Abundant phosphorus expected for possible life in Enceladus’s ocean

Abstract

Saturn’s moon Enceladus has a potentially habitable subsurface water ocean that contains canonical building blocks of life (organic and inorganic carbon, ammonia, possibly hydrogen sulfide) and chemical energy (disequilibria for methanogenesis). However, its habitability could be strongly affected by the unknown availability of phosphorus (P). Here, we perform thermodynamic and kinetic modeling that simulates P geochemistry based on recent insights into the geochemistry of the ocean–seafloor system on Enceladus. We find that aqueous P should predominantly exist as orthophosphate (e.g., HPO 4 2− ), and total dissolved inorganic P could reach 10 −7 to 10 −2 mol/kg H 2 O, generally increasing with lower pH and higher dissolved CO 2 , but also depending upon dissolved ammonia and silica. Levels are much higher than <10 −10 mol/kg H 2 O from previous estimates and close to or higher than ∼10 −6 mol/kg H 2 O in modern Earth seawater. The high P concentration is primarily ascribed to a high (bi)carbonate concentration, which decreases the concentrations of multivalent cations via carbonate mineral formation, allowing phosphate to accumulate. Kinetic modeling of phosphate mineral dissolution suggests that geologically rapid release of P from seafloor weathering of a chondritic rocky core could supply millimoles of total dissolved P per kilogram of H 2 O within 10 5 y, much less than the likely age of Enceladus’s ocean (10 8 to 10 9 y). These results provide further evidence of habitable ocean conditions and show that any oceanic life would not be inhibited by low P availability.

Putative Methanogenic Biosphere in Enceladus's Deep Ocean: Biomass, Productivity, and Implications for Detection

“…+ and phosphorus are both present in Enceladus's ocean, thus making it less likely that the growth of methanogens be limited by either of these nutrients (Cable et al 2021;Hao et al 2022).…”

A Bayesian Analysis of Technological Intelligence in Land and Oceans

“…The second major justifiable impediment is nutrient availability. A bevy of publications have proposed that a subset of worlds with surface (Wordsworth & Pierrehumbert 2013;Lingam & Loeb 2019b;Glaser et al 2020;Olson et al 2020) or subsurface (Zolotov 2007;Lingam & Loeb 2018b) oceans may evince a scarcity of dissolved phosphorus (a bioessential element for life as we know it) in the form of phosphates, while other studies have elucidated avenues that could raise phosphorus concentrations to Earth-like levels or higher (Pasek et al 2013;Syverson et al 2021;Brady et al 2022;Hao et al 2022;Pasek et al 2022). Looking beyond phosphorus, many other elements are critical for life as we know it (Fraústo Da Silva & Williams 2001;Wackett et al 2004), and even if one or a few of them are scarce, putative complex biospheres might be ruled out.…”

Moonraker: Enceladus Multiple Flyby Mission

“…While C, H, N, and O have been identified in Enceladus's plume, P-and S-bearing species are only tentatively detected, as phosphine and hydrogen sulfide, respectively, in the gas phase of the plume (Magee & Waite 2017). Phosphorus and sulfur may also be present in other chemical forms, such as phosphates (Hao et al 2022) and sulfates, which would be refractory. Nextgeneration instruments on board Moonraker would be capable of detecting trace abundances of these key species in the plume, whether in the vapor or solid phase.…”

“…Cassini data revealed Enceladus as one of the most promising objects for habitability in the solar system (Hemingway & Mittal 2019; Cable et al 2021;Hao et al 2022). The composition of the icy grains containing various salts (Postberg et al 2009(Postberg et al , 2011 demonstrates a subsurface liquid source for the plume material, i.e., an internal sea that is in contact with a rocky core.…”