites for a possible Europa Lander mission. As Schroeder explained:
“Because it’s closer to the surface, where you get interesting chemicals from space, other moons, and the volcanoes of Io, there’s a possibility that life has a shot if there are pockets of water in the shell. If the mechanism we see in Greenland is how these things happen on Europa, it suggests there’s water everywhere.”
Like Operation IceBridge, the Europa Clipper will rely on an ice-penetrating radar to study the interior structure of Europa’s ice sheet. This instrument is known as the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) and is overseen by a team that includes Schroeder as a co-investigator. This instrument will detect pockets of water using radio waves because of the way water reflects them one thousand times as brightly as ice.
This will allow the REASON team to create a vertical profile that maps the distribution of water pockets in the ice sheet. “We are another hypothesis on top of many – we just have the advantage that our hypothesis has some observations from the formation of a similar feature on Earth to back it up,” Culberg said. “It’s opening up all these new possibilities for a very exciting discovery.”
Identifying potentially habitable enclaves within the ice sheets also means that any astrobiology missions to Europa won’t need to enter the subsurface ocean to look for signs of life. In addition to increasing accessibility, exploring these pockets dramatically decreases the chances of contaminating potential biospheres in the moon’s interior ocean. As astrobiology missions progress, ensuring the safety of any extraterrestrial life we encounter will be paramount.
“It’s exciting, what it would mean if you have plenty of water within the ice shell,” Steinbrügge added. “It would mean the ice shell on Europa is extremely dynamic. It could facilitate exchange processes between the surface and the subsurface ocean. It could go in both directions.”
The study is yet another indication of how connected the study of Earth and the other Solar planets are. It will also lead to applications that could have significant impacts here at home. “This research will help us either use Earth to understand what we will see on Europa or, when we get to Europa, help us interpret what we see when we get there,” said Schroeder.