Detection of phosphates originating from Enceladus’s ocean

• Published in: Nature (London) Vol. 618; no. 7965; pp. 489 - 493
• Main Authors: Postberg, Frank, Sekine, Yasuhito, Klenner, Fabian, Glein, Christopher R., Zou, Zenghui, Abel, Bernd, Furuya, Kento, Hillier, Jon K., Khawaja, Nozair, Kempf, Sascha, Noelle, Lenz, Saito, Takuya, Schmidt, Juergen, Shibuya, Takazo, Srama, Ralf, Tan, Shuya
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 15.06.2023; Nature Publishing Group
• Subjects: 639/33/445/209; 639/33/445/847; Bicarbonates; Calcium carbonate; Calcium phosphates; Carbon dioxide; Cassini mission; Cosmic dust; Dust; Earth; Enceladus; Experiments; Geochemistry; Grains; Harbors; Humanities and Social Sciences; Ice; Ice cover; Ions; Laboratories; Mass spectra; Modelling; Moon; multidisciplinary; Ocean floor; Oceans; Orthophosphates; Phosphates; Phosphorus; Salt; Saturn satellites; Science; Science (multidisciplinary); Seawater; Snowline; Sodium; Sodium phosphate; Solubility; Solutes; Spacecraft; Spectral emittance
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/s41586-023-05987-9

Saturn’s moon Enceladus harbours a global 1 ice-covered water ocean 2 , 3 . The Cassini spacecraft investigated the composition of the ocean by analysis of material ejected into space by the moon’s cryovolcanic plume 4 – 9 . The analysis of salt-rich ice grains by Cassini’s Cosmic Dust Analyzer 10 enabled inference of major solutes in the ocean water (Na + , K + , Cl – , HCO 3 – , CO 3 2– ) and its alkaline pH 3 , 11 . Phosphorus, the least abundant of the bio-essential elements 12 – 14 , has not yet been detected in an ocean beyond Earth. Earlier geochemical modelling studies suggest that phosphate might be scarce in the ocean of Enceladus and other icy ocean worlds 15 , 16 . However, more recent modelling of mineral solubilities in Enceladus’s ocean indicates that phosphate could be relatively abundant 17 . Here we present Cassini’s Cosmic Dust Analyzer mass spectra of ice grains emitted by Enceladus that show the presence of sodium phosphates. Our observational results, together with laboratory analogue experiments, suggest that phosphorus is readily available in Enceladus’s ocean in the form of orthophosphates, with phosphorus concentrations at least 100-fold higher in the moon’s plume-forming ocean waters than in Earth’s oceans. Furthermore, geochemical experiments and modelling demonstrate that such high phosphate abundances could be achieved in Enceladus and possibly in other icy ocean worlds beyond the primordial CO 2 snowline, either at the cold seafloor or in hydrothermal environments with moderate temperatures. In both cases the main driver is probably the higher solubility of calcium phosphate minerals compared with calcium carbonate in moderately alkaline solutions rich in carbonate or bicarbonate ions. Cassini’s Cosmic Dust Analyzer mass spectra of ice grains emitted by Enceladus show the presence of sodium phosphates, suggesting that phosphorus is readily available in Enceladus’s ocean in the form of orthophosphates.