Is Enceladus Prebiotic?

Centauri Dreams regular Alex Tolley here examines a new paper with a novel take on Saturn’s moon Enceladus. Tempting us with its geysers and the organic compounds Cassini detected in their spray, Enceladus offers the prospect of life within its internal ocean. But are there other explanations for what we see, pointing to what may be a prebiotic environment? For that matter, what features of life’s chemistry could emerge on such a world without yet maturing into what we would recognize as living organisms? The paper Alex examines offers us quite an interesting take on a possible origin for life not just on Enceladus but elsewhere in the universe.

by Alex Tolley

Image: “Snow on Enceladus.” Credit: David Hardy.

The discovery of subsurface oceans in the icy moons of Europa and Enceladus has increased interest in the exploration of these moons. The logic of the mantra “Follow the water” implies that there may be extant life in these oceans, most excitingly from a unique genesis at hypothesized ocean hot vents that release the tidal heat. The Europa Clipper is one such Probe.

A new review article published in Astrobiology by Amit Kahana (Weizmann Institute of Science, Israel) and colleagues makes a case that rather than being biotic, Enceladus is prebiotic, a state such as has been speculated on Titan. As the authors state:

The enceladan subglacial ocean appears to be the first observationally discovered concrete example of what may well be a primeval soup.

They come to this conclusion via a chain of logic which follows.

The first piece of evidence is that the Cassini probe detected organic compounds in the plumes of Enceladus and Saturn’s rings. This should not be surprising as we might expect organic compounds as they are common on icy bodies like comets, which include the primordial compounds that can be converted to compounds that are presumed to be the basic building blocks of life on Earth. They tabulate 53 compounds identified in space, most of which are organic with up to 10 carbon atoms. There is also abundant insoluble organic matter (IOM), like kerogens that can be broken down into small molecules. The authors interpret the Cassini data to suggest the detected organic compounds may have up to 150 carbon atoms, similar in composition to the IOM detected elsewhere. The authors believe that the these organics are rich in carbon and hydrogen, and depleted in oxygen and nitrogen. This indicates methane and other alkanes, longer chain organics or aromatics, rather than the mix of organics that might be expected if life was extant..

On the other hand, the observations are consistent with a predominance of relatively long unsaturated hydrocarbon chains, aliphatic or aromatic, with a small number of polar heteroatoms.

The authors take pains to suggest that the origin of life research is dependent on the primordial compound mix and the energy sources to transform them to prebiotic compounds. For example, they counter the requirements of the classic Miller-Urey experiment that added electrical energy to reducing gas mixtures to create amino acids. They contrast the results of this experiment with the speculated abundance and composition of organics in the plumes of Enceladus to suggest other means of production. These organics could have been delivered by comets, or created ​de novo​ within Enceladus, beneath a thick ice crust, a very different environment from that on the early Earth. The authors paint a picture that favors Oparin’s primordial soup model in which heterotrophic early life feeds off the energy in the organic molecules in this soup. [Heterotrophic life consumes these “soup” compounds, rather than autotrophic life, such as methanogens which consumes inorganic CO2 and hydrogen to extract the energy from methane (CH4) synthesis, as we see in many Archaea extremophiles.]

A primordial soup is the model they have of Enceladus, with organics delivered by infall, with perhaps a rich layer of IOM between the ocean surface and the ice crust above (see figure 1 below).

If the ocean contains mainly high molecular weight organic material, then how are the lower molecular weight organics created? Experimental work suggests that it is the energy, and possibly mineral catalysts, at the hot vents that convert the IOM to the smaller molecules:

In parallel, high-temperature hydrous pyrolysis experiments of solid fossil fuels such as kerogen and bitumen, in the presence or absence of mineral catalysts, gave rise to a plethora of low-molecular-weight organics. The products included alkanes, alkenes, and isoprenoids (Huizinga et al., 1987) as well as long-chain fatty acids (Kawamura et al., 1986; Siskin and Katritzky, 1991). In another set of studies, hydrous pyrolytic degradation of petroleum sediments (kerogen) has been examined both in hydrothermal vents and in the laboratory (Leif and Simoneit, 1995). A major set of products constituted amphiphilic n-alkanones with chain lengths from C11 to C33. A similar set of results showed how hydrothermal pyrolysis leads to the formation of diverse polar compounds, including alkanoic acids and alcohols, isoprenoid ketones, and alkanoate esters, in the C9–C33 range (Rushdi and Simoneit, 2011). All these results are consistent with the possible generation of monomeric organics from insoluble polymers under conditions similar to those prevailing on Enceladus. Whether this actually happens on Enceladus could only be verified by future missions and analyses.

The authors’ model is that of an ocean rich in organics primarily from impactors. Much of this material is IOM. The silicate material detected in the plumes infers that there are hot vents on the ocean bottom that help break down the IOM into simpler, C, H rich molecules, which can then form the lipid membranes of micelles and vesicles. It is the mix of vent emissions and these lipids and IOM that Cassini detected in the plumes. The apparent paucity of nitrogen and oxygen in the plume organics leads the authors to suggest that the compounds likely have an abiotic origin, although they do not rule it out:

However, in the absence of evidence to the contrary, a biotic origin of Enceladus’ organics remains a remote possibility (Postberg et al., 2018).

Figure 1. Enceladus cross-section, showing the different potential components of the soup and the plumes. IOM stands for insoluble organic matter, which is taken to be synonymous with nondispersive organic matter. Such polymeric compounds can still be carried in the plume as small particles detached from insoluble organic layers at the top of the water layer. Lipid in water alludes to monomers and aggregates such as micelles and vesicles. Organic in water is dispersed, for example as a microemulsion. Inspired by the data in Postberg et al. (2018).

Figure 1 above shows the authors’ model of the organic chemistry of Enceladus.

Given the authors’ assumption that abundant lipids exist, and that they have a mechanism for creating them, what does this mean for abiogenesis and life? The two most dominant theories for the origin of life are the​ RNA First​ and Metabolism First​ theories. Both can exist without surrounding cell walls to contain either the RNA (as both catalysts and information molecules) or the metabolites for autocatalytic sets. However, some form of concentration and protection from degradation is eventually needed, and terrestrial life used lipid membranes to provide that solution. However, there are other theories, and Lipid First​ is more akin to the idea that lipids can form mono and bilayer membranes that can then encapsulate the metabolisms and replication machinery of the protocells. In such a world, we might expect to see vesicles or coacervates that concentrate the needed components for the transition to life.

The authors argue that the best origin of life model is one that fits with the observed chemical environment, rather than with a predetermined set of molecules and energy sources such as the Miller-Urey experiment required. They use their speculated set of lipid precursors to define the organic soups and from this favor the Lipid World hypothesis. In other words, having built up a case for long-chain unsaturated carbon molecules from their interpretation of the data, they use this as the given compound environment to base their origin of life scenario. They then argue that cell reproduction occurs by membrane splitting and that information about the membranes was added by the later incorporation of informational molecules like RNA. Thus their Lipid World would predate the RNA world and Metabolism World in this scenario.

Assuming that the authors are correct and Enceladus’s ocean contains a “Lipid World”, the next question they ask is this even prebiotic? It is generally accepted that lipids can form abiotically and organize into vesicles or micelles. Can the lipid membranes help catalyze reactions? A paper very recently published [1] suggests that amino acids can bind to lipid membranes, both stabilizing them and providing the means to catalyze reactions such as peptide formation. This would then allow the concentration of metabolites into these protocells. The experimental results from this paper would support a Lipid First model of the origin of life.

Figure 2. From soup to protocells, delineating the underpinnings of two different models, RNA First (RF, top) and Lipid First (LF, bottom). In RF, specific monomers are singled out from a heterogeneous chemical mixture and undergo polymerization, culminating in the emergence of a self-replicating polymer. This is then enclosed in a lipid bilayer, leading to protocells capable of selection and evolution. In LF, a large variety of amphiphiles spontaneously generates a plethora of assemblies, for example, micelles. The GARD model then predicts that very specific micellar compositions establish a mutually catalytic network, which may exhibit homeostatic growth. This, when followed by fission events, constitutes a reproduction system, capable of selection and evolution (see ‘‘How do lipids reproduce?’’ Section 8.4 and Lancet et al. [2018]). Subsequent prolonged evolution may lead to the emergence of RNA, proteins, and metabolism (See Section 8.5, ‘‘How would lipids beget full-fledged protocells’’ and Lancet et al. [2018] Section 11.1). Caption source: Kahana et al.

Figure 2 above shows the authors’ view of the Enceladus Lipid World hypothesis that would culminate in prebiotic protocells, but not fully living cells.

If their hypothesis is correct, then Enceladus is in a prebiotic state. The components to start life may be present, but mostly concentrated in protocells, but not in any form that we would call life. They may show some features of life, such as motility and replication by division, but no true metabolism to liberate energy for growth and reproduction, nor the information molecules needed to encode the instructions that can evolve under natural selection to drive these protocells to transition to living cells. Such a state would still be a fascinating one for study as it would offer a prebiotic environment that has long been lost to us once life emerged on Earth.

However, it should be borne in mind that their argument is based on a chain of logic. Any link that proved false could break that chain. The organic material may not be as depleted in nitrogen and oxygen as inferred. The organic material in the plumes may not be as complex as believed. The ocean may not have a lot of IOM as a feedstock, and reactions may not result in lipid chains, but rather a mix of different molecular weight organics, both linear and aromatic. The organics may therefore be primordial rather than prebiotic. The Cassini data are tantalizing and beg for further missions to elucidate the nature of this moon. Hopefully we may get further clues from the Europa Clipper mission.

The paper is Kahana A et all “Enceladus: First Observed Primordial Soup Could Arbitrate Origin-of-Life Debate”, Astrobiology v12 (10) 2019. Full Text.

References

1. Caitlin E. Cornell, Roy A. Black, Mengjun Xue, Helen E. Litz, Andrew Ramsay, Moshe Gordon, Alexander Mileant, Zachary R. Cohen, James A. Williams, Kelly K. Lee, Gary P. Drobny, and Sarah L. Keller. ​ Prebiotic amino acids bind to and stabilize prebiotic fatty acid membranes​. ​ PNAS 116 (35), (27 August, 2019), 17239-17244 (abstract).