Phosphorous detection strengthens case for Enceladus’ habitability

The Cassini spacecraft captured this image of the south polar region of Saturn’s moon Enceladus in 2009, showing geysers of icy particles spewing into space from a subsurface ocean. The plumes contain organic compounds and, researchers recently discovered, high concentrations of phosphorous, essential to life as it’s known on Earth. Image: NASA/JPL-Caltech/Space Science Institute

Researchers studying archived data from NASA’s Cassini probe have detected high concentrations of phosphorous in salt-rich ices spewing from the cracked crust of Saturn’s moon Enceladus. The data strengthen the case for a habitable environment in an ocean below the world’s frozen surface.

“We previously found that Enceladus’ ocean is rich in a variety of organic compounds,” said Frank Postberg, a planetary scientist at Freie Universität Berlin and leader of a study published in the journal Nature.

“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.”

Phosphorous is the least abundant of the elements considered essential to life as it’s known on Earth. It plays a role in the chemistry of DNA and energy-carrying molecules present in all known earthly life forms and is found in the bones of mammals and cell membranes.

Ice from the sub-surface ocean on Enceladus spews out into space through cracks in the crust over the south polar region, creating clearly visible geysers that provide the ice particles responsible for Saturn’s faint E ring.

NASA’s Cassini spacecraft flew through the geyser plumes and the E ring multiple times between 2004 and 2017 when the mission ended, and scientists were able to identify a variety of organic compounds and other elements and chemicals essential to life. They included sodium, potassium, chlorine and carbonate-containing compounds. The plumes included the ingredients of the amino acids so critical to life as we know it.

In the latest study, Postberg and his colleagues focussed on archived data from Cassini’s Cosmic Dust Analyser instrument. They were surprised to discover molecules of chemically-bound sodium, oxygen, hydrogen and phosphorous, that is, sodium phosphates.

“High phosphate concentrations are a result of interactions between carbonate-rich liquid water and rocky minerals on Enceladus’ ocean floor and may also occur on a number of other ocean worlds,” said co-investigator Christopher Glein, a planetary scientist and geochemist at Southwest Research Institute in San Antonio, Texas.

“This key ingredient could be abundant enough to potentially support life in Enceladus’ ocean. This is a stunning discovery for astrobiology.”

But the latest discovery does not mean life has been detected at Enceladus or anywhere else. At least, not yet.

“Having the ingredients is necessary, but they may not be sufficient for an extraterrestrial environment to host life,” Glein stressed. “Whether life could have originated in Enceladus’ ocean remains an open question.”