Europa is a moon of Jupiter with a quarter of the Earth’s radius, 13.4% of the Earth’s surface gravity, a thin oxygen atmosphere, a third of the Earth’s surface temperature, and a water-ice surface that is the smoothest of any solid body in the Solar system because of its youth. In September 2022, the Juno spacecraft flew within 320 kilometers of Europa’s surface.
At a depth of about 19–25 kilometers under the surface ice, lies a water ocean which is warmed by Jupiter’s tide, potentially hosting extraterrestrial life. The Hubble Space Telescope detected water vapor plumes similar to those observed on Saturn’s moon Enceladus, where water geysers emanate from cracks in the ice. This finding was supported by follow-up analysis of data obtained from the Galileo space probe, which flew in 1997 within 206 kilometers from Europa’s surface. The tidal force on Europa is about a thousand times stronger than the Moon’s tide on Earth. The estimated eruption rate at Europa is about 7 tons per second compared to about 0.2 tons per second for the plumes of Enceladus.
Is there life in Europa’s subsurface ocean? Does it involve only microorganisms or also fish?
The creative physicist, Freeman Dyson, suggested in a 1997 article that “an easy way to look for evidence of life in Europa’s ocean is to look for freeze-dried fish in the ring of space debris orbiting Jupiter. Sending a spacecraft to visit and survey Jupiter’s ring would be far less expensive than sending a submarine to visit and survey Europa’s ocean. Even if we did not find freeze-dried fish in Jupiter’s ring, we might find other surprises — freeze-dried seaweed, or a freeze-dried sea monster.”
Discovering subsurface life in Europa would imply that life is not restricted to the habitable zone around stars, as traditionally thought. There are many more frozen objects with liquid water — warmed by radioactive decays — under a shell of ice, in the outskirts of planetary systems than there are planets or moons in the habitable zones of stars. I discussed the potential prominence of subsurface life in a 2017 paper and a 2021 textbook “Life in the Cosmos,” that I wrote in collaboration with my former postdoc, Manasvi Lingam.
If the plumes eject ocean material out into space, that material could be detected by a spacecraft flyby. New models of the plumes have shown that a shock may develop in the plume interior as rising particles collide with particles falling back to the moon’s surface, limiting the plume’s altitude. A 2023 study investigated to what degree the limited extent of the shocked plumes reduces the future ability to detect plume molecules by ESA’s JUICE spacecraft, scheduled to arrive near Jupiter in 2031. If the spacecraft will fly close to the shock, the structure of the plume could be resolvable using the neutral mass spectrometer on JUICE, allowing to test models of the plume physics.
NASA’s Europa Clipper is the first mission designed to conduct a detailed study of Europa, scheduled to reach Jupiter in April 2030. It will orbit Jupiter, and conduct 49 close flybys of Europa. The plumes might be hard to detect even from up close. They could erupt sporadically, or have a geometry that is small and thin, given that Europa’s gravity is a dozen times stronger than Enceladus’. A single grain of ice ejected from Europa, if captured by Europa Clipper, could be enough to reveal alien bacteria when analyzed by the SUrface Dust Analyzer (SUDA). Europa Clipper’s instruments are not capable of identifying DNA, but SUDA could detect fatty acids and lipids, which can form biological cell membranes. In Earth’s oceans, the lipid membranes contribute to the thin film of ocean scum on the water’s surface.
The combination of data from Europa Clipper and JUICE holds the promise of uncovering the first conclusive evidence for extraterrestrial life. By providing a negative answer to the question “Are We Alone?,” the discovery of extraterrestrial life will raise the next question: “Who might reside beyond the Solar system?” and even more importantly: “Who is at the top of the Galactic food chain?” Everyone else would be a fair game in the restaurant menus of the top interstellar species.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
