Paris (AFP)
Researchers have established the "apparent presence" in the cloud layers of Venus of a gas which on Earth is associated with life, a discovery described by the NASA chief as "the most important event" in the search for life extraterrestrial.
The discovery, led by an astronomer at Cardiff University in the UK, was published Monday in the journal Nature Astronomy.
This is the first time that this compound has been discovered in one of the four terrestrial planets of our solar system, "the Earth apart," Jane S. Greaves, professor of astronomy and main author of the article.
Phosphine was detected by observing the Venusian atmosphere using two radio telescopes.
It "could come from unknown processes of photochemistry or geochemistry, or, by analogy with the biological production of phosphine on Earth, thanks to the presence of life", explains the study.
This compound is found in the giant gas planets of the solar system, such as Saturn, but it is not of biological origin, that is to say of living.
The traces of phosphine present in the Earth's atmosphere, on the other hand, come exclusively from human or microbial activity.
"Of life on Venus? The discovery of phosphine, a by-product of anaerobic biology, is the most important event to date in the search for life outside Earth," said Jim Bridenstine, administrator of The NASA.
"It's time to prioritize Venus," he said, as past life research missions are now focused on Mars, beset by American and other probes and robots.
For Professor Alan Duffy, an astronomer at Swinburne University in Australia, the discovery is "one of the most exciting signs I have ever seen of the possible presence of life outside of Earth."
The presence of phosphine, a highly toxic compound, does not mismatch in the hellish atmosphere of the second planet closest to the sun.
Also known as the Shepherd's Star, its atmosphere of carbon dioxide, at 97%, bathes in a surface temperature of around 470 ° C with a pressure more than 90 times greater than ours.
But it is in the thick layer of hyper acidic clouds, covering the planet up to around 60 km of altitude, that the team of Jane Greaves supposes that the molecules of phosphine can be found.
"There the clouds are + temperate + around 30 degrees Celsius", according to the study, which does not exclude that the gas forms at a lower and warmer altitude before rising.
- "Venusian clouds" -
But where does it come from?
Professor Greaves "hopes to have taken into account all the processes likely to explain its presence in the atmosphere of Venus".
Unless you identify a new one, there remains the hypothesis of a form of life.
If so, "we think (this life form) should be small, to float freely," says the scientist, whose study "insists that the detection of phosphine is not. a robust proof of life, only of an abnormal and unexplained chemistry ".
Phosphine is made up of one atom of phosphorus and three of hydrogen.
Phosphorus is one of the six chemical elements of living things, but "even if a planet contained an abundance of phosphorus, it might lack another necessary condition for life, such as other elements, or its environment might be too hot, or too dry, ”warns Jane Greaves.
A priori, the atmosphere of Venus, "extremely dehydrating and hyper acidic", is not conducive to life.
But maybe its cloud layer could be.
NASA also discovered a decade ago microbial life in the upper layers of the Earth's atmosphere.
This is why Prof. Greaves and her colleagues are calling for further observation of the phenomenon.
Ideally free from the "filter" of the Earth's atmosphere, thanks to a space telescope.
And why not with a new visit, by probe.
The renewed interest in this long neglected planet is general: two of the four missions competing for the next exploration program of the NASA solar system concern Venus, recalled Thomas Zurbuchen, scientific director of the agency.
The selection will take place next year.
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