Saturn's moon Enceladus has an ocean of liquid water beneath its crust, but its chemical composition remains unclear. However, chemical analyzes of ice particles thrown into space by Enceladus' geysers by NASA's Cassini spacecraft provided the first clues. A new analysis of this data now shows that the element phosphorus, which is important for life, is also present in the geysers and thus probably also in the subglacial ocean of Saturn's moon. Accordingly, its water could contain 100 times more phosphorus in the form of phosphates than the water in the Earth's oceans. This suggests that the liquid oceans beneath the icy crusts of this and other icy moons in the solar system contain all the necessary ingredients for life.
Saturn's moon Enceladus, which is only around 550 kilometers across, appears at first glance to be just a large ball of ice. But closer investigations with telescopes and space probes have provided evidence that an ocean of liquid water is hidden between its 30 to 40 km thick ice crust and a rocky core. Evidence of this includes numerous fountains of water vapor and tiny ice particles that shoot out of cracks in the ice crust in the south polar region of Saturn's moon. These geysers eject their material several hundred to several thousand kilometers into space – an average of around 300 liters of water per second. The water ejected by Enceladus spreads throughout its orbit and is instrumental in filling Saturn's E ring with ice particles. With the "Cosmic Dust Analyzer", NASA's Cassini probe had a mass spectrometer on board, with which it analyzed particles from the E ring and the geyser plumes. She detected various salts, including sodium chloride, potassium chloride and sodium carbonate. Overall, the salt content in the ice granules was between 0.5 and 2 percent.
Search for phosphorus in ejected ice grains
From this data, planetary scientists conclude that the liquid ocean beneath Enceladus' crust may be mineral-rich and fairly alkaline. Its water may resemble that of soda lakes or alkaline hydrothermal vents on Earth. The latter are considered to be possible cradles of life for the primeval earth, because in the vicinity of these submarine vents, almost all the chemical ingredients for simple life are available in addition to the energy required. Six elements are considered critical to water-based, organic life as we know it: carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. "Of these, phosphorus is the least common," explain Frank Postberg from Freie Universität Berlin and his colleagues. Therefore, this element is often considered a limiting factor in the search for life - even in space. "So far, phosphorus has not been detected on any of the ocean-bearing moons in the solar system," the researchers said. However, recent models of Enceladus' geochemistry suggest that phosphates should exist in the waters of its alkaline subglacial ocean.
That's why Postberg and his team analyzed the data from the Cassini probe again. They focused on the mass spectra of 345 ice particles from the E ring, whose sodium salt content indicated an origin from the ocean of Enceladus. The scientists found what they were looking for in nine of these ice particles: "Peaks were seen at the molecular masses 125, 165 and 187, the patterns of which proved to be indications of high concentrations of sodium phosphates in these ice grains," they report. "This suggests that phosphorus is abundantly available, at least in the upper regions of Enceladus' ocean." The team calculated how high the phosphate content in the water might be based on the proportion of these phosphates in the ice particles and the proportion of phosphate-containing ice particles in their total sample .
Hundreds of times more phosphorus than terrestrial seawater
The result: The water of the ocean under Enceladus' ice crust could contain 0.8 to 21 millimoles of phosphates. "This means that the phosphate concentrations are several hundred times higher than the average phosphate values in the Earth's oceans," explain Postberg and his colleagues. Additional laboratory tests confirmed that such phosphates are formed by geochemical interactions at the contact zone of rock and water. The more alkaline the water is, the less heat is required for the corresponding chemical reactions. “At pH 9.5, the water should be 80 degrees or less. If the pH value is 10.5, temperatures below 65 degrees would be enough,” reports the team. The detection of phosphates in the Enceladus ice plumes also supports the assumption that there may be alkaline hydrothermal vents at the bottom of the Enceladus ocean.
The scientists say these results shed new light on the potential habitability of the water beneath Enceladus' crust. Because contrary to what was previously assumed, at least the element phosphorus is no longer a limiting factor. At the same time, the detection of phosphates at Enceladus suggests that this element may also be more abundant than previously thought on other icy moons with subglacial oceans. "Enceladus' ocean could thus be a harbinger of high phosphorus availability in the oceans of the outer solar system beneath the crust," Postberg and his colleagues state. The planetary researcher Mikhail Zolotov from Arizona State University, who was not involved in the study, takes a similar view: "The results suggest that dissolved phosphates also occur on other celestial bodies with subglacial oceans, and thus increase the chances for their livability."
Source: Frank Postberg (Free University of Berlin) et al., Nature, doi: 10.1038/s41586-023-05987-9