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Mercury altitude map. Credit - NASA
Many celestial bodies, like Mercury, Luna, Ceres and many moons orbiting gas giants are almost completely covered with craters. Terraforming such places require a different technique.
Overall
Craters are created by impacts. If a celestial body is almost completely covered with craters, it means that its atmosphere does not have a significant effect in altering Geographic features or that the celestial body lacks any geological activity or both. The planet usually lacks a dense atmosphere and lacks an powerful internal dynamo.
Since craters are created by impacts, a significant (if not all) part of the impacting body is still there, compact or spread over a large area. Even for a planet close to its sun, as Mercury, there still is water at the poles, suggesting that the impacts there were partially made by comets. On a body full with craters, we would expect to find many minerals, from water and other volatiles trapped beneath surface to all ingredients needed for life and all metals needed for industry. Chemical composition of the rocks and mainly of subsurface materials vary significantly from place to place.
A crater has a central, low-elevation area, surrounded by higher borders or rims. Some craters have complex structures, while others are layered one on top of others. Water will tend to accumulate inside craters and will find hard to pass the outer rims.
Hydrological problems
Moon altitude map. Credit - NASA
Before terraforming, we must first acknowledge that for any terraformed planet hydrology is very important. Water balance is strongly correlated with climate patterns.
Look at this altitude map of the Moon. Suppose we fill all lowlands (those colored with blue) with water in an attempt to create an ocean. What would happen? We will get an ocean, we will get a main ocean and a multitude of smaller unconnected seas. Also, we will have a large continent and a multitude of islands and small continents. And here start the problems.
Endorheic problem
As one can see, there are many deep craters on the continent. Water will accumulate there and form lakes, without flowing to the oceans. As this happens and water accumulates there, the oceans will lose water and size. This automatically means that certain seas will be unconnected to the larger ocean. In the end, we will not have continents and oceans, but a complex system of endorheic basins.
In Geography, the word endorheic describes a river, a lake or a basin where water does not flow into the ocean and it leaves only by evaporation. Endorheic lakes are highly vulnerable to climate changes. If it rains less, they will dry, but if it rains more, they will increase in size. Changes in lake sizes will influence the climate, since air gets its humidity from oceans, seas and lakes.
If we don't do something to change Geographic patterns, a planet dominated by craters will end-up divided in many endorheic water basins and in an unstable equilibrium. This will not be good for settlers.
Extreme endorheic problem
Mercury altitude map. Credit - NASA
Now, take a look at the altitude map of Mercury. This planet is also dominated by craters. What catches the eye is that here, we see huge altitude differences. Each crater is surrounded by giant mountain rims and is very deep. This will block wind currents and will have a tremendous effect on climate patterns. On a Tidal Locked Planet, it is possible that even the air could freeze on the dark side, because it cannot return to the illuminated hemisphere to heat.
In time, small amounts of water can pass over natural barriers. This will slowly take water away from an endorheic basin and deposit it in another. Climate is no longer safe in these conditions.
Breaking obstacles
Craters will accumulate high amounts of water behind them. But, they also posses the risk of an uncontrolled dam. There will be craters where water passes over their top, through a tiny fissure. Erosion can occur and can increase the fissure and just like when a dam crashes it can produce a massive flood downstream.
Changing water surface
You can have the same amount of water in a large shallow ocean or in a small deep sea. Since endorheic basins are not stable, we can clearly see that there is a high chance for almost all of the water to accumulate inside limited areas, like at the poles, where evaporation is low.
Terraforming
It is clear that on a planet dominated by craters we have to do something to alter its Geographic features.
One way could be the use of large explosions and large-scale engineering. We can build canals to allow waters to flow from craters and to connect seas.
There is a terraforming process known as Noah Flood, which could do the work for us. The process requires the release of a huge amount of water in a short time. This can be done in 3 ways:
- On inner planets that can host a large enough atmosphere, like Venus, we can cool the atmosphere, forcing it to lose water. If the planet doesn't have enough water, we can add it by diverting comets. It is important that we cool the planet in a few years, forcing all water to rain in a short timeframe.
- On outer planets, water can be stored on the surface as ice. If we increase temperature and force the ice to melt, we can produce a massive flood.
- On planets and moons with low gravity, if we need water, we insert it by diverting comets. Each diverted comet must be targeted to brake a crater wall or to create a canal. Also, as these comets melt, they will provide the water for a limited local flood.
After terraforming
After terraforming, there will still be many endorheic basins and crater dam-lakes that might collapse.
However, there is another problem. Large areas will be covered with water, that will press the crust. It is possible that the excess weight will trigger volcanic and tectonic activity, but since planets covered with craters are usually dead bodies, the risk is negligible. Still, the crust will be compressed by the added weight of water. Also, if water existed as ice in another place, the crust will rise because of the removed weight.
This implies that the shores will not remain as they were. Also, there will be quakes even on a planet with a cold, solid core. However, in time, there will be less and less quakes and the shores will no longer change fast.