An artist’s conception shows a terraformed Mars in four stages of development.
Terraforming (literally, “Earth-shaping“) of a planet, moon, or other body is the hypothetical process of deliberately modifying its atmosphere, temperature, surface topography or ecology to be similar to the biosphere of Earth, in order to make it habitable by humans.

The term is sometimes used more generally as a synonym for planetary engineering, although some consider this more general usage an error. The concept of terraforming developed from both science fiction and actual science. The term was coined by Jack Williamson in a science-fiction story (“Collision Orbit“) published during 1942 in Astounding Science Fiction, but the concept may pre-date this work.

Based on experiences with Earth, the environment of a planet can be altered deliberately; however, the feasibility of creating an unconstrained planetary biosphere that mimics Earth on another planet has yet to be verified. 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 of altering the climate of Mars may fall within humanity’s technological capabilities, but at present the economic resources required to do so are far beyond that which any government or society is willing to allocate to the purpose. The long timescales and practicality of terraforming are the subject of debate. Other unanswered questions relate to the ethics, logistics, economics, politics, and methodology of altering the environment of an extraterrestrial world.

History of scholarly study

Carl Sagan, an astronomer, proposed the planetary engineering of Venus in an article published in the journal Science in 1961. Sagan imagined seeding the atmosphere of Venus with algae, which would convert water, nitrogen and carbon dioxide into organic compounds. As this process removed carbon dioxide from the atmosphere, the greenhouse effect would be reduced until surface temperatures dropped to “comfortable” levels. The resulting carbon, Sagan supposed, would be incinerated by the high surface temperatures of Venus, and thus be sequestered in the form of “graphite or some involatile form of carbon” on the planet’s surface. However, later discoveries about the conditions on Venus made this particular approach impossible. One problem is that the clouds of Venus are composed of a highly concentrated sulfuric acid solution. Even if atmospheric algae could thrive in the hostile environment of Venus’ upper atmosphere, an even more insurmountable problem is that its atmosphere is simply far too thickā€”the high atmospheric pressure would result in an “atmosphere of nearly pure molecular oxygen” and cause the planet’s surface to be thickly covered in fine graphite powder. This volatile combination could not be sustained through time. Any carbon that was fixed in organic form would be liberated as carbon dioxide again through combustion, “short-circuiting” the terraforming process.

Sagan also visualized making Mars habitable for human life in “Planetary Engineering on Mars” (1973), an article published in the journal Icarus. 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 McKay’s use of the word 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. Motivated by Lovelock’s book, biophysicist Robert Haynes worked behind the scenes to promote terraforming, and contributed the word ecopoiesis to its lexicon.

Beginning in 1985, Martyn J. Fogg began publishing several articles on terraforming. He also served as editor for a full issue on terraforming for the Journal of the British Interplanetary Society in 1991. In his book Terraforming: Engineering Planetary Environments (1995), Fogg proposed the following definitions for different aspects related to terraforming:

Fogg also devised definitions for candidate planets of varying degrees of human compatibility:

Fogg suggests that Mars was a biologically compatible planet in its youth, but is not now in any of these three categories, since it could only be terraformed with greater difficulty. Mars Society founder Robert Zubrin produced a plan for a Mars return mission called Mars Direct that would set up a permanent human presence on Mars and steer efforts towards eventual terraformation.

See also: