A mountain planet is a theoretical planet whose geography includes extreme altitude changes. The weather conditions included in this article are largely based off that of Mercury, since Mercury is the closest thing to a mountain planet that we know about.
Challenges[]
The challenges facing anyone who attempts the Terraforming of a mountain planet are immense. Extreme mountain chains and valleys would disrupt currents in said planets atmosphere and bodies of liquid (such as oceans) would be completely separated, creating extreme weather conditions.
Water[]
On a mountain planet, if water covers at least 50% of the surface, the oceans would need to be deep. Average ocean depth would be around 10,000 meters, while in an abyss, it can reach far more. Also, oceans are separated or poorly connected. This will have a strong influence in water currents. As salt water tends to go down and lighter fresh water tends to stay up, oceans will be separate into different layers. We can see this on Earth in the Black Sea, a sea that has no vertical currents, where deep water is salter, has no oxygen, and is therefore not populated by Krakens.
A body of water that is located close to the poles will get less light and will be covered by glaciers. Without a current to bring warmer water from the equator, polar seas will be permanently covered with ice. On the other hand, a sea that is close to the equator, will be warmed enough to reach high temperatures. As hot water is unable to dissolve enough oxygen, there will not be much life in water close to the equator.
Winds[]
A planet that rotates fast enough has a Coriolis effect, a common movement of air that brings a limited amount of heat from the equator to the poles, but also keeps the poles cold enough. A planet that is tidally-locked will have another atmosphere movement, where winds will continuously blow between the hot day-side and the cold night-side. In case of a mountain planet, atmospheric circulation is limited.
If the planet is rotating, polar air will remain trapped at the poles, while equatorial air will also remain trapped. Also, if the planet is tidal locked, air circulation between hemispheres will be blocked.
Climate[]
As shown above, the planet will be separated into different climate cells, each one with its own ecosystem.
Rotating planet[]
If the planet rotates around its axis, climate will be as follows: Around the poles, since there is no force to bring hot air or water, everything will be frozen. As ice reflects much of the sunlight, there will be no force to bring spring. Polar regions are not a place for settlers to go.
In temperate areas, climate will be harsher than on Earth. If the planet has a tilted axis, it will have seasons. Summers will be hotter and winters will be colder. If the water basin is large enough, it can absorb part of the summer heat and make a better climate. However, if the sea gets covered with ice, it will reflect sunlight and make springs come later than usual. High mountains are expected to have lower temperatures than the sea. This will sustain an air circuit between the central sea and surrounding mountains. Air vapors will circle from the sea to the mountains, sustaining a network of rivers. However, in hot summers, when the sea will be colder, the circuit will reverse and moisture will not get to the mountains, resulting in droughts.
In equatorial areas, it will be hot. A special climate will form. Winds will take water from the sea to the colder mountains surrounding, where it will rain. Probably at some altitude there will be a perfect climate for humans.
Slowly-rotating planet[]
If the planet is rotating slowly, like Mercury, each climate cell (except for the poles) will witness extreme temperature changes. During the long nights, everything will freeze, except water deep in the seas. During the long days, temperature can reach deadly values, even above 100 degrees Celsius. Still, deep inside the sea, water will not heat enough. In the mountains, in deep canyons or behind cliffs, land will be in shadow and their temperatures could sustain life. Humans would prefer to take cover underground.
Massive temperature changes will result in storms. However, a hurricane cannot exist on a planet fragmented by mountains. It will simply not have enough space to form.
Tidally-locked planet[]
On a tidal locked planet, some cells will be on an eternal night. Without major connections to the light hemisphere, temperatures will drop dramatically. In the center of the dark hemisphere, it is possible that temperatures will drop so low, that oxygen (or even nitrogen) can sublimate. If this happens, the atmosphere will slowly be deposited there, leaving the rest of the planet with rarefied air.
In center of the light hemisphere, temperatures will rise above boiling point. Heated air will slowly take all water over the mountains to nearby cells, leaving only a desert behind.
The best destination for settlers will be temperate places, with the required luminosity and temperature.
Long-term climate changes[]
Over long periods of time, some water moves from one cell to another. The change is little during a human lifetime, but significant over millennia. The result is that some cells will lose water and become deserts, while others (usually in polar regions or on the dark side) will gain water.
Special types of mountain planets[]
Depending on many conditions, mountain planets may vary.
Desert planets[]
A Desert planet has a limited amount of water. If a terraformed Mercury will have the same amount of water per surface as Earth, that water will get into deep craters and will cover less then 50% of the surface. On a desert Mercury, we can send more water to the poles, letting the rest to be dry and exposed to high temperatures. Ice will cover limited areas and will melt during daytime. If ice does not melt, we can extract it and take it to irrigate crops.
If the planet has a long daytime, it will have long cold nights. Almost all moisture will condense to the ground as snow and will support rivers for irrigation during the day. It will be harder for such a planet to enter a runaway ice age.
Outer planets[]
In the case of an Outer Planet, things are different. The layer of greenhouse gasses will be below the high mountains, resulting in a strange climate. We can add a different amount of greenhouse gasses to each cell, to get the desired temperature and we can treat each cell as a standalone ecosystem. Between cells, high mountains will experience temperatures below -100 or even -200 degrees Celsius. Since air movements are very slow, cells will not change much of their atmosphere and water.
Infrastructure[]
On a mountain planet, each cell will have its own infrastructure: roads, railways, irrigation systems, dams, energy production. Cities will be connected to each other. Connections between cells will be a hard task. Extreme engineering will be needed to cross mountain barriers with long tunnels. If needed, high altitude trains will have to be pressurized and heated.
Water and air management will be important. If somehow water is moving to other cells, settlers will have to move it back. To do this, the most easy way is to divert rivers from one cell, through a tunnel, to the other cell. Creating a tunnel to move sea water might not be a good idea, since it will also divert the salts within.
Another solution, if possible, might be to create canyons that will connect one cell to another. However, breaking through mountains reaching 10 km high, is not an easy task.
Ideal political system[]
Given the differences between each cell and the difficulties to connect one cell with another, the best way to governance a mountain planet is a federation. Each cell should have its local government, its institutions able to solve local problems, while a central government will represent the external politics to other planetary states.