Terraforming a planet without water is simply impossible. Also, life for Earth-like organisms is impossible without Water.
Depending on how much available water we have, the planet can become a Desert planet, a Shallow-oceaned planet, an Earth - like planet or an Oceanic Planet. The best scenario is where we can create an Earth-like planet. Some planets and moons have only very small amounts of water on their surface or trapped in their crust, while others are made over 50% of water. On the other hand, we can supply water by diverting comets, from the Kuiper Belt and from the moons of gas giants.
Volume of water Edit
Earth. All Earth's water is about 1.386 billion cubic meters. If we take all Earth's water and make a spherical object of it, it would have a radius of 693 km (or a diameter of 1385 km). This is comparable to Saturn's moons Rhea and Dione, with Uranus's moon Umbriel and with Pluto's moon Charon.
- Mercury is smalle then Earth, but it is a Mountain Planet, with high altitude differences. You will need more water to fill its deep craters, but given its small size, it will need between 60 and 90% of Earth's water.
- Venus is as big as Earth, but is more flat. It will become a Shallow-oceaned planet, requiring 20 to 30% of Earth's water.
- The Moon is far smaller then Earth and overall it has smaller changes in elevation. It might require less then 20% of Earth's water.
- Mars already has some water under the form of ice, but we don't know how much. Since it is smaller then Earth, it would require less water then Earth has. If we want to replenish its oceans, we might need about 35% of Earth's water, assuming there is no water left on the planet.
Ocean worlds. On the other hand, except for Io, all larger moons of the outer planets and all large Kuiper Belt objects seem to have an excess of water. Some of them appear to be made of over 50% water ice. For any planet or moon that has too much water, there are a few scenarios:
- Melt the ice and create an Oceanic Planet.
- Leave a small part of the ice for building.
- Creation of Artificial Continents.
- The use of Ground Insulation, which will allow us to build soil and oceans above the ice crust.
How to replenish water Edit
As shown above, all Earth's oceans have a volume equivalent to one of the largest moons of Uranus. Other planets and moons will need less water, but still a high amount. Diverting small comets would be impractical, we might need to use all small Kuiper Belt objects and still we would not achieve our goal.
And we must check the chemical composition of the water we want to bring in. Comets and icy moons are known to also contain salts and dissolved gasses in high amounts. We don't want to make the oceans too salty. Also, the amount of deuterium is important. So, we have to do research and find the best body for this.
Diverting a large Kuiper Belt object like Quaoar or an icy moon like Ariel is difficult and beyond current technology. See How to change orbit and rotation for details. If we push it too hard, it can break apart. If we build engines on the surface, they need to be in large numbers and spread all over the surface, so their force will not break the object. Maybe the best solution will be to alter the fabric of space.
The diverting process will take time. Let's suppose we divert Quaoar to replenish water for Mars, Venus and the Moon. At first, we slowly decrease Quaoar's speed, making it spiral towards the inner Solar System. As it passes the orbit of Jupiter, it becomes a comet and forms a huge tail, visible from anywhere in the solar System. The loss of water and gasses is negligible, giving the size of this object. Somewhere near Mars, we will drill to the core of Quaoar and bring in the most powerful atomic bomb ever created. This could be enough to break it into pieces. The explosion must be made in such a way that the fragments will be of the required size. Each fragment will be further diverted to the target planet.
As the designated fragments approach Mars, they must first enter into orbit around the red planet. A direct impact would have many unwanted consequences. They will enter orbit and will slowly spiral towards the planet. The impact will create a powerful blast on the surface and massive heat, similar to a Fire Flood. All water will evaporate and will form a huge greenhouse effect, melting and forcing to evaporate any water that might be on Mars. This will also force all carbon dioxide to evaporate from rocks and polar caps. Then, the planet will cool and all water will rain in a Noah Flood, replenishing the oceans.
For the Moon, things are different. The Moon is small. Without a significant gravity, it cannot sustain a thick boiling atmosphere. Also, after a big impact, fragments will escape into space. This means that not all water will make its way to lunar surface and what will reach the Moon will partially get lost into space. Also, runaway fragments might impact the Earth. So, everything must be well calculated. Fragments designed for the Moon will be first moved to a lunar orbit, then will be broken into smaller fragments and then they will be de-orbited, one by one, to impact. Each impact will be sent to alter the Geography, breaking crater rims and forming pathways for future valleys. If possible, a more advanced technology will try to make soft landing for the fragments into designated areas, then let them melt and replenish oceans.
In case of Venus, the scenario is much more different. Venus has a thick atmosphere that can absorb many impacts. All added water will be vaporized and will stay as a gas until we decrease temperature. Venus has sufficient mass to resist a strong impact, but this will trigger massive volcano eruptions. On the other hand, impacting Venus with a large object could change its rotation speed and could make a day shorter. It also is possible that fragments from the impact site will be lost into space, forming moons and asteroids. What is clear is that Venus will hold the newly inserted water as a gas in its atmosphere. And when we decrease temperature, all this water will fall on the planet, producing a Noah Flood.
In case of Mercury, we will need to send more water. Let's pretend that we divert Quaoar towards Mercury. A massive impact like this could force the planet to change its rotation speed, could restart volcanism and could create huge temperatures on the surface, like a Fire Flood. However, Mercury is unable to support a large, boiling atmosphere. We need to cool it down very fast, by inserting in high amounts Micro Helium Balloons. If we don't cool the planet fast enough, it will lose much of the water.
Producing water at the site Edit
Venus has sulfuric acid, which is made by sulfuric dioxide and water. If somehow we cool down the planet, we can extract sulfur and leave water and oxygen on the planet. In the same way, water can be extracted from other chemical compounds, like acids and hydroxides.
These technologies exist, but are expensive. Should be more easy to divert a dwarf planet from the Kuiper Belt or to use chemistry to extract water? Interesting question.
Removing excess water Edit
Some celestial bodies have excess of water. Such an object, if warmed-up, will become an Oceanic Planet. It had been suggested that we can extract excess water from there and bring it to the inner planets, where it's needed. This technology is very expensive and impossible with current technology. Water can be removed as a liquid or as ice from the surface, then transported to where it's needed.
An advanced civilization might try to extract water from the clouds of a gas giant.
Side effects Edit
By adding an extra weight like an ocean to a planet, quakes will occur. The crust will lower under pressure from the ocean. Volcanism can occur and tectonic movements can start again.
The impact to the planet will be so huge that it can alter Geographic patterns beyond recognition.
Other changes reflect the stability of orbits. The impact can alter orbit of the target planet or moons. The extra weight can also influence orbits. Changes will not be significant. If Earth were to replenish its water, then a large impact with an icy dwarf planet will only change the year length with a few hours. However, this can affect all asteroids that are in close resonance with Earth, quasi-satellites and passing asteroids. There will be a higher risk of future impacts.
Bringing the water will automatically mean that we bring the air. Gasses dissolved in ice will form an atmosphere if the planet had none. So, by creating oceans we are also Creating An Atmosphere.