Terraforming of Venus

Artist's conception of a terraformed Venus. The cloud formations are depicted assuming the planet's rotation has not been sped up.

The 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.[1][2][3] Terraforming Venus was first scholarly proposed by the astronomer Carl Sagan in 1961,[4] although fictional treatments, such as The Big Rain of The Psychotechnic League by novelist Poul Anderson, preceded it. Adjustments to the existing environment of Venus to support human life would require at least three major changes to the planet's atmosphere:[3] Reducing Venus's surface temperature of 462 °C (735 K; 864 °F), eliminating most of the planet's dense 9.2 MPa (91 atm) carbon dioxide and sulfur dioxide atmosphere via removal or conversion to some other form, and the addition of breathable oxygen to the atmosphere. These three changes are closely interrelated, because Venus's extreme temperature is due to the high pressure of its dense atmosphere, and the greenhouse effect.

History

Prior to the early 1960s, the atmosphere of Venus was believed by 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,[5] some scientists began to contemplate the idea of altering the atmosphere to make the surface more Earth-like. This hypothetical prospect, known as terraforming, was first proposed by Carl Sagan in 1961, as a final section of his classic article in the journal Science discussing the atmosphere and greenhouse effect of Venus.[4] Sagan proposed injecting photosynthetic bacteria into the Venus atmosphere, which would convert the carbon dioxide into reduced carbon in organic form, thus reducing the carbon dioxide from the atmosphere.

Unfortunately, the knowledge of Venus's atmosphere was still inexact in 1961, when Sagan made his original proposal for terraforming. Thirty-three years after his original proposal, 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:[6]

"Here's the fatal flaw: In 1961, I thought the atmospheric pressure at the surface of Venus was a few bars ... We now know it to be 90 bars, so if the scheme worked, the result would be a surface buried in hundreds of meters of fine graphite, and an atmosphere made of 65 bars of almost pure molecular oxygen. Whether we would first implode under the atmospheric pressure or spontaneously burst into flames in all that oxygen is open to question. However, long before so much oxygen could build up, the graphite would spontaneously burn back into CO2, short-circuiting the process."

Following Sagan's paper, there was little scientific discussion of the concept until a resurgence of interest in the 1980s.[7][8][9]

Proposed approaches to terraforming

A number of approaches to terraforming are reviewed by Martyn J. Fogg (1995)[2][10] and by Geoffrey A. Landis (2011).[3]

Eliminating the dense carbon dioxide atmosphere

The ground level pressure of Venus is 9.2 MPa (1,330 psi).

Biological approaches

The method proposed in 1961 by Carl Sagan involves the use of genetically engineered bacteria to fix carbon into organic compounds.[4] Although this method is still proposed[9] in discussions of Venus terraforming, later discoveries showed that biological means alone would not be successful.[11]

Difficulties include the fact that the production of organic molecules from carbon dioxide requires hydrogen, which is very rare on Venus.[12] 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 has already been removed.

Although it is generally conceded that Venus could not be terraformed by introduction of photosynthetic biota alone, use of photosynthetic organisms to produce oxygen in the atmosphere continues to be a component of other proposed methods of terraforming.

Capture in carbonates

In 1989, Smith proposed that Venus could be terraformed by lithosphere overturn, allowing crust to be converted into carbonates.[13] Landis 2011 calculated that it would require pulverizing the surface to a depth of over 1 km to produce enough rock surface area to convert enough of the atmosphere.[3] Modelling by Mark Bullock and David Grinspoon [14] of Venus's atmospheric evolution suggests that existing surface minerals, particularly calcium and magnesium oxides, could serve as a sink of carbon dioxide and sulfur 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 and Grinspoon was a 43 bars (620 psi) atmosphere and 400 K (127 °C) surface temperature.

Bombardment of Venus with refined magnesium and calcium 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.[15] 8×1020 kg is a few times the mass of the asteroid 4 Vesta (more than 500 kilometres (310 mi) in diameter).

Introduction of hydrogen

According to Birch,[16] 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 Venerian 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 extracting it from possible reservoirs in the core of the planet itself. According to some researchers the Earth's core might hold large quantities of hydrogen.[17] Since the inner structure of Earth and Venus are generally believed to be somewhat similar, the same might be true for the core of 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 further, in accordance with Henry's law.

Direct removal of atmosphere

The thinning of Venus's atmosphere could be attempted by a variety of methods, possibly in combination. Directly lifting atmospheric gas from Venus into space would probably prove difficult. Venus has sufficiently high escape velocity to make blasting it away with asteroid impacts impractical. Pollack and Sagan calculated in 1994[18] 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 because this is less than a thousandth of the total atmosphere and there would be diminishing returns as the atmosphere's density decreases, a very great number of such giant impactors would be required. Landis calculated[3] that to lower the pressure from 92 bar to 1 bar would require a minimum of 2,000 impacts, even if the efficiency of atmosphere removal was perfect. Smaller objects would not work, either, because more would be required. The violence of the bombardment could well result in significant outgassing that would replace removed atmosphere. Most of the ejected atmosphere would go into solar orbit near Venus, and, without further intervention, could be captured by Venus's 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 because 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.

In addition, if the density of the atmosphere (and corresponding greenhouse effect) were dramatically reduced, the surface temperature (now effectively constant) would probably vary widely between day side and night side. Another side effect to atmospheric-density reduction could be the creation of zones of dramatic weather activity or storms at the terminator because large volumes of atmosphere would undergo rapid heating or cooling.

Cooling planet by solar shades

Venus receives about twice the sunlight that Earth does, which is thought to have contributed to its runaway greenhouse effect. One means of terraforming Venus could involve reducing the insolation at Venus's surface to prevent the planet from heating up again.

Space-based

Solar shades could be used to reduce the total insolation received by Venus, cooling the planet somewhat.[19] A shade placed in the Sun–Venus L1 Lagrangian point also would serve to block the solar wind, removing the radiation exposure problem on Venus.

A suitably large solar shade would be four times the diameter of Venus itself if at the L1 point. This 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.[2]

Paul Birch proposed[16] 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 human-made object that has ever been brought into space or constructed in space.

Atmospheric or surface-based

Venus could also be cooled by placing reflectors in the atmosphere. 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[20] 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 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 Standard Temperature and Pressure (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-colored or reflective material on the surface (or at any level below the cloud tops) would not be useful, because Venus's surface is already completely enshrouded by clouds, and almost no sunlight reaches the surface. Thus, it would be unlikely to be able to reflect more light than Venus' already-reflective clouds, with Bond albedo of 0.77.[21]

Combination of solar shades and atmospheric condensation

Birch proposed that solar shades could be used to not merely cool the planet but that this could be used to reduce atmospheric pressure as well, by the process of freezing of the carbon dioxide.[16] This requires Venus's temperature to be reduced, first to the liquefaction point, requiring a temperature less than 304 K (31 °C; 88 °F) and partial pressures of CO2 to bring the atmospheric pressure down to 73.8 bar (carbon dioxide's critical point); and from there reducing the temperature below 217 K (−56 °C; −69 °F) (carbon dioxide's triple point). Below that temperature, freezing of atmospheric carbon dioxide into dry ice will cause it to deposit onto the surface. He then proposed that the frozen CO2 could be buried and maintained in that condition by pressure, or even shipped off-world (perhaps to provide greenhouse gas needed for terraforming of Mars or the moons of Jupiter). 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 icy moon of Saturn and bombarding Venus with its fragments to provide perhaps an average depth of 100 meters of water over the whole planet.

Artificial mountains

As an alternative to changing the atmosphere of Venus, it has been proposed that a large artificial mountain, dubbed the "Venusian Tower of Babel", could be built on the surface of Venus that would reach up to 50 kilometres (31 mi) into the atmosphere where the temperature and pressure conditions are similar to Earth and where a colony could be built on the peak of this artificial mountain. Such a structure could be built using autonomous robotic bulldozers and excavators that have been hardened against the extreme temperature and pressure of the Venus atmosphere. Such robotic machines would be covered in a layer of heat and pressure shielding ceramics, with internal helium-based heat pumps inside of the machines to cool both an internal nuclear power plant and to keep the internal electronics and motor actuators of the machine cooled to with in operating temperature. Such a machine could be designed to operate for years without external intervention for the purpose of building colossal mountains on Venus to serve as islands of colonization in the skies of Venus.[22]

Altering day–night cycle

Venus rotates once every 243 Earth days—by far the slowest rotation period of any known object in the Solar System. 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 probably accounts for the lack of a significant magnetic field.

Space mirrors

One proposal to compensate for the rotation rate is a system of orbiting solar mirrors which might be used to provide sunlight to the nightside of Venus and possibly shade to the dayside 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.[16]

Changing rotation speed

Increasing the speed of Venus's rotation would require energy many orders of magnitude greater than the construction of orbiting solar mirrors, or even than the removal of Venus's atmosphere. Birch calculates that increasing the rotation of Venus to an Earthlike solar cycle would require about 1.6 × 1029 Joules[23] (50 billion petawatt-hours).

Recent scientific research suggests that close flybys of asteroids or cometary bodies larger than 100 kilometres (60 mi) across could be used to move a planet in its orbit, or increase the speed of rotation.[24] The energy required to do this is large. In his book on terraforming, one of the concepts Fogg discusses is to increase the spin of Venus using three quadrillion objects circulating between Venus and the sun every 2 hours, each traveling at 10% of the speed of light.[2]

G. David Nordley has suggested, in fiction,[25] 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. A proposal by Birch involves the use of dynamic compression members to transfer energy and momentum via high-velocity mass streams to a band around the equator of Venus. He calculated that a sufficiently high-velocity mass stream, at about 10% of the speed of light, could give Venus a day of 24 hours in 30 years.[23]

More recent research has however shown that the current slow rotation rate of Venus is not detrimental to the planet's capability to support an Earth-like climate. Rather, the slow rotation rate enables the formation of thick cloud layers on the side of the planet facing the sun, which raises planetary albedo and acts to cool the global temperature to Earth-like levels, despite the greater proximity to the sun. According to calculations, maximum temperatures would be just around 35 °C, given an Earth-like atmosphere.[26][27] Speeding up the rotation rate would therefore be both impractical and detrimental to the terraforming effort. A terraformed Venus with the current slow rotation but a more Earth-like atmosphere would result in a global climate similar to warmer versions of Alaska or Siberia with "day" and "night" periods each about 58 days long. The "day" would resemble a short summer with warm humid climate, a heavy overcast sky and ample rainfall. The "night" would resemble a short winter with quite cold temperature and snowfall. There might be periods with more temperate climate and clear weather at sunrise and sunset.[26]

See also

References

  1. ↑ Adelman, Saul (1982). "Can Venus Be Transformed into an Earth-Like Planet?". Journal of the British Interplanetary Society. 35: 3–8. Bibcode:1982JBIS...35....3A.
  2. 1 2 3 4 Fogg, Martyn J. (1995). Terraforming: Engineering Planetary Environments. SAE International, Warrendale, PA. ISBN 1-56091-609-5.
  3. 1 2 3 4 5 Landis, Geoffrey (2011). "Terraforming Venus: A Challenging Project for Future Colonization" (PDF). doi:10.2514/6.2011-7215. Paper AIAA-2011-7215, AIAA Space 2011 Conference & Exposition, Long Beach CA, Sept. 26–29, 2011.
  4. 1 2 3 Sagan, Carl (1961). "The Planet Venus". Science. 133 (3456): 849–58. Bibcode:1961Sci...133..849S. doi:10.1126/science.133.3456.849. PMID 17789744.
  5. ↑ Greenhouse effect, clouds and winds. Venus express mission, European Space Agency.
  6. ↑ Sagan, Carl (1994). Pale Blue Dot (book). ISBN 0-345-37659-5.
  7. ↑ Oberg, James E. (1981). New Earths, Stackpole Books 1981; New American Library 1983. ISBN 0-8117-1007-6; ISBN 978-0-452-00623-2
  8. ↑ Marchal, C (1983). "The Venus-New-World Project". Acta Astronautica. 10 (5–6): 269–275. Bibcode:1983AcAau..10..269M. doi:10.1016/0094-5765(83)90076-0.
  9. 1 2 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.
  10. ↑ Landis, Geoffrey A., "Terraforming: Engineering Planetary Environments (review)" (also available here) (accessed 25 Dec, 2016).
  11. ↑ Fogg, M. J. (1987). "The Terraforming of Venus," Journal of the British Interplanetary Society, 40, pp. 551–564. (abstract).
  12. ↑ Kelly Beatty, J (ed.) (1999) The New Solar System, p176, CUP, ISBN 0-933346-86-7
  13. ↑ Smith, Alexander G (1989). "Transforming Venus by Induced Overturn". Journal of the British Interplanetary Society. 42: 571–576. Bibcode:1989JBIS...42..571S.
  14. ↑ Bullock, M.A.; Grinspoon, D.G. (1996). "The Stability of Climate on Venus" (PDF). J. Geophys. Res. 101: 7521–7529. Bibcode:1996JGR...101.7521B. doi:10.1029/95JE03862. Archived from the original (PDF) on 2004-09-20.
  15. ↑ Gillett, Stephen L. (1996). "Inward Ho!". In Stanley Schmidt; Robert Zubrin. Islands in the Sky: Bold New Ideas for Colonizing Space. John Wiley & Sons. pp. 78–84. ISBN 0-471-13561-5.
  16. 1 2 3 4 Birch, Paul (1991). "Terraforming Venus Quickly" (PDF). Journal of the British Interplanetary Society.
  17. ↑ Sakamaki, Tatsuya; Ohtani, Eiji; Fukui, Hiroshi; Kamada, Seiji; Takahashi, Suguru; Sakairi, Takanori; Takahata, Akihiro; Sakai, Takeshi; Tsutsui, Satoshi; Ishikawa, Daisuke; Shiraishi, Rei; Seto, Yusuke; Tsuchiya, Taku; Baron, Alfred Q. R. (1 February 2016). "Constraints on Earth's inner core composition inferred from measurements of the sound velocity of hcp-iron in extreme conditions". Science Advances. 2 (2): e1500802. Bibcode:2016SciA....2E0802S. doi:10.1126/sciadv.1500802 – via advances.sciencemag.org.
  18. ↑ Pollack, J.B.; Sagan, C. (1994). Lewis, J.; Matthews, M., eds. Resources of Near-Earth Space. Tucson: University of Arizona Press. pp. 921–950.
  19. ↑ Zubrin, Robert (1999). Entering Space: Creating a Spacefaring Civilization.
  20. ↑ Landis, Geoffrey A. (Feb 2–6, 2003). "Colonization of Venus". Conference on Human Space Exploration, Space Technology & Applications International Forum, Albuquerque NM.
  21. ↑ National Space Science Data Center (NSSDC), Venus Fact Sheet (retrieved 25 April 2017).
  22. ↑ Multiplanetary Society: A New Approach to Terraforming Venus, vol. 12. March 2018, Author WPM
  23. 1 2 Birch, Paul (1993). "How to Spin a Planet" (PDF). Journal of the British Interplanetary Society.
  24. ↑ Astronomers hatch plan to move Earth's orbit from warming sun, CNN.com
  25. ↑ Nordley, Gerald David (May 1991). "The Snows of Venus". Analog Science Fiction and Science Fact.
  26. 1 2 , STRONG DEPENDENCE OF THE INNER EDGE OF THE HABITABLE ZONE ON PLANETARY ROTATION RATE
  27. ↑ Way, M. J. (2016). "Was Venus the first habitable world of our solar system?". Geophysical Research Letters. 43: 8376–8383. arXiv:1608.00706. Bibcode:2016GeoRL..43.8376W. doi:10.1002/2016GL069790.
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