Key building block of life found on ocean world orbiting Saturn
A key chemical building block of life has been found on Saturn’s moon Enceladus.
Phosphorus was detected in salty ice grains that were released into space by plumes that erupt between the cracks of the moon’s ice shell.
An ocean exists beneath the thick, icy surface of Enceladus, and plumes of material regularly release from geysers at the moon’s south pole.
That material becomes incorporated into Saturn’s outermost E ring.
Scientists used data from NASA’s Cassini mission, which studied Saturn and its moons between 2004 and 2017. The spacecraft flew through the plumes of Enceladus and Saturn’s E ring many times, and Cassini’s Cosmic Dust Analyzer detected minerals and organic compounds necessary for life.
Previously, researchers detected the presence of sodium, potassium, chlorine and carbonate compounds in the ice grains collected and analyzed by Cassini. Now, scientists can add phosphorus to the list. A study detailing the findings was published Wednesday in the journal Nature.
“Phosphorus in the form of phosphates is vital for all life on Earth,” said lead study author Dr. Frank Postberg, a professor of planetary sciences at Freie Universität Berlin, in a statement. “It is essential for the creation of DNA and RNA, cell membranes, and ATP (the universal energy carrier in cells) for example. Life as we know it would simply not exist without phosphates.”
It’s the first time that phosphorus has been discovered in an ocean beyond Earth.
“Previous geochemical models were divided on the question of whether Enceladus’ ocean contains significant quantities of phosphates at all,” Postberg said. “These Cassini measurements leave no doubt that substantial quantities of this essential substance are present in the ocean water.”
A habitable ocean on another world
The data revealed high concentrations of sodium phosphates, or molecules that chemically bind sodium, oxygen, hydrogen and phosphorus together, inside the ice grains.
Dramatic plumes release ice particles, water vapor and organic compounds into space through cracks in the ice shell of Saturn's moon Enceladus. Beneath the ice shell is a salty ocean. - NASA/JPL/Space Science Institute
The collective detection of phosphorus and other organic compounds in Enceladus’ ocean suggests that it could be habitable for life, if it exists on the icy moon, the researchers said.
“By determining such high phosphate concentrations readily available in Enceladus’ ocean, we have now satisfied what is generally considered one of the strictest requirements in establishing whether celestial bodies are habitable,” said study coauthor Dr. Fabian Klenner, postdoctoral researcher at the University of Washington, in a statement.
Although the ocean exists beneath an ice shell, there are indications of hydrothermal environments along the seafloor that keep the ocean at a warmer temperature.
“The important part for habitability is that we found phosphates that have been dissolved in Enceladus’ ocean and, with that, are readily available for the formation of potential life,” Postberg said.
“In most cases, phosphates (on Earth and elsewhere in the solar system) are locked up in rocky minerals, but on Enceladus, they are dissolved in large quantities (as salts) in the ocean.”
The researchers also conducted lab experiments to model Enceladus’ salty ocean, and they determined the phosphate concentrations are at 100 times and possibly up to 1,000 times higher than in Earth’s oceans. That’s because “soda oceans,” or those rich in carbonates and carbon dioxide like the one on Enceladus, can dissolve large amounts of phosphates that are otherwise locked inside rocky minerals, Postberg said.
“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 study coauthor Christopher Glein, a planetary scientist and geochemist at Southwest Research Institute in San Antonio, Texas, in a statement. “This key ingredient could be abundant enough to potentially support life in Enceladus’ ocean; this is a stunning discovery for astrobiology.”
Wispy arcs of icy material expand out from Enceladus (brightest white dot above) into Saturn's E ring. - NASA/JPL/Space Science Institute
Some of the additional ocean world moons around Jupiter and Saturn include Europa, Titan and Ganymede. Future missions to Europa, like the European Space Agency’s Jupiter Icy Moons Explorer and NASA’s Europa Clipper, could determine more about the ingredients within these oceans.
Searching for signs of life
Although the Cassini mission ended by intentionally burning up in Saturn’s atmosphere in 2017, the data collected by its instruments is changing the way scientists understand Enceladus and similar ocean worlds, which may be the best bet for finding life beyond Earth across our solar system.
“This latest discovery of phosphorus in Enceladus’ subsurface ocean has set the stage for what the habitability potential might be for the other icy ocean worlds throughout the solar system,” said Linda Spilker, planetary scientist and Voyager project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, in a statement. Spilker, who served as Cassini’s project scientist, was not involved in the study.
“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,” Spilker said.
Although the building blocks of life and conditions for habitability exist on Enceladus, no actual life has been detected yet.
The Cassini spacecraft captured an image of Enceladus on July 14, 2005, revealing its craters and fractured icy crust. - NASA/JPL/Space Science Institute
“Having the ingredients is necessary, but they may not be sufficient for an extraterrestrial environment to host life,” Glein said. “Whether life could have originated in Enceladus’ ocean remains an open question.”
“The next step is clear — we need to go back to Enceladus to see if the habitable ocean is actually inhabited,” said study coauthor Dr. Nozair Khawaja, a planetary scientist and postdoctoral researcher at Freie Universität Berlin.
Sending a dedicated mission to Enceladus is a priority for astronomers. Plans are underway to design the Enceladus Orbilander, which would both orbit the moon and land on the surface. If the mission were to launch later this decade, it would likely arrive at the distant moon in the early 2050s.
A spacecraft capable of landing on plume deposits on Enceladus could perform a chemical analysis to further investigate the subsurface ocean, said planetary geochemist Mikhail Zolotov, a research professor at Arizona State University and author of an accompanying News & Views article to the Nature study. Zolotov did not participate in the study.
“Those plume deposits could be taken inside a lander, melted, and analyzed for dissolved salts, gases, organic compounds, and possible biosignatures that all will characterize oceanic chemistry and habitability,” Zolotov said.
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