All about Budgets: Humans (and animals) breathe oxygen, consume water, consume proteins and carbohydrates, and many trace minerals and vitamins. We exhale Carbon Dioxide (CO2), excrete water and salts (and other things). It will always be a bit expensive to ship the inputs up the gravity well to people in settlements or space stations, so astronauts must also be farmers. Plants and algae can convert our wastes and CO2 using light of certain wavelengths and water into food and oxygen. Animals can also convert plants and algae (and in some cases wastes) into protiens. Ships and stations have the tightest budgets...no more inputs other than light (others are very limited at best). Settlements and bases on planets, moons, and other bodies have some inputs, but they might need some kind of transformation. In any case, effecient farming (even on Earth) focuses on reuse, closing the cycle as much as possible. A thing to note about making an ecosystem, it takes lots of work to keep it balanced in a way we want. Near the sun we have ample sunlight (though maybe too much radiation..another topic), and likely ample energy (from solar, nuclear, etc.), so we assume we get these at need. We will then need to balance what humans need to take in. To do this mathematicaly we will need to find out how much each person takes in and lets out, and the same for any candidate species, including energy requirements (like heat, sunlight) and water. Ideally a profile of every species in the mix should be taken. For now lets try w/o the math. Lets assume we have a rotating space station (which provides some type of gravity) So what species should we consider for an example:
To Terraform is to make a place Earth-like ('Terra' = earth). Using various methods over (very) long periods of time places like the Moon or Mars could be made Earth-like, a home for humans away from Earth where Earth-like ecologies can flourish. No place will ever be exactly like Earth, but some places like Mars, given enough effort can become somewhat more friendly to people and Earth life. To look at terraforming one should look at what we call Earthlike. Earth's atmosphere at sea level has 78% nitrogen, 20% Oxygen, and around 0.04% Carbon dioxide, the rest in trace gasses such as Argon, at 1 atmosphere pressure (around 14.7 pounds per square inch). Also Earth's air additionally has to 5% Water Vapor ( or up to 100% relative humidity, humans need water vapor to breathe easily). Humans and other animals can tolerate lower air pressures if need be (Incas live in Peru at over 10000 feet altitude, around 9 psi). The terraformed air will have to clear enough of poison gasses (ex: Carbon monoxide, hydrogen sulfide, chlorine, sufuric acid, etc.) to allow people to live also.
Outdoor temperatures should also be tolarable, espcially if we intend to have farms, praries, or forests. The temperature shouldn't swing beyond -40 C to +50 C (i.e. -40 F to 130 F) and should be between 3 C and 40 C (or between around 40 F and 105 F)for at least a few months a year at some region of the planet. The temperature range will need to match that of any Earth ecosystem or crop set we wish to grow. The Atmosphere should be thick enough to balance out temperature swings and store some of the day's heat.
Ideally, a Terraformed planet would also have some liquid surface water, with salinities of less than 37 parts per thousand ( or 3.7% by mass, for salt water animals, algae, and plants) and less than 8 ppt for freshwater organisms.
There will have to be some kind of way to limit radiation exposure, but allow enough light (in the 680 to 700 nm range) during the day to grow plants. How much light depends on the algae or plant, but your eyes already know how much on average. If the average person can't tell a black thread from white, it is likely too dark. More technically, at least .03 Watts per square meter. Too much radiation will cause mutation and destroy cells. On Earth, the atmosphere, ionosphere, and Van Allen belts (w/magnetic field) shield Earth life from lethal solar or cosmic radiation. Our terraformed planet will need either a thick atmosphere, or some methods to keep radiation exposure within survivable levels. Radiation can some from soils and minerals (like Uranium) as well, and these will have to be livable as well.
Soils: Soil on Earth that grows plants chemically balanced (water, oxygen, acid/bases, minerals), and is actually a complex ecology itself, having bacteria, fungus, many micro-organisms and worms. Chemically balanced soil is the starting point. Various earth organisms can break it up from there. Even solid rock can be broken up by lichens over a long period of time. Plants, algae, and cyanobacteria in turn can build oxygen levels in the atmosphere.
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