Phosphates on Enceladus (2023): A Key Ingredient for Life Found

Frank Postberg and colleagues detected phosphates from the ocean of Saturn’s moon Enceladus in 2023, in ice grains sampled by the Cassini spacecraft. Phosphorus had never been found in an ocean beyond Earth, and concentrations are at least 100 times those in Earth’s oceans.

The question

Life on Earth is built mainly from six elements, known by the acronym CHNOPS: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. Phosphorus is the scarcest of them, yet indispensable. Enceladus was already a prime place to look for habitable conditions: Cassini had found an ocean under its ice rich in dissolved carbonates, organic molecules, and hydrogen from hydrothermal activity. But geochemical models disagreed on whether its ocean held significant phosphate at all.

What the paper found

• Sodium phosphates in ice grains. Mass spectra of ice grains from Enceladus showed the signature of sodium phosphate salts.
• From the ocean itself. The results show phosphorus is available in Enceladus’s ocean in the form of orthophosphates, dissolved phosphate ions.
• A first beyond Earth. Phosphorus, the least abundant of the bio-essential elements, had never been detected in an ocean outside our planet.
• Remarkably rich. The team inferred phosphate concentrations at least 100 times higher than in Earth’s oceans.

How they did it

Cassini carried a Cosmic Dust Analyzer, which recorded a mass spectrum, a chemical fingerprint sorted by molecular mass, for individual ice grains that hit it. The grains came from the plume spraying from Enceladus’s south pole and from Saturn’s faint E ring, which the plume feeds.

Some spectra carried an unfamiliar pattern. Co-author Fabian Klenner used a laboratory setup in Berlin that mimics the signal of an ice grain hitting the instrument, testing different chemical compositions and concentrations until phosphate solutions matched the data from space. Teams in Japan and the United States then ran lab experiments and models to explain how the ocean keeps so much phosphate dissolved.

Why it mattered

With this result, all six CHNOPS elements have now been inferred in or around Enceladus’s ocean. That strengthens the case that it is one of the most habitable places in the Solar System beyond Earth. It does not mean anything lives there. It means a key raw material is abundant rather than scarce.

The finding also widens where astrobiology can look. Planets with surface oceans must sit at just the right distance from their stars, but icy worlds with oceans under their crusts can exist across a much wider range of distances.

What we know now

Cassini’s mission ended in 2017, so this result came from archived data, years after the spacecraft’s last plume crossing. Enceladus is now a leading candidate for a future mission designed to look for life directly. The question that began with the Miller-Urey experiment of 1953, whether life’s ingredients and energy can come together naturally, now has a real ocean to test it in.

Who wrote it

Frank Postberg of the Freie Universität Berlin led the study. Yasuhito Sekine of the Tokyo Institute of Technology led the Japanese lab and modeling work, Fabian Klenner carried out the laboratory calibration, and Christopher Glein of the Southwest Research Institute led a U.S. modeling team. The paper appeared in Nature on 14 June 2023.

Read the original

Postberg, F., Sekine, Y., Klenner, F., et al. (2023). “Detection of phosphates originating from Enceladus’s ocean.” Nature 618, 489-493. doi:10.1038/s41586-023-05987-9

Free to read: publisher PDF at Nature.