- The ingredients for life are adrift in the Venusian clouds. Life is primarily composed of six chemical elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. These elements are found in a variety of biological molecules, including DNA and RNA. Probes, landers, and balloons have found signatures of these elements in the planet’s clouds, and they may be usable by life-forms.
- Some terrestrial organisms, known as extremophiles, can survive in conditions akin to the Venusian clouds. The surface of Venus is hellish, with temperatures as high as 890 °F (unlivable for all known terrestrial organisms) and crushing pressures reaching about 93 standard atmospheres (atm), equivalent to the pressure experienced at 3,000 feet below sea level on Earth. The clouds, however, are more hospitable, with temperatures ranging from 212 to -49 °F and pressures between 1.18 and 0.0336 atm. Here on Earth, microbial cells have been found dormant (not metabolically active but still alive) at temperatures as low as -49 °F. On the other end of the spectrum, some terrestrial archaea (single-cell organisms without nuclei) have been found to survive in temperatures as high as 251.6 °F
- In the ancient past, Venus may have been home to an ocean. Today, the clouds of Venus hold very little water vapor. However, climate models show that the planet’s surface likely boasted a shallow liquid water ocean for about 2 or 3 billion years after it formed and cooled. Any liquid water that may have existed is gone now, but its presence — a requirement for all known forms of life — means that life could have arisen on early Venus.
- A vast number of microorganisms are still waiting to be discovered. The clouds of Venus are composed primarily of sulfuric acid, with a caustic pH level of -1.5, so putative Venusian organisms would need to be polyextremophiles (able to thrive in multiple extremes) to survive these conditions. So far, a terrestrial polyextremophile that can survive in these conditions has not been found. But according to one estimate, around 1 trillion species of microorganisms live on Earth, and more than 99% have yet to be discovered. This means that, even though we haven’t found terrestrial life that could survive in the cloud conditions, its existence can’t be ruled out.
- Despite the harsh conditions, the metabolic processes necessary for life can still transpire on Venus. Iron- and sulfur-based metabolism, still used by some terrestrial microbes today, are some of the most ancient forms of chemosynthesis, the use of energy stemming from inorganic chemical reactions. The presence of sulfur and iron in the Venusian clouds reveals another potential pathway for life. Photosynthesis is also possible on Venus because enough solar energy in the photosynthetic wavelength range reaches the clouds.
- Scientists have already discovered possible signs of life on Venus. For example, methane was detected in the atmosphere of Venus in the 1990s. Its source is unknown; but on Earth, most methane has a biological origin: microbes known as methanogens. Methane can also be produced by natural processes that don’t involve life (i.e., abiotic). Phosphine, on the other hand, has no known abiotic sources, and in 2020, this gas was detected remotely in the clouds of Venus. However, this finding requires further study to confirm. And just because we haven’t discovered an abiotic source for phosphine doesn’t mean it doesn’t exist. There may be an unidentified form of chemistry taking place in the clouds of Venus. The only way to answer these questions is to go back. Three missions have recently been selected to return to Venus: NASA’s DAVINCI and VERITAS and the European Space Agency’s EnVision. These missions, and others proposed by international agencies, will contribute to our understanding of the habitability of Venus’s clouds
Jaime Cordova is a Ph.D. candidate in genetics who works in the Genome Evolution Laboratory. He studies the bacterial response to varying levels of oxygen and how those responses evolve. As a Solar System Ambassador for NASA and the Jet Propulsion Laboratory, he communicates to the public about the science of space exploration missions and discoveries.
This article was posted in Basic Science, Fall 2021, Front List and tagged astrobiology, extraterrestrial life, extremophile, Genetics, Jaime Cordova, methanogen, NASA, polyextremophile, Venus.