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Terraforming is the process of altering a planet to make it more suitable for life (habitable). Usually this means making the planet suitable for most, if not all, Earth life. However, if there is dormant or hidden life on the planet, terraforming will change conditions so that this life can possibly flourish. In terraforming, there are intermediate stages where the planet has become habitable, but only to organisms that can survive in extreme environments.

Until recently, the topic of terraforming Mars was considered more the subject of science fiction novels rather than serious scientific discussion. But it is now known that we can change the climate of a planet, as we are inadvertently doing it on Earth. In addition, it is thought that billions of years ago Mars did have a climate suitable for life. The main focus of current scientific studies of terraforming is the restoration of Mars to habitable conditions.

The Restoration of Mars

Mars can be made suitable for life by changing its climate; there is no need to alter its distance from the Sun, its rotation rate, or the tilt of its axis. Exploration of Mars indicates that it already has enough carbon dioxide, nitrogen, and water to build a biosphere . The challenge is to warm the planet and release those compounds. Mars is the only one of the inner planets that can be made habitable simply by changing its climate. It is not possible to move Venus or make it spin faster or to add an atmosphere to Mercury or the Moon to make them habitable. Mars is the only practical target for near-term terraforming.

The Habitability of Mars and Earth

In considering the possibility of restoring habitable conditions to Mars, it is important to define that term. The basic approach to this question is to look at Earth. Clearly, the present environment on Earth is habitable to microorganisms, plants, and animals. But Earth has not always been this way. For most of Earth's early history, oxygen was not present and carbon dioxide levels were much higher than they are today. This early environment was habitable for microorganisms and would be habitable for most plants but not for animals and humans, which require high oxygen levels and low carbon dioxide levels. On Mars the natural habitable condition is one with high carbon dioxide and only a little oxygen.

In a habitable state, Mars would have a thick atmosphere about one to two times sea-level air pressure on Earth. This atmosphere would be composed primarily of carbon dioxide, with lower levels of nitrogen and small amounts of oxygen produced by sunlight. There may be enough oxygen to create a thin but effective ozone shield, but there will not be enough for humans and animals to breathe. This restored environment would be similar

Day Length24h24h 37min 22.66sec117 days
Year365 days687 days225 days
Axis Tilt23°1225°122°36
Ave. Sunlight345 W/m2147 W/m2655 W/m2
Ave. Temperature+15°C60°C+460°C
Temperature Range60°C to +50°C145°C to +20°C460°C to +460°C
Pressure1 atm (101.3 kPa)1/120 atm95 atm
AtmosphereN2, O2CO2CO2

to what the Martian environment might have been like 3 to 4 billion years ago, when Mars may have had a biosphere.

Currently Mars is too cold (60°) and has an atmosphere that is too thin to allow liquid water on the surface; thus, it cannot support life. Therefore, the first step in making Mars habitable is to increase the temperature and the atmospheric pressure enough for liquid water to be stable. The most effective method is the use of super-greenhouse gases known as perfluoro-carbons (PFCs). These gases have a strong warming effect even at very low concentrations, as has been seen on Earth. PFCs are not toxic to plants and animals. Unlike chlorofluorocarbons, PFCs do not contain chlorine or bromine, and thus they would not destroy the ozone layer that would form as the atmosphere thickened.

There have been other suggestions of ways to warm Mars, such as placing large orbiting mirrors, sprinkling the poles with dark dust, and crashing asteroids and comets into the surface. Unlike the use of PFCs, none of these methods are practical with today's technology.

As the temperature on Mars increases, carbon dioxide gas will be released from the regolith and the polar cap as it melts (the south polar cap is composed of frozen carbon dioxide and ice). This carbon dioxide will thicken the atmosphere and augment greenhouse warming. This positive feedback between thickening the atmosphere, warming the surface, and releasing carbon dioxide will continue until all the carbon dioxide is in the atmosphere. Calculations indicate that in a concentration of a few parts per million, PFCs can trigger the outgassing of carbon dioxide. At this stage, Mars would be a warm, wet world if the regolith and polar regions have the amount of carbon dioxide and water ice it is thought they havebetween 100 and 1000 mbars.

If there is dormant life on Mars, it would expand rapidly into this recreated warm and wet world. The surface would once again be full of Martians. If there is no life on Mars, microorganisms and plants could be introduced from Earth.

Ecological Changes and the Martian Biosphere

The ecological changes on Mars as it warms up will be like hiking down a mountain: from barren frozen rock at the top, through alpine tundra and arctic and alpine grasses, and eventually to trees and forests.

The first Martian pioneers from Earth will be organisms that live in the coldest, driest, most Mars-like environment in the world. These are thecryptoendolithic microbial ecosystems found in Antarctica. In the cold, dry, ice-free regions of Antarctica,lichens ,algae , and bacteria live a few millimeters below the surface of sandstone rocks, where there is a warmer, wetter environment than exists on the surface of the rock. Enough sunlight penetrates through the rock to allow photosynthesis. Similar microorganisms in a rock habitat could survive on Mars when the air temperatures reached 10°C in the daytime for a few weeks during the warmest part of the year.

With further warming and extension of the growing season, alpine plants might survive and cover vast equatorial regions. The first introduction of photosynthetic microbial ecosystems and arctic and alpine tundra will be of biological interest. However, only with the development of ecosystems based on higher plants will the ecological development of Mars become significant in terms of the production of oxygen.

Although plants will be the major biological force on Mars, as they are on Earth, small animals also could play a key role. Insects and soil invertebrates, such as earthworms, would be important in the developing ecosystems. For example, pollination by flying insects would greatly increase the diversity of plants that can be grown on Mars at every stage of the process. Unfortunately, the minimum oxygen requirements and maximum carbon dioxide tolerance of flying insects at a third of Earth gravity remains unknown.

Although life-forms from Earth might be introduced to Mars in a careful sequence, this does not imply that the resulting biosphere will develop as predicted. As life on Mars interacts with itself and the changing environment, it will follow an independent evolutionary path that will be impossible to control. This should be considered a good thing. The resulting biological system is more likely to be stable and globally adapted to the altered environment than would any preconceived ecosystems, and studying such an independent evolutionary path will contribute to scientific knowledge.

By calculating the energy required to change Mars, it is possible to estimate how long the process might take. The results indicate that to warm Mars and introduce plant life would take about 100 years. It would take another 100,000 years for those plants to produce enough oxygen for humans to breathe. In the meantime humans would have to wear small oxygen masks but not pressurized space suits.

In the long term Mars will once again decay and lose its atmosphere as the carbon dioxide dissolves in water and is turned into carbonate . However, this will take 10 to 100 million yearslong enough for a biosphere to develop.

Ethical Issues

Although terraforming a planet is technologically feasible, is it ethically correct? Perhaps the most difficult issue is the possibility that life may already be present on the planet. In terraforming Mars, the first step would be creating a thick carbon dioxide atmosphere that supports a warmer and wetter planet. These conditions closely resemble those on early Mars, when any Martian life-forms would have developed, and therefore are the conditions they are adapted to. Terraforming Mars will make the planet more favorable to any present Mars organisms rather than having the unwanted effect of destroying a different life-form.

Terraforming has as its goal the spreading of life. The process can be seen as part of evolution, in which organisms expand into every available niche either by adapting or by changing the environment. Humans can help this spread of life and contribute in a positive way to the ecological development of the solar system.

see also Astrobiology (volume 4); Domed Cities (volume 4); Environmental Changes (volume 4); Exploration Programs (volume 2); Living on Other Worlds (volume 4); Mars (volume 2); Mars Bases (volume 4); Scientific Research (volume 4); Social Ethics (volume 4).

Christopher P. McKay andMargarita M. Marinova


Clarke, Arthur C. The Snows of Olympus. London: Victor Gollancz, 1995.

Fogg, Martyn J. Terraforming: Engineering Planetary Environments. Warrendale, PA:SAE, 1995.

McKay, Christopher P. "Bringing Life to Mars."Scientific American Presents 10, no.1 (1999):52-57.

McKay, Christopher P., and Margarita M. Marinova. "The Physics, Biology, and Environmental Ethics of Making Mars Habitable."Astrobiology 1 (2001):89-109.

McKay, Christopher P., Owen B. Toon, and James F. Kasting. "Making Mars Habitable."Nature 352 (1991):489-496.