Scientists have discovered high levels of phosphorus in the ocean spray of Saturn’s moon Enceladus, marking the first time that this essential ingredient for life has ever been detected in extraterrestrial seawater, reports a new study.
The breakthrough reveals that Enceladus is stocked with all right materials to support life as we know it, and hints that phosphorus may be present in other possible ocean worlds far from Earth, such as Neptune’s moon Triton or the dwarf planet Pluto. At this point, nobody knows if any of these places are actually inhabited by aliens, but the new results bolster the evidence that habitable conditions may be common in the solar system, and possibly beyond it.
Enceladus is a tiny ice world that measures about 300 miles across, but what it lacks in size it makes up for with its tantalizing subsurface ocean. Though its marine environment is hidden under the moon’s icy shell, plumes of seawater erupt into space from geysers on the surface. NASA’s Cassini spacecraft, which ended its mission in 2017, was fortuitously able to scoop up some of this frozen moon juice during its final years in orbit around Saturn, offering an unprecedented look at the contents of an extraterrestrial ocean.
Now, researchers led by Frank Postberg, a planetary scientist at Freie Universität Berlin, have discovered abundant supplies of phosphorus salts, known as sodium phosphates, in ice grains captured by Cassini from Saturn’s outer E-ring, which is primarily fed by the wellspring of Enceladus’ plumes. Moreover, the team found that phosphate concentrations are 100 times higher in Enceladus’ ocean than in Earth’s marine habitats, suggesting that there is no shortage of this key element in the moon’s subsurface sea.
“Enceladus was already considered a pretty habitable place before this,” Postberg said in a call with Motherboard. “The conditions in the ocean seem to be good for life. There are very likely hydrothermal vent systems at the bottom of the ocean that would be an energy source, so you don’t need sunlight. There is a rich variety of organic compounds we detected previously in the ice and vapor that is emitted in the plume.”
“The missing ingredient was phosphorus,” he continued. “That’s why phosphorus got a lot of attention, because it was the only element that had not been detected by Cassini, and was considered critical.”
Indeed, prior to the new study, scientists had shown that Enceladus’ ocean contains five of the six main elements that are needed for life as we know it—carbon, hydrogen, nitrogen, oxygen, and sulfur—leaving phosphorus as the absent piece of this bioessential puzzle. Phosphorus plays a major role in many life processes, such as the transfer of energy within cells and the stability of DNA and cell membranes, but it is also the most elusive element in life’s toolkit.
For that reason, the discovery of phosphates in Enceladus’ marine environment has satisfied “what is generally considered to be the strictest requirement of habitability” and suggests that “Enceladus’s ocean could be a harbinger of high phosphorus availability in subsurface oceans across most of the outer Solar System,” according to the team’s study, which was published on Wednesday in Nature.
“This was basically the last piece that was needed to finally, now, deem Enceladus’ ocean to be habitable without any doubt,” Postberg said. “Of course, habitable does not mean inhabited. This phosphorus is not something that comes from any lifeform. It’s not produced by life. It’s just an ingredient that, at least for Earth, was essential for the emergence of life.”
Postberg and his colleagues have spent years poring over data captured by a Cassini’s Cosmic Dust Analyzer (CDA), an instrument that was not equipped to search for life, but could examine the properties and compositions of extremely small particles with a high degree of precision.
Their detection of sodium phosphates was “one of the nice surprises we got,” Postberg said, though the high concentrations of the compounds in Cassini’s samples raised the question of how phosphorus came to be so much more abundant in Enceladus’ ocean compared to Earth’s seas.
To shed light on this mystery, Postberg’s team reconstructed an Enceladus-like environment in laboratory conditions using ocean water simulant and mineral-rich rocks. These experiments suggest that Enceladus has a so-called “soda ocean” rich in carbonates that speed up the dissolution of phosphorus from rocks into the marine environment.
These soda oceans may be common inside worlds that formed in the distant solar system, beyond a boundary called the carbon dioxide (CO2) snowline that marks where CO2 freezes into ice. As a result, planetary bodies that coalesce in these outer wilds often contain a lot of CO2 ice that eventually provides a source of carbonates for soda oceans, like the one inside Enceladus.
“A soda ocean where you have a lot of carbonates allows the release of phosphate from the rock into the ocean and with that, makes it available for the formation of life,” Postberg said.“It’s much harder, or almost impossible, for life to release the phosphate from the rock but if it’s dissolved in the ocean, that’s a critical thing for habitability.”
“That’s why it has implications beyond Enceladus,” he added. “If there are ocean moons elsewhere, far away from the Sun where they had CO2 as a building material, then these oceans should be rich in phosphate as well. This is generally good news for habitability in the universe, but specifically for the ocean worlds in our solar system.”
In other words, Enceladus is the first extraterrestrial world that we know contains marine phosphates, but it may not be the last place we find these life-giving materials. Jupiter’s moons, including Europa and Ganymede, are among the many worlds that might also bear habitable oceans. To that end, the European Space Agency recently launched its JUICE spacecraft to study Jupiter’s moons, while NASA’s Europa Clipper mission is due to depart for its namesake target in 2024.
Scientists have proposed new missions to Enceladus, this time with life-detecting instruments, but so far these concepts have not been greenlit. That said, if we do eventually mount a return to this tiny Saturnian moon, we could end up answering what is arguably the most sensational mystery in science: Are we alone in the universe?
“We could build a spacecraft with today’s technology and send it to Enceladus to answer the question: is this habitable place actually inhabited or not?” Postberg said. “So with the next mission, we will very likely get an answer to that question.”
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