Detection of phosphate in Enceladus’ ocean with implications for geochemistry and habitability in the outer solar system

Enceladus’s subsurface global ocean (1) can be probed by sampling the gaseous and icy material the moon expels into its cryovolcanic plume and - even further out - into Saturn’s E ring (2,3,4,5). Hydrothermal outflows caused by tidal heating (4,5,6), together with rich organic chemistry (7,8) imply that the moon appears to be one of most habitable places in our solar system. Among the critical elements C, H, N, O, P and S that are considered to be essential for life, all except phosphorous have either been identified (5,7,8) or - in the case of sulfur - tentatively detected (9). Recent geochemical modelling claims that P will be severely depleted in ocean worlds and thus P could be a bottle neck for the emergence of life in subsurface oceans (10).

Here we present results from a re-analysis of mass spectrometric data from Cassini’s Cosmic Dust Analyzer (CDA), showing proof of sodium-phosphate salts in ice grains originating from Enceladus’s subsurface ocean. We found a small number of ice grains whose spectra clearly indicate the presence of at least two sodium orthophosphates: Na₃PO₄ and Na₂HPO₄. These CDA spectra have been subsequently reproduced in the laboratory which enables the quantitative evaluation of CDA spectra (11). We infer phosphate concentrations in the Enceladus ocean in the order of a few mM, at least 100-times higher concentrations than in Earth’s ocean.

We carried out geochemical experiments and calculations showing that such high phosphate abundances can be achieved in Enceladus, either at the cold seafloor or in hydrothermal environments with moderate temperatures. The driver enabling the abundant availability of phosphate is the high observed concentration of dissolved carbonate species, which shift phosphate-carbonate mineral equilibria toward dissolution of solid phosphates into Enceladus’ ocean. We show that interactions between chondritic rocks and CO₂-rich fluids generally lead to conditions where dissolved phosphate concentrations tend to maximize. Therefore P-rich oceans would commonly occur in ocean worlds beyond in the outer Solar System beyond the CO₂ snow line.

These results demonstrate that Enceladus has a high availability of dissolved P, which is thus likely not a limiting factor for development of putative life on Enceladus and probably on other ocean worlds in the outer Solar System. Since phosphate plays many roles in organic synthesis (12), Enceladus and other icy bodies could moreover serve as natural analogs of P-rich environments on early Earth, where chemical evolution might have been promoted.

1 Thomas et al., Icarus 264 (2016), 2 Postberg et al., Nature 459 (2009), 3 Postberg et al., Nature 474 (2011), 4 Hsu et al., Nature 519 (2015), 5 Waite et al., Science 356 (2017), 6 Choblet et al. , Nat Astron 1 (2017), 7 Postberg et al., Nature 558 (2018) , 8 Khawaja et al., MNRAS 489 (2019), 9 Postberg et al., ISBN: 9780816537075 (2018), 10 Lingam & Loeb, Astron. J., 2018., 11 Klenner et al., Rapid Commun Mass Spectrom 33 (2019), 12 Powner et al., Nature 459 (2009)