Terraforming of Venus

There is a theoretical debate as to wheter or not Venusian terraforming for human habitation is possible. Terraforming Venus would require two major changes; removing most of the planet's dense 9 MPa carbon dioxide atmosphere and reducing the planet's 500 °C (770 K) surface temperature. These goals are closely interrelated, since Venus's extreme temperature is due to the greenhouse effect caused by its dense atmosphere.

Solar shades

Solar shades placed in the Sun-Venus L₁ point or in a more closely-orbiting ring could be used to reduce the total insolation received by Venus, cooling the planet somewhat. This does not directly deal with the immense atmospheric density of Venus, but could make it easier to do so by other methods. They could also serve double duty as solar power generators.

Construction of a suitably large solar shade is a potentially daunting task. The sheer size of such a structure would necessitate construction in space. There would also be the difficulty of balancing a thin-film shade at the Sun-Venus L₁ point with the incoming radiation pressure which would tend to turn the shade into a huge solar sail.

Other proposed cooling solutions involve comets, or creating artificial rings. A comet at the Sun-Venus L₁ point could produce a coma which could provide at least temporary shade for the planet, possibly allowing enough time for atmospheric processing to be done. Keeping a continuously decaying comet in a stable position could prove to be a difficult feat. Rings created by putting debris in orbit would provide some shade but to a lesser extent. The inclination of the rings would also need to be such that they present a significant amount of surface area to the Sun.

Space-based solar shade techniques are largely speculative due to the fact that they are beyond our current technological grasp. The vast sizes require material strengths and construction methods that have not even reached their infancy.

Cooling could be sustained by placing reflectors in the atmosphere or on the surface. Reflective balloons floating in the upper atmosphere could create shade. The number and/or size of the balloons would necessarily be great. Increasing the planet's albedo by deploying light color or reflective material on the surface could help keep the atmosphere cool. The amount would be large and would have to be put in place after the atmosphere had been modified already since Venus's surface is currently completely shrouded by clouds. The advantage of atmospheric and surface cooling solutions is that they take advantage of existing technology.

Removing atmosphere

Removal of Venus's atmosphere could be attempted by a variety of methods, possibly in combination. Directly lifting atmospheric gas from Venus into space would likely prove very difficult. Venus has sufficiently high escape velocity to make blasting it away with asteroid impacts impractical. Pollack and Sagan calculated in 1993 that an impactor of 700 km diameter striking Venus at greater than 20 km/s, would eject all the atmosphere above the horizon as seen from the point of impact, but since this is less than a thousandth of the total atmosphere and there would be diminishing returns as the atmosphere's density decreased a very great number of such giant impactors would be required. Smaller objects would not work as well, requiring even more. The violence of the bombardment could well result in significant outgassing that replaces removed atmosphere. Furthermore, most of the ejected atmosphere would go into solar orbit near Venus, eventually to fall right back onto Venus again.

Removal of atmospheric gas in a more controlled manner could also prove difficult. Venus's extremely slow rotation means that space elevators would be impossible to construct, and the very atmosphere to be removed makes mass drivers useless for removing payloads from the planet's surface. Possible workarounds include placing mass drivers on high-altitude balloons or balloon-supported towers extending above the bulk of the atmosphere, using space fountains, or rotovators. Such processes would take a great deal of technical sophistication and time, however, and may not be economically feasible without the use of extensive automation.

Converting atmosphere

Alternatively, Venus's atmosphere could be converted into some other form in situ by reacting it with externally supplied elements.

Bombardment of Venus with refined magnesium and calcium metal from Mercury or some other source, could sequester carbon dioxide in the form of calcium and magnesium carbonates. About 8×10²⁰ kg of calcium or 5×10²⁰ kg of magnesium would be required, which would entail a great deal of mining and mineral refining to get ahold of.

Bombardment of Venus with hydrogen, possibly from some outer solar system source and reacting 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 the resulting water would cover about 80% of the surface compared to 70% for Earth. The amount of water produced would amount to around 10% of the water found on Earth. A solar shade or equivalent would also be necessary, as water vapor is itself a greenhouse gas. Oceans on Venus would increase the planet's albedo and allow more incoming solar radiation to be reflected back into space. It would also be important to take into account water's capacity for absorbing CO₂ and O₂, and how much gas an ocean would hold. In addition to this Venus would still needs a significant percentage of a 'buffer gas' (meaning some inert gas, probably argon or nitrogen) in its new atmosphere. Nitrogen is present in the outer solar system in the form of NH₃ on comets and could be an important source of this gas.

A method proposed in 1961 by Carl Sagan involves the use of genetically engineered bacteria to fix carbon into organic forms. Although this method is still commonly proposed in discussions of Venus terraforming, later discoveries showed it would not be successful; organic carbon would be liberated into carbon dioxide again by the hot surface environment.

Though using the Bosch reaction to create more water would pave the way for microbes to survive in the atmosphere which would lower the amount of hydrogen we would need to import.

Cloud-top colonization

Geoffrey A. Landis proposes colonizing the cloud-tops of Venus. Initially, the image of floating cities may seem fanciful, but Landis' proposal points out that a Terran breathable air mixture (21:79 Oxygen-Nitrogen) is a lifting gas in the Venusian atmosphere. In effect, a gasbag full of human-breathable air would sustain itself and extra weight (such as a colony) in midair. At an altitude of 50 kilometers above Venusian surface, the environment is the most Earthlike in the solar system - a pressure of approximately 1 bar and temperatures in the 0-50 Celsius range. Because there is not a significant pressure differential between the inside and the outside of the breathable-air balloon, any rips or tears would not result in an explosive decompression, but rather would only diffuse at normal atmospheric mixing rates, giving time to repair any such defects.

Such colonies could be constructed at any rate desired, allowing a dynamic approach instead of needing any 'fell swoop' solutions. They could be used to gradually transform the Venusian atmosphere, with their impact directly related to the number of colonies in the atmosphere. As the constructed colonies increased, more solar panels could be used to absorb insolation and thus cool Venus; they could also be used to grow plant matter that would reduce the amount of carbon dioxide in the air. In the beginning, any impact on Venus would be insignificant, but as the number of colonies grew, they could transform Venus more and more rapidly.

Other modifications

Venus's extremely slow rotation rate would result in extremely long days and nights, which could prove difficult for most Earth life to adapt to. Speeding up Venus's rotation would require many orders of magnitude greater amounts of energy than removing its atmosphere would, and so is likely to be infeasible (at least by any current technology). Instead, a system of orbiting solar mirrors might be used to provide sunlight to the night side of Venus. Alternately, instead of requiring that Venus support life identical to Earth's, Earth life could instead be modified to adapt to the long Venusian day and night. However, Venusian cloud-top colonies can have the ability of having a faster period, even down to 24 days, by using anti-corrosive wind sails to propel it around the planet instead of depending on the planet's rotation or appendages.

Venus also lacks a magnetic field. It is thought that this may have contributed greatly to its current uninhabitable state, as the upper atmosphere is exposed to direct erosion by solar wind and has lost most of its original hydrogen to space. However, this process is extremely slow, and so is unlikely to be significant on the timescale of any civilization capable of terraforming the planet in the first place.