Terraforming (literally: Earth-shaping) is the proces of turning lifeless, uninhabitable planets, dwarf planets and other objects into Earth-like objects that can support human life. This can supposedly be done by modifying the atmosphere, surface temperature and removing or reducing harmful substances (such as the carbon dioxide and sulfuric acid in Venus' atmosphere). For example, carbon dioxide (if present) can be changed into oxygen using algae or other simple plants, which allows more advanced organisms including Earth animals and humans to thrive and reproduce. Although this may be possible in the far future, reviving the magnetic fields of certain planets (such as Mars' solid core) will be a much more difficult challenge.
(Rough) plans exist for terraforming Mars, and it might also become possible to terraform our Moon, Venus, Mercury, Ceres, Pluto and moons of other planets such as Europa, which is thought to actually have an ocean beneath the surface.
Challenges[]
There are many obstacles to terraforming that need to be overcome.
- Water is central to life, and can also be split to yield hydrogen and oxygen. In the absence of water, some proposals have suggested diverting an ice-comet in deep space, where a relatively small course adjustment could bring a collision course with the planet to be terraformed.
- Because no known life form can exist in the void of space for very long, all terraformed worlds would need an atmosphere of at least 15-30% oxygen, 60-75% nitrogen, and <1% carbon dioxide, to allow aerobic life to survive and to support carbon and nitrogen cycles. An atmosphere would also be necessary to heat up the surface of the planet, by means of greenhouse gasses. Any elements lacking on a planet would need to be imported from elsewhere.
Methods[]
Various methods have been proposed to terraform a planet or dwarf planet:
Albedo manipulation: A planet's albedo (the fraction of incident sunlight it reflects) is a value between 0.0 and 1.0. It can be radically changed using relatively little material. It is easier to reduce the albedo than increase it. Furrows in the ground, such as those made by ploughing, decrease the albedo, making the planet darker. A darker planet captures more solar energy, causing it to warm up.
Greenhouse gases: Greenhouse gases are gases that absorb and emit infrared radiation. The presence of such gases on a planet increases its ability to retain heat, and therefore increases its temperature. Examples of greenhouse gases are carbon dioxide and water vapour. A cold planet can be warmed up by adding greenhouse gases to its atmosphere. Some hot planets, such as Venus, are hot due to having excessive greenhouse gases, so these could be cooled down by removing most of these gases (e.g. through photosynthesis).
Removal of toxic substances: Many planets have substances toxic to Earth life, such as the sulfuric acid clouds in Venus' atmosphere and the perchlorate in Mars' soil. These would need to be removed. For example, perchlorate could be removed by using a perchlorate-degrading enzyme derived from bacteria.
Asteroid/comet redirection: If a planet lacks one or more of the substances needed for life, such as water, then these can be added to it by finding asteroids or comets with the necessary substances and redirecting them to hit the planet. Naturally, if there is anyone currently living on the planet, then this will be a potential hazard to them.
Artificial magnetosphere: Some planets, such as Mars, don't have a natural magnetosphere. This exposes their surfaces to the stellar wind, which strips away their atmospheres and is harmful to both living organisms and electronics. For such planets, an artificial magnetosphere would need to be constructed.
Timescale[]
Timescales for terraforming a Mars-like planet using only biological methods are likely to be measured in thousands of years. This is based on the amount of oxygen that a rainforest turns over per year. Use of technological methods could decrease the time needed, but it would still take decades at the very least.
Steps to Terraforming[]
In the medium term, the steps towards terraforming are perhaps more interesting.
We would need to show that we can reclaim deserts, right here on Earth. One fairly successful approach is to bury irrigation pipes underground to feed the roots of plants directly, thus minimising evaporative losses. Mixed plantings with varying plant height and root depth do best.
Long before terraforming is achieved, we would like to be able to live in closed environments, e.g. domed structures with their own ecosystems. This is a challenging goal, and many of the challenges are similar, but it is on a much more manageable scale. With a presence on the planet, we are in a much better position to adapt our terraforming strategy in the light of experience gained.
Specific Examples[]
Terraforming Mars[]
In many respects, Mars is the most earth-like of all the other planets in our Solar system.[13] Indeed, it is thought 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.[14]
Terraforming Venus[]
Terraforming Venus requires two major changes; removing most of the planet's dense 9 MPa carbon dioxide atmosphere and reducing the planet's 500 °C (773 K) surface temperature.
Terraforming Mercury[]
Mercury would be a challenging planet to terraform (though not as challenging as Venus) It has many features that are positive for terraforming, including a low surface gravity (about the same as Mars' [0.38 g]), an internal dynamo that produces a magnetic field (which is especially important as a radiation shield so close to the Sun) and considerable stores of water-ice and organic compounds in its polar craters. Initial human habitats could be built mostly underground, to allow the establishment of a continuous human presence on the planet before terraforming. However, the above-ground parts of these colonies would need to be rigorously self-contained and pressurized, like a space station. This is because Mercury's surface pressure is close to zero, as a result of its low gravity and proximity to the Sun.
Terraforming Ceres[]
As with the moons of Jupiter and Saturn, terraforming Ceres would first require that the surface temperature be raised in order to sublimate its icy outer layer. This could be done by using orbital mirrors to focus sunlight onto the surface, by detonating thermonuclear devices on the surface, or colliding small asteroids harvested from the Main Belt onto the surface.This would result in Ceres’ crust thawing and turning into a dense, water vapor-rich atmosphere. The orbital mirrors would once again come into play here, where they would be used to trigger photolysis and transform the water vapor into hydrogen and oxygen gas. While the hydrogen gas would be lost to space, the oxygen would remain closer to the surface.
Terraforming Pluto[]
As a ice-cold rocky dwarf planet that has been a few blocks away from the Sun, Pluto has five moons called Charon, Styx, Nix, Kerberos, and Hydra. Charon, the largest of the five moons, is mutually tidally locked with Pluto, and it is massive enough to pull the system's barycenter outside of Pluto and thus Pluto–Charon is sometimes considered a double planet.
Terraforming our Moon[]
Terraforming the Moon would require the importation of carbon and nitrogen, which are relatively scarce. Additionally, its low gravity means it would have difficulty holding onto an atmosphere. Paraterraforming may be a better method of making the Moon habitable than full terraforming.
Terraforming Europa[]
Europa, a moon of Jupiter, is a potential candidate for terraforming. One advantage to Europa is the presence of liquid water, which could be extremely helpful for the introduction of any form of life.