A habitat is an environment in which an organism, or group of organisms live in symbiosis.
The environmental conditions on Mars are hostile for humans without life support systems; examples of the factors which make the surface of Mars uninhabitable include the thin, oxygen-poor atmosphere, the lack of easily available liquid water and the extremely cold climate. The habitat of any human visitor to Mars would have to be set of self-contained artificial environments, including settlement facilities such as buildings, tunnels and greenhouses. Movement between habitats would require airtight rovers or space suits.
The lower gravity of Mars is an environmental factor that may or may not be important in the long run. If the lower gravity significantly affects the viability of a Mars settlement, it might be possible to build part of the habitat on a centrifuge. However, this may affect the economical viability of a surface settlement versus a rotating space habitat.
Terraforming has be suggested as a way of making the whole surface of Mars habitable. The more Mars is terraformed, thus increasing atmospheric pressure, the easier it is to built habitats and large domes, eventually huge domes spanning kilometers. Habitats can be categorized into four stages:
- First stage habitats: The initial habitats, made with elements from Earth, non permanent.
- Second stage habitats: The construction facilities for the next stage, permanent or non permanent.
- Third stage habitats: A permanent habitat, designed for long term use.
- Fourth stage habitats: Habitat for a partially or fully terraformed Mars.
First stage habitats
Transhab, a concept by NASA to produce inflatable habitats, producing more living space than traditional, solid habitats. This technology has been bought by Bigelow Aerospace, which is developing space hotels for tourism.
SpaceX proposes using the transportation vehicle as a habitat until surface installations are built.
Second stage habitats
These might be fabricated on Earth and installed on site, or fabricated using in-situ materials.
Third stage habitats
Tunnel bored habitat (TB-habitat) - Artificial cave
A TB-habitat is a habitat constructed using a tunnel boring machine (TBM) ,roadheaders, drill and blast or cut and cover techniques. A TBM will have to be specifically designed for this purpose taking weight into account if lifted from the Earth’s surface and landed on Mars´ surface. This will diminish its cutting diameter. Alternatively, it can be made from local resources, allowing bigger diameters.
This type of habitat is a 'third stage' habitat, as opposed to the habitat that will be used by the first party that lands on Mars. That habitat would be categorized as a 'first stage' habitat. And when a permanent ('second stage') settlement has been build, TB-habitats are to be used to expand the settlement subsurface in order to create really safe living and working rooms.
The approach would be, to drill a grid of intersecting tunnels, each closed of at the ends by a air tight wall or, on occasion, an airlock. Within this grid, sections can be closed of with non-load-bearing walls creating dormitory rooms, workspaces, technical facilities, labs, social hubs, underground greenhouses, and so on.
This approach requires the ability to make shotcrete(or gunite) on location. This means being able to make shotcrete from Martian regolith and available water. It probably will be necessary to strengthen the shotcrete with a spiral, winding steel wire. The tunnel will have to be able to carry the weight of the rock and soil above. And as Martian gravity is less than Earth's gravity, it will be physically less of a stretch to bear the weight. An alternative to shotcrete would be pre-fabricated sections of tunnel liners, as used on Earth for pressure tunnels and in poor quality ground.
Surface domes are often used to illustrate the concept of an enclosed habitat on Mars and its difference with an open space on Earth. However, domes are difficult to build, are not a very efficient space for occupation and most designs offer little, if any, radiation protection. Dome might be used as temporary housing, or a signature building with special functions, such as recreation domes. Magnetic shielding against radiation might make domes more useful as construction elements.
Fourth stage habitat
A fourth stage habitat necessarily comes after some terraforming effort, and therefore after the passage of up to a century of time after the initial settlement.
This stage takes into account changes in the environment that modify the habitat requirements, possibly making some of the third stage habitats redundant.