Astronomy Now - January 2021

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Phosphine-on-Venus discoverers fight back

hosphine is present in Venus’ atmosphere, but there’s less of it than initially thought, according

to the original discovery team led by Jane Greaves of the University of Cardiff.

In September 2020 it was announced to great fanfare that phosphine (PH3) had been detected in
Venus’ atmosphere, at an altitude of about 50–60 kilometres above the surface in a region where the
temperature and pressure is somewhat similar to that at Earth’s surface (see Is there life on Venus?, AN,
November 2020). On Earth, phosphine is produced only by anaerobic microbial life, and it had been
suggested by Clara Sousa-Silva of the Massachusetts Institute of Technology that phosphine could
act as a biosignature on other worlds. Given that there is no conceivable means of producing the
observed amounts of phosphine geochemically on Venus, thoughts turned towards a biological
origin.

However, the discovery was met with erce criticism (see News update, AN, December 2020) from
other scientists who argued that the data had been analysed incorrectly. Now Greaves’ team have hit
back at the arguments of an opposing team led by Geronimo Villanueva of NASA’s Goddard Space
Flight Center, who claimed that what Greaves and her colleagues had detected was not phosphine,
but the spectral line of sulphur dioxide (SO2), at an altitude of over 70 kilometres, higher than the
the atmospheric habitable zone.

Greaves’ team reject the assertion that they had mistaken the spectral lines. ey also point out that
the abundance of sulphur dioxide in Venus’ atmosphere is also not great enough to produce the
spectral signature that they see, and that the claim that the signal is coming from over 70 kilometres
high is in error.

However, it turns out that software at the Atacama Large Millimeter/submillimeter Array (ALMA)
in Chile, which Greaves’ team used along with the James Clerk Maxwell Telescope ( JCMT) in
Hawaii to observe the phosphine signature, had incorrectly calibrated the data that the discovery was
based upon. is data has now been recalibrated, and when Greaves’ team re-analysed it they found
that the abundance of phosphine is seven times less than at rst thought, averaging just one part per
billion, with a peak of ve parts per billion at most. is recalibrated ALMA data is now also at
odds with the JCMT results, which imply phosphine at 20 parts per billion. However, Greaves’ team
propose that there could be some variation with time and altitude, with greater abundances at 50
kilometres than at 60 kilometres.

Greaves and her colleagues have yet to respond to other critical analyses, led by Ignas Snellen of
Leiden University and Mark ompson of the University of Hertfordshire, who suggest that the
baseline of thermal noise in the data was incorrectly subtracted from the phosphine signal, and that
when they remove this baseline by different means, the signal disappears.

What is really required are further observations. Greaves and her team plan to re-observe Venus
with ALMA in 2021, and we await their ndings

Venus, imaged by Mariner 10 in 1974.

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Phosphine-on-Venus discoverers...

January 2021

Astronomy Now