Milestone as phosphorus discovered on a moon of Saturn

Using data collected by NASA’s Cassini mission, an international team of scientists has discovered phosphorus — an essential chemical element for life — locked inside salt-rich ice grains ejected into space from Enceladus, one of Saturn’s moons. Their breakthrough has been published in the journal Nature.

Phosphorus — the least abundant of the essential elements necessary for biological processes — is a building block for DNA and is present in the bones of mammals, cell membranes and ocean-dwelling plankton. Phosphorus is also a fundamental part of energy-carrying molecules present in all life on Earth; life wouldn’t be possible without it.

Enceladus is known to possess a subsurface ocean, and water from that ocean erupts through cracks in Enceladus’s icy crust as geysers at its south pole, creating a plume. The plume then feeds Saturn’s E ring (a faint ring outside of the brighter main rings) with icy particles. During its mission to Saturn from 2004 to 2017, the Cassini orbiter flew through the plume and E ring numerous times.

Previous analysis of Enceladus’s ice grains revealed concentrations of sodium, potassium, chlorine and carbonate-containing compounds, and computer modelling suggested the subsurface ocean is of moderate alkalinity — all factors that favour habitable conditions. For this latest study, the authors accessed NASA’s Planetary Data System, a long-term archive of digital data products returned from the agency’s planetary missions.

The authors focused on data collected by Cassini’s Cosmic Dust Analyzer instrument when it sampled icy particles from Enceladus in Saturn’s E ring. Many more ice particles were analysed when Cassini flew through the E ring than when it went through just the plume, so the scientists were able to examine a much larger number of compositional signals there. By doing this, they discovered high concentrations of sodium phosphates — molecules of chemically bound sodium, oxygen, hydrogen and phosphorus — inside some of those grains.

Co-authors in Europe and Japan then carried out laboratory experiments to show that Enceladus’s ocean has phosphorus, bound inside different water-soluble forms of phosphate, in concentrations of at least 100 times that of our planet’s oceans. Further geochemical modelling demonstrated that an abundance of phosphate may also be possible in other icy ocean worlds in the outer solar system, particularly those that formed from primordial ice containing carbon dioxide, and where liquid water has easy access to rocks.

Cassini took high-resolution images of Enceladus that were combined into this mosaic, which shows the long fissures at the moon’s south pole that allow water from the subsurface ocean to escape into space. Image credit: NASA/JPL/Space Science Institute.

“We previously found that Enceladus’s ocean is rich in a variety of organic compounds,” said study leader Frank Postberg, from Freie Universität Berlin. “But now, this new result reveals the clear chemical signature of substantial amounts of phosphorus salts inside icy particles ejected into space by the small moon’s plume. It’s the first time this essential element has been discovered in an ocean beyond Earth.”

“High phosphate concentrations are a result of interactions between carbonate-rich liquid water and rocky minerals on Enceladus’s ocean floor and may also occur on a number of other ocean worlds,” added co-investigator Christopher Glein, from Southwest Research Institute in Texas. “This key ingredient could be abundant enough to potentially support life in Enceladus’s ocean; this is a stunning discovery for astrobiology.”

Although the science team is excited that Enceladus has the building blocks for life, Glein stressed that life has not been found on the moon — or anywhere else in the solar system beyond Earth. “Having the ingredients is necessary, but they may not be sufficient for an extraterrestrial environment to host life,” he said. “Whether life could have originated in Enceladus’s ocean remains an open question.”

Linda Spilker, Cassini’s project scientist at NASA’s Jet Propulsion Laboratory, added, “This latest discovery of phosphorus in Enceladus’s subsurface ocean has set the stage for what the habitability potential might be for the other icy ocean worlds throughout the solar system. Now that we know so many of the ingredients for life are out there, the question becomes: is there life beyond Earth, perhaps in our own solar system? I feel that Cassini’s enduring legacy will inspire future missions that might, eventually, answer that very question.”

Top image caption: The icy crust at the south pole of Enceladus exhibits large fissures that allow water from the subsurface ocean to spray into space as geysers, forming a plume of icy particles. Image credit: NASA/JPL/Space Science Institute.