Scientists said on Monday they have detected in the harshly acidic clouds of Venus a gas called phosphine that indicates microbes may inhabit Earth's inhospitable neighbour, a tantalizing sign of potential life beyond Earth.
The researchers did not discover actual life forms but noted that on Earth phosphine is produced by bacteria thriving in oxygen-starved environments.
The international scientific team first spotted the phosphine using the James Clerk Maxwell Telescope in Hawaii and confirmed it using the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope in Chile.
"I was very surprised - stunned, in fact," said astronomer Jane Greaves of Cardiff University in Wales, lead author of the research published in the journal Nature Astronomy.
The existence of extraterrestrial life long has been one of the paramount questions of science. Scientists have used probes and telescopes to seek "biosignatures" - indirect signs of life - on other planets and moons in our solar system and beyond.
"With what we currently know of Venus, the most plausible explanation for phosphine, as fantastical as it might sound, is life," said Massachusetts Institute of Technology molecular astrophysicist and study co-author Clara Sousa-Silva.
Venus has not been the focus of the search for life elsewhere in the solar system, with Mars and other worlds getting more attention.
Phosphine - a phosphorus atom with three hydrogen atoms attached - is highly toxic to people.
This artistic impression depicts the planet Venus, where scientists have confirmed the detection of phosphine molecules /Reuters
Earth-based telescopes like those used in this research help scientists study chemistry and other characteristics of celestial objects.
Phosphine was seen at 20 parts-per-billion in the Venusian atmosphere, a trace concentration. Greaves said the researchers examined potential non-biological sources such as volcanism, meteorites, lightning and various types of chemical reactions, but none appeared viable. The research continues to either confirm the presence of life or find an alternative explanation.
Venus is Earth's closest planetary neighbour. Similar in structure but slightly smaller than Earth, it is the second planet from the sun. Earth is the third.
Venus is wrapped in a thick, toxic atmosphere that traps in heat. Surface temperatures reach a scorching 471°C, hot enough to melt lead.
The acid test
Some scientists have suspected that the Venusian high clouds, with mild temperatures around 30°C, could harbour aerial microbes that could endure extreme acidity. These clouds are around 90% sulphuric acid. Earth microbes could not survive that acidity.
On Earth, microorganisms in "anaerobic" environments - ecosystems that do not rely on oxygen - produce phosphine. These include sewage plants, swamps, rice fields, marshlands, lake sediments and the excrements and intestinal tracts of many animals. Phosphine also arises non-biologically in certain industrial settings.
To produce phosphine, Earth bacteria take up phosphate from minerals or biological material and add hydrogen.
"We have done our very best to explain this discovery without the need for a biological process. With our current knowledge of phosphine, and Venus, and geochemistry, we cannot explain the presence of phosphine in the clouds of Venus. That doesn't mean it is life. It just means that some exotic process is producing phosphine, and our understanding of Venus needs work," Sousa-Silva said.
Nasa Administrator Jim Bridenstine on Twitter called the new findings "the most significant development yet in building the case for life off Earth." Among missions that the US space agency is currently considering is one that would send an atmospheric probe to Venus. Source: Reuters