life
Communication
The Case (or Not) for Life in the Venusian Clouds
Dirk Schulze-Makuch 1,2,3,4
Citation: Schulze-Makuch, D. The
Case (or Not) for Life in the Venusian
Clouds. Life 2021,11, 255. https://
doi.org/10.3390/life11030255
Academic Editor: Jay Nadeau
Received: 7 March 2021
Accepted: 17 March 2021
Published: 20 March 2021
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1
Astrobiology Research Group, Center for Astronomy and Astrophysics (ZAA), Technische Universität Berlin,
2German Research Centre for Geosciences (GFZ), Section Geomicrobiology, 14473 Potsdam, Germany
3
Department of Experimental Limnology, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, (IGB),
12587 Stechlin, Germany
4School of the Environment, Washington State University, Pullman, WA 99163, USA
Abstract:
The possible detection of the biomarker of phosphine as reported by Greaves et al. in
the Venusian atmosphere stirred much excitement in the astrobiology community. While many in
the community are adamant that the environmental conditions in the Venusian atmosphere are too
extreme for life to exist, others point to the claimed detection of a convincing biomarker, the conjecture
that early Venus was doubtlessly habitable, and any Venusian life might have adapted by natural
selection to the harsh conditions in the Venusian clouds after the surface became uninhabitable. Here,
I first briefly characterize the environmental conditions in the lower Venusian atmosphere and outline
what challenges a biosphere would face to thrive there, and how some of these obstacles for life could
possibly have been overcome. Then, I discuss the significance of the possible detection of phosphine
and what it means (and does not mean) and provide an assessment on whether life may exist in the
temperate cloud layer of the Venusian atmosphere or not.
Keywords: Venus; life; atmosphere; clouds; extreme environment
1. Introduction
Ever since Morowitz and Sagan [
1
] suggested that life may exist in the lower Venu-
sian atmosphere, many authors looked into that possibility [
2
–
11
] and made incremental
progress understanding the environmental conditions in the Venusian atmosphere, and the
possibility of it being inhabited by microbial life. The presence of an aerial biosphere may
seem strange to an observer from Earth, because on Earth, the atmosphere appears to serve
only as a temporary habitat [
12
], as reproduction has not been demonstrated in Earth’s
atmosphere [
13
]. This assessment is further supported by a study from Amato et al. [
14
],
which indicated the lack of evidence for bacterial cell division in the atmosphere through
meta-transcriptomics analysis. However, an earlier field study seemed to indicate that
bacteria can grow and reproduce in cloud droplets [
15
] and earlier laboratory studies indi-
cated that limited cell divisions of the facultative anaerobic bacterium Serratia marcescens
occurred in an airborne state [
16
,
17
]. It would not be surprising if Earth’s atmosphere is not
utilized as a permanent habitat, because the environmental conditions on Earth’s surface
are well-suited for life and any microorganisms in the atmosphere would be deposited back
on the surface by precipitation, usually within a few days [
18
]. The situation is different
on Venus.
Whether there is or was life on Venus is a highly speculative assertion, simply because
we do not have much data from Venus and its atmosphere despite many missions to this
planet, especially during the Soviet era. Our understanding of Venus as an astrobiological
target did incrementally increase during the last decades, including a constant trickle
of papers, e.g., References [
1
,
3
–
11
], but only with the claimed detection of phosphine in
the Venusian atmosphere by Greaves et al. [
2
], pointing to the possibility of life, did that
question gain much attention, both in the scientific community and public discourse. A
flurry of papers has been produced since that announcement, so it seems timely to take
Life 2021,11, 255. https://doi.org/10.3390/life11030255 https://www.mdpi.com/journal/life
Life 2021,11, 255 2 of 11
a first stock of where we are standing and what should be the next steps for Venusian
research, about 7 months after the initial announcement.
2. The Environmental Conditions in the Venusian Clouds
In early Solar System history, Venus was in its habitable zone, and oceans likely existed
on the Venusian surface [
19
], as they did on Earth and likely Mars [
20
]. In fact, if life did not
originate on Venus independently under roughly similar conditions as on Earth at that time,
it could have been transported from either Earth or Mars [
19
]. It is unknown how long the
Venusian surface remained habitable, while the Sun’s luminosity increased, but more recent
studies [
9
,
21
] suggest that Venusian oceans may have possibly lasted until as recently as
715 million years ago. It has been speculated that microbial life could have adapted to
the extreme environmental conditions in the Venusian atmosphere by natural selection as
the last possible (borderline) habitat remaining on the planet [
6
]. Researchers focused on
the lower cloud deck as a potential microbial habitat where temperatures range between
40 and 90
◦
C at a pressure of roughly 1 bar [
22
], consistent with a habitat for thermophilic
microbes on Earth. The cloud decks of Venus are also much larger in extent, providing
more continuous and stable environments than clouds on Earth [
7
] and contain particles
(aerosols) that are projected to last for several months in the Venusian atmosphere [
3
]. The
Venusian atmosphere is in slight thermodynamic disequilibrium as oxygenated chemical
species, such as SO
2
and O
2
, coexist with reducing species, such as H
2
S—and possibly
also PH
3
based on the latest possible detection [
2
]. However, these chemical compounds
are very heterogeneously distributed in the Venusian atmosphere [
23
–
25
]. In addition,
methane has been detected at a concentration of 980 ppm at an altitude of about 50 km by
the Pioneer-Venus probe [
26
]. That detection has been questioned [
26
], however, and no
plausible explanation for its presence has been advanced [
23
], but if it is real, it would be a
potential additional atmospheric biosignature.
Not all environmental parameters in the lower Venusian atmosphere are favorable
to life, however. Quite the contrary. One critical parameter is water activity, which can
be used as a measure of available water from a microbial perspective. Water activities
range between 0 and 1, with 1 being the water activity of pure water. Salt water has a
water activity slightly lower than 1, because some of the water molecules are inaccessible
for microbes due to the dissolved salts within the water. Honey typically has a water
activity too low for microbial consumption. Therefore, it does not spoil, even though it
is highly nutritious. In regard to atmospheres, water activity is equal to the equilibrium
relative humidity divided by 100 and is usually measured in the field as such. Determining
the water activity in the Venusian atmosphere, where no direct measurements can be
taken, is obviously much more challenging. Estimates of the concentration of sulfuric
acid in the Venusian clouds range between 75% and 96% [
27
], which should reduce water
activities to values below 0.1, clearly out of the range for the three domains of life on
Earth [
28
]. The water activity limit of life on Earth seems to be about 0.605 [
29
], which is
indisputably a high, perhaps impossible, barrier to reach within the Venusian atmosphere.
Two studies may shed some additional insights. In one study, lichen grew under Martian
radiation-sheltered niche conditions, despite water activity being most of the time below
the known threshold for life on Earth [
30
], meaning that it is sufficient if the threshold is
overcome during a fraction of exposure time. In another study, the water activity in a liquid
asphalt lake was measured to be 0.49, yet life thrived in it [
31
]. While the overall measured
water activity was below the threshold, a whole ecosystem of bacteria was discovered in
microdroplets of water within the oil matrix [32]—the point being that we have to look at
the correct scale and search for possible microenvironments.
Nevertheless, extrapolated water activities in the Venusian atmosphere are so low that
it is hard to believe that any Earth organism would come even close to mastering them.
It might be that we have to invoke a different biochemistry to overcome the constraint
imposed by water activity. Limaye et al. [
33
] pointed out that the abundance of sulfuric
acid at the Venusian aerosols, which were proposed to harbor life, is based on indirect
Life 2021,11, 255 3 of 11
measurements and computer simulations, and recent insights seem to indicate the presence
of chemical compounds other than sulfuric acid. After all, at the locality and scale where
it counts, sulfuric acid concentrations may be lower and water activities higher than is
usually assumed.
Still, it is difficult to see how a viable range, from an Earth biology perspective, could
be reached. In addition, the hyperacidity of the cloud environment with sulfuric acid as
an extremely aggressive chemical compound, makes possible life under these conditions
a hypothesis hard to defend. Seager et al. [
11
], for example, pointed out that crucial
biochemical compounds essential to life on Earth are unstable in sulfuric acid, which
includes the major building blocks of life such as carbohydrates, nucleic acids, and proteins.
Are we back to the notion that we have to invoke a different biochemistry and that life as
we know it simply cannot meet the challenge?
Clearly, these are hurdles not easy to overcome. Nevertheless, several authors tried
to come up with biological solutions to the challenges for a putative Venusian biosphere.
For example, Schulze-Makuch et al. [
6
] suggested that putative microbial cells might be
protected by elemental sulfur, thus being insulated from the sulfuric acid environment
(elemental sulfur also being critical for photosynthesis, see below). Petrova [
34
] added that
elemental sulfur is not wetted by sulfuric acid, and thus could provide protection from the
concentrated sulfuric acid environment with cells only adhering to sulfuric acid droplets
rather than being entirely enveloped [
11
]. Nutrient requirements could also be a problem
for putative microbial life in the Venusian atmosphere, especially trace metals (Figure 1).
However, it seems that phosphorus may be common, perhaps as common as sulfur in the
Venusian atmosphere, and there are examples on Earth of microbes that obtain all their
carbon and nitrogen needs from the atmosphere [
11
]. X-ray fluorescence measurements
by the Venera 13 and 14, and Vega 1 and 2 descent probes not only found sulfur, but also
phosphorus, chlorine, and iron—with up to as much phosphorus as sulfur in the lower
clouds below 52 km [
27
], most likely being in the form of phosphoric acid (H
3
PO
4
) [
35
].
Cockell [
4
] pointed out that, in terms of elemental requirements for life (e.g., C, N, P), the
lower clouds of Venus are attractive sites for biology, though available hydrogen may be
a problem.
Life 2021, 11, x FOR PEER REVIEW 3 of 11
sulfuric acid at the Venusian aerosols, which were proposed to harbor life, is based on
indirect measurements and computer simulations, and recent insights seem to indicate
the presence of chemical compounds other than sulfuric acid. After all, at the locality and
scale where it counts, sulfuric acid concentrations may be lower and water activities
higher than is usually assumed.
Still, it is difficult to see how a viable range, from an Earth biology perspective, could
be reached. In addition, the hyperacidity of the cloud environment with sulfuric acid as
an extremely aggressive chemical compound, makes possible life under these conditions
a hypothesis hard to defend. Seager et al. [11], for example, pointed out that crucial
biochemical compounds essential to life on Earth are unstable in sulfuric acid, which
includes the major building blocks of life such as carbohydrates, nucleic acids, and
proteins. Are we back to the notion that we have to invoke a different biochemistry and
that life as we know it simply cannot meet the challenge?
Clearly, these are hurdles not easy to overcome. Nevertheless, several authors tried
to come up with biological solutions to the challenges for a putative Venusian biosphere.
For example, Schulze-Makuch et al. [6] suggested that putative microbial cells might be
protected by elemental sulfur, thus being insulated from the sulfuric acid environment
(elemental sulfur also being critical for photosynthesis, see below). Petrova [34] added
that elemental sulfur is not wetted by sulfuric acid, and thus could provide protection
from the concentrated sulfuric acid environment with cells only adhering to sulfuric acid
droplets rather than being entirely enveloped [11]. Nutrient requirements could also be a
problem for putative microbial life in the Venusian atmosphere, especially trace metals
(Figure 1). However, it seems that phosphorus may be common, perhaps as common as
sulfur in the Venusian atmosphere, and there are examples on Earth of microbes that
obtain all their carbon and nitrogen needs from the atmosphere [11]. X-ray fluorescence
measurements by the Venera 13 and 14, and Vega 1 and 2 descent probes not only found
sulfur, but also phosphorus, chlorine, and iron—with up to as much phosphorus as sulfur
in the lower clouds below 52 km [27], most likely being in the form of phosphoric acid
(H3PO4) [35]. Cockell [4] pointed out that, in terms of elemental requirements for life (e.g.,
C, N, P), the lower clouds of Venus are attractive sites for biology, though available
hydrogen may be a problem.
Figure 1. Major arguments for and against life on Venus, and some scientific speculations as to
how some of the habitability challenges could be addressed by putative life in the Venusian cloud
layer. In addition to resolving the question of the presence of organic compounds and phosphine,
there are many other open questions regarding the Venusian environment, which would need to
be resolved. Image shows cloud structure and presence of UV absorber (black streaks) as revealed
by the Venus-Pioneer probe in 1979 (Credit: NASA).
Figure 1.
Major arguments for and against life on Venus, and some scientific speculations as to how some of the habitability
challenges could be addressed by putative life in the Venusian cloud layer. In addition to resolving the question of the
presence of organic compounds and phosphine, there are many other open questions regarding the Venusian environment,
which would need to be resolved. Image shows cloud structure and presence of UV absorber (black streaks) as revealed by
the Venus-Pioneer probe in 1979 (Credit: NASA).
Life 2021,11, 255 4 of 11
Clearly, the suggestion that the lower cloud deck is a habitat, and one that is hosting
life, is a difficult one to maintain, especially if we base our analyses on life as we know
it and the biochemical adaptations we are familiar with. Furthermore, microbes do not
live as individuals, but are part of a larger biosphere, which increases our challenges even
more, because we would have to propose a biosphere that can thrive in the Venusian
clouds despite their extreme environmental conditions. Nevertheless, hypotheses have
been put forward.
3. Proposed Adaptations of Microbial Life to the Venusian Cloud Environment
In the last decades of analog research on Earth, we learned much on how microbes are
able to use their environmental resources to an amazing degree, circumnavigate challenges,
or even co-opt in principle detrimental factors to their advantage [
36
,
37
]. One of these po-
tentially detrimental factors is UV irradiation, particularly in an atmospheric environment.
However, for putative life on Venus, UV irradiation may actually be an asset. The reason
is that the Venusian cloud layer contains large amounts of elemental sulfur, particularly
cycloocta sulfur (S
8
) [
38
,
39
] (see also Figure 2 in Reference [
6
]), which has the intriguing
property to adsorb UV irradiation and re-radiate it in the visible light spectrum.
The aerosols, also called mode 3 particles, which are present in the lower cloud deck of
Venus seem to be coated to a large degree by elemental sulfur based on spectral analyses [
40
].
It has been proposed that these aerosols could be microbes that use elemental sulfur to
power anaerobic photosynthesis reactions ([
6
], Equation (1)), a highly energetic pathway
that could be able to sustain a permanent microbial biosphere in the Venusian clouds. The
authors further elaborated that the sulfur that is oxidized during photosynthesis might later
be reduced by chemoautrophic microorganisms to close the nutrient cycle. Limaye et al. [
10
]
supported that notion by reporting that more than half of the UV irradiation that the
Venusian atmosphere receives is absorbed by an unknown mechanism and speculated that
this phenomenon could be the result of an energy capture process by an aerial biosphere.
The reason is that none of the abiotic explanations of the UV absorber (including S
8
) can
fully explain its abundance and spectrum. The possibility of anaerobic photosynthesis
as a main energy capture process would also be supported by the super-rotation of the
Venusian atmosphere, which cuts the nighttime significantly, allowing shorter periods
between light and dark [41].
2 H2S + CO2+ light →CH2O+H2O+S2(1)
Seager et al. [
11
] added and elaborated on the advanced hypothesis by Schulze-
Makuch et al. [
6
], pointing out that the coating of the cells would also have to include
hydrophilic filaments in addition to the elemental sulfur to allow the putative microorgan-
ism to uptake critical liquids. They further suggested a life cycle to address the problem of
microbial cells falling through the clouds towards the surface, where they would eventually
die and be permanently removed from the atmosphere. In their conceptual model, the
microbial cells would dry out during settling and become desiccated spores, which later
would be returned to the cloud layer by gravity waves. Once back in the cloud layer, they
would rehydrate by cloud condensation to complete the cycle.
That model would have the advantage that the loss rate of microbial cells would
be much lower compared to the hypothesized microbial ecosystem by Schulze-Makuch
et al. [
6
], in which the reproduction rate would have to compensate for the rate at which
the aerosols drop out of the cloud layer. The constraints on the required reproduction rate
to maintain a constant microbial population in the Venusian clouds would be less limited
in the Seager et al. model [
11
], but it would require that the proposed microorganisms can
form spores to counter the extreme environmental conditions when sinking into the lower
haze layer of the Venusian atmosphere.
Life 2021,11, 255 5 of 11
4. The Claimed Detection of Phosphine
Phosphine (PH
3
) was claimed to be detected in the Venusian atmosphere at a concen-
tration of about 20 parts per billion, using thorough spectral analysis and observations
from two different Earth-based telescopes in 2017 and 2019 [
2
]. More recently, the detection
has been corrected down to peak concentrations of 5–10 ppb and a global concentration of
1–4 ppb by the authors from the original paper [
42
]. However, concerns about the spectral
analysis [
43
], in particular that the PH
3
detection could be misidentified sulfur dioxide,
have been raised [
44
–
46
], mostly because the absorption lines of PH
3
(266.94 GHz) and
SO
2
(~267.5 GHz) are close. The original authors continued to insist that their analytical
methods are correct, and that the phosphine detection is valid [
47
]. SO
2
is the third most
abundant gas in the lower atmosphere of Venus and usually occurs at least in the ppm
range, but the Greaves et al. [
2
] study claimed it was below their detection threshold and
interpreted the identified spectral peak to be PH
3
. Rimmer et al. [
48
] suggested that the
lack of SO
2
is due to its dissolution in the clouds because of the presence of hydroxide
salts. However, this does not necessarily verify the PH
3
detection, and no resolution of the
scientific controversy about the validity of the detection is to be expected any time soon. In
principle, more than enough phosphorus should be present (in oxidized form) within the
Venusian atmosphere to explain the reported detection of PH
3
[
49
]. From a viewpoint of P
as purely a nutritional requirement, enough phosphorus should be present to allow the
presence of microbial life [
50
], but certainly P availability is only one of many constraints
for life.
Sousa-Silva et al. [
51
] argued for PH
3
being a suitable biosignature for an oxidized
atmosphere such as Venus, because of the apparent lack of “abiotic” false positives. They
also claimed that phosphine has uniquely identifiable spectral features in the infrared range.
However, this is not the spectral range Greaves et al. [
2
] used to detect phosphine. As the
phosphine detection has been marred in controversy [42–48], an independent verification
of the claimed PH
3
detection is warranted. Especially insightful would be if a different
methodology could be employed, such as trying to detect phosphine in the infrared range
of the spectrum. An independent confirmation could also derive from a re-analysis of the
Large Probe Neutral Mass Spectrometer (LNMS) on board of the Pioneer-Venus mission,
as done by Mogul et al. [
52
]. In their recently published paper, they claimed that their
analysis is suggestive of PH
3
and H
2
S being present in the middle clouds, based on a peak
fitting model that uses data points within the LNMS dataset to estimate the full-width
half-maximum and peak heights of chemical reference and target species. Their re-analysis
of the original data also suggested the presence of other reduced chemical compounds
such as carbon monoxide, ethane, and nitrogen species in various oxidation states, thus
pointing to more complex redox disequilibria than only from the presence of PH
3
and H
2
S.
Phosphine is a colorless gas, which is toxic to aerobic organisms including humans,
and on Earth it is associated with anaerobic life [
53
–
55
]. Exact processes involved are
unclear, but Bains et al. [
56
] suggested that phosphate-reducing bacteria could obtain
energy from the reduction of phosphate (HPO
42−
) to phosphite (HPO
32−
) by coupling
phosphate-reduction to NADH oxidation. The phosphine would then be produced by the
combined action of phosphate reducing and phosphite disproportionating bacteria.
The claimed detection of phosphine in the oxidizing Venusian atmosphere is surpris-
ing, because, if real, it would mean that there has to be a pathway that consistently produces
phosphine given its reactivity with other chemical compounds and its vulnerability of
being broken apart by UV irradiation. Greaves et al. [
2
] and Bains et al. [
57
] considered
a total of 74 potential abiotic phosphine production pathways, including gas- and cloud-
phase reactions, reactions with sulfur haze, and reduction of phosphate minerals at the
surface of the planet or as dust in the atmosphere. Their emphasis was on possible gas-
and cloud-phase reactions, which included reduction reactions of H
3
PO
4
to PH
3
, reduction
reactions of P
4
O
10
to PH
3
, reduction reactions of P
4
O
6
to PH
3
, disproportionation of H
3
PO
3
(in cloud droplets) and P
4
O
6
in the gas phase, and reduction reactions of H
3
PO
3
in the
droplets to PH3.
Life 2021,11, 255 6 of 11
Bains et al. [
57
] also looked at potential photochemical pathways, lightning as a poten-
tial source, subterranean sources such as volcanic outgassing, and asteroid or cometary
impacts, but none explained the amount of detected phosphine. After completion of their
investigation, they concluded that their analyses left either (A) an unknown geochemical
or photochemical pathway or (B) biology in the clouds of Venus as the only explanations.
Cockell et al. [
58
] reiterated that the low water activity makes the Venusian clouds unin-
habitable to known life, while Izenberg et al. [
59
] concluded that life on Venus cannot be
excluded as an option. They came up with a non-zero likelihood, to be exact between
10−8and 10%, depending on the underlying assumptions.
Regarding Option A, it seems fair to state that potential abiotic geochemical or photo-
chemical pathways are likely to exist, given the rich chemical endowment of Venus and the
possibility of many heterogeneous reactions. For example, Catling [
60
] suggested that PH
3
might be made via pathways from phosphorus trioxide (P
4
O
6
), which may go through
a phosphorous acid (H
3
PO
3
) intermediary. Specifically, he suggested that the following
reaction pathways may be involved:
P4O6+ 6H2O→PH3+ 3H3PO4(2)
or, similarly, with hydrochloric acid instead of water:
P4O6+ 6HCl →2 H3PO3+ 2PCl3(3)
From the reaction described in Equation (3), phosphorous acid (H
3
PO
3
) can decom-
pose into phosphoric acid (H
3
PO
4
) and phosphine (PH
3
). Bains [
61
] suggested that there
would be too little water at the prevailing temperatures in the atmosphere, given an esti-
mated relative humidity of 0.015%, for reaction (2) to be favorable. It would require about
200 kJ/mol under Venus atmospheric conditions. The feasibility of Equation (3) is difficult
to assess, because the PCl
3
concentration in the Venusian atmosphere is unknown. It has
to be realized that an amount of 1–4 ppb for phosphine is relatively low, so an abiotic
pathway seems certainly possible, or even likely. In that context, we also have to realize
that Venus is largely still an alien planet, and we do not know much about the chemistry
in its lower atmosphere. The majority of the missions to Venus took place during the
Soviet era (1960–1980s), and there is a lot of uncertainty regarding both atmospheric gas
compositions and abundances.
Regarding Option B, we have to acknowledge that life in the extreme environmental
conditions of the lower Venusian atmosphere seems to be extremely challenging. As
pointed out above, the hyperacidity is much stronger than in any natural environment on
Earth, and no organism on Earth is known to thrive at water activities estimated to exist
in the Venusian clouds. No organisms on Earth could withstand the acidity of an even
75% solution of sulfuric acid, which would reduce organic carbon compounds without any
protection into elemental carbon. Thus, in order for life to thrive at these conditions, either
some adaptation has to be proposed that has no analog in Earth’s biology or a different
biochemistry has to be involved.
The latter possibility seems unlikely as life probably originated under similar condi-
tions on Earth and Venus, or it might have been transported from Earth to Venus early
in Solar System history. Microbial life might have existed in early Venusian oceans, but
whether life could adapt to the current conditions is unclear. It depends to some degree on
the natural history of the planet and how fast the runaway greenhouse effect occurred on
our neighboring planet. If it was triggered by a cataclysmic event such as a huge asteroid
impact or sequence of several of such impacts (the retrograde rotation of Venus may still
be a consequence of such an encounter [
62
]), or by a global volcanic trigger [
21
], then
it would he highly doubtful that an early biosphere could have survived. However, if
there was enough time for natural selection to come up with better and better solutions to
cloud-based life and increasingly acidic conditions, then this may be a possibility.
Life 2021,11, 255 7 of 11
There are only a few hyper-acidic locations on Earth, such as the Dallol geothermal
area, and even this area is not as hyper-acidic and is much wetter than the Venusian
atmosphere [
63
], so there would be no reason for microbial life on our planet to evolve
the needed capability. But could life adapt to these hyper-acidic conditions? It would
have to be a multi-extremophilic microbe. Enough energy should be available through
photosynthesis to come up with potential energy-requiring biochemical pathways, but we
do not know how they would work. However, that does not mean they cannot exist.
There is one other intriguing detail about the new finding by Greaves et al. [
2
] that
deserves mentioning. The phosphine distribution was heterogeneous, it was detected near
the temperate latitudes, but not in the polar area. As the authors point out, this would be
consistent with a biological explanation (but certainly no proof), because the atmospheric
circulation patterns in the mid-latitudes would offer the most stable environment for life.
However, assuming the phosphine detection was real, the main problem with the
abiotic explanation (and perhaps in favor of a biological explanation) is that PH
3
is ex-
tremely difficult to produce in the oxidizing Venusian atmosphere because the strongest
natural reducing agent is H
2
. Biology, however, can use agents that are more reducing,
such as iron-sulfur proteins [
57
]. An iron-sulfur redox metabolism in the clouds of Venus
was also suggested by Limaye et al. [
10
]. Of course, that requires a significant amount of
extra energy, but again, photosynthesis as a metabolic pathway could easily provide the
needed energy.
5. Discussion and Next Steps
So where does this leave us? The claimed presence of phosphine in the oxidizing
atmosphere of Venus is just astounding, especially because the gas has not been detected
previously on any other terrestrial planet besides Earth. However, that does not prove the
presence of biology. There are many unknowns about our “twin planet”, which remains
largely alien to us. Many processes and chemical reactions that are likely occurring in the
Venusian atmosphere and also on the Venusian surface are not well understood. There
are many puzzles and open questions, no matter whether we adhere to an unknown
chemical pathway or biology as possible solutions to the observations made. For example,
if these observations are due to abiotic chemistry, how could PH
3
be continuously produced
in the oxidizing atmosphere of Venus? If they are due to biology, this is: how could life
permanently cope not only with the difficulties of being airborne, but also with hyperacidity,
extreme lack of water, and a possible lack of critical nutrients? These challenges to life are
very high, and thus Cockell et al. [
64
] argued that there is no good reason to entertain the
biological hypotheses. They advised that the original authors should not have evoked it,
but by doing so, Greaves et al. [
2
] caused a lot of media hype. However, as pointed out
above, Venus likely had oceans on its surface in the past [
9
] and if life did not originate
on Venus independently, it could have been transported from Earth into another habitable
environment on Venus [
13
]. Therefore, I consider it reasonable that life existed on Venus at
some point in the distant past, the more difficult hypothesis to defend is whether it could
have adapted to the currently existing environmental conditions in the lower atmosphere.
In my view, we have an anomaly in the sense of Cleland [
65
], that in principle could
be caused by biology, even if seemingly unlikely, and we need to further investigate
this “anomaly”.
The first step of the ensuing investigation should be to independently verify the detec-
tion of PH
3
. This is critical, because otherwise the possibility that SO
2
was misidentified
as PH
3
looms large. Aside from trying to detect phosphine in the infrared range and
by confirming it with LNMS mass spectra, I would suggest searching for diphosphine
(P
2
H
4
) in the Venusian atmosphere. P
2
H
4
is an intermediate in the photolysis reaction of
phosphine to phosphorus and hydrogen [
66
], and thus should be present in the Venusian
atmosphere if PH
3
is present. Thus, efforts should be undertaken to also search for P
2
H
4
in
Venusian spectra.
Life 2021,11, 255 8 of 11
We also have to think about possible mechanisms of how life could thrive in the
extremely challenging environmental conditions in the Venusian clouds. The efforts by
Schulze-Makuch et al. [
6
], Limaye et al. [
10
], and Seager et al. [
11
] are first steps toward
this goal. Life under hyper-arid conditions on Earth, analogous to Martian environments,
has resulted in amazing evolutionary adaptations, such as relying on deliquescence as
a sole source of water for microbes [
67
]. What mechanisms could be envisioned in the
Venusian clouds, especially on how to adapt to hyperacidity and the extreme lack of
liquid water? Experiments are conducted to assert how far microorganisms can adapt to
Martian environmental conditions, particularly high perchlorate concentrations [
68
], to
assess the feasibility of life on Mars. Analogously, experiments should be conducted on
how far selected acidophilic organisms can adapt from one generation to the next to higher
and higher sulfuric acid concentrations. Also, it would be interesting to find out how
microorganisms adapt to a lack of trace metals required for life as we know it. What are the
microbial needs and are there ways to compensate for a lack of say magnesium, manganese,
and even molybdenum? Are there biochemical ways we have not considered yet how
microbes might cope to low water-activity environments? Can we at least theoretically
envision how microbial life, despite the obvious challenges, could get its critical water in
the Venusian atmosphere? Our results could be that we have to invoke not only biochemical
pathways unknown from life as we know it, but an entirely new biochemistry. To do so
would become highly speculative, yet we also have to recognize our limitation of knowing
only one type of life (ours), even if it is incredibly diverse. But even from this limited
dataset, it is obvious that life is amazingly adaptive to environmental changes. It will be
one of the most exciting scientific endeavors to find out what these limitations of life are,
no matter whether life exists at Venus or not.
In the last decades, there have not been many missions to Venus as astrobiologists and
planetary scientists were focused on Mars and icy moons in the outer Solar System when
searching for extraterrestrial life. Our next-door neighbor does, however, represent an
intriguing target, especially since Venus used to be located in the habitable zone around our
Sun, and may have had oceans and possibly also living organisms on its surface, perhaps
for a very long time. The last possible outpost of life on Venus can only be in the temperate
cloud decks of the lower Venusian atmosphere—given the even more extreme conditions
on the surface and in the subsurface of that planet—and if it is there, it has to be in the
form of aerosol particles floating in the more benign cloud layer (which is still incredibly
extreme). A mission is overdue to analyze the interior of these aerosols to find out about
their composition (after an earlier attempt failed with NASA’s Pioneer-Venus mission when
the pyrolyzer jammed). If those aerosols contain organic compounds, a sample return
mission should be launched to bring these particles back for further analysis on Earth.
This can be done with a Stardust-type mission as suggested earlier [
5
], or with some other
mission architecture that involves, for example, balloons, tethers [
69
], or aerial platforms to
do the sample collection [
70
,
71
]. Whether Venus holds life or not, it is an intriguing planet
to investigate.
Funding:
Part of this Research was funded by the European Research Council Advanced Grant
“Habitability of Martian Environments” (HOME, no. 339231).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Date is contained within the article.
Acknowledgments:
I’m thankful to Toby Samuels from the University of Tübingen and four anony-
mous reviewers for their constructive comments, which significantly improved this paper.
Conflicts of Interest: The author declares no conflict of interest.
Life 2021,11, 255 9 of 11
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