Terraforming of Venus

Artist's conception of a terraformed Venus. (credit: Daein Ballard)

There is a theoretical debate as to whether or not Venus can be terraformed for human habitation. It would require several major changes:

• Reducing Venus's 500 °C (770 K) surface temperature.
• Eliminating most of the planet's dense 9 MPa (~90 atm) carbon dioxide atmosphere, via removal or conversion to some other form.
• Establishing a day/night light cycle shorter than Venus's current 116.75 day solar day.

These goals are closely interrelated, since Venus's extreme temperature is due to the greenhouse effect caused by its dense atmosphere.

Solar shades

Space based

Solar shades placed in the Sun-Venus L₁ Lagrange 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 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. If the shade were left at the L1 point, the pressure would add too much force to the sunward side and necessitate moving the shade even closer to the Sun than the L₁ point. The size of the shade would be twice the diameter of Venus itself if at the L₁ point.

Modifications to this solar shade design have been suggested, allowing a change of placement and a reduction in size of the shade. One proposal is a system of orbiting solar mirrors which might be used to provide sunlight to the night side of Venus and possibly shade to the day side surface. Paul Birch has proposed a slatted system of mirrors near the L₁ Lagrange point between Venus and the Sun. The shade's panels would not be perpendicular to the sun's rays, but instead at an angle of 30 degrees, such that the reflected light would strike the next panel, negating the photon pressure. Each successive row of panels would be +/- 1 degree off the 30-degree deflection angle, causing the reflected light to be skewed 4 degrees from striking Venus. Furthermore, a rotating circular soletta mirror in a polar orbit would reflect light (including some light redirected from the sunshade) onto Venus itself producing a 24-hour light cycle, as well as blocking the solar wind.

Another suggested method of bringing the shade closer to Venus and reducing its size, would be to use polar orbiting, solar-synchronous mirrors that reflect light toward the back of the sunshade, from the non-sunward side of Venus. Photon pressure would push the support mirrors to an angle of 30 degrees away from the sunward side.

Other proposed cooling solutions involve the creation of an artificial Planetary ring. Rings created by placing 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.

It may be possible to cool Venus down enough by mining aluminum from Earth's Moon and shooting it into orbit around Venus. The aluminum dust would reflect sunlight and cool down Venus. The aluminum dust should be released simultaneously from orbiting containers. The dust would not remain in orbit for long, so timing of a total comprehensive plan would be required.

Space-based solar shade techniques are largely speculative due to the fact that they are beyond our current technological grasp. The vast sizes require a quantity of material that is many orders of magnitude greater than we can currently transport into space.

Atmospheric or Surface based

See also: Colonization of Venus § Aerostat habitats and floating cities

Cooling could also be effected 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. Geoffrey A. Landis has suggested that if enough floating cities were built, they could form a solar shield around the planet, and could simultaneously be used to process the atmosphere into a more desirable form, thus combining the solar shield theory and the atmospheric processing theory with a scalable technology that would immediately provide living space in the Venusian atmosphere. If made from carbon nanotubes (recently fabricated into sheet form) or graphene (a sheet-like carbon allotrope), then the major structural materials can be produced using carbon dioxide gathered in situ from the atmosphere. The recently synthesised amorphous carbonia might prove a useful structural material if it can be quenched to STP conditions, perhaps in a mixture with regular silica glass. According to Birch's analysis such colonies and materials would provide an immediate economic return from colonizing Venus, funding further terraforming efforts.

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.

An advantage of atmospheric and surface cooling solutions is that they take advantage of existing technology. A disadvantage is that Venus already has highly reflective clouds (giving it an albedo of 0.65), so any approach would have to significantly surpass this to make a difference. Also floating cities covering the entire planet would be kinda hard to build.

Eliminating dense carbon dioxide atmosphere

Converting atmosphere

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

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. The production of organic molecules from carbon dioxide requires an input of hydrogen, which on Earth is taken from its abundant supply of water but which is nearly nonexistent on Venus. Since Venus lacks a magnetic field, the upper atmosphere is exposed to direct erosion by solar wind and has lost most of its original hydrogen to space.

Furthermore, 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 has already been removed. Thirty years later, in Pale Blue Dot, Sagan conceded that his original proposal for terraforming would not work due to the fact that the atmosphere of Venus is far denser than was known in 1961.

Bombarding 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. (Loss of hydrogen due to the solar wind is unlikely to be significant on the timescale of terraforming.) Birch suggests disrupting an ice-moon of Saturn and bombarding Venus with its fragments to provide perhaps 100 metres/sq. metre of water. The resulting water would cover about 80% of the surface compared to 70% for Earth, although the water would amount to only roughly 10% of the water found on Earth, due to the shallowness of potential Venusian oceans.

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. 8×10²⁰ kg is a few times the mass of the asteroid 4 Vesta (more than 300 miles in diameter). Or, to put it another way, terraforming Venus in this way would require mankind to be able to carefully mine all of the material in the state of Alabama, extending all the way from the surface to the center of the earth, launch it into outer space, and then land it on another planet.

Birch's proposal involves using a solar shade to cool Venus down sufficiently to permit liquefaction, from a temperature less than 304.18 K and partial pressures of CO₂ down to 73.8 bar (carbon dioxide's critical point) and then down to 5.185 bar and 216.85 K (carbon dioxide's triple point). Below that temperature, freezing of atmospheric carbon dioxide into dry ice will cause it to deposition onto the surface, after which the frozen CO₂ would be buried and maintained in that condition by pressure, or shipped off-world. After this process was complete, the shades could be removed or solettas added, allowing the planet to partially warm again to temperatures comfortable for Earth life. A source of hydrogen or water would still be needed, and some of the remaining 3.5 bar of atmospheric nitrogen would need to be fixed into the soil.

Floating colonies could gradually transform the Venusian atmosphere: for example, their reflectivity could alter the overall albedo of Venus. Colonies could also grow plant matter, if water or another source of hydrogen was imported, which would gradually sequester carbon dioxide in the air. However, it would take an enormous number of such colonies, and large quantities of introduced hydrogen, to have a significant atmospheric impact, as there is over the 1.2×10²⁰ kg of carbon in Venus's atmosphere.

Removing atmosphere

The 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 be captured by Venus' gravitational field and become part of the atmosphere once 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 as the planet's geostationary orbit lies an impractical distance above the surface; and the very thick 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.

Rotation

Venus rotates once every 243 days – by far the slowest rotation period of any of the major planets. A Venusian sidereal day thus lasts more than a Venusian year (243 versus 224.7 Earth days). However, the length of a solar day on Venus is significantly shorter than the sidereal day; to an observer on the surface of Venus the time from one sunrise to the next would be 116.75 days. Nevertheless, Venus's extremely slow rotation rate would result in extremely long days and nights, which could prove difficult for most known Earth species of plants and animals to adapt to. The slow rotation also likely accounts for the lack of a significant magnetic field.

One proposal is a system of orbiting solar mirrors which might be used to provide sunlight to the night side of Venus and possibly shade to the day side surface. Paul Birch has proposed a combination of a slatted system of mirrors near the L1 point between Venus and the Sun, and a rotating soletta mirror in a polar orbit, which would produce a 24-hour light cycle, as well as blocking the solar wind.

Speeding up Venus's rotation would require many orders of magnitude greater amounts of energy than construction of orbiting solar mirrors, or even than the removal of Venus's atmosphere. Recent scientific research suggests that close fly-bys of asteroids or cometary bodies larger than 60 miles across could be used to move a planet in its orbit, or increase the speed of rotation. G. David Nordley has suggested, in fiction, that Venus might be spun-up to a day-length of 30 Earth-days by exporting the atmosphere of Venus into space via mass drivers. This concept was also explored more rigorously by Birch.

See also

• Colonization of Venus
• Terraforming of Mars

External links

• An approach to terraforming Venus
• Terraformers Society of Canada
• Visualizing the steps of solar system terraforming
• The Terraforming Information Pages
• A fictional account of the terraformation of Venus
• Venus Unveiled: A terraformed Venus, one thousand years from now -- by Chris Wayan, 2003-4.
• Terraform Venus (discussion on the New Mars forum)
• Terraforming Venus - The Latest Thinking (discussion on the New Mars forum)