Phosphate availability and implications for life on ocean worlds

The search for extraterrestrial life centers on identifying environments with liquid water, essential nutrients, and energy sources. Numerous moons, including Europa and Enceladus, are believed to harbor subsurface oceans. Spacecraft observations, particularly from the Cassini mission to Enceladus, provide evidence supporting potentially habitable conditions, including suitable temperatures, pH, and the presence of essential elements like carbon, hydrogen, nitrogen, and oxygen. However, the availability of phosphorus, often a limiting nutrient for biological productivity on Earth, remains a critical question. Phosphorus, in the form of orthophosphate, is a crucial component of DNA, RNA, and ATP. Its limited availability in some regions of Earth's oceans highlights its potential to restrict life's abundance and productivity on other celestial bodies. This study aims to address this question by modeling the equilibrium dissolved phosphate concentrations resulting from water-rock reactions in ocean worlds, considering a wide range of compositional and environmental parameters.

Previous research has explored the potential limitations of phosphate availability on ocean worlds. Lingam and Loeb (2021) examined the crucial processes needed to produce granite and the rainfall required for granite weathering, concluding that these processes are unlikely to exist on small, ice-covered satellites. In contrast, geochemical evidence and theoretical models suggest that hydrothermal circulation, a potential sink for phosphate, may be active on Enceladus and Europa. Lingam and Loeb also postulated that acidic conditions on Europa could yield higher silicate dissolution rates and a phosphate-rich ocean, contrasting with potentially low phosphate release in Enceladus' alkaline ocean. Hao et al. (2022), focusing on Enceladus, suggested higher phosphate concentrations based on equilibrium with the ocean's inferred temperature, dissolved inorganic content, and redox state. This study builds upon this existing work by incorporating a broader range of chemical equilibria and dynamic controls on phosphate abundance.

The study employs equilibrium geochemical models using the EQ3/6 software to calculate phosphate concentrations during aqueous alteration of various rock compositions under different conditions. The models consider carbonaceous chondrites, representing primitive solar system material, as a basis for initial compositions, acknowledging the variability among chondrite types. Calculations simulate the reaction of chondritic materials with minimal fluid (neutral, anoxic water with trace elements) across temperatures ranging from 1 to 300 °C and water-rock ratios (W:R) from 0 to 10+. The impact of redox conditions on phosphate concentrations is assessed by varying the oxidation state of carbon. Simulations include a range of phosphate and phosphate-bearing mineral species. The resulting equilibrium dissolved phosphate concentrations are then analyzed and compared to phosphate requirements and uptake kinetics of Earth's aquatic microorganisms to assess the potential for supporting life. The biological implications are evaluated by comparing modeled phosphate concentrations to the phosphorus content and uptake rates of various aquatic cells. Doubling times for hypothetical cell populations are calculated using Prochlorococcus phosphate uptake kinetics as a conservative representation of phosphate-limited microbial growth.

The equilibrium models reveal that dissolved phosphate concentrations range from 10⁻¹¹ to 10⁻¹⁴ mol/kg for the range of carbonaceous chondrite compositions and reaction conditions considered. However, under most plausible scenarios, concentrations are generally >10⁻⁵ mol/kg. These concentrations are significantly higher than the minimum values observed in phosphate-limited hydrothermal vents on Earth, implying that they would be sufficient to support microbial life. CI chondrites, considered representative of solar system formation, yield the highest phosphate concentrations. Redox conditions show minimal impact on phosphate abundance. The variability across chondrite types impacts total dissolved phosphate concentrations, with CI chondrites producing the most. Comparing modeled sodium concentrations with observations from Enceladus' E-ring suggests a water-rock ratio between 0.1 and 6, corresponding to equilibrium phosphate concentrations consistently >10⁻² mol/kg for CI chondrite composition. For Europa, a lower bound on ocean salinity translates to a water-rock ratio <40 and consistently >10⁻¹ mol/kg phosphate for CI chondrite composition. High-temperature (>200 °C) reactions yield lower phosphate concentrations, but observational and modeling constraints suggest that these high-temperature regimes are unlikely to dominate ocean chemistry. Considering Earth-like cell phosphorus content and uptake kinetics, modeled phosphate concentrations could support cell densities from 10⁶–10⁹ cells/m³, comparable to or exceeding those found in Earth's deep oceans. Hypothetical doubling times are calculated as days to weeks, indicating rapid potential for population growth. The study highlights that bulk phosphorus abundance is less relevant than the processes governing its partitioning into the ocean.

The findings demonstrate that phosphate availability is unlikely to be a major limiting factor for life in the subsurface oceans of at least some ocean worlds. The high equilibrium phosphate concentrations, even under various conditions, suggest that phosphorus would be readily available to support microbial populations, potentially exceeding those found in Earth’s deep oceans. The rapid hypothetical doubling times further support this conclusion. These results challenge previous studies emphasizing phosphate limitation. While the model focuses on equilibrium processes, it acknowledges the role of dynamic processes in influencing phosphate availability. Future work should incorporate these dynamic factors to refine the predictions. The study's implications are relevant for future missions like Europa Clipper, guiding the search for biosignatures and informing the assessment of habitability.

This study demonstrates that phosphate is likely not a limiting factor for the establishment and growth of substantial populations of Earth-like life in ocean worlds. Equilibrium modeling reveals sufficient phosphate concentrations across a broad range of conditions, challenging previous concerns about phosphate limitation. The findings emphasize the need to consider other factors in assessing the habitability of these environments. Future research should investigate the interplay between equilibrium and dynamic processes, as well as the specific physiological adaptations of potential extraterrestrial life.

The study primarily focuses on equilibrium processes, neglecting the potential influence of kinetic factors and biological activity on phosphate cycling. The model assumes Earth-like cell physiology and phosphorus uptake kinetics, which might not accurately reflect the characteristics of potential extraterrestrial life. The analysis uses carbonaceous chondrites as proxies for ocean world compositions, acknowledging that these compositions may not perfectly represent the actual composition of ocean world interiors.