Scientists have been unable to find any signs of life on Venus despite claims the planet’s dense atmosphere could house living organisms.
Sulphur in the clouds of the second planet from the Sun was thought to be able to support life and act as a potential food source. But the unusual behavior of the sulphur in Venus’ atmosphere – that suddenly disappears in the outer layers – cannot be explained by an “aerial” form of extraterrestrial life, according to a new study.
Researchers at the University of Cambridge in the United Kingdom tested the “life in the clouds” hypothesis, which astronomers have speculated about for decades. They report life cannot explain the make-up of the Venusian atmosphere now why sulphur is “sucked out” of the air.
Any life form in sufficient abundance is expected to leave chemical fingerprints on a planet’s atmosphere as it consumes food and expels waste. But the scientists found no evidence of sulphur-eating life hiding in the clouds of Venus.
Scientific models looked at a particular feature of the Venusian atmosphere: the abundance of sulphur dioxide (SO2). On Venus, which is the brightest natural object in Earth’s night sky after the Moon, there are high levels of SO2 lower in the clouds. But it somehow gets “sucked out” of the atmosphere at higher altitudes, scientists explain.
On Earth, most SO2 in the atmosphere comes from volcanic emissions.
“If life is present, it must be affecting the atmospheric chemistry,” says study co-author Dr. Oliver Shorttle, from Cambridge’s Department of Earth Sciences and Institute of Astronomy, in a statement. “Could life be the reason that SO2 levels on Venus get reduced so much?”
Researchers used a combination of atmospheric and biochemical models to study the chemical reactions expected to occur, given the known sources of chemical energy in Venus’s atmosphere. The models included a list of metabolic reactions that the life forms would carry out in order to get their “food,” and the waste byproducts.
Scientists ran the model to see if the reduction in SO2 levels could be explained by these metabolic reactions. They found that the reactions can result in a drop in SO2 levels, but only by producing other molecules in very large amounts that are not seen.
The results set a hard limit on how much life could exist on Venus without blowing apart our understanding of how chemical reactions work in planetary atmospheres.
“We looked at the sulphur-based ‘food’ available in the Venusian atmosphere – it’s not anything you or I would want to eat, but it is the main available energy source,” says first author Sean Jordan from Cambridge’s Institute of Astronomy. “If that food is being consumed by life, we should see evidence of that through specific chemicals being lost and gained in the atmosphere.
“If life was responsible for the SO2 levels we see on Venus, it would also break everything we know about Venus’s atmospheric chemistry,” he continues. “We wanted life to be a potential explanation, but when we ran the models, it isn’t a viable solution. But if life isn’t responsible for what we see on Venus, it’s still a problem to be solved – there’s lots of strange chemistry to follow up on.”
Although there’s no evidence of sulphur-eating life hiding in the clouds of Venus, the researchers say their method of analyzing atmospheric signatures will be valuable when The James Webb Space Telescope (JWST) – the successor to the Hubble Telescope – begins returning images of other planetary systems later this year.
Some of the sulphur molecules in the current study are easy to see with JWST, so learning more about the chemical behavior of our next-door neighbor could help scientists figure out similar planets across the galaxy.
“To understand why some planets are alive, we need to understand why other planets are dead,” says Dr. Shorttle. “If life somehow managed to sneak into the Venusian clouds, it would totally change how we search for chemical signs of life on other planets.”
Even if Venus is devoid of life, the researchers suggest their results could be useful for studying the atmospheres of similar planets throughout the galaxy, and the eventual detection of life outside our solar system.
“We’ve spent the past two years trying to explain the weird sulphur chemistry we see in the clouds of Venus,” comments co-author Dr. Paul Rimmer, from Cambridge’s Department of Earth Sciences and Cavendish Laboratory. “Life is pretty good at weird chemistry, so we’ve been studying whether there’s a way to make life a potential explanation for what we see. Even if ‘our’ Venus is dead, it’s possible that Venus-like planets in other systems could host life.
“We can take what we’ve learned here and apply it to exoplanetary systems – this is just the beginning,” he adds.
The research was funded by the Simons Foundation and the Science and Technology Facilities Council (STFC), part of UK Research and Innovation (UKRI).
The study is published in the journal Nature Communications.
South West News Service writer Chris Dyer contributed to this report.