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Terraforming of Venus

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Artist's conception of a terraformed Venus.

Terraforming of Venus is the hypothetical process of engineering the global environment of the planet Venus in such a way as to make it suitable for human habitation. Terraforming Venus was first seriously proposed by the astronomer Carl Sagan in 1961.[1] The minimum adjustments to the existing environment of Venus to support human life would require three major changes to the planet:

  • Reducing Venus's 450°C (850°F) surface temperature.
  • Eliminating most of the planet's dense 9 MPa (~90 atm) carbon dioxide atmosphere, via removal or conversion to some other form.
  • Addition of breathable oxygen to the atmosphere.

These three goals are closely interrelated, since Venus's extreme temperature is due to the greenhouse effect caused by its dense carbon-dioxide atmosphere. In addition, two additional changes would be highly desirable:

  • Establishing a day/night light cycle shorter than Venus's current solar day (currently equal to 116.75 Earth days).
  • Establishing a planetary magnetic field or substitute for protection against solar and cosmic radiation.


Solar shades

Space based

Solar shades could be used to reduce the total insolation received by Venus, cooling the planet somewhat.[2] A shade placed in the Sun-Venus L1 Lagrange point also serves to block the solar wind, removing the radiation exposure problem on Venus.

Construction of a suitably large solar shade is a daunting task. The size of the shade would be four times the diameter of Venus itself if at the L1 point. This size would necessitate construction in space. There would also be the difficulty of balancing a thin-film shade perpendicular to the Sun's rays at the Sun-Venus Lagrangian 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 force to the sunward side and necessitate moving the shade even closer to the Sun than the L1 point.

Modifications to the L1 solar shade design have been suggested to solve the solar sail problem. One suggested method is 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.[3]

Paul Birch proposed[4] a slatted system of mirrors near the L1 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.

Solar shades could also serve as solar power generators. Space-based solar shade techniques, and thin-film solar sails in general, are only in an early stage of development. The vast sizes require a quantity of material that is many orders of magnitude greater than any man-made object that has ever been brought into space or constructed in 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[5] 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.

Eliminating the dense carbon dioxide atmosphere

Biological approaches

A method proposed in 1961 by Carl Sagan involves the use of genetically engineered bacteria to fix carbon into organic forms.[1] 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. 23 years later, in Pale Blue Dot, Sagan conceded that his original proposal for terraforming would not work because the atmosphere of Venus is far denser than was known in 1961.[6]

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 were 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 1.2×1020 kg of carbon in Venus's atmosphere.

Introduction of hydrogen

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.) Due to the 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.[4]

Capture in carbonates

Bombardment of Venus with refined magnesium and calcium metal 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.[7] 8×1020 kg is a few times the mass of the asteroid 4 Vesta (more than 300 miles in diameter).

Modelling by Mark Bullock[8] of Venus' atmospheric evolution suggests that existing surface minerals, particularly calcium and magnesium oxides, could serve as a sink of carbon dioxide and sulphur dioxide. If these could be exposed to the atmosphere then the planet would cool and its atmospheric pressure decline somewhat. One of the possible end states modelled by Bullock was a 43 bar atmosphere and 400 K surface temperature.

Direct liquefaction and sequestration

Birch's proposal[4] 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 deposit 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. Birch suggests disrupting an ice-moon of Saturn and bombarding Venus with its fragments to provide perhaps an average depth of 100 meters of water over the whole planet.

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 difficult. Venus has sufficiently high escape velocity to make blasting it away with asteroid impacts impractical. Pollack and Sagan calculated in 1993[6] 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 either, requiring even more. The violence of the bombardment could well result in significant outgassing that replaces removed atmosphere. Most of the ejected atmosphere would go into solar orbit near Venus, and, without further intervention, could 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 very difficult 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.

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. In addition to his suggestion of slatted system of mirrors near the L1 point between Venus and the Sun, Paul Birch has proposed a rotating soletta mirror in a polar orbit, which would produce a 24-hour light cycle.[4]

Increasing the speed of 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.[9] G. David Nordley has suggested, in fiction[10], 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.[11]

See also

References

  1. ^ a b Sagan, Carl (1961). "The Planet Venus". Science.
  2. ^ Zubrin, Robert (1999). Entering Space: Creating a Spacefaring Civilization.
  3. ^ Fogg, Martyn J. (1995). Terraforming: Engineering Planetary Environments. SAE International, Warrendale, PA. ISBN 1560916095.
  4. ^ a b c d Birch, Paul (1991). "Terraforming Venus Quickly". Journal of the British Interplanetary Society. {{cite journal}}: External link in |title= (help)
  5. ^ Landis, Geoffrey A. (2003). "Colonization of Venus". Conference on Human Space Exploration, Space Technology & Applications International Forum, Albuquerque NM. {{cite journal}}: Unknown parameter |month= ignored (help)
  6. ^ a b Carl Sagan, Pale Blue Dot: A Vision of the Human Future in Space, 1994, ISBN 0345376595
  7. ^ Gillett, Stephen L. (1996). "Inward Ho!". In Stanley Schmidt and Robert Zubrin (ed.). Islands in the Sky: Bold New Ideas for Colonizing Space. John Wiley & Sons. pp. 78–84. ISBN 0-471-13561-5.
  8. ^ Bullock, M.A., and D.G. Grinspoon, The Stability of Climate on Venus, J. Geophys. Res. 101, 7521-7529, 1996.
  9. ^ Astronomers hatch plan to move Earth's orbit from warming sun, CNN.com
  10. ^ Nordley, Gerald (1991). "The Snows of Venus". Analog Science Fiction and Science Fact. {{cite journal}}: Unknown parameter |middle= ignored (help); Unknown parameter |month= ignored (help)
  11. ^ Birch, Paul (1993). "How to Spin a Planet". Journal of the British Interplanetary Society.
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