Life forms on the planet Venus
The surprise discovery of a gas that could indicate the presence of life on Venus
has rekindled scientific interest in Earth's "twin" planet,
and researchers and space agencies around the world
are now competing to aim their instruments - both on Earth and in space - at the planet of clouds.
The news dates back to September 14 and is now in the public domain,
already commented on Insane Curiosity by another nice video that you can find here.
More than a month has passed since that day, and now we want to come back to the subject
to better specify the terms of the question, to tell you the reactions of the scientific world
(some of them very skeptical) and above all to answer the question that everyone
is now asking themselves, namely: okay, maybe there is microbial life on Venus,
but now what happens, how do we confirm it? Let's proceed in order and calmly
go through the whole story. It is the week from 9 to 16 June 2017.
We are on the Mauna Kea volcano in Hawaii, where some of the largest telescopes in the
world are crowded at an altitude of 4000 meters. The James Clerk Maxwell Telescope, a 15-meter
diameter radio telescope (the largest ever designed for the study of the universe through
submillimetric waves), is conducting surveys in the atmosphere of Venus, when incredibly the team
led by astrophysicist Jane Greaves, realizes that the spectrum is showing, despite much
background noise, a peak at 1.123 millimeters ... The reason of such wonder by astronomers is soon
said: to that wavelength is associated the presence of phosphine, that is a compound of
phosphorus and hydrogen that on Venus should not theoretically exist! Let's see why.
Phosphine is a molecule formed by one atom of phosphorus and three of hydrogen (PH3). Also
called hydrogen phosphide, it is a colorless gas, very unpleasant smell and highly toxic. On our
planet it is produced only synthetically and used in agriculture as a pesticide,
while in nature there are only very weak traces of it, almost entirely produced by
anaerobic bacteria (i.e. that do not need oxygen to live) in putrescent environments such as
marshes, animal intestines, guano, etc. ... It is known, in fact, that some terrestrial
bacteria are able to absorb phosphate minerals, add hydrogen and finally expel phosphine,
in a process that requires a lot of energy. The reason why they do this is not yet clear,
but it is assumed that it may be a waste product or a strategy to drive away rival microorganisms.
In other words: there are no known processes in nature that can produce phosphine
without the involvement of a large bacterial colony!
And this obviously explains the disbelief of the James Clerk Maxwell Telescope astronomers
in finding that specific and smelly molecule in the clouds of Venus!
A discovery made even more incredible by the measured amount: about 20 ppb (parts per billion),
a thousand times higher than that found in the Earth's atmosphere!
Rather high concentrations of phosphine had already been found elsewhere in the Solar System,
in particular in the atmosphere of Jupiter and Saturn, but under such extreme conditions of
pressure and temperature that it is quite unlikely to meet them on a rocky planet...
The fact is so sensational that Greaves has been forced to have all the doubts of the world. Have
they done something wrong? Is the background noise too loud? Are those measurements really reliable?
A confirmation is obviously necessary, and there is only one other instrument in the world with
a resolution that can confirm or deny the observations of the Hawaiian instrument:
ALMA, the radio telescope complex located in the Chilean Atacama desert. But it takes time... these
are instruments that have a very busy schedule and only almost two years later - on March 5,
2019 - 44 of ALMA's 66 antennas are able to target Venus. With a clear result:
the phosphine peak is confirmed, with the addition of other valuable information. ALMA reveals in
fact that the amount of phosphine decreases at the poles and reaches the maximum value (20 ppb) at
medium latitudes and at an altitude of about 60-70 km (where pressure and temperature are similar to
terrestrial ones and the acidity from sulphuric acid is modest compared to lower altitudes).
The resolving power of ALMA is also so great to be able to exclude that in Hawaii they
could make confusion with spectral lines very close to that of phosphine (such as that of
sulphur dioxide at 1.121 mm or that of deuterated water at 1.126 mm), and at this point there can be
no more doubts: in the atmosphere of Venus, at a height where temperatures resemble the
summer terrestrial ones, there is a huge excess of phosphine, but now we must somehow justify it!
That's why Jane Greves and colleagues, before writing for Nature the article that would make
public their revolutionary result, reviewed any known chemical or geological process
that could give reason for the unexpected presence of phosphine. Nothing is left out...
volcanic eruptions, which are still possible on Venus, electrical discharges in the atmosphere,
mineral outcrops on the surface, strange chemical processes in progress in the clouds... Nothing!
None of these phenomena can justify such an amount of phosphine.
The calculations of the authors show in fact that these sources, let's say "natural",
produce phosphine in a drastically lower quantity than the biological action. Someone has proposed
that they may be exogenous sources, such as the fall of meteorites from space. Yes because it
is well known that ferrous meteorites contain a particular type of phosphorus (sheibersite)
that can produce phosphine by contact with water. It is estimated that in this way they can produce
about 10 tons per year, which seems a lot, but in reality is a quantity of several orders
of magnitude lower than that found on Venus. Other researchers have advanced the hypothesis
that the solution of the mystery may come from the very high Venusian surface temperature
(about 400°C), capable of triggering one of the classic reactions for the formation of phosphine,
namely the thermal decomposition at about 300°C of phosphites, which are transformed into
phosphine + phosphates. On Earth, phosphates and phosphites are the main compounds of phosphorus
and are of volcanic origin. There is no reason to think that the past
eruptive activity of Venus has not enriched the surface of the planet with these salts.
But there is a problem: the Venusian environment is so oxidizing that the presence of phosphites
is almost completely excluded... and therefore this heat is also a hypothesis to be discarded.
At this point, to say it with Sherlock Holmes, "once all the impossible is excluded,
what remains, however improbable, can only be the truth"... So that the authors of the research,
having exhausted every possible chemical-natural explanation, at the end of their article could
not do without (scientists never like to expose themselves with too "pop" hypothesis) to mention
the possibility that the Venusian phosphine may be of bacterial origin; that is produced by
anaerobic microorganisms that were born on Venus when the surface was covered by oceans (up to 1-2
billion years ago) have progressively retreated to higher atmospheric altitudes when the water
disappeared due to the greenhouse effect triggered by the massive presence of carbon dioxide.
Although the region of the cloud cover at an altitude of 60 km is one of the least extreme
in terms of temperature, it remains very difficult due to the presence of an exceptionally acidic
and highly dehydrating environment. But on the other hand, these are conditions that organisms
similar to terrestrial extremophiles (living in exasperated conditions,
at temperatures higher than the boiling water or in an extremely acidic environment,
even without light or oxygen) could be able to cope with without problems.
"It's true - concludes Jane Greaves in the article on Nature - that the bacteria floating in the
clouds is a really extreme hypothesis, which can be confirmed or denied, in my opinion, only
at the end of this decade, when we will be able to fly in the clouds of Venus a kind of airship
able to take and analyze atmosphere samples". But maybe we won't have to wait that long.
Before seriously considering this possibility, astronomers will have to make sure that phosphine
is really present on Venus. Not everyone, in fact, agree on the data obtained by the Hawaii team,
and this is partly due to the fact that so far has been identified only one of the
spectral lines of phosphine (the 1.123 mm). The most immediate project, therefore,
will be to start other observations with other telescopes, such as the NASA Infrared Telescope
Facility in Hawaii; the other is the famous SOFIA, a NASA stratospheric observatory for
infrared astronomy mounted on a BOEING 747. Observations in the infrared and other parts
of the spectrum will allow to look for other absorption lines associated with phosphine,
providing a way to definitively verify its presence; and could also reveal
where phosphine is more concentrated and how its concentration may vary over time.
This is true for telescopes on Earth, but what about interplanetary probes?
Three missions are scheduled to fly close to Venus in the coming months:
the European and Japanese spacecraft BepiColombo , travelling to Mercury, the Solar Orbiter of
the European Space Agency and the solar probe Parker of NASA , both travelling to the Sun.
The observations produced by these ships would be very important because they would not be
limited by the Earth's atmosphere, but their instruments are designed for other purposes,
such as the study of the sun or the surface of Mercury, and therefore it is not clear
whether they have the right sensitivity to detect phosphine in the Venusian atmosphere.
There is also a spacecraft currently orbiting Venus: the Akatsuki mission
of Japan, which since 2015 is studying the meteorological phenomena and volcanism of
the planet. Although it lacks the necessary instrumentation to directly detect phosphine,
it could help in the study of the atmosphere. More promising will probably be the missions
still under development, those that would still be done in time to modify.
The Indian Space Research Organization, for example, is preparing an orbiter for Venus called
Shukrayaan-1, with the planned launch in 2025.
And the United States and Europe are also contemplating missions that could provide
useful data on the potential habitability of the planet - or even look directly for signs of life.
There are also several other missions that are still being studied, which could be modified for
this purpose. Two of them, the DAVINCI+ and the VERITAS, could arrive on Venus as early as 2026.
The best solution to explore the high atmosphere of Venus would be to send one or more balloons
able to resist for a while in the clouds of the planet. In practice, probe balloons
capable of analyzing the biomolecules of the atmosphere of Venus. Exactly what the Venus
Flagship mission provides, a proposal for the long-term study of Venus, its climate,
its geological activity and its atmosphere, which would make use of several elements (orbiter,
lander, balloons). A mission, however, that even if approved, will hardly materialize before 2030.
To close, we are one step away from a truly revolutionary and completely unexpected discovery,
which if confirmed would not only change our vision of the Solar System, but could even give
us a key to open the doors of extrasolar systems. If indeed no non-biological process was able to
produce significant concentrations of this gas, then the detection of phosphine in the atmosphere
of an extrasolar terrestrial planet could become an almost certain indication of the presence of
life at least on a microbial level. And all this is really magnificent.