Enceladus, one of Saturn’s small icy moons, spans just 300 miles (500 kilometers) — yet despite its modest size, it has become a star in the search for life beyond Earth. From cracks near its south pole, the moon blasts towering geysers of water vapor, ice and organic molecules into space, which are tantalizing hints of a hidden ocean that could, in theory, be habitable.
But new research presented this week at a planetary science conference in Finland shows that many of the organic molecules detected in these plumes could also form right on the moon’s surface, driven by relentless radiation from Saturn’s magnetic field. The results cast doubt on whether the plumes truly carry whispers of alien life, or merely echoes of lifeless chemistry on the frozen shell.
“Although this doesn’t rule out the possibility that Enceladus’ ocean may be habitable, it does mean we need to be cautious in making that assumption just because of the composition of the plumes,” study lead Grace Richards of Italy’s National Institute for Astrophysics said in a statement.
For their experiment, Richards and her colleagues recreated conditions on Enceladus in miniature inside a specialized laboratory in Hungary. Using an ice chamber, the team froze mixtures of water, carbon dioxide, methane and ammonia to a bone-chilling –420 degrees Fahrenheit (-253 degrees Celsius), mimicking frigid conditions near the moon’s surface. The ices were then bombarded with high-energy “water-group ions,” the same charged particles trapped around Saturn that constantly irradiate Enceladus.
To monitor the chemical changes induced by radiation, the researchers used infrared spectroscopy to observe the molecular “fingerprints,” or spectra, of the ices. As radiation interacted with the samples, the spectra shifted, signaling the formation of new molecules.
Each of the five experiments produced carbon monoxide, cyanate, and ammonium — compounds that were detected in Enceladus’ plumes by NASA’s Cassini spacecraft in 2005. When the samples were gently warmed, more complex organics appeared, including carbamic acid, ammonium carbamate and potential amino acid precursors including methanol and ethanol, as well as molecules like acetylene, acetaldehyde and formamide, which are building blocks that could contribute to the chemistry of life.
“Although many of these products have not previously been detected on Enceladus’ surface, some have been detected in Enceladus’ plumes,” Richards and her colleagues wrote in the paper. This leads to “questions about whether plume material is formed within the radiation-rich space environment or whether it originates in the subsurface ocean.”
Crucially, the timescales necessary for radiation to drive these chemical reactions are comparable to how long ice remains exposed on Enceladus’ surface or in its plumes, so distinguishing ocean-sourced organics from surface-born ones may be difficult, the study notes.
“It is likely that the composition of the subsurface ocean may not be accurately reflected by the composition of the emergent plume, or by material deposited on the surface immediately adjacent to the plume,” the paper reads.
For astrobiologists, the results are both sobering and exciting. On one hand, they complicate the story that organics in the plumes are definitive signs of a life-friendly ocean. On the other, they highlight that rich, potentially life-relevant chemistry can thrive even in extreme, radiation-battered environments, thereby expanding the ways scientists think about where prebiotic molecules might form and why Enceladus remains a prime target for exploration.
NASA’s Cassini mission, which ended in 2017 with a dramatic plunge into Saturn’s atmosphere, gave humanity its first and only direct “taste” of Enceladus’ geysers. But instruments onboard the spacecraft weren’t designed to distinguish between molecules forged in the moon’s presumably deep ocean and those cooked up in the icy shell.
These answers could come in the coming decades with future missions. One concept under consideration as part of the European Space Agency’s Voyage 2050 program envisions a dedicated probe that could land on the surface and collect material ejected from the moon’s hidden ocean. NASA has also previously studied an “Orbilander” concept designed to sample Enceladus’ plumes from orbit.
Meanwhile, China is exploring a multi-part mission architecture that would include an orbiter, a lander and a deep-drilling robot that would attempt to reach the subsurface ocean to search for potential biosignatures.
This research is described in a paper published in the October 15 edition of the journal Planetary & Space Science.