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 , the surface of Venus is above the melting point of lead. Meanwhile, its 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. The rapid growth in carbon capture and storage since the beginning of the century presents another plausible catalyst to terraforming.
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" Stapledon, Olaf "Last and First Men" Methuen 1930, chapter 12 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,Greenhouse effect, clouds and winds. Venus express mission, European Space Agency. 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:
Following Sagan's paper, there was little scientific discussion of the concept until a resurgence of interest in the 1970s and 1980s.Oberg, James E. (1981). New Earths, Stackpole Books 1981; New American Library 1983. ; 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).Berry, Adrian (1984) "Venus, The Hell-World," and "Making it Rain in Hell," Chapters 6 & 7 in The Next Ten Thousand Years, New American Library. Marchal proposed colonization of the polar regions in 1983 via the "pulverization" of nearby asteroids with nuclear processes.
Atmosphere reduction
A number of approaches to terraforming are reviewed by Martyn J. Fogg (1995)Landis, Geoffrey A., "Terraforming: Engineering Planetary Environments (review)" (also available here) (accessed 25 Dec, 2016). and by Geoffrey A. Landis (2011).
The main problem with Venus today, from a terraformation standpoint, is the very thick carbon dioxide atmosphere. The ground level pressure of Venus is . 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.Kelly Beatty, J (ed.) (1999) The New Solar System, p176, CUP, 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.
Capture in carbonates
On Earth nearly all carbon is sequestered in the form of carbonate minerals or in different stages of the carbon cycle, while very little is present in the atmosphere in the form of carbon dioxide. On Venus, the situation is the opposite. Much of the carbon is present in the atmosphere, while comparatively little is sequestered in the lithosphere. Many approaches to terraforming therefore focus on getting rid of carbon dioxide by chemical reactions trapping and stabilising it in the form of carbonate minerals.
Modelling by astrobiologists Mark Bullock and David Grinspoon of Venus's atmospheric evolution suggests that the equilibrium between the current 92-bar atmosphere and existing surface minerals, particularly calcium and magnesium oxides, is quite unstable, and that the latter could serve as a sink of carbon dioxide and sulfur dioxide through conversion to carbonates. If these surface minerals were fully converted and saturated, then the atmospheric pressure would decline and the planet would cool somewhat. One of the possible end states modelled by Bullock and Grinspoon was an atmosphere of and a surface temperature of . To convert the rest of the carbon dioxide in the atmosphere, a larger portion of the crust would have to be artificially exposed to the atmosphere to allow more extensive carbonate conversion. In 1989, Alexander G. Smith proposed that Venus could be terraformed by lithosphere overturn, allowing crust to be converted into carbonates. Landis 2011 calculated that it would require the involvement of the entire surface crust down to a depth of over 1 km to produce enough rock surface area to convert enough of the atmosphere.
Natural formation of carbonate rock from minerals and carbon dioxide is a very slow process. Recent research into sequestering carbon dioxide into carbonate minerals in the context of mitigating global warming on Earth however points out that this process can be considerably accelerated (from hundreds or thousands of years to just 75 days) through the use of catalysts such as polystyrene microspheres. It could therefore be theorised that similar technologies might also be used in the context of terraformation on Venus. It can also be noted that the chemical reaction that converts minerals and carbon dioxide into carbonates is exothermic, in essence producing more energy than is consumed by the reaction. This opens up the possibility of creating self-reinforcing conversion processes with potential for exponential growth of the conversion rate until most of the atmospheric carbon dioxide can be converted.
Bombardment of Venus with refined magnesium and calcium from off-world could also sequester carbon dioxide in the form of calcium and magnesium carbonates. About 8 kg of calcium or 5 kg of magnesium would be required to convert all the carbon dioxide in the atmosphere, which would entail a great deal of mining and mineral refining (perhaps on Mercury which is notably mineral rich). 8 kg is a few times the mass of the asteroid 4 Vesta (more than in diameter).
Injection into volcanic basalt rock
Research projects in Iceland and the US state of Washington have shown that potentially large amounts of carbon dioxide could be removed from the atmosphere by high-pressure injection into subsurface porous basalt formations, where carbon dioxide is rapidly transformed into solid inert minerals.
Other studies predict that one cubic meter of porous basalt has the potential to sequester 47 kilograms of injected carbon dioxide. According to these estimates a volume of about 9.86 billion km3 of basalt rock would be needed to sequester all the carbon dioxide in the Venusian atmosphere. This is equal to the entire crust of Venus down to a depth of about 21.4 kilometers. Another study concluded that under optimal conditions, on average, 1 cubic meter of basalt rock can sequester 260 kg of carbon dioxide. Venus's crust appears to be thick and the planet is dominated by volcanic features. The surface is about 90% basalt, and about 65% consists of a mosaic of volcanic lava plains. There should therefore be ample volumes of basalt rock strata on the planet with very promising potential for carbon dioxide sequestration.
Research has also demonstrated that under the high temperature and high pressure conditions in the mantle, silicon dioxide, the most abundant mineral in the mantle (on Earth and probably also on Venus) can form carbonates that are stable under these conditions. This opens up the possibility of carbon dioxide sequestration in the mantle.
Introduction of hydrogen According to Birch, bombarding Venus with hydrogen and reacting it with carbon dioxide could produce elemental carbon (graphite) and water by the Bosch reaction. It would take about 4 × 1019 kg of hydrogen to convert the whole Venusian atmosphere, and such a large amount of hydrogen could be obtained from the gas giants or their moons' ice. Another possible source of hydrogen could be somehow extracting it from possible reservoirs in the interior of the planet itself. According to some researchers, the Earth's mantle and/or core might hold large quantities of hydrogen left there since the original formation of Earth from the nebular cloud. Since the original formation and inner structure of Earth and Venus are generally believed to be somewhat similar, the same might be true for Venus.
Iron aerosol in the atmosphere will also be required for the reaction to work, and iron can come from Mercury, asteroids, or the Moon. (Loss of hydrogen due to the solar wind is unlikely to be significant on the timescale of terraforming.) Due to the planet's relatively flat surface, this water would cover about 80% of the surface, compared to 70% for Earth, even though it would amount to only roughly 10% of the water found on Earth.
The remaining atmosphere, at around 3 bars (about three times that of Earth), would mainly be composed of nitrogen, some of which will dissolve into the new oceans of water, reducing atmospheric pressure in accordance with Henry's law. To further reduce the pressure even more, nitrogen could also be fixated into nitrates.