I'm working on a project involving terraforming, and I'm compiling a list of techniques for various goals. Most of them are well-documented in terraforming circles, but one area I've found that seems totally unaddressed is reducing the amount of water on a planet.

If humans landed on a water-world and needed to lower the sea level enough to create some usable land, how might they go about that? It's unimportant what happens to the water after the process is complete (destroyed, removed, etc), but the process needs to be planet-scale and result in the permanent reduction in sea level, and it needs to work for LARGE quantities of water (i.e. a planet where the entire surface is covered in deep oceans).

One last note: I'm looking for a realistic plan. I am open to some sci-fi technologies, as long as they stay grounded in reality (e.g. "orbital mirrors" yes, "transdimensional gateways" no).

  • $\begingroup$ Sandtrout. Of course you'll have to ask Frank Herbert how best to get them off of Arakkis. $\endgroup$ – Cort Ammon - Reinstate Monica Aug 28 '15 at 19:25

A lot depends on the planet, but there are theories that as much as 3X the amount of water in the oceans is bound in mineral particles in the mantle. On Earth, this water is trapped in sediments and dragged into the mantle via the process of subduction, as sections of continental plates slide under other ones, only to reappear millions or billions of years later when magma bubbles back up to the surface in the form of volcanic eruptions.

If the planet in question already has active plate tectonics, then the solution would be to change the chemical and mineral nature of the sediments in the subduction zones so they could absorb a lot more water. This would allow some immediate lowering of the oceanic levels as the seabed becomes a sponge, than long term "storage" of the water as the sodden sediments are subducted into the mantel. The colonists will notice much more water vapour in volcanic eruptions @ 1,000,000 years in the future, but hopefully they will have used the time productively to prepare...

For a planet without active plate tectonics, the core and mantel wold need to be "reactivated", which would involve depositing huge amounts of energy deep below the crust. Since water itself is very good at absorbing energy, just setting up gigantic lasers or particle beam accelerators in orbit will result in boiling oceans, but not a lot of new tectonic activity. In the book "The Forge of God" by Greg Bear, malevolent aliens drop a slug of antimatter compressed to neutronium density into the Earth's core, which provides a fiery end to the Earth, but if you were to calibrate this properly and the energy release was less than the gravitational binding energy, then you should be able to melt the core and begin the process of plate tectonics (note, it will probably take thousands to millions of years for the heat to work its way from the core to the mantle and melt everything, so once again, this is a sort of long term project.

One thing which is not entirely clear is the role of the water in the upper mantel. Some scientists believe it acts as a sort of lubricant. If this is true then in the first case, the amount of movement between the plates could speed up (OMG, the continent has accelerated to 10 mm/year!), causing more earthquakes. In the second case, as heat percolates upwards the plates will remain locked together until a considerable amount of force is placed on them, resulting in even more massive earthquakes, until enough water is subducted into the mantle and a chemical equilibrium is reached.


There really isn't a practical way unless you are discussing truly massive scaled terraforming.

Freeze it out

Set up a sunshade arrangement to drop the planetary temperature low enough to freeze the water, then "mine" it and launch it into space.

Understand that you'll need a super efficient method of surface - space transport. You'll need something as efficient as a space elevator and this structures efficiency could be improved by simultaneously transporting goods from space - surface and using that energy to power the lifting of water.

Burn it off

My other suggestion would be the opposite approach. Use a massive orbital mirror to concentrate sunlight on the surface of the planet and "burn" the water off.

Note that this wouldn't work very well over short periods of time for a planet as massive as Earth. However, if the planet is roughly half as massive, its hold on water is a bit tenuous and you might be able to heat the water enough to significantly its outflow over non-geologic time scales. I haven't run the math though so I might be incorrect.

In general, this is going to wreck the biosphere more thoroughly than the "freeze it out" method. However, it is a less active approach - no mining and transport workers required, just apply the giant space heater.

Note: this approach works better if you focus that sunlight into a very narrow beam and heat a small column of water to a tremendously high temperature rather than baking the entire planet at some lower temperature. I believe the Jean's escape mechanism scales linearly to the maximum gas temperature. It also helps to apply this heat to the top levels of the atmosphere rather than lower down - so perhaps regularly "salting" the upper atmosphere with low albedo dust would help the process.


Trivially, you can lower the global sea level by freezing more of the planet's hydrosphere in its polar ice caps, which you could accomplish by global cooling. There are any number of ways you could theoretically cool the planet, and many of these "geoengineering" techniques are being actively debated in the scientific community today as a possible way of counteracting the effects of global warming.

An non-exhaustive list of these techniques includes: direct chemical capture and sequestration of carbon dioxide from the atmosphere or ocean; seeding the ocean to encourage carbon capture by microorganisms; and artificially increasing the cloud cover by releasing substances into the atmosphere. And, perhaps more fancifully, there's the "orbital mirror" example mentioned in the question itself.

  • $\begingroup$ And since you, unlike most would-be geoengineers, actually want to get rid of water, maybe you could sequester the carbon you capture by combining it with hydrogen obtained through electrolysis of water to make hydrocarbons, then pump that nasty stuff into underground caverns. : ) $\endgroup$ – Doug Warren Aug 28 '15 at 18:14

English is not my first langage, feel free to correct the mistakes !

Evaporate all of that ?

Not a good idee. You can make water evaporate with an orbital mirror or any other way to warm a part or the whole planet, but you want to change the whole world, where will the steam go ? A little further, and then fall in rain. So you will have to keep steam in the air with a constant warming and your atmosphere will be... wet. And rainy. And on our world, less than 5% of the water stay in the atmosphere. Maybe that's enough ?

Freeze that all ?

Much better option in my opinion. With 2 questions :

  • How to freeze it ?
  • What can you do with all that ice ?

One idea : Sending water in space will freeze it and you will get rid of that, but it's a difficult thing. With actual space technology, sending water into space would be expensive and difficult but, hey, wait, you have a lot of hydrogen to spend as fuel in the water around. You just need electricity... (do you have tidal power station on your super oceanic planet ?). Or use wathever near-future technology you want to send this water far away.

Another one : Freeze it on the planet. Build one or two giant freezer (on the poles) and, if needed, take a big pump to bring water on the top of the new big iceberg. A freezer need a heat source, and big pipes with gas inside (quick google check - ammonia for example). Doesn't seem too expensive to make a world habitable. My personal choice !

Find some interesting chemical use of water

Again with electrolise. You can easily turn your water into hydrogen and oxygen, and maybe find an use for it. Or burn it (pretty cool source of energy, might compense the electricity used to separate the gas. Or send them into space (perhaps easier than sending liquid or solid water)

There should be other chemical reaction you can use.


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