In order for an atmosphere to support Earth-like life (and also human life), it must be similar to Earth's.
Requirements for Earth life Edit
Pressure. An atmosphere must have a surface pressure similar to Earth's. However, it is shown that humans, animals and plants can survive if the pressure is not the same. Those who climb in the upper Himalayas are suffering from a lack of oxygen and not from a lack of pressure. Humans can adapt to a pressure of 0.2 atm. Also, it is known that divers can adapt to pressures of up to 10 atm. fast and in a longer timeframe to pressures up to 50 atm. It is still questionable if a person, used to a pressure of 50 atm. will ever be able to adapt to 0.2 atm.
Oxygen. From all atmospheric gasses, oxygen is the most important. On Earth, we have 20% oxygen. Humans and animals can live in an atmosphere made of 100% oxygen without problems, assuming the same pressure. However, in such conditions, fires will be more dangerous. Living in an environment with 100% oxygen and a pressure of 0.2 atm. is the same as living in an atmosphere with 20% oxygen and a pressure of 1 atm. This shows that a rarefied atmosphere is breathable if it has more oxygen.
Astronauts breath pure oxygen, but in their costumes pressure is reduced at 0.25 to 0.3 atm. This way, they are as active as in normal conditions, but there is less pressure inside their costumes.
On the other hand, if we have a denser atmosphere, with a pressure of 5 atm., we cannot keep an oxygen density of 4% (which would mean the same amount of oxygen as 20% a 1 atm.) because it will not be enough.
If atmospheric pressure is higher and the oxygen density remains at 20%, risks of fire hazards is much higher.
Nitrogen. Plants need nitrogen, but they cannot extract it from air. Nitrates are vital for plant life. They are brought to the soil and water by lightning and bacteria. A planet without nitrogen will not be able to support plant life. Also, in case of an Outer Planet, where storms are more rare, the amount of nitrates on the ground will be smaller, but also plant growth, with less light, will be much slower.
A planet can support life with an atmosphere composed 30% of nitrogen and probably even less.
Helium and other inert gasses. It was proposed that a planet without enough nitrogen can use helium instead. Helium is one of the Anti Greenhouse Gasses and will lower temperature a bit. However, it is lighter and will tend to separate and escape into outer space. The use of helium can be suitable if the atmosphere has strong currents and we have enough gravity. Also, the use of other noble gasses, like argon or neon is possible, if they are found in enough quantities. The planet will still need some nitrogen to create nitrates.
Carbon dioxide. This gas is vital for plant life, but in larger amounts, it is toxic. Also, it has a greenhouse effect.
Ozone. This gas is naturally produced from oxygen. It will be produced in higher or smaller amounts, depending on the amount of ionizing radiation found on the planet.
Ionization. A ionosphere is vital for life. Experiments done in environments without ionized gasses showed that life is strongly affected. The ionosphere is created by wind currents and by ionizing radiations. In case of an Inner Planet, we expect strong ionospheres to form, but on an Outer Planet, it will be almost absent.
Water. From a dry world to a place saturated with vapors, each planet and each place on a planet will be different.
Initial atmospheres Edit
The initial atmosphere or the atmosphere built from diverted comets, is different from what we need. It is not breathable. It contains of:
- Carbon dioxide (sometimes over 90%)
- Nitrogen (also, sometimes over 50%)
- Methane and other hydrocarbons
- Tenuous amounts of hydrogen and helium
- Oxygen (found in atmospheres around icy moons)
- Sulfuric acid and carbonic acid (in hot environments)
- Other gasses, in trace amounts.
Transforming carbon dioxide Edit
If the atmosphere is hot, we have to cool it down somehow. When we do this, water and sulfuric acid will rain to the surface, decreasing the greenhouse effect. And also, this will split carbonic acid into carbon dioxide and water.
Carbon dioxide is the most important gas that needs to be transformed. We need to lower its concentration and at the same time we need to produce oxygen. There are two ways to do this: with the help of plants and with the help of technology.
Using plants Edit
Plants use carbon dioxide to produce organic matter, but when they die, they are decomposed or eaten by animals and all carbon is released back into the atmosphere. We have to store the carbon somewhere. On Earth, carbon is efficiently stored in fossil fuel, which is not suitable for animal food. Also, plants use light with little efficiency of 1 to 1.5%.
The best way to transform carbon dioxide is the use of genetically modified algae. These algae would first produce organic compounds, then extract hydrogen and oxygen atoms, leaving behind only inert Atomic Carbon, which, being heavy, will sink to the bottom of the ocean.
But how efficient will be this process? A forest produces an yearly average 3 cubic meters per hectare , while grain crops usually produce 4000 kg per hectare . However, in case of grains, the seeds are a small part from the entire plant. Grass production appears to be over 10000 kg per hectare .
In case of artificial algae, the record appears to be of an average 50000 kg dry mass per hectare , by far greater then superior plants. This means a production of 5 kg per square meter. Carbon dioxide is both carbon and oxygen, while organic compounds consist both of carbon and other elements. However, we can approximate that 1 kg of carbon dioxide can be transformed into 1 kg of organic matter. If all Earth's atmosphere were made of carbon dioxide, assuming a mass of 10000 kg per square meter, these algae will need 2000 years to remove all carbon. However, commercial algae used for biofuel have a low efficiency in using light. By increasing the efficiency from 1% to 10%, we reduce the time to 200 years. With an efficiency of 50%, the time is shortened to 40 years. But if we consider a planet that also has other gasses (and carbon dioxide only makes for 30%), the time required is only 13 Earth years.
With improved algae, using light with a 50% efficiency, we could transform atmospheres on inner planets within reasonable timeframes. Venus has more carbon dioxide, but it is closer to the Sun and receives more light. In case of Mars, it will have a less denser atmosphere. Mars could have a breathable atmosphere in 20 Earth years.
Not the same will be in case of outer planets and moons. Around Saturn, where solar luminosity is 1% of Earth's, the process will take 100 tomes more. Instead of 13 years, we would need 1300.
The use of plants has a risk. Genetically modified algae can use all carbon dioxide. It would be difficult to kill all the algae before we insert other living organisms.
Using technology Edit
Artificial photosynthesis was first used by Nazi Germany during world war II, when they ran out of fuel. The technology exists today, much improved. By using electricity, we can, from carbon dioxide and water, produce simple organic compounds. Then, with less energy, we can extract atomic carbon. On a planetary scale, the process can transform all the carbon dioxide into oxygen and atomic carbon. However, it will require huge amounts of energy.
It is possible to do this using light, but it will require large surfaces to be covered and huge costs.
This might be the only solution for outer planets within a reasonable timeframe.
Methane and other hydrocarbons can burn in newly produced oxygen. This can happen when lightning occurs or when we ignite the atmosphere.
Ammonia can naturally decompose with the help of bacteria or we can use technology. Nitrogen will be released in the atmosphere, while hydrogen will burn and become water.
Many toxic compounds can be burned this way. Once we have enough oxygen, the process will happen almost by itself.
Side effects Edit
As we remove carbon dioxide, the planet will cool. We will need to remove part of the anti-greenhouse devices and increase amounts of greenhouse gasses. Also, oxygen will have a strong impact on all rocks, interacting with minerals that were never exposed to it. As this happens, the amount of oxygen will decrease a bit and other gasses (mainly carbon dioxide) will be released into the atmosphere, but in a much smaller amount then before.
Once we created an atmosphere, reached the require temperature and created an ocean, the next vital step in terraforming is to ameliorate the atmosphere. After this step, we can focus on inserting Earth-like life forms.