Evaluating Alternatives to Water as Solvents for Life: The Example of Sulfuric Acid

Abstract

The chemistry of life requires a solvent, which for life on Earth is water. Several alternative solvents have been suggested, but there is little quantitative analysis of their suitability as solvents for life. To support a novel (non-terrestrial) biochemistry, a solvent must be able to form a stable solution of a diverse set of small molecules and polymers, but must not dissolve all molecules. Here, we analyze the potential of concentrated sulfuric acid (CSA) as a solvent for biochemistry. As CSA is a highly effective solvent but a reactive substance, we focused our analysis on the stability of chemicals in sulfuric acid, using a model built from a database of kinetics of reaction of molecules with CSA. We consider the sulfuric acid clouds of Venus as a test case for this approach. The large majority of terrestrial biochemicals have half-lives of less than a second at any altitude in Venus’s clouds, but three sets of human-synthesized chemicals are more stable, with average half-lives of days to weeks at the conditions around 60 km altitude on Venus. We show that sufficient chemical structural and functional diversity may be available among those stable chemicals for life that uses concentrated sulfuric acid as a solvent to be plausible. However, analysis of meteoritic chemicals and possible abiotic synthetic paths suggests that postulated paths to the origin of life on Earth are unlikely to operate in CSA. We conclude that, contrary to expectation, sulfuric acid is an interesting candidate solvent for life, but further work is needed to identify a plausible route for life to originate in it.

Constraints on the Production of Phosphine by Venusian Volcanoes

“…When we attempt to account for these factors by running a full 1D photochemical model, we find a required PH 3 production rate of 26 kg/s, compared to 1 kg/s for [16] (to produce 1 ppb PH 3 ). We note that this probably under-estimates the necessary PH 3 production rate, as PH 3 is oxidized efficiently by liquid concentrated sulfuric acid [13], a sink not accounted for in any models for lack of kinetic data. This is not to discount the usefulness of such calculation; indeed, we executed a variant of it ourselves ( [11] and Section 3.2.4.1 in ref.…”

“…Ref. [11] shows that a biological source in the clouds is not ruled out by thermodynamics, but comment that Venus' clouds are an extremely challenging environment due to high acidity and very low water activity, some of which have been partially addressed in follow-up work [13][14][15].…”

Introducing the Venus Collection—Papers from the First Workshop on Habitability of the Cloud Layer

“…The presumed low water activity in the venusian clouds has been considered a challenge for life (Cockell et al, 2021), though Bains et al (2021a) argue that concentrated sulfuric acid is a possible solvent for life. The reported detection of phosphine by Greaves et al (2020c) and the suggestions that it may have biological origins prompted Hallsworth et al (2021) to consider the water activity from the scant measurements of water vapor abundances and assumed cloud droplet composition (dilute sulfuric acid).…”

Water–Sulfuric Acid Foam as a Possible Habitat for Hypothetical Microbial Community in the Cloud Layer of Venus

“…According to the data obtained, the clouds of Venus contain up to 75% sulfuric acid [2][3][4]17]. Under such extreme conditions, a strong selection for extreme acidophilic organisms might have occurred [23][24][25][26]. Prior to catastrophic climate change on Venus, all the types of extremophilic microorganisms would have occupied the appropriate ecological niches.…”