Friday, August 31, 2012

Paraterraforming: Creating Habitable Worlds

It’s really incumbent upon us as life’s agents to extend life to another planet. I think that being a multi-planet species will significantly increase the richness and scope of the human experience.
- Elon Musk, founder of SpaceX, interview in Ad Astra, 2006

The quality of a civilization is measured not by what it has to do, but by what it wants to do.
- Bruce Murray, research scientist, Exploring Space, 1991

Terraforming is the process of modifying a planet, moon or any other suitable object in order to make it habitable for humans. The word terraforming literally means “Earth-shaping”. Mars is often regarded as the first candidate for terraforming because it is the most Earth-like planet in the Solar System. Early in its history, Mars is believed to be a lot more like Earth with a significantly thicker atmosphere and abundant liquid water on its surface. Today, transforming Mars into an Earth-like world through terraforming will require thickening its atmosphere, warming up the planet and keeping the atmospheric constituents from escaping into space. Such an endeavour will require numerous technological breakthroughs and demand huge economic resources. Furthermore, terraforming Mars is a gradual process which is expected to occur over a timescale that is likely to exceed a human lifespan and the lack of gratification in return for investment will deter initial investors. For these reasons, paraterraforming serves as an attractive intermediate step before full terraforming is achieved.

Figure 1: Artist’s impression of a terraformed Mars.

 Figure 2: Artist’s impression of a terraformed Venus.

Paraterraforming involves the construction of a pressurised habitat in the form of an air-tight roof over a particular area on a planet where an Earth-like environment is completely enclosed within the habitat. A modular, “pay-as-you-go” approach of expansion is possible for paraterraforming and this allows it to appropriately meet increasing demand. As the population increases, more pressurised habitats can be constructed over the surface of a planet until most of the planet’s surface is covered or until full terraforming of the planet is eventually achieved.

A typical habitat can consists of an ultra-strong membrane that is primarily supported by the air pressure contained within it. The membrane acts as a roof over the surface of the planet, allowing sunlight in and preventing the atmosphere from escaping. At regular intervals, tension cables or support towers can anchor the membrane to the surface of the planet. To support a more Earth-like hydrological cycle, the membrane needs to be at least a few kilometres above the planet’s surface so as to provide sufficient altitude for clouds to form. A higher membrane height is also advantages for aerial transport within the habitat. To deal with the occasional meteor strike, the membrane should be designed to be capable of self-repairing.

An object that is somewhat less massive than Mars will be unsuitable for terraforming since its gravity will be too weak to hold on to an Earth-like atmosphere for long. Therefore, one key advantage of paraterraforming over terraforming is that paraterraforming can be done on objects much less massive than Mars since the atmosphere is contained by the overlying membrane rather than by gravity. As a result, even small objects such as asteroids can be paraterraformed since an Earth-like atmosphere can be contained around an asteroid by a membrane which completely envelops the asteroid. This creates a habitable environment which fully encompasses the asteroid. A paraterraformed asteroid will be rather interesting as a habitat due to its very low gravity environment.

Paraterraforming also allows the creation of Earth-like habitable environments on worlds that are too cold to support life. Such places include asteroids beyond the orbit of Mars, the satellites of the gas giant planets, comets and Kuiper Belt Objects (KBOs). The paraterraforming membranes that are used to envelope such objects can be designed to provide the necessary super-greenhouse effect to create Earth-like habitable environments at large distances from the Sun. Many of these objects are small enough that a single membrane is sufficient to completely envelope such an object. A paraterraformed comet completely enveloped within a membrane that provides a strong greenhouse effect will melt and eventually settle as a sphere of water with denser rocky material gravitationally settling in its core.