life on Venus
Historical ideasThe similarity in size between Venus and Earth and the presence, evident even to early telescopic observers, of a substantial atmosphere on the second planet encouraged speculation about Venusian life. Temperatures on Venus were assumed to be generally higher than those on Earth but not so high as to pose a major obstacle to habitation, as Richard Proctor, writing in 1870, affirmed:1
It is clear that, merely in the greater proximity of Venus to the sun, there is little to render at least the large portion of her surface uninhabitable by such beings as exist upon our earth. This undoubtedly would render [the sun's] heat almost unbearable in the equatorial regions of Venus, but in her temperate and subarctic regions a climate which we should find well suited to our requirements might very well exist ... I can find no reason ... for denying that she may be considered the abode of creatures as far advanced in the scale of creation as any which exist upon the earth.
In 1918, the Swedish chemist and Nobel laureate Svante Arrhenius concluded that:
[E]verything on Venus is dripping wet... A very great part of the surface ... is no doubt covered with swamps corresponding to those on the Earth in which the coal deposits were formed... The constantly uniform climatic conditions which exist everywhere result in an entire absence of adaptation to changing exterior conditions. Only low forms of life are therefore represented, mostly no doubt, belonging to the vegetable kingdom; and the organisms are nearly of the same kind all over the planet.
This evocative portrait of a world trapped in a kind of Carboniferous time-warp was the first of a number of intriguing Venusian dioramas to emerge in the first half of the 20th century.
Seas and swamps
During the 1920s, spectroscopic searches were carried out to try to detect water vapor in the Venusian clouds but, to everyone's surprise, none was found. What did show up were large quantities of carbon dioxide. This seemed to put paid to the swamp theory and a radically new picture emerged in which the clouds of Venus were assumed to be dust and the surface a dry, windswept desert. According to another theory the clouds were made of methanal (formaldehyde), prompting the comment that Venus was not only dead but pickled.
More speculation followed. In 1955, the American astronomers Frank Whipple and Donald Menzel2 argued that the Venusian atmosphere might be rich in ice crystals which would not show up spectroscopically. For them, Venus was a world completely covered by a carbonated ocean – a globe-encircling seltzer sea. Any protruding land masses, they claimed, would have removed most of the carbon dioxide from the atmosphere and fixed it in the rocks in the form of carbonates (as has happened on our own world). Encouraged by this vision of a planet-wide Pacific, it became fashionable to speculate on a world inhabited by marine organisms, similar perhaps to those which existed on Earth in the Cambrian era, 500 million years ago. Isaac Asimov gave the aqueous theory center stage in his 1954 novel Lucky Starr and the Oceans of Venus.
Yet any hopes of finding even a puddle of surface water on Venus, let alone antediluvian life, quickly evaporated in the late 1950s and early 1960s. Measurements, first from radio telescopes on the ground, and later from spacecraft on flyby missions, revealed that, far from being balmy, the Venusian climate is hostile in the extreme. Astronomers had previously thought that much of the extra heat arriving at Venus because of its proximity to the Sun would be reflected back into space by the enveloping white clouds. But it soon became clear that this is not the case. The dense carbon dioxide atmosphere, 90 times thicker than Earth's, creates a runaway greenhouse effect which traps the Sun's radiation and drives the surface temperature up to around 600°C, easily hot enough to melt lead – or fry flesh. Far from being a haven for life, Venus is a planet from hell. See Venus, atmosphere.
Life after all?Or so it seems. Yet, in 2002, at the European astrobiology conference in Graz, Austria, Dirk Schulze-Makuch and Louis Irwin, from the University of Texas at El Paso, suggested that high clouds in the Venusian atmosphere contain chemicals that hint at the presence of some kind of biological activity.3 Based on data from the Russian Venera space missions and also the Pioneer Venus and Magellan probes, Schulze-Makuch and Irwin pointed to oddities in the chemical composition of water droplets in the Venusian clouds which, they argue, could be explained by the presence of microbes. The researchers found hydrogen sulfide and sulfur dioxide – two gases that react with each other, and are not seen in the same place unless something is producing them. They also said that, despite solar radiation and lightning, the atmosphere contains hardly any carbon monoxide, suggesting that something is removing the gas. One possibility is that microbes living in the Venusian clouds could be combining sulfur dioxide with carbon monoxide and possibly hydrogen sulphide or carbonyl sulphide in a metabolism similar to that of some early terrestrial microorganisms. Given that the temperature on Venus was once much cooler, there may once have been oceans on the planet. Life could have started there and retreated to stable niches once the runaway greenhouse effect began. This idea seems all the more reasonable following the discovery by Austrian scientists, also in 2002, bacteria living and reproducing within clouds on Earth. The Venusian clouds are high in the atmosphere, where the temperature and pressure are quite Earth-like.
Another problem could be UV radiation from the Sun. But Schulze-Makuch suggests that Venusian bacteria could make use of a natural chemical sunscreen there based on sulfur compounds. It's possible that organisms have evolved ways of making use of the UV, much like Earth plants use visible light for photosynthesis.
The next step will be to send a probe to test these ideas. One favored option involves a collector tethered to a balloon-like floating spacecraft. This would gather samples of the cloud droplets and then be returned to Earth for analysis. See also Venus probes.
1. Procter, Richard. Other Worlds Than Ours. New York: P. F.
Collier (1900) (first published 1894).
2. Whipple, F. L., and Menzel, D. H. "The Case for H2O Clouds on Venus." Publications of the Astronomical Society of the Pacific, 67, 161 (1955).
3. Schulze-Makuch, D., and Irwin, L. N. "Reassessing the possibility of life on Venus: Proposal for an Astrobiology Mission," Astrobiology, 2(2), 197-202 (2002).