Many worlds in the Solar System have clouds. You find them on rocky planets like Earth and Venus as well as around the gas giants Jupiter and Saturn, and even on some of their moons. Clouds can tell us about the water activity of the planets below. New research has looked into which of these bodies may have the friendlier clouds for potential Earth-like life in terms of water content and temperature, and it appears that Jupiter is cozier than Venus.
Last September, researchers announced the possible detection of phosphine on Venus, a molecule that on Earth is associated with the decay of organic matter. Since then many have speculated if some extreme life-form might exist in the sulphuric acid clouds of Earth's hellish twin. More follow-ups are necessary to confirm if that detection is actually correct, and the many new missions to Venus recently announced will expand our understanding further.
However, the new paper, published in Nature Astronomy, looked at the water activity in the clouds of Venus and found that life on that planet is not plausible. By devising a way of determining the water activity of the atmospheres of various planets they found that Venus has a water limit well below what is needed for life to exist.
The researchers placed the water activity that allows metabolism and reproduction to take place on a scale from 1 to 0. The minimum that Earthly life requires is 0.585. The highest value estimated for the clouds of Venus is less than 0.004 at about 60 kilometers (37 miles) from ground level. That’s over 150 times lower. If there’s life in the clouds of Venus it will be extremely different to what we are used to on Earth.
The team also looked at the water activity in the clouds of Mars and found the value there is 0.537, still below habitability requirements but much closer than Venus. This is similar to the level found in Earth’s stratosphere.
The really big surprise appears to be Jupiter. The planet’s clouds maintain a water activity that could support Earth-like life forms across a wide range of temperatures and altitudes. Jupiter’s clouds tick two life-friendly boxes of water and temperatures but this doesn’t mean they have every ingredient for life to evolve and thrive among them.
“Our research shows that the sulphuric acid clouds in Venus have too little water for active life to exist, based on what we know of life on Earth," lead author Dr John Hallsworth from Queen’s University Belfast said in a statement sent to IFLScience. "We have also found that the conditions of water and temperature within Jupiter’s clouds could allow microbial-type life to subsist, assuming that other requirements such as nutrients are present.”
It is important to stress that when we talk about "life" we humans have only one very broad and complex example, so it is possible that alien life forms are radically different from what we might expect, requiring radically different environments.
However, “This is a timely finding given that NASA and the European Space Agency just announced three missions to Venus in the coming years," Dr Hallsworth added. "One of these will take measurements of Venus’s atmosphere that we will be able to compare with our finding.”
The team believes that this approach could be used also to assess the likelihood of conditions for life on exoplanets.