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Under the term Snowball Earth is described a planet that is terraformed or under terraforming process, that has undergone an ice age. The term is not official and is often used by terraformers (usually those using Universe Sandbox).
Terraforming is a highly complex process that involves many risks. There are many unknown processes that can occur on a planet. Sometimes, there might be something unexpected in the chemistry of the planet, sometimes it might be all about a human error, an ice age can be caused by a change in the orbit or the behavior of the parent star. In some cases, once terraformed, there are some maintenance works needed to keep the planet habitable. Also, some planets can have far more fragile ecosystems then Earth. Pollution and wars can destroy them more easily. In the following, we will talk about everything that can send a planet into a runaway ice age.
Large-scale natural disasters Edit
In some cases, the planet might be on a collision course with a dwarf planet or even with another planet. Its orbit might be unstable or its parent star might be unstable too (for example a flare star). It doesn't sound likely that anyone would try to terraform a planet or a moon before checking its orbit parameters. Terraforming will take many years and will certainly be done at high costs, much more then the time and budget needed to check if the orbit is safe.
Still, there are a few natural disasters with lower risk that we can live with. For example, if the planet or moon has an active core, we know there is a risk of a supervolcano. If there is a nearby asteroid belt, we know there is a risk of a collision. If we calculate that the risk is manageable (less then one in a thousand years), we can continue with terraforming, assuming that future settlers will come with a solution against these threads.
During terraforming process Edit
A planet can become a snowball Earth before it is even inhabited. Suppose the Terraforming Plant does not get all the funds it needs, its workers will return home and the entire process will be stopped. Not all greenhouse gasses will be produced, to increase temperature up to required values.
Also, something might go wrong during the terraforming process. For example, if the planet contains some dangerous chemicals, like dioxins, underground. When ground waters will start to dissolve them, this might kill life in the oceans, destroying all ecosystems.
It was said that on Earth the Gulf Stream protects us from a future ice age. On a planet that is undergoing terraforming processes, similar air and water currents might exist, only that, since its environment is changing fast, currents will also shift fast. We might see huge climate changings, that will require fast human intervention and a lot of planning. A land that is a desert for 10 Earth years, it might become very soon ice-covered, then, as currents will change again, it might become a jungle. If these changes are not controlled, there is a high chance the planet will undergo a runaway ice-age effect.
After terraforming Edit
No planet is safe, even Earth. On a long time scale, nature will change many things. Also, human activity can send a planet towards a runaway ice-age.
Currents can send a planet towards an ice age. Air and water currents are very important for climate changing. Of particular importance, this is for a Tidal Locked Planet, where ices can accumulate on the night side, until all water and maybe a significant part of the atmosphere will be removed from the day side. Still, currents are reversible and at some point a natural ice age might end.
Atmosphere is slowly lost into space (see Atmosphere around small bodies for more details). This decreases the greenhouse effect. On the other hand, as the atmosphere escapes, so might do the greenhouses gasses used for terraforming.
Greenhouse gasses also have a limited lifetime. Over time, they are dissolved in the oceans, lost in space, they react with other molecules or are destroyed by lightning and radiation. So, on an Outer Planet, maintenance is required.
In case of outer planets, there is also another risk. A hole in the greenhouse layer can be caused by a meteorite or a volcanic eruption. The hole will let heat escape into cosmos. Temperature will drop fast in the area, probably down to -50 C or even lower. This will create strong wind currents that might disturb the greenhouse layer over the entire planet, further creating other holes. This might decrease temperature even further. At some point, a new equilibrium will form, but the planet will already be in an ice age.
Man-made disasters Edit
An Outer Planet has a very fragile ecosystem. These planets will have very low air currents and their temperature will be almost uniform. In this environment, the greenhouse gas layer must be maintained at nearly the same thickness. A big city or a large industrial complex can create too much heat, to make a vertical current that can disturb the greenhouse gas. This will result in a hole in the greenhouse layer. The area below will lose heat into outer space. As air cools down, it will contract and will create strong winds, able to create other holes in the greenhouse later.
The same can be done with a bomb If you want to destroy an outer planet, use some atomic bombs, they will do all the work for you. After some time, the planet will come to a new atmospheric equilibrium, but its average temperature will be very low, below freezing point.
A special kind of snowball Earth can be obtained even from an Inner Planet. Such a planet requires heat shielding, with the use of anti-greenhouse gasses or space mirrors. If they are used in excess, you can send the planet into an ice age.
On a Snowball Earth Edit
Ice reflects most of the light and will stay cold. Also, ice absorbs a lot of heat when it melts. So, a snowball planet will tend to remain a snowball Earth. Making it back habitable will be difficult. You will need to carefully add greenhouse gasses until the ice melts. Then, you will need to decrease the amount of greenhouse gasses fast, or you will have a Fireball Earth.
Still, there are two scenarios where you would like to keep a snowball Earth as it is.
First example is if the terraforming was stopped because of a lack of funds. Suppose the planet is small and a wide atmosphere will escape into space. In that scenario, terraformers might decide to send the planet into an extreme ice age, so that its gasses will partially snow to the surface, so that the atmosphere will not be lost, until they will have enough money to continue the terraforming processes. This is a good example of space conservation.
A second example is if scientists find our that the terraforming process is not sustainable. For example, they might found huge amounts of dioxins and other toxic chemicals naturally existing beneath surface. If terraforming continues, all dioxins can be dissolved by ground waters and might kill all life in the oceans and maybe also on ground. Until technology reaches a level where we can transform all chemicals into something less toxic, terraformers might decide to freeze the planet.