Last fall the announcement that phosphine was discovered in Venus's upper atmosphere stunned
geeks.1 This announcement generated over 4,700 news stories around the
2 The reason why is that many astronomers consider phosphine to be a signature for the
existence of life. Is the signature genuine? Have scientists discovered life on Venus?
Phosphine Discovery Claim
Phosphine is a gas. It is a phosphorus atom bonded to
three hydrogen atoms (PH3). On Earth, phosphine is emitted by certain microbes and as
fumigants from industrial sources.
Between 50 and 65 kilometers above Venus's surface, the atmospheric pressure and temperature
approximate those of Earth's surface. (It is not so on Venus's surface, where the average
temperature is 864°F.) It was at this height above
Venus's surface that a team of 19 astronomers led by Jane Greaves claimed to have detected
phosphine. The detection, according to Greaves' team, could be an indication that microbes exist in
Venus's upper atmosphere since nonbiological
routes to producing phosphine in Venus's upper atmosphere, though not impossible, are challenging.
Greaves and her colleagues used the James Clerk Maxwell Telescope (JCMT) in Hawaii to observe a
spectral line at millimeter wavelengths where both phosphine and sulfur dioxide (SO2)
absorb light. Follow-up observations with
the Atacama Large Millimeter Array (ALMA) led them to announce last September that they found
phosphine in Venus's upper atmosphere at an abundance level of 20 parts per billion.
Phosphine Discovery Rebuttals
I wrote about the possibility of life existing in the
upper clouds of Venus exactly one month before Greaves' team announced their discovery.3
I explained why, outside of Earth,
Venus's upper clouds provide the most "life-friendly" locale in the solar system. I also explained
how microbes from Earth have been transported to Venus's upper clouds by natural means. However,
Venus's upper atmosphere lacks the
resources to sustain microbial metabolism and offers no protection from deadly solar and cosmic
radiation. At best, all researchers can hope to find in Venus's upper atmosphere are the remains of
Earth life, remains that will be at
an extremely low abundance level.
When I first learned of the phosphine detection, I was skeptical. Phosphine abundance at 20 parts per
billion seemed too high. Furthermore, a detection level of 20 parts per billion based on just
one spectral line that is very close
to a sulfur dioxide line, in my opinion, made the detection marginal. Greaves' team came close to
acknowledging the marginal nature of their detection by noting in their paper that "other
PH3 spectral features should be
It is the nature of science that discoveries will be tested and reanalyzed—even by the same
researchers. The first published critique in the scientific literature of the claimed phosphine
discovery came from a team of 26 astronomers
led by Geronimo Villanueva.5 These astronomers independently calibrated and analyzed
Greaves' team's ALMA data using different interferometric analysis tools. In all cases, they
observed no evidence for phosphine.
A second response—on the same day—came from 10 astronomers that
included Greaves and was led by Thérèse Encrenaz. They published results
online of their search for possible spectral signatures
of phosphine at infrared wavelengths.
6 Their measurements established an upper limit (at a 99.7% confidence level) on the
abundance of phosphine in Venus's upper atmosphere of only 5 parts per billion. Clearly,
the earlier detection claim at 20 parts per
billion was, at best, a serious overestimate.
Third, in an online response to Villanueva, 10 of the 19 astronomers who were part of the first
Greaves team reported on their reanalysis of their data.7 Reanalysis of their ALMA data
suggested an average phosphine abundance
in Venus's upper atmosphere of 1–4 parts per billion. This measure is reconcilable with their
JCMT detection of 20 part per billion only if there is a factor of 10 temporal variation in Venus's
upper atmosphere phosphine abundance—a
highly unlikely outcome. In their conclusion, the ten astronomers wrote that their measured values
were "at the limit of observatory capabilities."8
In a fourth critique a month later, a team of five astronomers led by Ignas Snellen conducted an
independent reanalysis of Greaves' team's data.9 They processed the data using the same
methods employed by Greaves' team and
concluded that the published "ALMA data provide no statistical evidence for phosphine in the
atmosphere of Venus."
Fifth, in the February 1 issue of Astrophysical Journal Letters four astronomers led by Alex
Akins presented the most detailed reanalysis of the Greaves
11 First, they explained that even if phosphine were being released into Venus's
atmosphere, astronomers should not expect to measure any significant amount. Venus's known
atmospheric chemistry will rapidly photochemically
destroy any phosphine that is released.
Akins' team then re-reduced the ALMA observations of Greaves' team following both the initial and
revised calibration procedures used by Greaves and her colleagues. Though Akins' team did note a
spectral feature that could be a phosphine
spectral line in the narrowband data using the initial calibration procedures, they could not find
the spectral feature in the narrowband data using the revised calibration, nor could they find it in
the wideband data.
What to Make of Venusian Phosphine
These five published rebuttals to the "discovery"
of phosphine in Venus's upper atmosphere are all strong enough to conclude that phosphine has not
been discovered in Venus's upper atmosphere.
Thus, the solar system's most "life-friendly" location outside of Earth still measures to be
Yet, I do not believe that it is completely sterile. When large meteorites strike Earth, microbes on
Earth get ejected into interplanetary space. Many of these microbes get transported to Venus's upper
atmosphere. However, though the temperature
and pressure conditions 50–65 kilometers above Venus's surface are similar to those on Earth's
surface, the chemical conditions are not. The chemistry in Venus's upper atmosphere is hostile to
even extremophile (an organism living
in extreme conditions) microbial life. Hence, what will be present in Venus's upper atmosphere are
remains of microbes rather than viable microbes. These remains will result in
little, if any, biogenic phosphine in Venus's upper atmosphere—probably no more than 1 part
Given that microbes on Earth have been transported to Venus's upper atmosphere, it is inevitable that
chemical signatures of the remains of Earth's life will be found there. However, discovering
such signatures will be challenging.
It will take a new generation of advanced telescopes to reliably detect the chemical signatures of
life's remains at about 1 part per billion or less. When those detections are achieved, they will
testify to the abundance, diversity,
and longevity of life that God has created on Earth.
Featured image: Artist's Rendition of Venus Sans Atmosphere Based on Radar Reflection Data