Ice grains from ocean geysers on Enceladus contain phosphates

Geysers of frozen sea spray from Saturn’s moon Enceladus provide tantalizing clues about the chemistry of the water ocean locked beneath its frozen surface. Now scientists have detected phosphorus in ice grains originating from these seas, indicating that Enceladus has all the essential elements to support life as we know it (Nature 2023, DOI: 10.1038/s41586-023-05987-9).

There’s currently no evidence of life on Enceladus, yet scientists have probed the moon’s habitability using both telescopes based on Earth and NASA’s Cassini spacecraft, which sampled ice grains from the ocean geysers during a mission that ended in September 2017. Through these observations, researchers determined that Enceladus’ subsurface water is liquid, alkaline, and contains carbon, hydrogen, nitrogen, oxygen, and sulfur. That leaves out just one of the critical ingredients for organic biochemistry on Earth—phosphorus.

Of the six elements of known life—nicknamed CHNOPS—phosphorus is the least cosmologically abundant in our universe, and is often locked up in rocks that make the precious element inaccessible for biology, says Frank Postberg, a planetary scientist at the Free University of Berlin. Early geochemical models suggested that low-phosphorus availability on Enceladus would likely create a bottleneck for the emergence of life, he says.

Postberg and his colleagues discovered this missing piece buried in data collected by Cassini’s Cosmic Dust Analyzer. During its voyage around Enceladus, this instrument measured the mass spectra of salty ice grains originating from the moon’s hidden seas by passing through the erupting plume directly and sampling material left behind in a portion of Saturn’s rings. While sifting through thousands of spectra from this ring region, Postberg and his team noticed a handful of unusual signals. They identified nine ice grains originating from Enceladus that contained the spectral signatures of sodium phosphate ions mixed in with other salts. “In these few ice grains, the signature is unmistakable,” Postberg says.

To investigate, the researchers reverse engineered their own recipe for extraterrestrial seawater to replicate the phosphate fingerprint in the lab. Next, they simulated the geochemistry of the rock-water interactions in the alkaline lunar ocean using samples of a carbonaceous meteorite with a similar composition to Enceladus’ rocky core, which contains a greater abundance of carbonates compared to Earth. The researchers found that these carbonates play a key role in liberating the phosphorus trapped in mineral formations.

“Our geochemical experiments show that there’s a clear correlation between the abundance of carbonates and the solubility of phosphates,” Postberg says. The researchers were surprised to find that the newly estimated concentration of phosphorus in Enceladus’ ocean is several hundred times higher than in Earth’s own oceans. There are plenty of factors that could still render Enceladus inhospitable to life, but a lack of phosphorus is probably not one of them, Postberg says.

“This is the last one of that set we call CHNOPS to be confirmed for Enceladus,” says Morgan Cable, a chemist and research scientist at the NASA Jet Propulsion Laboratory who was not involved in the study. “This result is really critical because it adds another important data point to how we might constrain potential biochemistry in this environment,” she says. The findings also create a blueprint for seeking out phosphorus in other water worlds of the outer solar system.