The terraforming of Venus is the hypothetical planetary engineering procedure that would transform Venus from a planet hostile to life to one that could sustainably host humans and other lifeforms free of protection or mediation. Necessary adjustments to the existing environment of Venus to support human life would require a reduction of the planet's surface temperature and a substantial elimination of the planet's dense atmosphere. At 737 K (464 °C; 867 °F), the surface of Venus is above the melting point of lead. Meanwhile, its 9.2 MPa (91 atm) carbon dioxide and sulfur dioxide atmosphere is almost 100 times as dense as Earth's. These facts are closely interrelated, as Venus's extreme temperature is due to the high pressure of its dense atmosphere and the greenhouse effect. Terraforming would require the removal or conversion of the atmosphere to some other form, alongside the addition of breathable oxygen to the atmosphere. Proposals to modify the planet include "veiling" the planet from the sun, thus dropping the temperature low enough to condense or solidify carbon dioxide, which would then need to be removed or stored in some way.
History of the idea Debate over "cosmic pluralism," or life on other worlds, dates to at least ancient Greece, gaining currency with the Renaissance thinker Giordano Bruno. The planet features in virtually every recorded human culture, given its visibility. Olaf Stapledon, a philosopher and author, described the terraforming of Venus in his 1930 novel "Last and First Men" In the 1950s, colonization of Venus appeared alongside proposals for the Moon and Mars before it gradually fell out of fashion when the prohibitive costs and engineering hurdles became apparent. Poul Anderson, a successful science fiction writer, had proposed the idea in his 1954 novelette "The Big Rain", a story belonging to his Psychotechnic League future history. The first known suggestion to terraform Venus in a scholarly context was by the astronomer Carl Sagan in 1961. Prior to the early 1960s, the atmosphere of Venus was believed by many astronomers to have an Earth-like temperature. When Venus was understood to have a thick carbon dioxide atmosphere with a consequence of a very large greenhouse effect, some scientists began to contemplate the idea of altering the atmosphere to make the surface more Earth-like. In the final section of an article in the journal Science in 1961, Carl Sagan discussed these effects and proposed injecting photosynthetic bacteria into the atmosphere of Venus. He argued that this would convert the carbon dioxide into reduced carbon in organic form, thus reducing the carbon dioxide from the atmosphere. The knowledge of Venus's atmosphere was still inexact when Sagan first prosed this. Thirty-three years later, in his 1994 book Pale Blue Dot, Sagan conceded his original proposal for terraforming would not work because the atmosphere of Venus is far denser than was known in 1961:
"Here's the fatal flaw: In 1961, I thought the atmospheric pressure at the surface of Venus was a few bars ... We now know it to be 90 bars, so if the scheme worked, the result would be a surface buried in hundreds of meters of fine graphite, and an atmosphere made of 65 bars of almost pure molecular oxygen. Whether we would first implode under the atmospheric pressure or spontaneously burst into flames in all that oxygen is open to question. However, long before so much oxygen could build up, the graphite would spontaneously burn back into CO2, short-circuiting the process." Following Sagan's paper, there was little scientific discussion of the concept until a resurgence of interest in the 1970s and 1980s. Despite its formidable atmosphere, the bulk mass and density characteristics of Venus relative to Earth make it in many ways a more attractive terraforming target than Mars or the solar system's natural satellites. Adrian Berry devoted chapters to the subject in his The Next Ten Thousand Years: A Vision of Man's Future in the Universe (1974). Marchal proposed colonization of the polar regions in 1983 via the "pulverization" of nearby asteroids with nuclear processes.
Proposed approaches to terraforming A number of approaches to terraforming are reviewed by Martyn J. Fogg (1995) and by Geoffrey A. Landis (2011).
Eliminating the dense carbon dioxide atmosphere The main problem with Venus today, from a terraformation standpoint, is the very thick carbon dioxide atmosphere. The ground level pressure of Venus is 9.2 MPa (91 atm; 1,330 psi). This also, through the greenhouse effect, causes the temperature on the surface to be several hundred degrees too hot for any significant organisms. Therefore, all approaches to the terraforming of Venus include somehow removing almost all the carbon dioxide in the atmosphere.
Biological approaches The method proposed in 1961 by Carl Sagan involves the use of genetically engineered algae to fix carbon into organic compounds. Although this method is still proposed in discussions of Venus terraforming, later discoveries showed that biological means alone would not be successful. Difficulties include the fact that the production of organic molecules from carbon dioxide requires hydrogen, which is very rare on Venus. Because Venus lacks a protective magnetosphere, the upper atmosphere is exposed to direct erosion by the solar wind and has lost most of its original hydrogen to space. And, as Sagan noted, any carbon that was bound up in organic molecules would quickly be converted to carbon dioxide again by the hot surface environment. Venus would not begin to cool down until after most of the carbon dioxide had already been removed.
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