Enceladus – the sixth largest of Saturn’s 146 moons – has a liquid ocean with a rocky bottom beneath the clear, white and icy surface. Ice volcanoes spew frozen grains of material into space, generating one of the many rings that orbit the planet.
Now a team of researchers has discovered that those icy granules contain phosphates. They found them using data from Cassini, a joint NASA-European orbiter that completed its study of Saturn, its rings and moons in 2017. It is the first time that phosphorus has been found in an ocean beyond Earth. The results, which add to the prospect that Enceladus is home to extraterrestrial life, were published Wednesday in the journal Nature.
“We did not expect this. We didn’t look for it,” said Frank Postberg, a planetary scientist at the Free University of Berlin who led the study. He described the realization that they had found phosphates (chemicals containing the element phosphorus) as a “pungent moment”.
With the discovery of phosphorus on the ocean world, scientists say they have now found all the elements essential for life as we know it. Phosphorus is a key ingredient in human bones and teeth, and scientists say it’s the rarest bioessential ingredient in the cosmos. Planetary researchers had previously detected the other five key elements on Enceladus: carbon, hydrogen, nitrogen, oxygen and sulfur (the last of which has been tentatively detected).
Previous research indicated that phosphorus should be scarce on alien ocean worlds, which could hold back the formation of life elsewhere in the solar system or galaxy.
But on Enceladus, the researchers found “quite the opposite,” said Dr. Post mountain. Rather than being deficient in phosphates, he said, the frigid sea was “enriched by a factor of 1,000 or so compared to Earth’s oceans.”
Dr. Postberg and his colleagues came to this conclusion by conducting an in-depth examination of 345 ice grains that Cassini studied as they flew through Saturn’s “E ring,” which is formed by Enceladus’ emissions. They measured the composition of dust particles that resulted from the collisions of these grains with the metal plate of an instrument on the spacecraft, the Cosmic Dust Analyzer. Nine of the ice particles, they found, had molecular masses that indicated the presence of phosphates.
To make sure they didn’t misinterpret Cassini’s measurements, they set up a series of experiments in the lab, testing different states and concentrations of phosphorus. “And after taking a lot of measurements, we hit the mark,” said another study author, Fabian Klenner, who is now an astrobiologist at the University of Washington. “We found one perfect match with the data from space.”
But the researchers still couldn’t explain how Enceladus had such high concentrations of phosphate in its ocean. Some of the study’s researchers investigated this at the Tokyo Institute of Technology by simulating the geochemical interactions between the ocean’s water and the rocky bottom.
They found answers in the alkaline waters of Enceladus, which are rich in carbonates. “You could call it a ‘soda ocean,'” said Dr. Post mountain.
Phosphorus occurs naturally most often in solid minerals, such as those found in asteroids and comets. “And if it’s locked in a rock, it’s hard to harvest for a lifetime,” said Dr. Postberg, because it must be soluble in order to be used biologically. “But we find that this soda water can dissolve phosphates very well.”
Mikhail Zolotov, a planetary geochemist at Arizona State University who wrote a perspective article on the study for Nature, was not surprised by this explanation. “It was clear before, from studies of Earth’s soda lakes, that we would expect large amounts of phosphorus in natural soda lakes,” he said.
In addition to Enceladus, Dr. Postberg, this discovery may indicate that other outer solar system ocean worlds, such as Jupiter’s moon Europa or the dwarf planet Pluto, are rich in phosphates – and thus potentially habitable.
He and fellow researchers hope to analyze a larger sample of Cassini data to bolster their results. But a definitive search for life on Enceladus will require another mission a decade or two away, if it’s ever approved.
“We don’t yet know if this highly habitable place is actually inhabited,” said Dr. Post mountain. “But it’s definitely worth a look.”