Life on Venus? We joked!
Do you remember last September 14th? That day the scientific world was shaken
by really unexpected news. It seemed that in the cloud cover surrounding the planet Venus
a substance, phosphine, had been discovered, which here on Earth is produced only by processes
of biological origin. If confirmed, this would have led to an astonishing conclusion:
there is life on Venus! Microbial life, of course, but it would
have been truly epoch-making news... After decades of searching for a sign of alien life on Mars,
the planet most similar to the Earth in terms of environmental conditions, the miracle
happens in the infernal atmosphere of Venus! Is it possible? If it were true, that would be
proof that life can settle anywhere, even in the most hostile environments of the Solar System...
So there was great enthusiasm... but just a month and a half later, something began to go wrong...
What went wrong? Well, to understand it, it's better to go through the story very briefly first
(which is already covered in our video: "Life forms on Venus?").
On 14 September last, an international team of astronomers led by astrophysicist
Jane Greaves announced that they had detected traces of phosphine, a toxic gas composed of
phosphorous and hydrogen (PH3), in the Venusian atmosphere, in quantities a thousand times greater
than those found in the Earth's atmosphere. To succeed, the team used the observations
of two different radio telescopes: the James Clerk Maxwell Telescope in Hawaii
and the Atacama Large Millimeter / submillimeter Array in Chile, which showed a characteristic
absorption of light at a specific wavelength corresponding to that of phosphine.
On Earth, phosphine is produced almost entirely by microbes and only negligible traces come
from non-biological processes; and the amount of phosphine found on Venus is much greater than what
could theoretically be produced by non-biological mechanisms. Phosphine can be produced by
lightning or cosmic rays hitting the upper atmosphere, but these small amounts are destroyed
as quickly as they are created. In short, something on Venus is producing phosphine much
faster than it can be destroyed - ten thousand to one million times faster. What can it be?
In the article, published in the scientific journal Nature Astronomy,
the authors list the different geochemical and atmospheric mechanisms potentially responsible
for the presence of the molecule and discard them all one after the other, concluding in the end
that phosphine is most likely produced by microorganisms that have colonized
the more temperate part of Venus' atmosphere. The key to everything lies in that "most
likely"... Jane Greaves is convinced of the goodness of her observations,
but of course she also knows that the discovery must be confirmed by other researchers and other
data before being consigned to history. Planet scientist Carl Sagan, who was the
first to speculate about the possibility of microbial life on Venus back in the 1960s,
loved to say that "extraordinary claims demand extraordinary evidence". Extraordinary evidence
that in this case, as the authors themselves acknowledge, does not exist at the moment.
But one could also say that "extraordinary affirmations create extraordinary expectations"!
And, despite the enormous caution with which Greaves presents her discovery to the world,
the media immediately take over the news, fuelling the enthusiasm of millions of people
fascinated by the problem of life in the universe. But from dreams, one can wake up abruptly... And
in fact, as has often happened with other similar "great news", the detection of phosphine in the
atmosphere of Venus is soon questioned. In the past - you will remember - the same
fate had touched upon the hopes raised by the biological experiment of the Viking probes,
which seemed to have revealed the presence of bacteria in the sands of Mars; those of the
methane found in the atmosphere (also of Mars), which was believed to be of biological origin;
and those for the recurring signs of running water found on the surface...
All hopes then sadly dashed. ..Until the biggest of all, when in 1996 it was almost certain
that a fragment of the martian meteorite Allan Hills 84001, precipitated on Earth 13.000 years
ago, showed evident fossil traces of bacteria. There was a scientific dispute over the years...
Then most scientists rejected the Martian biological hypothesis, offering much evidence
of how similar structures could have formed without life, or how the meteorite could have
been contaminated by terrestrial bacteria. Of course, it was a terrible disappointment,
but that's the way science works. Someone makes a hypothesis,
says they have discovered something new, brings their evidence... and then other scientists check,
compare and come to a common judgment. This time, however, it seems that with
the phosphine affair things could go much better. A few days after the release of the article, in
fact, thanks to the possibility of researching the digital archives that preserve the observations of
previous years, it is possible to verify that clues of possible microbial life forms on Venus
were already present in the spectra filmed by the Pioneer Venus Multiprobe,
launched by NASA in 1978! In practice, data more than 40 years old show and confirm the
presence of phosphorus atoms in the clouds of the medium Venusian atmosphere, with spectral lines
very similar to those of phosphine! So it seemed that Jane Greaves's hypothesis
started to have important confirmations... but soon after, the bad news definitely took over
and in scientific circles it was immediately known that the discovery of phosphine was
questioned by three independent scientific analyses, still awaiting publication...
An initial team of scientists, led by Therese Encrenaz of the Paris Observatory, judged the
data from the Pioneer Venus Multiprobe to be too old and preferred to check those collected by a
telescope not used for initial observations, NASA's Infrared Telescope Facility in Hawaii.
And unfortunately, in these surveys, which date back to 2015, a phosphine level of up to a quarter
of that declared by the Greaves team is found! The absence of new evidence, however,
is not a denial of the initial work but rather an indication of a complex situation.
According to Clara Sousa-Silva, Jane Greaves' colleague, the instruments may not have
investigated deep enough in the clouds of Venus to find higher doses. In other words, those data
could refer to a wrong atmospheric band where phosphine is normally absent. But there may also
be other valid reasons for not detecting it, such as variability in time that is currently unknown.
Two other teams instead tried to rework the original data of the study, reviewing the
observations of the James Clerk Maxwell Telescope and ALMA telescopes; especially to understand if
the phosphine signal was correctly extracted from the background "noise" typical of such surveys.
A first study indicates that the James Clerk Maxwell Telescope detected a signal at the
right frequency, but that that same spectral line could also be traceable to sulfur dioxide,
abundant in the atmosphere of Venus. Another one reconsidered the data of ALMA
(Atacama Large Millimeter/submillimeter Array), a network of radio telescopes that produces
very high-resolution observations but which, due to its sensitivity, also captures a lot
of background "noise", especially from bright and close objects like Venus. In this case, to obtain
a clean signal, astronomers had to remove the radio noise produced by the Earth's atmosphere,
the brightness of the planet itself, and even the observatory equipment. To do this,
the researchers in the original study used a technique called “polynomial fitting”, in practice
a mathematical filter which in theory should be able to eliminate only the chaotic signal.
But other astronomers are skeptical about this type of processing.
According to them, to extract the phosphine signal from a messy data set, Greaves and colleagues
subtracted the background noise using a high-order polynomial, which means that more variables than
normal were used to model the data. Also, they would model the background noise by observing
portions of the spectrum outside the area where they expected to find a signal from phosphine,
a method that normally prevents unknown noise from obscuring a possible signal. However,
the combination of a high-order polynomial with a noisy data set makes it possible to
artificially create a false signal. Another analysis of ALMA data,
conducted by Ignas Snellen and colleagues from the Leiden Observatory, also failed to find any
signs of phosphine. Similarly, this group also reported that the higher-order polynomial fitting
could create spurious spectral lines. "You can always improve adaptation to a
data set by adding other variables, but you need to define a metric that tells you when to stop,"
says Meredith MacGregor, an astronomer at the University of Colorado at Boulder. "At some point,
you end up adapting noise signals and inflating signals that aren't real.
Greaves and his team refused to comment on the new analysis of ALMA's observations, at least until
the observatory has finished processing the data. But detecting a faint signal of a specific
molecule on another planet is a complex process, and the authors of the original
study are not surprised that other scientists are putting their work under the magnifying glass.
"It's normal. That's how science works. If phosphine could be seen by looking at this data
with the naked eye, it would have been discovered a long time ago," Clara Sousa-Silva said
very diplomatically. "I am relieved that someone else is finally looking at the data as well,
and that it is not just us who are putting our reputation on the line.
However, if researchers were also to reach a consensus that phosphine exists on Venus,
this would not automatically mean that there could be life on the planet. Phosphine is
certainly a potential biological signature, but always with a large margin of error.
Phosphine is produced on Earth by bacteria that live in wastewater, swamps, paddy fields
and animal intestines, but we know that it is also produced in some industrial applications
and on gaseous planets with extreme environmental conditions such as Saturn and Jupiter, where it is
thought that life cannot survive. As far as what is happening in the case of Venus is concerned,
we do not know enough about the planet to completely rule out the possibility
that some strange chemical process responsible for the production of phosphine,
even if it were to be there. The same is true for other potential
biological signatures that we have discovered in the Solar System. Methane, for example,
is produced by many types of bacteria on Earth, but is also vomited by volcanoes.
Breathable oxygen (in the form of O2) was created by the Earth's first cyanobacteria, but strange
reactions involving sunlight and a mineral called "titania" also produce it on other worlds...
In short, no single clue observed by the Earth can be concrete proof of life on distant worlds…
It would always be necessary to send automatic probes to check.
Yes, the only way to answer these questions is to go there.