Enceladus’ ocean may not have produced precursor chemicals for life

Enceladus’ chilly geysers spew chemical compounds that hint at the potential for life in the moon of Saturn’s subsurface ocean.

But some compounds within the plumes may have formed via high-energy radiation above ground, a new study suggests. Researchers described the findings in a September 9 presentation at the Europlanet Science Congress–Division for Planetary Sciences of the American Astronomical Society joint meeting in Helsinki.

“We need to be careful when we’re thinking about Enceladus’ habitability, because it may not necessarily originate from the subsurface ocean,” says planetary scientist Grace Richards of the National Institute for Astrophysics in Rome. Richards and colleagues also report the work in the Oct. 15 Planetary and Space Science.  

When NASA’s Cassini spacecraft, which surveyed Saturn and its moons from 2004 to 2017, flew through plumes of ice and water sprayed from geysers near Enceladus’ south pole, the probe found organic compounds — carbon-based building blocks of life. “What’s exciting about Enceladus is we have detected all the … elements that we expect that we definitely need to find life,” Richards says.

Living things are thought to need liquid water, and the geysers provide a way to probe the ocean beneath the moon’s icy shell for signs of life or its precursors. But Richards wanted to determine whether some substances in and around the plumes originated from outside the ocean. Incoming radiation from Saturn’s surroundings could spark similar chemical reactions above ground.

Richards and colleagues created ices made of water, carbon dioxide, methane and ammonia at about –200° Celsius, the moon’s average surface temperature, and blasted them with charged particles. Afterward, the team heated the irradiated ices to around –100° C to mimic warmer spots on the moon, turning the ice to gas and exposing molecules trapped within.

The gas components and wavelengths of light absorbed by the ices revealed that the experiments produced some simple organic compounds, such as acetylene. Chemical reactions with these components can create complex molecules that could lead to life, Richards says.

Some of the lab-created molecules matched ones that were detected within Enceladus’ plumes, suggesting that these substances may not have come from the moon’s subsurface ocean after all. That doesn’t rule out the possibility of microbial life arising on Enceladus, Richards says. But researchers should take caution when interpreting molecules that could suggest life — at least some may not be associated with biology.