Serpentinisation-driven liberation of bioessential phosphite (P(III)) in Europa-relevant lithologies.

As a plausibly habitable Solar System “ocean world”, Jupiter’s moon Europa is a key target for ongoing and future exploration [1]. A fundamental habitability requirement is the provision of bioavailable Phosphorus (P), one of six elements (CHNOPS) essential for all known life that is vital for the formation of phospholipids, the structural framework of DNA and RNA, and energy production and transfer. However, the delivery, abundance, and speciation of P into Europa’s subsurface ocean is currently entirely unconstrained.

Phosphorus is a limiting nutrient on Earth [2], as P is less abundant than other CHNOPS elements and P reservoirs are dominated by the poorly soluble phosphate (P(V) as PO₄^3-) mineral apatite. While present at lower abundances, reduced P species phosphite (P(III)) has significantly greater solubility and reactivity, and hence bioavailability, than phosphate [2-3]. As such, phosphite has been argued to represent an important P source in early Earth and extraterrestrial aqueous environments [3-5]. Several geological pathways exist to produce and liberate phosphite. Most pertinent to Europa is serpentinisation [5], where water-rock reaction of ultramafic and mafic lithologies at the mantle-ocean interface may i) reduce lattice-bound phosphate substituting for SiO₄ in olivine to phosphite and/or ii) liberate magmatic phosphite lattice-bound in Mg-Fe silicates. Though ultramafic and mafic rocks possess low bulk P contents, their likely lithological dominance in differentiated icy moons means serpentinisation is a viable mechanism for bioavailable P delivery to subsurface oceans [5].

Following the methods of [3] and using loss of ignition (LOI) as a proxy for water/rock ratios and degree of serpentinisation, we constrain P speciation in variably serpentinised ultramafic to mafic lithologies from the Troodos ophiolite, Cyprus. Preliminary data reveal variable but ubiquitous phosphite in mantle and crustal samples (P(III)/P(V) ≤0.01 to 0.45). In variably serpentinised harzburgites, increasing P(III)/P(V) correlates with LOI, supporting serpeninisation-driven liberation of phosphite, via either reduction of phosphate or preferential release of magmatic phosphite. Furthermore, an observed decrease in P(III)/P(V) above 15 % LOI supports the thermodynamic models of [5], which imply an upper limit to the water/rock ratios permissive for these reactions. These data indicate serpentinisation-driven reduction of phosphate and/or liberation of magmatic phosphite is a resolvable and ubiquitous process in natural materials analogous to Europa’s rocky ocean floor, providing further constraints on the habitability of Europa’s subsurface ocean.

[1] Vance et al. (2023), Space Science Reviews 219, 81. [2] Duhamel (2024) Nature Reviews Microbiology 23, 239-255. [3] Baidya et al. (2024), Communications Earth & Environment 5(1), 491. [4] Baidya et al. (2025) Nature Communications 16, 4825. [5] Pasek et al. (2022), GCA 336, 332-340.