Alternative solvents for life: framework for evaluation, current status and future research

Abstract: Life is a complex, dynamic chemical system that requires a dense fluid solvent in which to take place. A common assumption is that the most likely solvent for life is liquid water, and some researchers argue that water is the only plausible solvent. However, a persistent theme in astrobiological research postulates that other liquids might be cosmically common, and could be solvents for the chemistry of life. In this paper we present a new framework for the analysis of candidate solvents for life, and deploy this framework to review substances that have been suggested as solvent candidates. We categorize each solvent candidate through four criteria: occurrence, solvation, solute stability and solvent chemical functionality. Our semi-quantitative approach addresses all the requirements for a solvent not only from the point of view of its chemical properties but also from the standpoint of their biochemical function. Only the protonating solvents fulfil all the chemical requirements to be a solvent for life, and of those only water and concentrated sulfuric acid are also likely to be abundant in a rocky planetary context. Among the non-protonating solvents liquid CO2 stands out as a planetary solvent, and its potential as a solvent for life should be explored. We conclude with a discussion of whether it is possible for a biochemistry to change solvents, as an adaptation to radical changes in a planet's environment. Our analysis provides the basis for prioritizing future experimental work exploring potential complex chemistry on other planets.

• The paper presents a novel framework for evaluating alternative life solvents based on occurrence, solvation, solute stability, and chemical functionality.
• Methodological analysis compares protonating solvents like water and concentrated sulfuric acid with non-protonating candidates such as liquid carbon dioxide.
• The study emphasizes the need for future empirical research to explore solvent replacement scenarios and broaden astrobiology paradigms.

An Evaluation Framework for Alternative Life Solvents: A Methodological Assessment and Future Directions

The paper "Alternatives to Liquid Water: A Framework for Evaluation, Current Status, and Future Research" presents a comprehensive framework for evaluating alternative solvents for life, challenging the prevailing paradigm that liquid water is the sole solvent capable of supporting biochemistry. This work is pivotal in the domain of astrobiology, where it addresses the requirements of a solvent that could theoretically support life forms distinct from terrestrial analogues.

The study begins by outlining a robust framework for solvent assessment based on four critical criteria: occurrence, solvation capacity, solute stability, and chemical functionality. This approach diverges from traditional methods that primarily emphasize the inherent chemical properties of the solvent itself, instead focusing on the biochemical functionality required by a solvent to sustain life.

Framework Criteria for Solvent Assessment

1. Occurrence: The solvent must be present in significant quantities and stable as a liquid on a planetary surface or within the crust and atmosphere. This criterion naturally favors environments where the solvent can persist over geological timescales without requiring biogenic influences.
2. Solvation: The solvent needs to solvate a diversity of compounds selectively, including polymers and macro molecules, while not resulting in the dissolution of all substances indiscriminately, thereby preserving molecular integrity necessary for life's processes.
3. Solute Stability: Stability of the solutes within the solvent is imperative to ensure that the molecular constituents required for life do not degrade rapidly. The solvent’s temperature range is critical here, as it influences the rate of chemical reactions.
4. Solvent Chemical Functionality: It is desirable for the solvent to participate directly in biochemical reactions, akin to water's role in hydrolysis and redox reactions. This feature, while beneficial, is not viewed as critical as the other criteria.

Evaluation of Candidate Solvents

The assessment covers multiple candidate solvents, contrasting their merits against the outlined framework. Noteworthy among these are:

Protonating Solvents: Only protonating solvents such as water and concentrated sulfuric acid meet all the chemical requirements essential for life, and their abundance is realistic given certain planetary conditions. Concentrated sulfuric acid, in particular, surprises as a viable candidate due to its prevalence on Venus, potentially offering a medium for exotic life forms.

Non-Protonating Solvents: Liquid carbon dioxide emerges as an intriguing possibility amongst non-protonating solvents. Although it fulfills solvation and solute stability criteria, its absence of chemical functionality presents a significant hurdle. The study suggests further exploration into its capacity for supporting novel biochemical processes.

Implications and Future Research Directions

The implications of this research are profound for astrobiology, as they broaden the conceptual spectrum for life's potential habitats beyond the confines of Earth-centric models. The paper raises the possibility of life adapting to different solvents over geological time, promoting the exploration of solvent replacement scenarios should environmental conditions drastically change.

For future research, the framework posits an opportunity to prioritize both theoretical calculations and empirical experimentation on these proposed solvents to elucidate their feasibility further. Specifically, the potential for life to utilize sulfuric acid or carbon dioxide under varying planetary scenarios warrants deeper investigation, as does the exploration of other solvent mixtures which might exhibit novel chemical properties not present in solitary solvents.

Conclusion

This research establishes a novel evaluative framework that effectively expands the scope of solvents considered for supporting life. In doing so, it challenges preconceived notions tied largely to water and compellingly advocates for a more comprehensive approach towards understanding life's potential chemical diversity. The methodological rigor of this work provides a solid foundation for ensuing studies and underscores the potential for revolutionary revelations about life beyond our Earth-centric conceptions.