Abstract
A recent breakthrough publication has reported complex organic molecules in the plumes emanating from the subglacial water ocean of Saturn's moon Enceladus (Postberg et al., 2018, Nature 558:564–568). Based on detailed chemical scrutiny, the authors invoke primordial or endogenously synthesized carbon-rich monomers (<200 u) and polymers (up to 8000 u). This appears to represent the first reported extraterrestrial organics-rich water body, a conceivable milieu for early steps in life's origin (“prebiotic soup”). One may ask which origin-of-life scenario appears more consistent with the reported molecular configurations on Enceladus. The observed monomeric organics are carbon-rich unsaturated molecules, vastly different from present-day metabolites, amino acids, and nucleotide bases, but quite chemically akin to simple lipids. The organic polymers are proposed to resemble terrestrial insoluble kerogens and humic substances, as well as refractory organic macromolecules found in carbonaceous chondritic meteorites. The authors posit that such polymers, upon long-term hydrous interactions, might break down to micelle-forming amphiphiles. In support of this, published detailed analyses of the Murchison chondrite are dominated by an immense diversity of likely amphiphilic monomers. Our specific quantitative model for compositionally reproducing lipid micelles is amphiphile-based and benefits from a pronounced organic diversity. It thus contrasts with other origin models, which require the presence of very specific building blocks and are expected to be hindered by excess of irrelevant compounds. Thus, the Enceladus finds support the possibility of a pre-RNA Lipid World scenario for life's origin.
Highlights
A recent breakthrough publication has reported complex organic molecules in the plumes emanating from the subglacial water ocean of Saturn’s moon Enceladus (Postberg et al, 2018, Nature 558:564–568)
Contemporary laboratories can synthesize practically every molecule of life, but it appears necessary to make a distinction between organic molecules that could form abiogenically and such that could only form within evolving protocellular entities
We describe here some inferences derived from the large water body on Enceladus, recently shown by direct experimental approaches to contain a diversity of organic compounds from several likely sources (Postberg et al, 2018)
Summary
Acentral objective of any astrobiology roadmap is finding out how life began and evolved (Des Marais et al, 2008; Horneck et al, 2016). Since a widely accepted definition is ‘‘life is that which replicates and evolves’’ (Nowak and Ohtsuki, 2008), life’s origin appears to entail a transition from the abiogenic availability of various organic compounds to chemical entities capable of producing their own copies Organic compounds are those that contain covalently bound combinations of carbon with one or more of the five other main life elements, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. It appears that molecular size and complexity are necessary but not sufficient conditions for being relevant to life, as some very large carbon-based molecules are abiotic (Fig. 1). Different origin-of-life models differ vastly in this respect
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