Small terraformable planets and moons are found more frequently then an Earth-size planet. In Solar System, we have only Earth and Venus as large rocky bodies, but also Mercury, Luna, Mars, Io, Europa, Ganymede, Callisto, Titan and probably Triton are terraforrmable. Basically, this article is about planets and moons that are roughly 3000 to 7000 km in diameter.
As shown above, we have 9 small bodies in Solar System, compared to only two larger rocky planets. In other solar systems, we might conclude that smaller bodies are more common too, only that for the moment our technology does not allow us to detect many of them. The way to terraform a small body is different from a larger planet.
Small planets and moons have a weaker gravity. This means that they can harder hold an atmosphere. As shown in many places on this wiki, terraforming them is possible, but with different techniques, varying from a planet to the other. Same technologies can be used for other solar systems.
Atmosphere & Climate Edit
A small planet behaves different from an Earth-like planet. First of all, because of its small size, air currents circle much faster around them. While on Earth air currents need to travel over 3000 km to bring moisture from the oceans to the dried places in Central Asia, on a small planet 3000 km might be a ride from the equator to the poles. Unless there is a giant continent covering a whole hemisphere, air currents will more easily reach seas to take water vapors over continents. So, the amount of deserts should be smaller then on Earth.
Since distance from the equator to the poles is smaller, air currents will need less time to travel and the atmosphere will circle more easy. This will bring heat from the equator to the poles and cold air from the poles to the equator. If polar ice caps exist, they will be smaller or only seasonally. Around the equator, changes will be not so strongly sensed, but close to the poles, there will be less tundra and more land with warmer temperatures. On a planet with same year length and same axial tilt as the Earth, but with a diameter of only 4000 km, winters will have less snow and more rain, while summers will be less hot.
Atmosphere on a small body needs to be wider, to have the same pressure. At Titan, at ground level, atmospheric pressure is 1.5 times that of Earth's, even if it stretches 100 km higher above. The amount of gas per surface area (per square meter) is 7.3 times higher on Titan then on Earth . This means that if Titan had Earth's gravity, the effect should be felt like if the atmosphere were 7.3 times denser then Earth's. In case of a terraformed Moon, to get a ground atmosphere pressure similar to Earth's, we will need about 10 times more gas per square km then on Earth. Moon's atmosphere will rise over 100 km above ground. This will have strong climate implications. The large atmosphere will have a stronger greenhouse effect. On the other hand, there will be high altitude ice clouds that will partially reflect light. Differences between day and night, as well as differences between seasons, will not be felt like on Earth, but much smoother. On the Moon, a day lasts about 14 Earth days, followed by other 14 days of darkness. On Earth, in Africa, that would cause temperature to rise above 100 degrees Celsius and fall way below freezing in night. On the Moon, there will be no freezing in Africa-related regions.
Air currents will be different, since the atmosphere will be much wider. High mountains might not be such a blockade they are on Earth.
Since ultraviolet light will encounter 5 to 10 times more oxygen molecules on their way to surface, the ozone layer will be much wider and will better protect the planet. Not the same can be said about other kinds of radiation. Small bodies lack of strong magnetic fields. So, unless they are in orbit around a giant planet, there is little protection against solar and cosmic radiation. Still, the enlarged atmosphere might offer some protection. On Pluto, New Horizons found that solar wind doesn't completely penetrate Pluto's atmosphere, it stops around 1000 km above the surface . On a long time scale, solar winds will erode the atmosphere, but at least for a human lifetime, we don't need to worry.
On a small planet, many interesting things will happen. The reduced gravity will allow water to behave like never expected. It will be more easy for winds to carry clouds over mountains to the heart of the continents. There will be less rain on the mountains, but more rain in steppe and desert. When it will rain, water drops will be larger and will fall slower then on Earth. Also, superficial tensions will allow more water to stick to objects in the air, to tree branches and grass.
Plants will have an easier life, since there will be less power needed to pump water from ground into tall trees. Ground water will also infiltrate more easy towards the surface.
Rivers will behave different then on Earth. They will flow much smaller. A waterfall will fall like in slow motion, but with larger drops. It will be very interesting to see rivers flow slowly enough even on high declivity. Decreased flow speed means that rivers will be much wider. Also, they will produce less erosion to the ground. River power plants will produce less energy. The amount of energy decreases with the square of gravity. So, since Moon has 1/6 of Earth's gravity (16.7%), a dam will produce only 2.78% of the amount of electricity it would produce on Earth. Also, a river on Moon would produce only 2.78% of the erosion it produces on Earth.
Lakes and oceans will also behave different. In a world with low gravity, waves will be higher by the square of gravity. So, we can expect huge waves to form. However, they will move much slower then on Earth and will carry a much smaller impact energy. A tsunami on the Moon will have far less force then on Earth.
On a low gravity world, we could even walk on water. On Europa or Moon, where a human body will weight less then 10 kg, if you move fast enough, you will not have time to sink into water. This doesn't mean that if you don't move you don't get in water completely. Anyway, swimming will be an interesting experience. You could jump above water like a dolphin. Well, it will not be like a dolphin, it will all be like in slow motion.
Tides on a small planet will be strong. If Moon were exposed to Earth's tidal forces, then water would rise with at least 10 meters with a record of 100 meters.
A small planet, unless is gravitationally stressed by something, should have its core cooler then Earth's. There should be less volcanoes (or even none) and far less seismic activity. Still, after terraforming operations, that resulted in formation of new oceans that might stress the crust, at least some quakes are expected to happen, as the crust gets a new equilibrium profile. Close to oceans, altitude should slowly decrease as ocean weight pulls down the crust. By opposite, if there were polar ice caps that have been melted, ground should slowly rise there.
Main article: Future races.
Small Earth race is what we might expect to see on Mars, Mercury, Moon, Jovian Galilean moons and on Titan. People from this race will have less strong bones, less muscular tissue and will be taller then us. Still, they will look similar to us. Bones will not get thinner, only that they will have less strength. These people will behave very similar to us. They will walk, they will eat and drink in the same way we do. Their society will look very similar to our. Also, they will, in most of their worlds, be able to live in open air, compared with other lighter races, who will not have this opportunity. Compared to us, they will be more healthy. Reduced gravity will result in less stress to the heart and blood vessels. Also, they will have less broken legs, even if their bones will have less calcium. Their worlds will have a thinner atmosphere, so their lungs will need to work harder. This might not be a problem, since the lack of gravity will result in less effort.
Constructions & Economy Edit
On a small planet, there will be far more easy to build. Buildings will weight less and that will require smaller foundation and resistance structures, that will also result in even lesser weight. Sky scrappers should rise to heights never reached on Earth. There will be more easy to build suspended highways, bridges, domes or arcades. All industries will benefit from the low gravity. Also, agriculture should benefit, since plants will need less energy for their own strength and will have more energy available for fruits and seeds.
Space Connection Edit
On a small planet, the escape velocity is far smaller then on Earth. So, sending a spacecraft into orbit will be more cheaper. It will be much more easy to launch satellites and ships and to trade with other planets and moons.
Far In The Future Edit
As shown above, small planets are far abundant then Earth-like planets. They will offer a better climate, will be a better place for economical development, but will need maintenance to replenish their atmospheres. The point is that, assuming a Bloom colonization will occur, at some point majority of humans will be settled on small planets and moons. Since gravity will transform settlers after generations, the Small Earth Race might become the dominant human race.