Living Creatures Need Water. But Could They Make Do With Sulfuric Acid?

New results cast doubt on the presence of life in the Venusian atmosphere.

The Danakil Depression in northwest Ethiopia has the sulfuric acid-richest environments on Earth. (Achilli Family | Journeys / flickr)
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One of the big questions in astrobiology is whether life can use solvents other than water to facilitate biochemistry. We know how critical water is for life on our planet, and how all biological activity—starting with processes in the cells of microbes, plants, and animals—depends on the availability of water.

Yet water as a solvent also has drawbacks. Especially problematic is how life could originate in water, because it makes organic synthesis more difficult and requires energy. However, because liquid H2O is so abundant on our planet, life found a way to work around its more challenging properties. And because it’s also abundant in the Universe, most life elsewhere would also be expected to find water a convenient solvent.

Yet the intriguing question remains: Could other liquids work as well? In a recent paper, William Bains and his colleagues at MIT looked into the possibility of concentrated sulfuric acid as a potential solvent for life. That substance, as most high school chemistry students are well aware, is known for its ability to dissolve organic compounds. I had my own personal experience with that when I accidently splashed some on my legs when I was a student doing lab work. Although I ran right away to the emergency shower, I saw white clumps—proteins—forming on my legs due to the instant reactivity of the acid with my skin.

For their study, Bains and his colleagues modeled the stability of a wide range of organic compounds exposed to sulfuric acid, in concentrations similar to what we think exists in the Venusian clouds. Their results were sobering. Two-thirds to three-quarters of terrestrial biochemicals are unstable at any altitude in the Venusian clouds, with half-lives of less than a second! In other words, they start to dissolve instantly. Only synthetic chemicals and certain silicon compounds were found to be relatively stable. This means that the chemical compounds thought to be instrumental in the origin of life on Earth would not work in concentrated sulfuric acid.

Bains and colleagues also evaluated the possibility, which I think more likely, that microbes might have adapted from using a water-based solvent to using sulfuric acid as environmental conditions changed on Venus. But the researchers couldn’t see a way for this to happen—not when Venusian microbes would have had to overhaul more than two-thirds of their metabolic processes, which seems an enormous evolutionary hurdle, even given a time span of several hundred million years.

I should mention that some of the authors of this paper also were authors on a controversial paper suggesting the presence of phosphine in the Venus atmosphere, which got a lot of attention in the press. I think the approach taken in this new study has to be praised. They came up with a hypothesis, evaluated the ramifications, and concluded that it’s hard to envision Venusian life using sulfuric acid as a solvent. That finding, no question, undermines their earlier speculation about possible life in the lower Venusian atmosphere. Nevertheless, the team published their results, which is a great example of how science should work. We have to follow the evidence no matter where it leads, and whether we like the results or not.

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