An international team of astronomers announced the discovery of a rare molecule — phosphine — in the clouds of Venus. On Earth, this gas is only made industrially or by microbes that thrive in oxygen-free environments. Astronomers have speculated for decades that high clouds on Venus could offer a home for microbes — floating free of the scorching surface but needing to tolerate very high acidity. The detection of phosphine could point to such extra-terrestrial ‘aerial’ life.
“When we got the first hints of phosphine in Venus’s
spectrum, it was a shock!,” says team leader Jane Greaves of Cardiff University
in the UK, who first spotted signs of phosphine in observations from the James
Clerk Maxwell Telescope (JCMT), operated by the East Asian Observatory, in
Hawai’i. Confirming their discovery required using 45 antennas of the Atacama
Large Millimeter/submillimeter Array (ALMA) in Chile, a more sensitive
telescope in which the European Southern Observatory (ESO) is a partner. Both
facilities observed Venus at a wavelength of about 1 millimetre, much longer than
the human eye can see — only telescopes at high altitude can detect it
effectively.
The international team, which includes researchers
from the UK, US and Japan, estimates that phosphine exists in Venus’s clouds at
a small concentration, only about twenty molecules in every billion. Following
their observations, they ran calculations to see whether these amounts could
come from natural non-biological processes on the planet. Some ideas included
sunlight, minerals blown upwards from the surface, volcanoes, or lightning, but
none of these could make anywhere near enough of it. These non-biological
sources were found to make at most one ten thousandth of the amount of
phosphine that the telescopes saw.
To create the observed quantity of phosphine (which
consists of hydrogen and phosphorus) on Venus, terrestrial organisms would only
need to work at about 10% of their maximum productivity, according to the team.
Earth bacteria are known to make phosphine: they take up phosphate from
minerals or biological material, add hydrogen, and ultimately expel phosphine.
Any organisms on Venus will probably be very different to their Earth cousins,
but they too could be the source of phosphine in the atmosphere.
While the discovery of phosphine in Venus’s clouds
came as a surprise, the researchers are confident in their detection. “To our
great relief, the conditions were good at ALMA for follow-up observations while
Venus was at a suitable angle to Earth. Processing the data was tricky, though,
as ALMA isn’t usually looking for very subtle effects in very bright objects
like Venus,” says team member Anita Richards of the UK ALMA Regional Centre and
the University of Manchester. “In the end, we found that both observatories had
seen the same thing — faint absorption at the right wavelength to be phosphine
gas, where the molecules are backlit by the warmer clouds below,” adds Greaves,
who led the study published in Nature Astronomy.
Another team member, Clara Sousa Silva of the
Massachusetts Institute of Technology in the US, has investigated phosphine as
a “biosignature” gas of non-oxygen-using life on planets around other stars,
because normal chemistry makes so little of it. She comments: “Finding
phosphine on Venus was an unexpected bonus! The discovery raises many questions,
such as how any organisms could survive. On Earth, some microbes can cope with
up to about 5% of acid in their environment — but the clouds of Venus are
almost entirely made of acid.”
The team believes their discovery is significant
because they can rule out many alternative ways to make phosphine, but they
acknowledge that confirming the presence of “life” needs a lot more work.
Although the high clouds of Venus have temperatures up to a pleasant 30 degrees
Celsius, they are incredibly acidic — around 90% sulphuric acid — posing major
issues for any microbes trying to survive there.
ESO astronomer and ALMA European Operations Manager
Leonardo Testi, who did not participate in the new study, says: “The
non-biological production of phosphine on Venus is excluded by our current
understanding of phosphine chemistry in rocky planets’ atmospheres. Confirming
the existence of life on Venus’s atmosphere would be a major breakthrough for
astrobiology; thus, it is essential to follow-up on this exciting result with
theoretical and observational studies to exclude the possibility that phosphine
on rocky planets may also have a chemical origin different than on Earth.”
More observations of Venus and of rocky planets
outside our Solar System, including with ESO’s forthcoming Extremely Large
Telescope, may help gather clues on how phosphine can originate on them and
contribute to the search for signs of life beyond Earth.
Source: https://www.eso.org/public/news/eso2015/
Journal article:
https://www.nature.com/articles/s41550-020-1174-4
https://www.liebertpub.com/doi/10.1089/ast.2020.2244
https://www.liebertpub.com/doi/10.1089/ast.2018.1954
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