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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