Bodies with small gravity might be hard to terraform. In this section, we discuss only about the Atmosphere, not about other features.
If gravity is too low, an atmosphere will tend to escape. Already some small bodies in Solar System are experiencing this. Pluto and Triton are losing gasses. Small bodies usually lack of magnetic fields. If somehow they were terraformed, it is better to be within the magnetic shield of a larger body (like the moons of Saturn or Jupiter).
They will have a very large atmosphere. This material shows that a breathable atmosphere around Pluto will be wider then the planet's radius. So, it will have more gas then Earth's atmosphere, creating a big greenhouse effect. This is because, without enough gravity, weight of outer gas layers is smaller, so they don't develop enough pressure to compress inner layers.
A wide atmosphere will be in higher risk of getting lost. Huge amounts of gas will be needed to compensate the loss. However, in case of a larger body, like Luna or Jovian moons, the loss will be not that much. In the need of keeping an Atmosphere down, heavy inert gases could be used.
There are a few high-tech methods to help these tiny worlds become new homes:
- Paraterraforming - covering them with a transparent shield
- Let them covered with water and cover the water with a transparent floating shield (also to create greenhouse effect)
- Continuously feeding the atmosphere with new gasses
- Artificial gravity (maybe in the far future).
The following is a list of bodies that are included in this category. They will lose atmosphere and will need continuous maintenance work. Safe are the bodies that might resist over 1000 years without maintenance, risky (with details) are bodies that will need continuous engineering and hard - bodies that are unlikely to be terraformed in a classic way.
- Mercury - easy
- Luna - easy
- Ceres - hard (no magnetic field, too low gravity, an atmosphere will stretch over 3000 km)
- Io - risky (small size, strong gravity pull from Jupiter)
- Europa - risky (small gravity, more easy could be an ocean covered with a shield)
- Ganymede - easy
- Callisto - easy
- Mimas - hard (small size, strong gravity pull from Saturn)
- Enceladus - hard (small size, could be an ocean world covered with a shield)
- Tethys - risky (low gravity, will need gas replenishment)
- Dione - risky (low gravity, will need gas replenishment)
- Rhea - risky (low gravity, will need gas replenishment)
- Titan - easy
- Hyperion - hard (too low gravity)
- Iapetus - risky (low gravity, will need gas replenishment)
- Phoebe - hard (better use an oceanic shield)
- Oberon - risky (low gravity, will need gas replenishment)
- Titania - risky (low gravity, will need gas replenishment)
- Ariel - hard (too small to hold an atmosphere without a shield)
- Umbriel - hard (too small to hold an atmosphere without a shield)
- Miranda - hard (too small to hold an atmosphere without a shield)
- Triton - risky (low gravity, will need gas replenishment)
- Pluto - risky (low gravity, will need gas replenishment)
- Charon - hard (too small to hold an atmosphere without a shield).
Update After Pluto Encounter Edit
On July 14th 2015, New Horizons reached the distant Pluto. What the tiny spacecraft found, will make us re-consider many of our existing theories. Images and data from Pluto has left scientists puzzled.
One of the most important findings, at least for future terraforming, is the strange looking of Pluto's atmosphere. As the image shows, Pluto has a gigantic atmosphere, stretching as far as its outer moons. And all this atmosphere is not enough to compress the lower layers up to a pressure that humans will support.
Pluto has the big advantage of a very low solar wind. If the solar wind was as strong as it is around Earth, for sure there would be almost no atmosphere. This comes with a very hard question: Can such a small celestial body be terraformed? Is it possible to compress or to replenish all that gas?
Adding extra gas into Pluto's atmosphere will not be a hard job. After all, the Plutonian crust is full with frozen gasses. Just make some greenhouse effect and all will start to sublimate or boil. Bringing extra gasses from other Kuiper Belt Objects might also be possible. However, keeping them fixed, will be very hard.
Pluto's atmosphere has a very low pressure: 0.2 to 0.3 Pa (1/400 000 of Earth's). And even at this very low surface pressure, the escape rate is alarming: 500 tons per hour! Guess that if the atmosphere were to have the same density as Earth's, the escape rate will be 55 000 tons per second. At that huge escape rate, you will need to crash 4 of Rosetta's 67P/Churyumov–Gerasimenko comets every Earth year, to replenish the gas losses. Heating all that material, so far away from the Sun, will require an artificial source of heat. As the new gasses boil into the atmosphere, they create an anti-greenhouse effect. and also, we must not forget that gasses ejected from a comet are not breathable: carbon monoxide and dioxide, methane, ammonia and others. They must be refined, to replenish exactly what is lost: oxygen, nitrogen and water.
As the atmosphere is pushed away, also the greenhouse gasses will be lost into outer space.
In case of Pluto in particular, there is another problem. Some gasses (oxygen, nitrogen) will be gas at low temperature, but when they are at higher elevations, they will be out from the reach of major greenhouse gasses. As so, a part of the runaway gasses will condensate and will snow down to the planet. What will that mean? When a snow of -180 C is flowing through the lower atmosphere, it will heat-up, cooling everything around. Pluto will require a huge amount of greenhouse gasses in order to increase temperature to a value acceptable for humans. This will trigger another problem. As gasses cool down, they create clouds. First, will be clouds of water ice. Far above them, clouds of oxygen and of nitrogen will get formed. In such conditions, there will be less light. This will make life for plants even harder, if artificial light is not introduced.
In this example, we used Pluto, but if we consider another small celestial body, things will be more different. A smaller celestial body will tend to lose its atmosphere even faster. Solar winds will be an even bigger problem, but not for moons of Jupiter and Saturn (end even for moons of Uranus and Neptune), where strong magnetic fields can be of great help. On the other hand, Pluto has the advantage of a lack of external gravity. Triton, located close enough to Neptune, cannot develop such a fluffy atmosphere, because its upper gas layers will exit the Roche sphere (therefore, will get into Neptune's gravity field).
Is it possible to use something heavier to compress the atmosphere of Pluto?
An idea will be to use heavier gasses. Earth's atmosphere has nitrogen and oxygen, with some small amounts of noble gasses. Well, NASA found out that exactly the nitrogen is flying away from Pluto in stronger amounts. Xenon is the heaviest of all noble gasses (radon is the heaviest, but it's radioactive and has a short half-life). By replenishing nitrogen with xenon, we might get a heavier atmosphere. This will imply 3 major problems. First, xenon is rare. Finding and transporting so much xenon will be a hard job. Second, the heavy xenon will make other lighter gasses to separate. Stratification will occur. Much lighter oxygen will be pushed up and lost into space. Third, nitrogen is essential for life. without it, there will be no proteins.
Will future settlers on Pluto agree to crash 4 comets every Earth year to replenish their atmosphere? Or will they prefer Paraterraforming instead?
Future reading: Atmosphere Parameters