Terraforming

Terraforming (literally: Earth-shaping) is the proces of turning lifeless, uninhabitable planets, moons and other objects into Earth-like objects that can support human life. This can supposedly be done by modifying its atmosphere, surface temperature, and completely removing or reducing harmful gases or chemicals (such as the extreme carbon dioxide atmosphere or the 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, 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 colision 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, in order to breathe in and support carbon and nitrogen cycles. An atmosphere would also be necessary to heat up the surface of the planet, by means of greenhouse gasses.  Possible lack of Carbon and Nitrogen may require that materials to be brought from Earth. The material would need to be harvested from high orbit to reduce the 'lifting cost'.

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 as made by plowing decrease the albedo, making the planet appear darker. The darker planets captures more solar energy, causing it to warm up.

Timescale
Timescales for terraforming a planet such as Mars using only biological methods are likely to be measured in thousands of years. This is based on considerations of the amount of Oxygen that a rain forest turns over per year.

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 - so minimising evaporative losses. Mixed plantings with varying height of plant and root depth do best. This would allow many plants which are suited to survive in more harsh environments to survive, such as a Eucalyptus tree. This would then create more water due to the planets absorbing oxygen and slowly by introducing species from the bottom of the eco-system you could eventually turn a Desert into a Jungle.

Long before terraforming is achieved we would like to be able to live in closed environments, e.g. domed structures with their own ecosystem. 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.