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Terraforming or terraformation ("Earth-shaping") is the hypothesis process of deliberately modifying the atmosphere, temperature, surface topography or ecology of a planet, moon, or other body to be similar to the environment of Earth, with the goal of making it habitable for humans.
The concept of terraforming developed from both science fiction and actual science. Carl Sagan, an astronomer, proposed the planetary engineering of Venus in 1961, which is considered one of the first accounts of the concept. The term was coined by Jack Williamson in a science-fiction short story ("Collision Orbit") published in 1942 in Astounding Science Fiction.
Even if the environment of a planet could be altered deliberately, the feasibility of creating an unconstrained planetary environment that mimics Earth on another planet has yet to be verified. While Venus and the Moon have been studied in relation to the subject, Mars is usually considered to be the most likely candidate for terraforming. Much study has been done concerning the possibility of heating the planet and altering its atmosphere, and NASA has even hosted debates on the subject. Several potential methods for the terraforming of Mars may be within humanity's technological capabilities, but according to Martin Beech, the economic attitude of preferring short-term profits over long-term investments will not support a terraforming project. "The present economic fashion of favoring short-term gain over long term investment will never be able to support a terraforming project."
The long timescales and practicality of terraforming are also the subject of debate. As the subject has gained traction, research has expanded to other possibilities including biological terraforming, para-terraforming, and modifying humans to better suit the environments of planets and moons. Despite this, questions still remain in areas relating to the ethics, logistics, economics, politics, and methodology of altering the environment of an extraterrestrial world, presenting issues to the implementation of the concept.
Sagan also visualized making Mars habitable for human life in an article published in the journal Icarus, "Planetary Engineering on Mars" (1973). Three years later, NASA addressed the issue of planetary engineering officially in a study, but used the term "planetary ecosynthesis" instead. The study concluded that it was possible for Mars to support life and be made into a habitable planet. The first conference session on terraforming, then referred to as "Planetary Modeling", was organized that same year.
In March 1979, NASA engineer and author James Oberg organized the First Terraforming Colloquium, a special session at the Lunar and Planetary Science Conference in Houston. Oberg popularized the terraforming concepts discussed at the colloquium to the general public in his book New Earths (1981). Not until 1982 was the word terraforming used in the title of a published journal article. Planetologist Christopher McKay wrote "Terraforming Mars", a paper for the Journal of the British Interplanetary Society. The paper discussed the prospects of a self-regulating Martian biosphere, and the word "terraforming" has since become the preferred term.
In 1984, James Lovelock and Michael Allaby published The Greening of Mars. Lovelock's book was one of the first to describe a novel method of warming Mars, where chlorofluorocarbons (CFCs) are added to the atmosphere to produce a strong greenhouse effect.
Motivated by Lovelock's book, biophysicist Robert Haynes worked behind the scenes to promote terraforming, and contributed the neologism Ecopoiesis,Haynes, RH (1990), "Ecce Ecopoiesis: Playing God on Mars", in MacNiven, D. (1990-07-13), Moral Expertise: studies in practical and professional ethics, Routledge. pp. 161–163. . forming the word from the Greek οἶκος, oikos, "house",. and ποίησις, poiesis, "production".. Ecopoiesis refers to the origin of an ecosystem. In the context of space exploration, Haynes describes ecopoiesis as the "fabrication of a sustainable ecosystem on a currently lifeless, sterile planet". Fogg defines ecopoiesis as a type of planetary engineering and is one of the first stages of terraformation. This primary stage of ecosystem creation is usually restricted to the initial seeding of microbial life. A 2019 opinion piece by Lopez, Peixoto and Rosado has reintroduced microbiology as a necessary component of any possible colonization strategy based on the principles of microbial symbiosis and their beneficial ecosystem services. As conditions approach that of Earth, plant life could be brought in, and this will accelerate the production of oxygen, theoretically making the planet eventually able to support animal life.
Fogg also devised definitions for candidate planets of varying degrees of human compatibility:Fogg, 1996
Fogg suggests that Mars was a biologically compatible planet in its youth, but is not now in any of these three categories, because it can only be terraformed with greater difficulty.
Classifications of the criteria of habitability can be varied, but it is generally agreed upon that the presence of water, non-extreme temperatures, and an energy source put broad constraints on habitability. Other requirements for habitability have been defined as the presence of raw materials, a solvent, and clement conditions, or CHON (such as carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur), and reasonable physiochemical conditions. When applied to organisms present on the earth, including humans, these constraints can substantially narrow.
In its astrobiology roadmap, NASA has defined the principal habitability criteria as "extended regions of liquid water, conditions favorable for the assembly of complex , and energy sources to sustain metabolism."
Much of Earth's biomass (~60%) relies on photosynthesis for an energy source, while a further ~40% is Chemotroph. For the development of life on other planetary bodies, chemical energy may have been critical, while for sustaining life on another planetary body in the Solar System, sufficiently high solar energy may also be necessary for phototrophic organisms.
Once conditions become more suitable for life of the introduced species, the importation of microbial life could begin. As conditions approach that of Earth, plants could also be brought in. This would accelerate the production of oxygen, which theoretically would make the planet eventually able to support animal life.
The exact mechanism of this loss is still unclear, though three mechanisms, in particular, seem likely: First, whenever surface water is present, carbon dioxide () reacts with rocks to form , thus drawing atmosphere off and binding it to the planetary surface. On Earth, this process is counteracted when plate tectonics works to cause volcanic eruptions that vent carbon dioxide back to the atmosphere. On Mars, the lack of such tectonic activity worked to prevent the recycling of gases locked up in sediments.Forget, Costard & Lognonné 2007, pp. 80–2.
Second, the lack of a magnetosphere around Mars may have allowed the solar wind to gradually erode the atmosphere. Convection within the core of Mars, which is made mostly of iron, originally generated a magnetic field. However the dynamo theory ceased to function long ago,Schubert, Turcotte & Olson 2001, p. 692 and the magnetic field of Mars has largely disappeared, probably due to "loss of core heat, solidification of most of the core, and/or changes in the mantle convection regime." (2025). 9780124158450 ISBN 9780124158450 Results from the NASA MAVEN mission show that the atmosphere is removed primarily due to coronal mass ejection events, where outbursts of high-velocity protons from the Sun impact the atmosphere. Mars does still retain a limited magnetosphere that covers approximately 40% of its surface. Rather than uniformly covering and protecting the atmosphere from solar wind, however, the magnetic field takes the form of a collection of smaller, umbrella-shaped fields, mainly clustered together around the planet's southern hemisphere.Solar Wind, 2008
Finally, between approximately 4.1 and 3.8 billion years ago, asteroid impacts during the Late Heavy Bombardment caused significant changes to the surface environment of objects in the Solar System. The low gravity of Mars suggests that these impacts could have ejected much of the Martian atmosphere into deep space.Forget, Costard & Lognonné 2007, pp. 80.
Terraforming Mars would entail two major interlaced changes: building the atmosphere and heating it.Faure & Mensing 2007, p. 252. A thicker atmosphere of greenhouse gases such as carbon dioxide would trap incoming solar radiation. Because the raised temperature would add greenhouse gases to the atmosphere, the two processes would augment each other. Carbon dioxide alone would not suffice to sustain a temperature above the freezing point of water, so a mixture of specialized greenhouse molecules might be manufactured.
Venus's atmosphere currently contains little oxygen, so an additional step would be to inject breathable O2 into the atmosphere. An early proposal for such a process comes from Carl Sagan, who suggested the injection of floating, Photosynthesis bacteria into the Venusian atmosphere to reduce CO2 to Organic carbon, and increase the atmospheric concentration of O2 in the atmosphere. This concept, however, was based in a flawed 1960s understanding of Venus's atmosphere as much lower pressure; in reality, the Venusian atmospheric pressure (9300 kPa) is far higher than early estimates. Sagan's idea is therefore untenable, as he later conceded.
An additional step noted by Martin Beech includes the injection of water and/or hydrogen into the planetary atmosphere; this step follows after sequestering CO2 and reducing the mass of the atmosphere. In order to combine hydrogen with O2 produced by other means, an estimated 4×1019 kg of hydrogen is necessary; this may need to be mined from another source, such as Uranus or Neptune.
Despite being much smaller than Mars, Mercury has an escape velocity only slightly less than that of Mars due to its higher density and could, if a magnetosphere prevents atmospheric stripping, hold a nitrogen/oxygen atmosphere for millions of years.
To provide one atmosphere of pressure, roughly 1.1×1018 of gas would be required;Roy, Kenneth (2015). "Terraforming Mercury". In Inner Solar System: Prospective Energy and Material Resources, pp. 421-435. Springer International Publishing. pdf retrieved 29 Dec. 2023. or a somewhat lower amount if lower pressure is acceptable. Water could be delivered from the outer Solar System. Once this water has been delivered, it would split the water into its constituent oxygen and hydrogen molecules, possibly using a photo-catalytic dust, with the hydrogen rapidly being lost to space. At an oxygen pressure of 20-30 kPa, the atmosphere would be breathable and nitrogen may be added as required to allow for plant growth in the presence of .
Temperature management would be required, due to the equilibrium average temperature of ~159°C. However, millions of square kilometers at the poles have an average temperature of 0-50°C ( i.e., an area the size of Mexico at each pole with habitable temperatures). The total habitable area could be even larger if the planetary albedo were increased from 0.12 to ~0.6, potentially increasing the habitable area. Roy proposes that the temperature could be further managed by decreasing the solar flux at Mercury to near the terrestrial value by solar sails reflecting sunlight. He calculates that 16 to 17 million sails, each with an area of one square kilometer would be needed.
The moons are covered in ice, so heating them would make some of this ice sublimate into an atmosphere of water vapour, ammonia and other gases. For Jupiter's moons, the intense radiation around Jupiter would cause radiolysis of water vapour, splitting it into hydrogen and oxygen. The former would be rapidly lost to space, leaving behind the oxygen (this already occurs on the moons to a minor extent, giving them thin atmospheres of oxygen). For Saturn's moons, the water vapour could be split by using orbital mirrors to focus sunlight, causing photolysis. The ammonia could be converted to nitrogen by introducing bacteria such as Nitrosomonas, Pseudomonas and Clostridium, resulting in an Earth-like nitrogen-oxygen atmosphere. This atmosphere would protect the surface from Jupiter's radiation, but it would also be possible to clear said radiation using orbiting tethers or radio waves.
Challenges to terraforming the moons include their high amounts of ice and their low gravity. If all of the ice were fully melted, it would result in deep moon-spanning oceans, meaning any settlements would have to be floating (unless some of the ice was allowed to remain, to serve as land). Low gravity would cause atmospheric escape over time and may cause problems for human health. However, atmospheric escape would take place over spans of time that are long compared to human lifespans, as with the Moon.
One proposal for terraforming Ceres would involve heating it (using orbital mirrors, detonating thermonuclear devices or colliding small asteroids with Ceres), creating an atmosphere and deep ocean. However, this appears to be based on a misconception that Ceres' surface is icy in a similar way to the gas giant moons. In reality, Ceres' surface is "a layer of mixed ice, silicates and light strong phases best matched by hydrated salts and clathrates". It is unclear what the result of heating this up would be.
As synthetic biology matures over the coming decades it may become possible to build designer organisms from scratch that directly manufacture desired products efficiently. Lisa Nip, Ph.D. candidate at the MIT Media Lab's Molecular Machines group, said that by synthetic biology, scientists could genetically engineer humans, plants and bacteria to create Earth-like conditions on another planet.
Gary King, microbiologist at Louisiana State University studying the most extreme organisms on Earth, notes that "synthetic biology has given us a remarkable toolkit that can be used to manufacture new kinds of organisms specially suited for the systems we want to plan for" and outlines the prospects for terraforming, saying "we'll want to investigate our chosen microbes, find the genes that code for the survival and terraforming properties that we want (like radiation and drought resistance), and then use that knowledge to genetically engineer specifically Martian-designed microbes". He sees the project's biggest bottleneck in the ability to genetically tweak and tailor the right microbes, estimating that this could take "a decade or more" to be solved. He also notes that it would be best to develop "not a single kind of microbe but a suite of several that work together".
DARPA is researching the use of photosynthesizing plants, bacteria, and algae grown directly on the Mars surface that could warm up and thicken its atmosphere. In 2015 the agency and some of its research partners created a software called DTA GView, in which Genome of several organisms can be pulled up on the program to immediately show a list of known genes and where they are located in the genome. According to Alicia Jackson, deputy director of DARPA's Biological Technologies Office, they have developed a "technological toolkit to transform not just hostile places here on Earth, but to go into space not just to visit, but to stay".
Potential targets for paraterraforming include Mercury, the Moon, Ceres and the gas giant moons.
On the pro-terraforming side of the argument, there are those like Robert Zubrin, Martyn J. Fogg, Richard L. S. Taylor, and the late Carl Sagan who believe that it is humanity's moral obligation to make other worlds suitable for human life, as a continuation of the history of life-transforming the environments around it on Earth.Robert Zubrin, , pp. 248–249, Simon & Schuster/Touchstone, 1996, Fogg 2000 They also point out that Earth would eventually be destroyed if nature takes its course, so that humanity faces a very long-term choice between terraforming other worlds or allowing all terrestrial life to become extinction. Terraforming totally barren planets, it is asserted, is not morally wrong as it does not affect any other life.
The opposing argument posits that terraforming would be an unethical interference in nature, and that given humanity's past treatment of Earth, other planets may be better off without human interference. Still others strike a middle ground, such as Christopher McKay, who argues that terraforming is ethically sound only once it is completely certain that an alien planet does not harbor life of its own; but that if it does, it should not try be reshaped to fit humans' own use, but rather to engineer its environment to artificially nurture the alien life and help it thrive and co-evolve, or even co-exist with humans.Christopher McKay and Robert Zubrin, "Do Indigenous Martian Bacteria have Precedence over Human Exploration?", pp. 177–182, in On to Mars: Colonizing a New World, Apogee Books Space Series, 2002, Even this would be seen as a type of terraforming to the strictest of ecocentrists, who would say that all life has the right, in its home biosphere, to evolve without outside interference.
A related concept from science fiction is xenoforming – a process in which aliens change the Earth or other planets to suit their own needs, already suggested in the classic The War of the Worlds (1898) of H.G. Wells.
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