エンケラドスの海には、生命を維持するために不可欠な元素であるリンが存在することが初めて明らかになりました。(For the first time, astronomers have found life-supporting molecules called phosphates on Enceladus)

2023-06-16 ニューサウスウェールズ大学(UNSW)

＜関連情報＞

• https://newsroom.unsw.edu.au/news/science-tech/first-time-astronomers-have-found-life-supporting-molecules-called-phosphates
• https://www.nature.com/articles/s41586-023-05987-9

エンケラドスの海に由来するリン酸塩を検出 Detection of phosphates originating from Enceladus’s ocean

Frank Postberg,Yasuhito Sekine,Fabian Klenner,Christopher R. Glein,Zenghui Zou,Bernd Abel,Kento Furuya,Jon K. Hillier,Nozair Khawaja,Sascha Kempf,Lenz Noelle,Takuya Saito,Juergen Schmidt,Takazo Shibuya,Ralf Srama & Shuya Tan
Nature  Published:14 June 2023
DOI:https://doi.org/10.1038/s41586-023-05987-9

Abstract

Saturn’s moon Enceladus harbours a global¹ 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^4,5,6,7,8,9. The analysis of salt-rich ice grains by Cassini’s Cosmic Dust Analyzer¹⁰ enabled inference of major solutes in the ocean water (Na⁺, K⁺, Cl^–, HCO₃^–, CO₃^2–) and its alkaline pH^3,11. Phosphorus, the least abundant of the bio-essential elements^12,13,14, 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 abundant¹⁷. 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 CO₂ 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.