[note: this post is an excerpt from a literature review paper on Venusian Astrobiology that I wrote last year.]
3. Extant Life in the Clouds
Data returned by Mariner II in 1962 drastically changed the picture of conditions on Venus, causing most to lose interest in it as a harbor for life. Nevertheless, within just a few years scientists were speculating that life
might still survive on Venus in the clouds.
Harold Morowitz and Carl Sagan (1967) published a brief article in
Nature containing a great deal of speculation about the nature of a life form
they imagined could survive in such an environment: an organism constructed as
a float bladder filled with molecular hydrogen for buoyancy. This macroorganism would collect water from
rain or by contact with droplets in the clouds, acquire nutrients from minerals
picked up from the surface by the powerful winds, and produce its own lifting
gas as a by-product of photosynthesis.
Given what they knew at the time, they claimed such life in the Venus clouds “can be envisaged which operates entirely on
known terrestrial principles.”
More realistic hypotheses involving cloud-borne
microorganisms have followed that are compatible with our current knowledge of
the Venusian atmosphere. These
hypotheses should be taken seriously in light of bacteria found actively
growing and reproducing—at temperatures below 0° C—in cloud droplets collected at high altitude on Earth (Sattler et
al. 2001).
3.1 Conditions in the
Clouds
Venus may be a terribly inhospitable place on or near its
surface, but the conditions at altitudes between 50 and 60 km are remarkably
Earth-like. The pressure is close to 1
bar, the temperature is in a range where water is liquid (0-100° C), there is abundant solar energy,
and the atmosphere contains the primary materials required for life: carbon,
oxygen, nitrogen, and hydrogen (Landis 2003).
Also present: sulfur, phosphorus, chlorine, fluorine, and iron
(Grinspoon and Bullock 2007).
3.1.1
Attributes that Favor
Life
In addition to the general conditions above, the following
attributes are favorable for supporting life in the clouds:
- Aqueous environment: It is certainly not abundant, but water vapor
concentrations approach a few hundred parts per million in the cloud layers
(Ingersoll 2007)
- Continuous clouds: the
clouds on Venus are much larger, more continuous, and more stable than those of
Earth, which provides an ongoing habitat for microorganisms (Schulze-Makuch et
al. 2004).
- Superrotation: The
clouds of Venus make a complete rotation about the planet once every 4-6 days
(van den Berg et al. 2006), providing a day-night cycle for life in the clouds
that is much shorter than the 117-day cycle experienced at the planet’s surface
(Ingersoll 2007). This enhances the
potential for photosynthetic reactions by reducing the duration of “night”
(Grinspoon and Bullock 2007).
- Atmosphere in disequilibrium: O2,
H2, H2S, and SO2 coexist, providing the basis
for energy-yielding redox reactions that could be harvested by microbial life (Schulze-Makuch
and Irwin 2002)
3.1.2 Challenges for cloud-hosted life
Ultraviolet (UV) radiation from the Sun presents a
challenge for life in the clouds of Venus.
UV is damaging to biological macromolecules, and any surviving organisms
must adapt to it in some fashion. Using
Earth-based organisms for reference, several examples are available: there are
organisms that use pigments such carotenoids and scytonemin for protection,
others grow beneath the safety of soil or water, and some make a shield from
organic compounds derived from dead cells.
A more elaborate example are microbes such as cyanobacteria that possess
internal mechanisms for repairing DNA and resynthesize UV-sensitive proteins
(Schulze-Makuch et al. 2004). Charles
Cockell (1999) points out that the UV flux in the upper clouds of Venus is
comparable to the surface flux on the Archean Earth, the time when life is
believed to have appeared.
The acidity of the clouds of Venus (pH=0) has been raised
as a possible obstacle to life (Cockell 1999).
Nevertheless, acidophile organisms have been found on Earth, such as Ferroplasma acidarmanus which thrives at
pH 0 (Schulze-Makuch et al. 2004), Picrophilus
oshimae, which showed optimal growth at pH 0.7, but still grew at pH 0
(Schleper et al. 1995), and the green alga Dunaliella acidophila which can
survive at Ph 0, but prefers pH 1 for maximum growth (Grinspoon and Bullock
2007).
3.2
Speculations on
potential life forms
Venus researchers have
proposed feasible forms that life might take to survive in the clouds. Wickramasinghe and Wickramasinghe (2008)
suggest that hydrogenogens, a group of terrestrial bacteria and archaea that
can grow anaerobically using CO as their sole carbon source, are good analogs
for cloud-borne organisms on Venus. They
note that the lightning present on Venus (mentioned in section 2.3) could generate
large amounts of CO from the predominantly CO2 atmosphere. They imagine a scenario occurring within the
three cloud layers of Venus where “(a) bacteria nucleate droplets containing
water and nutrients, (b) colonies grow within the droplets, (c) droplets fall
into regions of higher temperature where they evaporate releasing spores to
convect upwards to yield further nucleation.”
Dirk Schulze-Makuch, David Grinspoon, and
colleagues (2004) propose that microbial life forms, in response to the high
doses of ultraviolet radiation received in the upper atmosphere, could shroud
themselves in elemental sulfur, possibly a layer of cycloocta-sulfer (S8). It is a strong UV absorber, and Venusian organisms
could produce elemental sulfur via a simple photochemical reaction combining H2S and CO,
just as some organisms on Earth do.
In another paper co-authored by Schulze-Makuch
and Louis Irwin (2006), they proposed phototrophic organisms in the Venusian
atmosphere that could employ a photosystem based on the oxidation of sulfur, as
many terrestrial organisms thriving in warm seas and hot springs do.
3.3
Possible evidence for
life in the clouds
Is there any current evidence that could suggest the
existence of cloud-borne organisms on Venus?
There is more than one might think.
Of particular interest are the larger droplets or particles (referred to
as “mode 3” particles) found only in the lowest of Venus’ three cloud layers
(Grinspoon and Bullock 2007). They are
non-spherical (indicative of a solid core), and comparable in size to Earth
bacteria. Their composition is currently
unknown, but they could represent even small bacteria colonies.
The dark regions plainly visible on UV images of Venus are
caused by an unknown UV absorber. The
Venus Monitoring Camera aboard the Venus Express spacecraft took wide-angle
images at the characteristic wavelength of the UV absorber, and determined that
the brightness variation is the result of compositional differences, not
elevation differences (Titov et al. 2008).
Elemental sulfur in the form S8 is a strong UV absorber, and could be
the cause of the dark regions. It has
been proposed that the potential S8 in the Venusian clouds could be a byproduct
of microbiological processes (Schulze-Makuch and Irwin 2006).
Compounds positively identified in the Venusian atmosphere
could also indicate the presence of organisms.
The presence of oxygenated gases such as O2 and SO2, observed at the
same time with reduced gases such as H2S and H2, indicates the atmosphere is in
a state of disequilibrium. Some active
process is working to maintain this situation, and it may be biological (Landis
2003). The second-most common sulfur gas
in the Venusian atmosphere, Carbonyl
sulfide (COS), is considered a possible indicator for life since its sources on
Earth are almost entirely biological (Landis 2003; Schulze-Makuch and Irwin
2002).
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1487-1501.
Grinspoon, D. H., & Bullock, M. A. (2007). Astrobiology and venus
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Ingersoll, A. P. (2007). Venus: Express dispatches. Nature, 450(7170),
617-618.
Landis, G. A. (2003). Astrobiology-the case for venus. Journal of the
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Morowitz, H. (1967). Life in the clouds of venus? Nature, 215,
1259-1260.
Sattler, B., Puxbaum, H., & Psenner, R. (2001). Bacterial growth in
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Schleper, C., Puehler, G., Holz, I., Gambacorta, A., Janekovic, D.,
Santarius, U., . . . Zillig, W. (1995). Picrophilus gen. nov., fam. nov.: A
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Schulze-Makuch, D., & Irwin, L. N. (2006). The prospect of alien life
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