Terraforming of Venus
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There is a theoretical debate as to whether or not terraforming Venus for human habitation is possible. It would require two major changes; removing most of the planet's dense 9 MPa (~90 atm) carbon dioxide atmosphere and reducing its 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.
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[edit] 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 as solar power generators.
Construction of a suitably large solar shade is a daunting task. The sheer size of such a structure would necessitate construction in space. If it were placed at the Sun-Venus L1 point it would also act as a solar sail due to the radiation pressure, making it difficult to maintain its position. This could be compensated for by moving it somewhat closer to the Sun, or shaping it to merely deflect sunlight from a direct path to Venus rather than reflecting it directly backwards or absorbing it.
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 a quantity of material that is many orders of magnitude greater than we can currently transport into space.
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.
[edit] 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 as the planet's geostationary orbit lies an impractical distance above the surface; 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.
[edit] 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.[1]
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. That amount is the equivalent of millions of years of hydrogen output from the Sun via solar winds, so another way would be needed to bring the hydrogen to Venus or find it there in another molecular form. The total amount of water that could be 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. Venus's atmosphere is 3.5% nitrogen and so over 3 bar would remain after carbon dioxide was removed. So, nitrogen fixing bacteria could play a role in scrubbing excess nitrogen from the atmosphere. Also, terraforming Mars needs a significant percentage of a 'buffer gas' (meaning some inert gas, probably argon or nitrogen) in any new atmosphere made by terraforming, so Venus could export excess nitrogen via dynamic space elevator designs like Paul Birch's orbital rings. Nitrogen is also present in the outer solar system in the form of NH3 on comets, and in the atmosphere of Titan (98% nitrogen), and both could be important sources of this gas.
A method proposed in 1961 by Carl Sagan involves the use of genetically engineered bacteria to fix carbon into organic forms.[2] Although this method is still commonly proposed in discussions of Venus terraforming, later discoveries showed it would not be successful without the production of H2O and thus Venus lacks sufficient hydrogen to convert its atmosphere. Also the rate at which Carbon was removed from the atmosphere would not be sufficient to cool Venus before the organic carbon would be liberated into carbon dioxide again by the hot surface environment.
However 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 needed for the transformation.
It has also been proposed to use a sunshade or other methods to cool the planet far below the temperature of the Earth, to the freezing point of carbon dioxide (−78 °C at 1 atmosphere, though higher at higher pressure, and temperature and pressure would both cover a large range during the process), so that the atmosphere undergoes deposition into dry ice, and then introducing additional material to bury the dry ice and maintain it in that condition by pressure, before allowing the planet to partially warm again to temperatures comfortable for Earth life.
[edit] Cloud-top colonization
Geoffrey A. Landis proposes colonizing the cloud-tops of Venus.[3] 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, with half the lifting power helium has on Earth. 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 km above Venusian surface, the environment is the most Earthlike in the solar system - a pressure of approximately 1 bar and temperatures in the 0°C-50°C range. Because there is not a significant pressure difference between the inside and the outside of the breathable-air balloon, any rips or tears would cause gases to diffuse at normal atmospheric mixing rates, giving time to repair any such damages. In addition, humans would not require pressurized suits when outside, merely air to breathe and a protection from the acidic rain. Without a pressurized suit, low pressure, like on Mars, would cause a person's blood to boil.
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 insulation 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.
[edit] Rotation
Venus's extremely slow rotation rate would result in extremely long days and nights, which could prove difficult for most known species of plant and animal to adapt to. Also, the slow rotation likely accounts for the lack of a significant magnetic field. Ideally, the rotation could be modified by some sort of precisely directed cometary or asteroid impact(s). Current technology could be used to slightly modify the orbit of near Venus asteroids and comets, but comets and asteroids of sufficient mass and velocity would have to be identified. And researchers do not know if the interior of Venus has sufficient iron to create a magnetic field even if the planet were somehow made to rotate.
Since Venus 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. Conversely, if Venus were somehow be made to generate a magnetic field it would be able to trap significant amounts of Hydrogen ejected from the Sun via its Solar Winds.
[edit] References
- ^ Gillett, Stephen L. (1996). “Inward Ho!”, Stanley Schmidt and Robert Zubrin: Islands in the Sky. John Wiley & Sons, 78-84.
- ^ Sagan, Carl (1961). "The Planet Venus". Science.
- ^ Landis, Geoffrey A. (Feb. 2-6 2003). "Colonization of Venus". Conference on Human Space Exploration, Space Technology & Applications International Forum, Albuquerque NM.
