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The concept relies on the assumption that the environment of a planet can be altered through artificial means. In addition, the feasibility of creating a planetary biosphere on Mars is undetermined. There are several proposed methods, some of which present prohibitive economic and natural resource costs, and others that may be currently technologically achievable.[1]
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Motivation and ethics
See also: Ethics of terraforming
Future population growth and demand for resources may necessitate human colonization of objects other than Earth, such as Mars, the Moon, and nearby planets. Space colonization will facilitate harvesting the Solar System's energy and material resources.[2]In many respects, Mars is the most Earth-like of all the other planets in the Solar System. It is thought[3] that Mars once did have a more Earth-like environment early in its history, with a thicker atmosphere and abundant water that was lost over the course of hundreds of millions of years. Given the foundations of similarity and proximity, Mars would make one of the most efficient and effective terraforming targets in the Solar System.
Ethical considerations of terraforming include the potential displacement of indigenous life, even if microbial, if such life exists.
Challenges and limitations
See also: Colonization of Mars
The Martian environment presents several terraforming challenges to
overcome and the extent of terraforming may be limited by certain key
environmental factors.Low gravity
See also: Effects of low gravity on humans
The surface gravity on Mars is 38% of that on Earth. It is not known if this is enough to prevent the health problems associated with weightlessness.[4]Additionally, the low gravity (and thus low escape velocity) of Mars may render it more difficult for it to retain an atmosphere when compared to the more massive Earth and Venus.[5] Earth and Venus are both able to sustain thick atmospheres, even though they experience more of the solar wind that is believed to strip away planetary volatiles. Continuing sources of atmospheric gases on Mars might therefore be required to ensure that an atmosphere sufficiently dense for humans is sustained in the long term.
Countering the effects of space weather
See also: Health threat from cosmic rays
Mars lacks a magnetosphere,
which poses challenges for mitigating solar radiation and retaining
atmosphere. It is believed that fields detected on Mars are remnants of a
magnetosphere that collapsed early in its history.The lack of a magnetosphere is thought to be one reason for Mars's thin atmosphere. Solar-wind-induced ejection of Martian atmospheric atoms has been detected by Mars-orbiting probes. Venus, however, clearly demonstrates that the lack of a magnetosphere does not preclude a dense atmosphere.
Earth abounds with water because its ionosphere is permeated with a magnetosphere. The hydrogen ions present in its ionosphere move very fast due to their small mass, but they cannot escape to outer space because their trajectories are deflected by the magnetic field. Venus has a dense atmosphere, but only traces of water vapor (20 ppm) because it has no magnetic field. The Martian atmosphere also loses water to space. Earth's ozone layer provides additional protection. Ultraviolet light is blocked before it can dissociate water into hydrogen and oxygen. Because little water vapor rises above the troposphere and the ozone layer is in the upper stratosphere, little water is dissociated into hydrogen and oxygen.
The Earth's magnetic field is 31 µT. Mars would require a similar magnetic-field intensity to similarly offset the effects of the solar wind at its distance further from the Sun. The technology for inducing a planetary-scale magnetic field does not currently exist.
The importance of magnetosphere has been brought into question. In the past, Earth has regularly had periods where the magnetosphere changed direction,[6] yet life has continued to survive. A thick atmosphere similar to Earth's could also provide protection against solar radiation in the absence of a magnetosphere.[7]
Advantages
See also: Atmosphere of Mars
The lack of both a magnetic field and geologic activity on Mars may be a result of its relatively small size, which allowed the interior to cool more quickly than Earth's, though the details of such a process are still not well understood.
It has been suggested that Mars once had an environment relatively similar to that of Earth during an earlier stage in its development.[8] Although water appears to have once been present on the Martian surface, water appears to exist at the poles just below the planetary surface as permafrost. On September 26, 2013, NASA scientists reported the Mars Curiosity rover detected "abundant, easily accessible" water (1.5 to 3 weight percent) in soil samples at the Rocknest region of Aeolis Palus in Gale Crater.[9][10][11][12][13]
The soil and atmosphere of Mars contain many of the main elements[which?] needed for life.[citation needed]
Large amounts of water ice exist below the Martian surface, as well as on the surface at the poles, where it is mixed with dry ice, frozen CO2. Significant amounts of water are stored in the south pole of Mars, which, if melted, would correspond to a planetwide ocean 11 meters deep.[14] Frozen carbon dioxide (CO2) at the poles sublimates into the atmosphere during the Martian summers, and small amounts of water residue are left behind, which fast winds sweep off the poles at speeds approaching 400 km/h (250 mph).[citation needed] This seasonal occurrence transports large amounts of dust and water vapor into the atmosphere, giving potential for Earth-like cirrus clouds.[citation needed]
Most of the oxygen in the Martian atmosphere is present as carbon dioxide (CO2), the main atmospheric component. Molecular oxygen (O2) only exists in trace amounts. Large amounts of elemental oxygen can be also found in metal oxides on the Martian surface, and in the soil, in the form of per-nitrates.[15] An analysis of soil samples taken by the Phoenix lander indicated the presence of perchlorate, which has been used to liberate oxygen in chemical oxygen generators.[16] Electrolysis could be employed to separate water on Mars into oxygen and hydrogen if sufficient liquid water and electricity were available.[citation needed]
Proposed methods and strategies
| Mars | Earth | |
|---|---|---|
| Pressure | 0.6 kPa (0.087 psi) | 101.3 kPa (14.69 psi) |
| Carbon dioxide (CO2) | 96.0% | 0.04% |
| Argon (Ar) | 2.1% | 0.93% |
| Nitrogen (N2) | 1.9% | 78.08% |
| Oxygen (O2) | 0.145% | 20.94% |
The tremendous air currents generated by the moving gases would create large, sustained dust storms, which would heat the atmosphere (by absorbing solar radiation).[citation needed]
Carbon dioxide sublimation
There is presently enough carbon dioxide (CO2) as ice in the Martian south pole and absorbed by regolith (soil) on Mars that, if sublimated to gas by a climate warming of only a few degrees, would increase the atmospheric pressure to 30 kilopascals (0.30 atm),[17] comparable to the altitude of the peak of Mount Everest, where the atmospheric pressure is 33.7 kilopascals (0.333 atm). Although this would not be breathable by humans, it is above the Armstrong limit and would eliminate the present need for pressure suits. Phytoplankton can also convert dissolved CO2 into oxygen, which is important because Mars's low temperature will, by Henry's law, lead to a high ratio of dissolved CO2 to atmospheric CO2 in the flooded[clarification needed] northern basin.Importing ammonia
Another more intricate method uses ammonia as a powerful greenhouse gas. It is possible that large amounts of it exist in frozen form on minor planets orbiting in the outer Solar System. It may be possible to move these and send them into Mars's atmosphere.[18] Because ammonia (NH3) is mostly nitrogen by weight, it could also supply the buffer gas for the atmosphere. Sustained smaller impacts will also contribute to increases in the temperature and mass of the atmosphere.The need for a buffer gas is a challenge that will face any potential atmosphere builders. On Earth, nitrogen is the primary atmospheric component, making up 78% of the atmosphere. Mars would require a similar buffer-gas component although not necessarily as much. Obtaining sufficient quantities of nitrogen, argon or some other comparatively inert gas is difficult.
Importing hydrocarbons
Another way to create a martian atmosphere would be to import methane or other hydrocarbons,[19][20] which are common in Titan's atmosphere (and on its surface). The methane could be vented into the atmosphere where it would act to compound the greenhouse effect.Methane (or other hydrocarbons) could be helpful to increase atmospheric pressure. These gases also can be used to produce water and CO2 for the Martian atmosphere:
This reaction could probably be initiated by heat or by Martian solar UV irradiation. Large amounts of the resulting products (CO2 and water) are necessary for photosynthesis, which would be the next step in terraforming.
Importing hydrogen
Hydrogen could be imported for atmosphere and hydrosphere engineering.[21] For example, hydrogen could react with iron(III) oxide from the Martian soil, which would give water as a product:Depending on the level of carbon dioxide in the atmosphere, importation and reaction of hydrogen would produce heat, water and graphite via the Bosch reaction. Alternatively, reacting hydrogen with the carbon dioxide atmosphere via the Sabatier reaction would yield methane and water.
Use of fluorine compounds
Because long-term climate stability would be required for sustaining a human population, the use of especially powerful fluorine-bearing greenhouse gases, possibly including sulfur hexafluoride or halocarbons such as chlorofluorocarbons (or CFCs) and perfluorocarbons (or PFCs), has been suggested.[22] These gases are the most cited candidates for artificial insertion into the Martian atmosphere because they produce a strong effect as a greenhouse gas, thousands of times stronger than CO2. This can conceivably be done relatively cheaply by sending rockets with payloads of compressed CFCs on collision courses with Mars.[15] When the rockets crash onto the surface they release their payloads into the atmosphere. A steady barrage of these "CFC rockets" would need to be sustained for a little over a decade while Mars changes chemically and becomes warmer.In order to sublimate the south polar CO2 glaciers, Mars would require the introduction of approximately 0.3 microbars of CFCs into Mars's atmosphere. This is equivalent to a mass of approximately 39 million metric tons. This is about three times the amount of CFC manufactured on Earth from 1972 to 1992 (when CFC production was banned by international treaty). Mineralogical surveys of Mars estimate the elemental presence of fluorine in the bulk composition of Mars at 32 ppm by mass vs. 19.4 ppm for the Earth.[22]
A proposal to mine fluorine-containing minerals as a source of CFCs and PFCs is supported by the belief that because these minerals are expected to be at least as common on Mars as on Earth, this process could sustain the production of sufficient quantities of optimal greenhouse compounds (CF3SCF3, CF3OCF2OCF3, CF3SCF2SCF3, CF3OCF2NFCF3, C12F27N) to maintain Mars at 'comfortable' temperatures, as a method of maintaining an Earth-like atmosphere produced previously by some other means.[22]
Use of orbital mirrors
Mirrors made of thin aluminized PET film could be placed in orbit around Mars to increase the total insolation it receives.[1] This would direct the sunlight onto the surface and could increase Mars's surface temperature directly. The mirror could be positioned as a statite, using its effectiveness as a solar sail to orbit in a stationary position relative to Mars, near the poles, to sublimate the CO2 ice sheet and contribute to the warming greenhouse effect.Albedo reduction
Reducing the albedo of the Martian surface would also make more efficient use of incoming sunlight.[23] This could be done by spreading dark dust from Mars's moons, Phobos and Deimos, which are among the blackest bodies in the Solar System; or by introducing dark extremophile microbial life forms such as lichens, algae and bacteria. The ground would then absorb more sunlight, warming the atmosphere.If algae or other green life were established, it would also contribute a small amount of oxygen to the atmosphere, though not enough to allow humans to breathe. The conversion process to produce oxygen is highly reliant upon water. The CO2 is mostly converted to carbohydrates.[24] On 26 April 2012, scientists reported that lichen survived and showed remarkable results on the adaptation capacity of photosynthetic activity within the simulation time of 34 days under Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).[25][26]
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