Terraforming

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Terraforming (literally, "Earth-shaping") is the process of modifying a planet, moon or other body to a more habitable atmosphere, temperature or ecology.

Contents

History

In fiction

The term first appeared in a science fiction novel, Seetee Shock (1949) by Jack Williamson, but the actual concept pre-dates this work. Olaf Stapledon's First and Last Men (1930) provides an example in fiction in which Venus is modified, after a long and destructive war with the original inhabitants, who naturally object to the process. Early fictional accounts of the process are frequently handicapped by the inaccurate contemporary knowledge of the actual conditions, as in the Stapledon example, which had Venus covered in oceans.

A more recent example, using the actual conditions on Mars as revealed by planetary probes to that time, is the Mars trilogy by Kim Stanley Robinson. The three volumes provide a lengthy description of a fictional terraforming of Mars, and very evidently result from a massive amount of research by the author.

Scholarly study

In the 1960s the astronomer and popularizer of science Carl Sagan proposed terraforming the planet Venus by seeding its atmosphere with algae, which would remove carbon dioxide and reduce the greenhouse effect until surface temperatures dropped to "comfortable" levels. Later discoveries about the conditions on Venus made this particular approach impossible, however: Venus simply has too much atmosphere to process and sequester. Even if atmospheric algae could thrive in the hostile and arid environment of Venus' upper atmosphere, any carbon that was fixed in organic form would be liberated as carbon dioxide again as soon as it fell into the hot lower regions.

Today Mars seems the most feasible local planet for terraforming. Robert Zubrin, the founder of the Mars Society, has presented a well-elaborated plan for this. He wants to start with a Mars return mission called Mars Direct.

The principal reason given to pursue terraforming is the creation of worlds suitable for habitation by human beings and of an ecology to support them. However, some researchers believe that space habitats will provide a more economical means for supporting space colonization.

If research in nanotechnology and other advanced chemical processes continues apace, it may become feasible to terraform planets in centuries rather than millennia. On the other hand, it may become reasonable to modify humans so that they don't require an oxygen/nitrogen atmosphere in a 1g gravity field to live comfortably. That would then reduce the need to terraform worlds, or at least the degree to which other worlds' environments would need to be altered.

Theoretical methods of terraforming

Mars

  • Orbiting mirrors: large mirrors made of extremely thin aluminized mylar could be placed in orbit to increase the total insolation Mars receives. This would increase the planet's temperature directly, and also vaporize water and carbon dioxide to increase the planet's greenhouse effect.
  • Moving ammonia-containing asteroids: Since ammonia is a powerful greenhouse gas, and it is possible that nature has stockpiled large amounts of it in frozen form on asteroidal sized objects orbiting in the outer solar system, we could move these and send them into Mars atmosphere.
  • Producing halocarbons on Mars: Halocarbons (such as CFCs) are powerful greenhouse gases, and are stable for lengthy periods in atmospheres. They could be produced by genetically engineered aerobic bacteria or by mechanical contraptions scattered across the planet's surface.

There is some scientific debate over whether it would even be possible to terraform Mars, or how stable its climate would be once terraformed. It is possible that over geological timescales - tens or hundreds of millions of years - Mars could lose its water and atmosphere again, possibly to the same processes that reduced it to its current state.

Indeed, it is thought that Mars once did have a relatively Earthlike envrionment early in its history, with a thicker atmosphere and abundant water that was lost over the course of hundreds of millions of years. The exact mechanism of this loss is still unclear, though several mechanisms have been proposed. The lack of a magnetosphere surrounding Mars may have allowed the solar wind to erode the atmosphere, the relatively low gravity of Mars helping to accelerate the loss of lighter gasses to space. The lack of plate tectonics on Mars is another possibility, preventing the recycling of gasses locked up in sediments back into the atmosphere. The lack of magnetic field and geologic activity may both be a result of Mars' smaller size allowing its interior to cool more quickly than Earth's, though the details of such processes are still sketchy. However, none of these processes are likely to be significant on the timescale of human civilization, or even over the typical lifespan of most animal species, and the slow loss of atmosphere could possibly be counteracted with ongoing low-level artificial terraforming activities.

Venus

Terraforming Venus requires two major changes; removing most of the planet's dense 9 MPa carbon dioxide atmosphere and reducing the planet's 737K surface temperature. These goals are closely interrelated, since Venus' extreme temperature is due to the greenhouse effect caused by its dense atmosphere.

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.

Removal of Venus' atmosphere could be attempted by a variety of methods, possibly in combination.

Removing atmosphere

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 700km diameter striking Venus at greater than 20km/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' 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' 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, to sequester carbon dioxide in the form of calcium and magnesium carbonates.

Bombardment of Venus with hydrogen, possibly from some outer solar system source, reacting with carbon dioxide to produce elemental carbon (graphite) and water by the Bosch reaction. It would take about 4�1019kg of hydrogen to convert the whole Venerian atmosphere, and the resulting water would cover about 80% of the surface compared to 70% for Earth. A solar shade or equivalent would also be necessary, as water vapor is itself a greenhouse gas.

Other modifications

Venus' 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' rotation would require many orders of magnitude greater amounts of energy than removing its atmosphere would, and so is likely to be infeasible. 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.

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.

Paraterraforming

Also known as the "worldhouse" concept, paraterraforming involves the construction of a habitable enclosure on a planet which eventually grows to encompass most of the planet's usable area. The enclosure would consist of a transparent roof held one or more kilometers above the surface, pressurized with a breathable atmosphere, and anchored with tension towers and cables at regular intervals. A worldhouse can be constructed with technology known since the 1960s.

Paraterraforming has several advantages over the traditional approach to terraforming. For example, it provides an immediate payback to investors; the worldhouse starts out small in area, but those areas provide habitable space from the start. The paraterraforming approach also allows for a modular approach that can be tailored to the needs of the planet's population, growing only as fast and only in those areas where it is required. Finally, paraterraforming greatly reduces the amount of atmosphere that one would need to add to planets like Mars in order to provide Earthlike atmospheric pressures. By using a solid envelope in this manner, even bodies which would otherwise be unable to retain an atmosphere at all (such as asteroids) could be given a habitable environment. The environment under an artificial worldhouse roof would also likely be more amenable to artificial manipulation.

It has the disadvantage of requiring a great deal of construction and maintenance activity, the cost of which could be ameliorated to some degree through the use of automated manufacturing and repair mechanisms. A worldhouse could also be more susceptible to catastrophic failure in the event of a major breach, though this risk can likely be reduced by compartmentalization and other active safety precautions.

Geoengineering

Geoengineering is the deliberate modification of Earth's environment on a large scale, in a sense "terraforming" Earth itself. Currently a great deal of debate takes place over the notion that human civilization has already inadvertently modified Earth's climate through the industrial emission of greenhouse gases, and proposals have been made to counter any such effects through further deliberate geoengineering. For example, some have proposed to put large mirrors in orbit which would modify the insolation received by Earth - either increasing or decreasing it, as the need arises. Large-scale sequestration of carbon dioxide inside geological formations or ocean sediment, modification of Earth's albedo with reflective or absorbtive materials spread over portions of its surface, or the alteration of rainfall patterns through the creation of artificial seas are other examples which have been seriously considered.

Terraforming in Science Fiction

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