The Search for Slow Sulfur Sinks

Abstract

Earth's earliest epochs are shrouded by billions of years of planetary and biological evolution. As a result, many questions surround the origins of life, ranging from what surface conditions prevailed to where and how key prebiotic precursors formed and combined to give rise to life as we know it. Stanley Miller and Harold Urey performed some of the first laboratory explorations of those questions with their spark discharge experiments (Miller, 1953, 1955; Miller & Urey, 1959). Six decades on, researchers are describing plausible mechanisms that can form the building blocks of life (e.g., Becker et al., 2019) from molecules anticipated to exist in a prebiotic atmosphere (Cleaves et al., 2008) and ocean (Rimmer & Shorttle, 2019). These advances are all important components of the long voyage toward discovering how life originated on Earth. And while the ultimate destination is set, the route is not fully mapped, nor has the ship been fully assembled.

Part of that missing map is due to the limits in our understanding of sulfur, as is discussed by Ranjan et al. Sulfur is thought to play critical roles in prebiotic chemistry, volatile cycling, and climate in large part due to its ability to gain and lose electrons and participate in chemistry as a gas, dissolved in liquids, or as a solid. Sulfur aerosols, formed from either sulfuric acid (H₂SO₄) or elemental sulfur (S_n), contribute to the present-day climates of both Venus (e.g., Taylor & Grinspoon, 2009) and Earth (e.g., Storelvmo et al., 2016), and substantial hazes may have appeared at multiple points in Earth's (e.g., Kasting et al., 1989) and Mars' (e.g., Tian et al., 2010) histories.

The work of Ranjan et al. helps to draw a more complete map and furnishing a seaworthy vessel in three ways. First, their efforts are advancing our understanding of the earliest part of Earth's history. Because of the potential effect sulfate and elemental sulfur have on the climate of the early Earth, as well as the secondary effect that those species have on other sulfur-bearing molecules in the atmosphere (e.g., Kasting et al., 1989), understanding how sulfite reacts is an important region of the molecular map. Sulfate is also critical in several proposed pathways to form prebiotic molecules (e.g., Becker et al., 2019), making it a vital component of the vessel traversing the abiotic to biotic seascape.

Second, the experiments described are neither quick nor easy to perform, which matters when many of the remaining problems are difficult to accomplish on “graduate student lifetime” timescales. Of all the atoms that stock the prebiotic shipyard, including carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur, sulfur is perhaps the thorniest. Given its moderate electronegativity, sulfur can (and does) form numerous distinct molecules with other species, but this is also a double-edged sword. Sulfur compounds tend to be more reactive and unstable, posing hazards to lab personnel and equipment (e.g., Raab & Feldmann, 2019). Additionally, much like carbon, sulfur can form numerous allotropes, but molecular sulfur species are notorious for clogging instrumentation and gumming up experiments. All these effects together mean that sulfur reactions are some of the most poorly constrained (if at all), and given sulfur's chemical flexibility, that's a large fraction of the prebiotic chemical toolbox that is inaccessible or unknown. The experiments of Ranjan et al. in particular probe slow reactions, which are competing with other processes that are orders of magnitude faster, but are a necessary component if we are to understand the environment in which life arose on the early Earth.

Lastly, Ranjan et al.’s results and their evaluations are presented in a clear, compelling, and forthright fashion, representing a textbook example of the scientific method in action. Their work should be held up as a demonstration for not just enterprising young scientists but to those of us who occupy more than one discipline or that struggle to communicate with a new audience. Laying out the reasons why an experiment should be performed, developing a proof-of-concept to demonstrate the desired outcome, and then coming to terms with what did and didn't work so that the next experiment (whether yours or someone else's) showcases praise-worthy professionalism.

Notably, persistent issues with maintaining anoxic conditions prevented the experiments from determining the rate of sulfur disproportionation, but the long-term constraints provided by these new data strongly suggest this process is indeed slow. As a result, we have a better idea of where we might expect the disproportionation rate to fall and where the major experimental hurdles exist in moving from order-of-magnitude estimates to clear constraints. Furthermore, the broader implication of these constraints will anchor ongoing laboratory prebiotic chemistry experiments to plausible physical conditions for the early Earth. Finally, the consideration of photolysis as dominating over disproportionation over a large fraction of the parameter space highlights further laboratory studies that will be needed to inform our understanding of the possible climate and atmospheric states on the early Earth.

The authors declare no conflicts of interest relevant to this study.