Biosignatures & Paleobiology

Distinguishing Potential Organic Biosignatures on Ocean Worlds from Abiotic Geochemical Products using Thermodynamic Calculations

By Keith Cowing

Status Report

chemrxiv.org

May 9, 2024

Filed under abiotic biosignature, biosignature, biotic biosignature, chemrxiv.org, Enceladus, geochemistry, ocean worlds, organic chemistry, tricorder, water worlds

Distinguishing Potential Organic Biosignatures on Ocean Worlds from Abiotic Geochemical Products using Thermodynamic Calculations — (A) Schematic of assumptions concerning the outer layers of Enceladus, where H2 (and other solutes) and H2O erupt from the subsurface ocean as a plume of gas. As solutes and H2O vaporize and travel through the tiger stripes, H2O condenses onto the surrounding ice shell as it travels outward, enriching the plume in solutes. Once out of the ice shell conduit, the ratio of solutes to H2O remains the same as the gases expand into the vacuum of space and are detected by Cassini’s INMS. (B) Calculated log aqueous activities (at 273.15 K) of constrained species, resulting from the process illustrated in the left panel using data from Table 1, are displayed as functions of tiger-stripe temperatures. The dashed curve represents a hypothetical organic molecule detected at 1 ppm in the plume gas and then converted into log aqueous activity. — chemrxiv.org

The search for life in our solar system often involves efforts to detect organic molecules, which have been found on many extraterrestrial bodies, including planets, moons, meteorites, comets, and asteroids. These chemical signatures are not typically thought of as biosignatures because we know that organic synthesis can occur through abiotic processes.

Therefore, development of methods for distinguishing biotic and abiotic biosignatures would enable interpretation of data collected from habitability and life-detection missions. Life on Earth harnesses energy-releasing reactions to power biosynthesis reactions, which often require energy. Using thermodynamic data, we can quantify the energy required for organic synthesis. If an organic molecule is detected in an abundance that is thermodynamically unstable, then it is possible that life coupled its synthesis to other energy-releasing reactions.

On the other hand, if an organic molecule is detected in an abundance that is thermodynamically stable, then abiotic synthesis was plausible. This sorting framework can be applied to the search for life wherever we have geochemical data. One such example is Saturn’s moon Enceladus. Small compounds involving the elements that comprise the majority of biomass were detected by the Cassini spacecraft in the plume gas erupting from the subsurface ocean.

Using Enceladus as an example, we demonstrate the utility of thermodynamic calculations for distinguishing biosignatures and show that organic synthesis is often favorable using the carbon sources available on Enceladus. While these results may lead us to conclude that hypothetical organic signatures on Enceladus are abiotic, this framework can be applied to other environments in the search for genuine biosignatures.

Distinguishing Potential Organic Biosignatures on Ocean Worlds from Abiotic Geochemical Products using Thermodynamic Calculations, chemrxiv.org (open access)

Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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