Finding the right conditions for life off the Earth justifiably drives many a researcher’s work, but nailing down just what might make the environment beneath an icy moon’s surface benign isn’t easy. The recent wave of speculation about Enceladus revolves around the discovery of phosphorus, a key ingredient for the kind of life we are familiar with. But Alex Tolley speculates in the essay below that we really don’t have a handle on what this discovery means. There’s a long way between ‘habitable’ and ‘inhabited,’ and many data points remain to be analyzed, most of which we have yet to collect. Can we gain the knowledge we need from a future Enceladus plume mission?
by Alex Tolley
There has been abundant speculation about the possibility of life in the subsurface oceans of icy moons. Europa’s oceans with possible hydrothermal vents mimicking Earth’s abyssal oceans and the probable site of the origin of life, caught our attention now that Mars has no extant surface life. Arthur C Clarke had long suggested Europa as an inhabited moon in his novel 2010: Odyssey Two. (1982). While Europa’s hot vents are still speculative based on interpretations of the surface features of its icy crust, Saturn’s moon, Enceladus, showed visible aqueous plumes at the southern pole. These plumes ejected material that contributes to the E-Ring around Saturn as shown below.
While most searches for evidence for life focus on organic material, it has been noted that of the necessary elements for terrestrial life, Carbon, Hydrogen, Oxygen, Nitrogen, Sulfur, and Phosphorus (CHONSP), phosphorus is the least abundant cosmically. Phosphorus is a key component in terrestrial life, from energy management (ATP-ADP cycle) and information molecules DNA, and RNA, with their phosphorylated sugar backbones.
If phosphorus is absent, terrestrial biology cannot exist. Phosphorus is often the limiting factor for biomass on Earth, In freshwater environments phosphorus is the limiting nutrient . Typically, algae require about 10x as much nitrogen as phosphorus. If the amount of available nitrogen is increased, the algae cannot use that extra nitrogen as the amount of available phosphorus now determines how large the algal population can grow. The biomass-to-phosphorus ratio is around 100:1. When phosphorus is the limiting nutrient, then the available phosphorus will limit the biomass of the local plants and therefore animals, regardless of the availability of other nutrients like nitrogen, and other factors such as the amount of sunlight, or water. Agriculture fertilizer runoff can cause algal blooms in aqueous environments and may result in dead zones as oxygen is depleted by respiration as phytoplankton blooms die or are consumed by bacteria.
While nitrogen can be fixed by bacteria from the atmosphere, phosphorus is derived from phosphate rocks, and rich sources of phosphorus for agriculture were historically gleaned from bird guano.
A recent paper in Nature about the detection of phosphorus in the grains from the E-ring by the Cassini probe’s Cosmic Dust Analyzer (CDA) suggested that phosphorus is very abundant. As these grains are probably sourced from Enceladus’ plumes, this implies that this moon’s subsurface ocean has high levels of dissolved phosphorus.
The authors of the paper have modeled, and experimentally confirmed the model, and make the claim that Enceladus’ ocean is very rich in phosphorus:
…phosphorus concentrations at least 100-fold higher in the moon’s plume-forming ocean waters than in Earth’s oceans.
around 100-fold greater than terrestrial phosphorus abundance levels. They show that the CDA spectrum [figure 1) is consistent with a solution of disodium phosphate (Na2HPO4) and trisodium phosphate (Na3PO4) (figure 2) The source of these salts on Enceladus is likely from the hot vents chemically releasing the material from the carbonaceous chondritic rocky core and the relatively alkaline ocean. Contrary to intuition, the greater CO2 concentrations in cold water with the hydroxyapatite-calcite and whitlockite-calcite buffer system maintain an alkaline solution that allows for the high phosphate abundance in the plume material that produces the grains in Saturn’s E-ring.
Figure 1. CDA cation spectrum co-added from nine baseline-corrected individual ice grain spectra. The mass lines signifying a high-salinity Type 3 spectrum are Na + (23 u) and (NaOH)Na + (63 u) with secondary Na-rich signatures of (H2O)Na + (41 u) and Na 2+ (46 u). Sodium phosphates are represented by phosphate-bearing Na-cluster cations, with (Na3 PO4)Na + (187 u) possessing the highest amplitude in each spectrum followed by (Na2HPO4 )Na + (165 u) and (NaPO3)Na + (125 u). The first two unlabelled peaks at the beginning of the spectrum are H + and C +, stemming from target contamination 3 (source nature paper). a.u., arbitrary units.
Figure 2. Spectrum from the LILBID analogue experiment reproducing the features in the CDA spectrum. An aqueous solution of 0.420 M Na2HPO4 and 0.038 M Na3PO4 was used. All major characteristics of the CDA spectrum of phosphate-rich grains (Fig. 1) are reproduced at the higher mass resolution of the laboratory mass spectrometer (roughly 700 m/?m). Note: this solution is not equivalent to the inferred ocean concentration. To derive the latter quantity, the concentration determined in these P-rich grains must be averaged over the entire dataset of salt-rich ice grains. (source Nature paper).
Fig. 3: Comparison of observed and calculated concentrations of ΣPO43– in fluids affected by water–rock reactions within Enceladus. a, Relation between ΣPO43– and ΣCO2 at a temperature of 0.1 °C for the hydroxyapatite-calcite buffer system (solid lines) and the whitlockite-calcite buffer system (dashed lines). Constraints on ΣCO2 obtained in previous studies are indicated by the blue shaded area. The area highlighted in pink represents the range of ΣPO43– constrained in this study from CDA data. b, Dependence of ΣPO43– on temperature for the hydroxyapatite-calcite buffer and different values of pH and ΣCO2. A similar diagram for the whitlockite-calcite buffer can be found in Extended Data Fig. 11.
The simple conclusion to draw from this is that phosphorus is very abundant in the Enceladan ocean and that any extant life could be very abundant too.
While the presence of phosphorus ensures that the necessary conditions of elements for habitability are present on Enceladus, it raises the question: “Does this imply Enceladus is also inhabited?”
On Earth, phosphorus is often, the limiting factor for local biomass. On Enceladus, if phosphorus was the limiting factor, then one would not expect it to be detected as inorganic phosphate, but rather in an organic form, bound with biomolecules.
But suppose Enceladus is inhabited, what might account for this finding?
1. Phosphorus is not limiting on Enceladus. Perhaps another element is limiting allowing phosphates to remain inorganic. In Earth’s oceans, where iron (Fe) can be the limiting factor, adding soluble Fe to ocean water can increase algal blooms for enhanced food production and possible CO2 sequestration. On Enceladus, the limiting factor might be another macro or micronutrient. [This may be an energy limitation as Enceladus does not have the high solar energy flux on Earth.]
2. Enceladan life may not use phosphorus. Some years ago Wolfe-Simon claimed that bacteria in Mono Lake used arsenic (As) as a phosphorus substitute.  This would have been a major discovery in the search for “shadow life” on Earth. However, it proved to be an experimental error. Arsenic is not a good substitute for phosphorus, especially for life already evolved using such a critical element, and as is well-known, arsenic is a poison for complex life.
3. The authors’ modeling assumptions are incorrect. Phosphorus exists in the Enceladan ocean, but it is mostly in organic form. The plume material is non-biological and is ejected before mixing in the ocean and being taken up by life. The authors may also have wildly overestimated the true abundance of phosphorus in the ocean.
Of these explanations allowing for Enceladus to be inhabited, all seem to be a stretch that life may be in the ocean despite the high inorganic phosphorus abundance. Enceladan biomass may be constrained by the energy derived from the moon’s geochemistry. On Earth, sunlight is the main source of energy maintaining the rich biosphere. In the abyssal darkness, life is very sparse, although it can huddle around the deep ocean’s hot vents.
However, if life is not extant, then the abundance of inorganic phosphorus salts is simply the result of chemical equilibria based on the composition of Enceladus rocky core and abundant frozen CO2 where it formed beyond the CO2 snow line.
While the popular press often conflate habitability with inhabited, the authors are careful to make no such claim, simply arguing that the presence of phosphorus completes the set of major elements required for life:
Regardless of these theoretical considerations, with the finding of phosphates the ocean of Enceladus is now known to satisfy what is generally considered to be the strictest requirement of habitability.
With this detection, it would seem Enceladus should be the highest priority candidate for a search for life in the outer solar system. Its plumes would likely contain evidence of life in the subsurface ocean and avoid the difficult task of drilling through many kilometers of ice crust to reach it. A mission to Enceladus with a suite of life-detecting instruments would be the best way to try to resolve whether life is extant on Enceladus.
The paper is Postberg, F., Sekine, Y., Klenner, F. et al. Detection of phosphates originating from Enceladus’s ocean. Nature 618, 489–493 (2023). https://doi.org/10.1038/s41586-023-05987-9
1. Smil, V (2000) Phosphorus in the Environment: Natural Flows and Human Interferences. Annual Review of Energy and the Environment Volume 25, 2000 Smil, pp 53-88
2. Wolfe-Simon F, et al (2010) “A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus. SCIENCE 2 Dec 2010, Vol 332, Issue 6034 pp. 1163-1166