Venus has clouds of concentrated sulfuric acid, but life could still survive

The surface of Venus is like a scene from Dante Hell – “Abandon all hope, you who enter here!” and so on. The temperature is hot enough to melt lead, the air pressure is nearly a hundred times that of Earth at sea level, and there are even rain clouds of sulfuric acid! But at about 48 to 60 km (30 to 37.3 miles) above the surface, temperatures are much cooler and air pressure is roughly equal to Earth’s at sea level. Therefore, scientists have speculated that life may exist above the cloud deck (possibly in the form of microbes) as on Earth.

Unfortunately, these clouds aren’t composed of water but concentrated sulfuric acid, making the likelihood that life could survive among them dubious. However, a new study by scientists at the Massachusetts Institute of Technology (MIT) reveals that the basic building blocks of life (nucleic acid building blocks) are stable in concentrated sulfuric acid. These findings indicate that Venus’s atmosphere could support the complex chemistry necessary for life to survive, which could have profound implications for the search for habitable planets and extraterrestrial life.

The study was led by Professor Sara Seager, an astrophysicist and planetary scientist in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS) and MIT’s Department of Aeronautical and Astronautical Engineering (MIT AeroAstro); and Dr. Janusz J. Petkowski, astrobiologist and EAPS MIT Research Affiliate. They were joined by researchers from Nanoplanet Consulting, Rufus Scientific Ltd., Worcester Polytechnic Institute (WPI) and several universities. The paper presenting their findings was recently published in Proceedings of the National Academy of Sciences.

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Before the Space Age and the first robotic missions began flying past Venus, astronomers harbored the idea that it might be Earth-like. Its impenetrable cloud deck was believed to be evidence of lingering rainstorms, and some have even speculated that the planet was a lush tropical environment. In 1961 and 62, the Soviet Venus 1 AND 2 and NASA sailor 1 AND 2 the probes became the first missions to buzz the planet and examine its atmosphere. These revealed an incredibly dense, hot and isothermal atmosphere, meaning there is little temperature variation between day and night and throughout the year.

As Dr. Petkowski told Universe Today via email, there are clouds of sulfuric acid with very low water content. “Therefore, if there is life in the clouds of Venus, and it still uses water as a solvent, it must have adaptations that are unprecedented here on Earth.” In an earlier paper, Seager and his coauthors explored various features of Venus’ clouds that would affect habitability, whether or not it was “Earth-like.” This included energy needs (chemical and solar), radiation, lack of available hydrogen, metals, acidity and low water content.

As Petkowski explained, they concluded that the conditions in the Venusian clouds do not rule out life, although they do rule out Earth-like life:

“This is an important distinction, as opinions about habitability and the possibility of life in Venusian clouds vary widely, from calling clouds ‘habitable’ to categorically stating that life cannot exist there. We decided to quantitatively review what we know about Venus and compare this knowledge with the basic principles of life, all of this to see if life on Venus is worth looking for (for example, missions focused on astrobiology), or if life could be categorically excluded”.

This is a key consideration for astrobiologists: the possibility that life could and does exist in environments radically different from Earth. This complicates matters as scientists can’t look for evidence of life that doesn’t follow the same patterns as life here on Earth. One possible way around this is to consider “what life does rather than what it is” (processes that do not depend on specific molecules) or constitute life at the most basic level. That was the focus of their recent paper, Petkowski said, in which they explored whether organic chemistry is possible in concentrated sulfuric acid.

“Here, we were able to demonstrate that the nucleic acid bases adenine, thymine, guanine, cytosine and uracil are stable in concentrated sulfuric acid with a concentration of 80 to 98% at room temperature. We confirmed the stability by UV spectroscopy and NMR spectroscopy and there was no detectable reactivity and degradation after incubation in concentrated sulfuric acid for up to 2 weeks. This result is significant because concentrated sulfuric acid is often thought to be sterile for any interesting organic chemistry. We set out to demonstrate that this is not the case. Our newspaper is the first step towards doing so”.

They note that organic chemistry is not the same as life, but no life can occur without it. So while this result does not indicate that life is present in Venus’s atmosphere, it does confirm that the organic processes necessary for life certainly can be. In recent years, the apparent detection of phosphine in Venus’ atmosphere has sparked a debate as to whether these chemical signatures (if confirmed) are the result of ongoing biological processes (i.e., a biosignature). This debate is still unresolved, and scientists look forward to future missions to investigate further.

These include the Deep Atmosphere Venus Investigation of Noble gas, Chemistry, and Imaging proposed by NASA (DAVINCI+) and the Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy (VERITAS) probes, both of which could explore Venus by 2028. China has also announced plans to send a Venus Volcano Imaging and Climate Explorer (VOICE) probe to explore the geological history, evolution and potential habitability of Earth’s “Sister Planet”, which will launch in 2026 and arrive by 2027. There are also proposals such as the Rocket Lab mission to Venus (scheduled for launch in January 2025), the first private mission to explore the clouds of Venus and look for signs of organic matter.

This will be followed by MIT’s Morning Star Missions to Venus program, a series of mission concepts that evolved from the Venus Life Finder Mission Concept study led by Professor Seager. This program will send a series of probes to Venus to study its cloud for signs of life and investigate habitability conditions. The program will culminate in 2041 with the launch of an Atmospheric and Cloud Sample Return Mission (ACSRM), which will return samples of Venus’ atmosphere to Earth for detailed analysis. As Petkowski noted, these and other Venus missions could have significant implications when it comes to the search for extraterrestrial life in the Solar System and beyond:

“Although the ribose phosphate backbone of DNA is unstable in concentrated sulfuric acid, other DNA-like polymers could be stable and potentially support the idea that concentrated sulfuric acid in the clouds of Venus can support life. This would be instead of water, concentrated sulfuric acid would become the primary liquid medium for life to thrive.

“It is currently unknown whether concentrated sulfuric acid could be the solvent for life, but if it could, then it would have huge implications for the habitability of Venus’ clouds and the habitability of other planets in the galaxy (exoplanets). There are very likely many other planets like Venus out there. Who knows? Perhaps many of them are also habitable for some very exotic life forms.”

Further reading: PNAS