Can Europa’s massive, interior ocean contain the building blocks of life, and even support life as we know it? This question is at the forefront of astrobiology discussions as scientists continue to debate the possibility for habitability on Jupiter’s icy moon. However, a recent study presented at the 55th Lunar and Planetary Science Conference (LPSC) might put a damper in hopes for finding life as a team of researchers investigate how Europa’s seafloor could be lacking in geologic activity, decreasing the likelihood of necessary minerals and nutrients from being recycled that could serve as a catalyst for life.
Here, Universe Today speaks with Henry Dawson, who is a PhD student in the Department of Earth, Environmental, and Planetary Sciences at Washington University in St. Louis and lead author of the study, about his motivation behind the study, significant results, follow-up studies, and whether Dawson believes there’s life on Europa. So, what was the motivation behind this study?
Dawson tells Universe Today, “A large portion of the community has been looking at the habitability potential of the seafloor, and looking at processes that might occur at seafloor hydrothermal vents, or at water–rock interaction chemistry. However, it was never established that there would actually be any fresh rock exposed at the seafloor, or if the tectonic processes that drive hydrothermal vents would be present. The silicate interior of Europa is a similar size to that of Earth’s Moon, which is largely geologically dead on the surface.”
For the study, Dawson and his colleagues examined the likelihood for geologic activity occurring on Europa’s seafloor through analyzing data on Europa’s geophysical characteristics and comparing them with known geologic parameters and processes, including the strength of potential fault lines and fractures within Europa’s rocky interior, how the strength of this rock changes with depth, and how the rock could react to ongoing stresses, commonly known as convection. Using this, they conducted a series of calculations to ascertain whether the seafloor crust could drive geologic activity. Therefore, what were the most significant results from this study?
“It looks a lot more difficult to expose fresh rock (which is required to drive the reactions that life would exploit) to the ocean,” Dawson tells Universe Today. “Tidal forces do not seem able to cause motion along faults, like it can on the surface, and so the seafloor is most likely still. All the rock that water is able to interact with through porosity was likely altered hundreds of millions to billions of years ago, and so the ocean and rock are in chemical equilibrium. This means that there is no present day, continuous input of nutrients into the ocean from the rocky core, and so any possible life would likely have to exploit nutrient input from the icy shell above the ocean.”
While this study focused on geologic stresses related to fractures and fault lines, Europa’s interior ocean is produced from another type of geologic stress known as tidal heating, which is induced from the constant stretching and compressing as Europa orbits the much more massive Jupiter. This same tidal process occurs between the Earth and its Moon, and we see this in action in the rising and falling of the Earth’s waters around the globe. For Europa, over the course of thousands to millions of years, the stretching and compressing leads to friction in Europa’s inner rocky core, which leads to becoming heated and melting the inner ice into the interior ocean that exists today. It is in this ocean that astrobiologists hypothesize that life could exist, possibly even life as we know it.
However, given these study’s unfortunate findings, Dawson and his colleagues give dire implications for the potential habitability on Europa, noting their calculations estimate that geologic activity on Europa’s seafloor is limited enough to indicate habitable conditions within Europa’s interior ocean could be limited, as well. However, the study was quick to note that other geologic processes could be examined to explain the present state of Europa’s seafloor geologic activity, including processes known as serpentinization and thermal expansion anisotropy.
“As rock is exposed to water and chemically alters, the new minerals that form may have a different molar volume than the unaltered minerals in the original rock,” Dawson tells Universe Today. “Serpentinization specifically is the process where peridotite, a typical mantle rock, is altered to serpentinite. This reaction has a net volume increase, which introduces new stresses. These stresses might lead to the fracturing of the rock, fresh rock faces exposed, and more alteration, leading to a self-propagating cycle. On the other hand, the new minerals might cement up pre-existing fractures, preventing further exposure, and creating a negative feedback loop. Thermal expansion anisotropy describes the process where different minerals have varying degrees of expansion upon heating. Thus, when a rock is heated or cooled, the mineral grains inside will push against each other, introducing porosity and interior stresses.”
Regarding the tidal forces responsible for producing Europa’s interior ocean, this icy moon and the Earth’s Moon are not the only planetary bodies in the solar system that could experience these unique forces. Others include Jupiter’s third Galilean Moon, Ganymede, Saturn’s icy moon, Enceladus, and Saturn’s largest moon, Titan, all of which are currently hypothesized to possess interior oceans from tidal heating. Like Europa, Ganymede exhibits a predominantly crater-free surface, which is indicative of frequent resurfacing, and Enceladus was observed on numerous occasions by NASA’s Cassini spacecraft to have geysers on its south pole region that frequently shoots out water into space.
Additionally, Cassini flew through these geysers to obtain data on the ejecta’s composition, discovering organic molecules. For Titan, Cassini data revealed that an interior ocean exists beneath its surface, which is currently hypothesized to contain a combination of ammonia and salts. But regarding this most recent research, what follow-up studies are currently being conducted or planned?
Dawson tells Universe Today, “I’m currently using the same model to estimate whether tidal forces are able to cause fracturing on other icy moons in the outer solar system, such as Ganymede, Enceladus, Titan, and the mid-size Uranian moons. Based on my preliminary results that I presented at LPSC, it appears that tidal forces are insufficient on those moons as well. In addition, our collaborator Austin Green is looking at whether seafloor volcanism might occur, based on the forces that volcanic dikes can exert on the rock that they are propagating through. For Europa, the lithosphere is too deep and too strong for magma to reach the seafloor, and so any melt that forms in the mantle stalls out at depth.”
Despite being discovered by Galileo Galilei in 1610, the fascination for finding life within Europa’s ocean has only come within the last few decades, thanks largely to the NASA Voyager missions, with Voyager 1 and Voyager 2 flying through the Jupiter system in 1979 and imaged the Galilean Moons up close and in detail for the first time, hinting that Europa was currently geologically active. This is because Europa has almost no visible craters throughout its entire surface, indicating specific processes are responsible for reshaping the small moon and covering up evidence of past impacts. Europa, being the second Galilean Moon, shares these traits with the first and third Galilean Moons, Io and Ganymede, respectively, while the fourth Galilean Moon, Callisto has a surface that is almost entirely covered by craters.
Thanks to further data obtained from proceeding missions, including NASA’s Galileo spacecraft, Hubble Space Telescope, and Juno, scientists are almost entirely convinced that an interior ocean lies beneath Europa’s icy crust, with some estimates putting the volume of liquid water at double of Earth’s oceans. Therefore, as we see on Earth, liquid water means life, which is why Europa’s interior ocean is a target for astrobiology research. But does Henry Dawson think there’s life on Europa?
Dawson tells Universe Today, “I think there’s still a lot more that I would like to understand before I make a yes or no statement on that. While I believe that Europa is one of the most likely candidates to host life, alongside Enceladus, the chance of life remains small, and this research reduces the probability even more.”
This study comes as NASA prepares to launch the Europa Clipper spacecraft this October with a planned arrival date of April 2030 and is designed to explore the habitability potential of Europa and its interior ocean. During its 3.5-year mission, Clipper will perform up to 44 close flybys of Europa ranging between 25 and 2,700 kilometers (16 to 1,678 miles) as the spacecraft will perform elongated orbits to keep from staying within Jupiter’s powerful magnetic field for too long. To assess Europa’s habitability potential, Clipper will carry a powerful suite of scientific instruments designed to analyze Europa’s chemistry, surface geology, and interior ocean characteristics.
Additionally, the European Space Agency’s Jupiter Icy Moons Explorer (JUICE) mission was launched in April 2023 with a planned orbital insertion at Jupiter in July 2031, followed by a departure from Jupiter and an orbital insertion around Ganymede in December 2034. Like Clipper, JUICE is designed to investigate the habitability potential of the icy moon, but will also examine Ganymede and Callisto, as well.
“Get excited for the Europa Clipper and JUICE missions! Dawson exclaims to Universe Today. “While it will still be 6 years before they reach Jupiter, once they arrive, we will be able to learn much more about what is going on at Europa. While they will not be able to directly measure the interior, observations of the ice shell, gravity field, and tidal forcing on Europa will help to constrain future models. As well, always be careful about the assumptions you make for other planetary bodies. While Europa may be covered with ice, it is truly a rocky world that happens to have a deep ocean, and the processes occurring at depth may not reflect what we see at Earth’s seafloor.”
Is Europa’s seafloor geologically active, and what new insights will Europa Clipper and JUICE make about this astonishing and intriguing icy moon in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!