In many cases, during terraforming processes, the oceans of a planet might be too salty for Earth-like life forms. On such a planet, terraforming will require a different approach.
In our Solar System, many planets have subsurface oceans. However, these oceans are different from what we find on Earth. For example, Titan's ocean appears to be as salty as the Dead Sea . Research on Enceladus have shown that its subsurface ocean is also salty and alkaline . Again, Europa was measured to find an acid ocean, not suitable for Earth life . And things are not ending here. In case of some waterless worlds, like Mercury, Venus, Moon or even Mars, we have to bring water to them. This water can easily be delivered by diverting comets. However, research from ESA's Rosetta probe revealed that comets are also salty, containing sodium and magnesium . On the other hand, we know that even in waterless worlds, salt exists. We have detected water flowing on Mars, but researchers say this is in fact salt water . So, there is plenty of salt surrounding us and much of this salt is not like in Earth's oceans.
Ocean planet Edit
Many celestial bodies are covered with a layer of ice. If this melts, they will become an Oceanic Planet. This would force us to build Artificial Continents or to use Ground Insulation. Based on this, we have 3 different scenarios.
First scenario: on an oceanic planet, if there are no strong currents (tidal forces and internal heating), the ocean will have weak currents. In this case, water will slowly separate. On surface, water with less salt will accumulate, mainly coming from rain. If this process happens, then we will see stratification in a similar way to Earth's Black Sea, where water close to surface is less salty and oxygenate, while deep water is more salty and toxic for fish life. Majority of moons orbiting Jupiter, Saturn and Uranus show to have a white ice crust, with a low concentration of salts. We might conclude that in ancient times, when these moons were hotter, surface water was less salty.
If settlers decide to build artificial continents floating, then we will have rivers with freshwater, that will slowly flow into the oceans. These rivers will accelerate the separation of a less salty surface layer.
Second scenario: On an oceanic planet with strong currents (exposed to tidal forces, significant temperature differences between hemispheres or with strong subsurface volcanoes), terraforming will be harder. In these conditions, stratification will not occur or will be not enough to allow salt-resistant fish to survive. It is also possible that the ocean will contain some extremely toxic salts, like compounds of arsenic or cadmium. In this scenario, construction of artificial continents will be a hard task. Still, at least in theory, it could be possible to create a global floating layer and to build shallow seas on top of it. At least the artificial islands can have lakes and ponds on their surface, to sustain life.
The best solution would be to try to transform the salts into something not soluble. Some solutions exist, but are expensive and limited to a few chemical elements. It is possible, in case of heavy metals, like chrome, to change oxidation state, making them not soluble. In other cases, we can create complex ions, heavier and less soluble. For the most common salt, sodium chloride, a solution would be to link both sodium and chlorine to organic molecules, that are not soluble. This could be done with the help of genetically modified bacteria. Replacing hydrogen with chlorine is easy to do in organic chemistry. And both sodium and magnesium are found in organic compounds. The problem is that chemical compounds have a limited lifetime and after some time they might disintegrate, releasing the salts.
Third scenario: The use of ground insulation can be a better solution. This technique requires covering the ice with a material that does not conduct heat and keeps the ice beneath cold. This could allow us to build continents and oceans (with as much salt as we need) above the insulation layer. In this way, the ice crust does not need to be melted and the subsurface ocean, with all its salts, can remain undisturbed. However, if at some point the insulation layer brakes, water from the oceans can reach the ice and start melting it. On a long timescale, heat from above will at some point reach through the insulation, but this would take more time then a human civilization needs to develop.
The use of a ground insulation might be cheaper, faster and safer then previous techniques, because of the time saved for melting the ice and the time needed for water layers to separate.
Rocky planet Edit
A rocky planet does not have a global ocean. In fact, it is more like Earth. A rocky planet may have lost part of its water (and the salt is now concentrated) or might have almost no initial water (but settlers brought water by diverting comets). In the second case, salt might have existed on the surface before terraforming or it was brought to the planet inside the comets. Regardless the cause, terraformers will have to find a way to decrease salinity.
In choosing the solution for terraforming, the Geography and the hydrologic regime are key elements. We might have a few solutions for our problem:
Sacrifice an ocean would be the solution if continents separate water bodies completely or almost completely. On Earth, we see this on Caspian Sea. Its salinity is very low compared to the oceans. However, Caspian Sea has a small gulf, named Kara-Bogaz. Since the connection between the two water bodies is very narrow, water from Caspian Sea enters into Kara-Bogaz taking its salt, but never returns to the sea (in fact, it evaporates). This is why Kara-Bogaz is a very salty water body.
On Earth, it is hard to find a suitable place for dumping all excess salt. The sacrificed ocean (or sea) must be separated from other oceans (naturally or artificially) Also, it must have a very small water inflow from rivers and rain. The only suitable places on Earth would be Mediterranean Sea, Red Sea and Persian Gulf, but to make them swallow at least half of Earth's oceanic salt, we will need to wait for millennia.
After the salt was diverted into the sacrificed sea, we have to do something to prevent it from returning. We have to always keep some water above the salt, or winds will blow it away, destroying agricultural crops. We see this today around Aral Sea, where salt from the exposed seabed is blown by dust storms over agricultural land. So, we will have to divert a river to flow there. In time, the river will carry sediments and will slowly cover the salt. This is similar to how salt deposits formed on Earth.
Save a sea. This concept is different and is the best solution if the ocean salt cannot be drained into a sacrifice sea (like on Earth). The concept requests construction of dams on existing gulfs. Then, freshwater from rivers will slowly push the salt out into the oceans. On Earth, this can be done on the Baltic Sea, by constructing a dam between Denmark and Sweden. The isolated sea must receive excess water from a river. Also, the Black Sea, which receives more water then it loses, can be isolated by a dam on Bosporus Strait. Excess water can be diverted towards the Caspian Sea, to increase is size.
In addition, inland seas can be created where natural mountain ranges block the course of a river. Water from a large river can be diverted towards a desert region to fill it. This would occur in Africa, by diverting part of the Niger and Nile into depressions that exist in Sahara.
Changing salt composition is another technique, that will require huge costs. The way to do this is described above, for oceanic planets.
Blocking the salt from ever reaching the ocean could be a solution. If the planet initially had no surface water, we can take the salt towards safer places (like underground or into a sacrificed endorheic basin). We also can cover salt pans with impermeable layers.
If we bring water through comets and we know that the comets have a certain amount of salt, we can use a different technique. We would send the comets on a crash course to a sacrifice area. There, water will evaporate, while salt will remain on the ground. During impact, the heat generated is high enough to make all water evaporate, while the salt will remain trapped. The sacrificed impact area must be an endorheic basin (not connected by rivers to the oceans). On Earth, we have endorheic basins in Sahara, Central Australia or Central Asia. Those are also desert areas, so the water will evaporate quick, if any reaches the surface without melting.
Adapting to hipersaline environments Edit
Another way is to create genetically modified plants and animals, able to survive in environments with high concentrations of salt or in water with toxic compounds. It is, however, questionable if these plants and animals will be suitable for human food.
Many terraformed worlds will have excess of salts in their oceans. Finding the best solution differs from a place to another. Terraformers and settlers will have to chose the best technique and use it.