Frontiers Physics World  October 2020

Signatures of life found in the clouds of Venus

Cloudy with a chance of life? Astronomers have discovered phosphine – an essential building block of RNA and DNA – in the atmosphere of Venus. (NASA/JPL-Caltech)

Astronomers have announced the detection of phosphine in the clouds of Venus. The gas, which on Earth is produced exclusively by microbes, is considered a strong signature of life on other worlds. The phosphine was observed by a team led by Jane Greaves of Cardiff University, UK, using the James Clerk Maxwell Telescope in Hawaii before following up with the Atacama Large Millimeter/submillimeter Array in Chile (Nature Astronomy 10.1038/s41550-020-1174-4).

Phosphine – a phosphorus atom bonded to three hydrogen atoms – is an essential building block of RNA and DNA. On Earth it is produced by anaerobic bacteria, which are microbes that do not require oxygen. They absorb phosphate minerals and combine them with hydrogen, releasing phosphine in the process. Importantly, phosphine is not produced by any known geological process, at least not on Earth. “In terms of the most distinctive biomarkers [in the solar system] where we can’t find a geological explanation, this is very strong,” Greaves told Physics World.

Venus’s dense atmosphere is almost entirely made of carbon dioxide, laced with clouds of sulphuric acid. It is possible that some unknown chemical reaction in these extreme conditions could be producing the phosphine, but one problem is the lack of hydrogen. In the outer solar system, Jupiter and Saturn are able to produce phosphine via a non-biological process. These planets are hydrogen rich, however, and with so much hydrogen available at the high temperatures and pressures deep within their interiors, it is a relatively straightforward process for them to produce phosphine, which can then be dredged into their upper atmosphere by convection currents.

Venus, on the other hand, has very little hydrogen, having lost it to space long ago, along with most of the planet’s water. Carbon-rich and without free hydrogen, it is therefore difficult to conceive of a non-biological process to create phosphine. “That doesn’t mean that the biological origin is the correct idea,” says Greaves. “It just means that we can’t find a really viable geological process.” NASA is considering a future mission to Venus that may be able to search for the presence of phosphine in the atmosphere.

Keith Cooper