“Most of those are speculative possibilities at this stage. The ones with real evidence are Europa, Ganymede (tentative), Callisto, Enceladus, and Titan,” said Showman, who tragically passed away during the reporting of this article.
Like other scientists, Showman identified Europa and Enceladus as the most promising candidates. Both of these moons are tidally heated through gravitational interactions with their host planets (Jupiter and Saturn, respectively) and their nearby fellow moons. In both cases, models of these moons’ interior structures indicate that their oceans are sandwiched between an icy layer above and a rocky layer below. “This opens the possibility of water-rock interactions that could introduce biologically interesting chemicals into the ocean,” said Showman.
Life on such worlds could resemble the myriad bacteria, tube worms, clams, and other life-forms that thrive around hydrothermal vents on Earth’s ocean floors. These creatures draw their sustenance from nutrients expelled by the vents, not from sunlight.
The icy shells of Europa and Enceladus are relatively thin. In 2005, NASA’s Cassini spacecraft returned spectacular images of jets venting salty-ice particles into space from Enceladus’ south polar region. And in 2014, the Hubble Space Telescope first spotted evidence of plumes emanating from Europa’s south pole. In 2019, researchers released the best evidence yet that Europa’s plumes contain water. All these discoveries suggest that biogenic materials could exist at or near the surface.
NASA’s planned Europa Clipper mission will fly by its namesake moon 44 times starting in the early 2030s. Scientists expect it to confirm the existence of the world’s ocean, measure the ice shell’s thickness, and help researchers ascertain the moon’s biological potential. Green says that the spacecraft will be able to sample material in Europa’s plumes for possible evidence of organics and other indicators of life. This material, he says, might be coming from hydrothermal vents at the bottom of its global subsurface ocean. The spacecraft won’t be detecting actual life-forms, but it could conceivably provide compelling evidence that life is possible. And the results could, in turn, motivate the launch of a lander to sample surface material or even drill or melt through the ice to explore the ocean directly.
Finding life on a solar system ocean world would be an astrobiologist’s dream come true. It would imply a universe brimming with life — for, as Summers says, “Ocean worlds are probably commonplace in the galaxy.”
NASA is also planning to fly a robotic rotorcraft named Dragonfly on Saturn’s large moon Titan in 2034. Titan is the only satellite with a thick atmosphere, which is similar to Earth’s in terms of its surface pressure and nitrogen-dominated composition. Better yet, Cassini radar found dozens of lakes on Titan filled with liquid methane and ethane (C2H6). It is a world oozing with organic compounds.