Colonists on Mars would be almost entirely confined to their habitats: radiation sickness and dust storms would make venturing outside treacherous. Any foray outside of the tunnels would require them to wear a spacesuit. Terraforming Mars would greatly reduce the dangers that colonists faced on the planet, and would pave the way for mass settlement. In addition, the psychological benefits of terraforming would be huge. While terraforming Mars would be a gargantuan task, it would be potentially achievable if undertaken in the series of steps detailed below.
The first problem that would need to be fixed is the almost non-existent magnetic field of Mars. When first formed, Mars had a liquid core, similar to Earth. The movement of fluids within this core created a strong magnetic field capable of the deflecting the solar wind. However, Mars is much smaller than Earth, and so its core cooled soon after the planet formed, ~4 billion years ago. As a result, it lost its magnetic field, and its atmosphere was stripped away by the solar wind as a result. This exposes the ground to much higher radiation levels than on Earth, causing several problems outlined below. In addition, the solar wind would quickly remove the gas we added to the atmosphere, requiring an enormous constant supply of gases to maintain a high-pressure atmosphere.
While melting Mars' core would require a ridiculously large amount of energy, an artificial magnetic shield could be created. This would entail placing a powerful magnetic dipole near to Mars, at the L1 Lagrange point (a Lagrange point is a point where an object placed there remains the same distance from two larger orbiting bodies; in this case, it would allow the magnetic shield to always remain in place between the Sun and Mars). The magnetic field would deflect the solar wind, creating a "magnetotail" behind it where the solar radiation is much less severe, drastically reducing the radiation in this area (where Mars is). Unfortunately, this design still doesn't protect against cosmic radiation from objects other than the Sun, but it's a massive improvement over the current state of affairs.
This would drastically reduce the rate at which the solar wind removes the atmosphere, which over a period of thousands of years would lead to an increase in atmospheric pressure, due to the sublimation of dry ice at the poles during the summer. Currently, atmospheric pressure is much too low, at roughly 610Pa (Earth's atmospheric pressure at sea level is ~100,000Pa). As a result, liquid water is not able to exist on Mars, even if the temperature is high enough (which is a very rare occurrence). Both of these problems could be solved by adding large quantities of greenhouse gases to the Martian atmosphere.
The only greenhouse gases present in significant quantities on Mars are carbon dioxide and water, both trapped in solid form at the poles, with smaller reserves of carbon dioxide also found in the Martian soil. Luckily, when the planet is heated, these reserves should slowly be converted to gaseous form, increasing the heating of the planet, and leading to a release of even more carbon dioxide. The initial heating of the planet can be accomplished by a variety of means.
One of these methods entails placing large orbital mirrors at a Lagrange point of Mars and the Sun, such that sunlight is reflected onto the south pole of Mars. This would then cause evaporation of the polar ice cap over time, causing heating and starting a runaway greenhouse effect. These mirrors would have a radius of 100km each and would weigh 200,000 tonnes. The material required (a thin layer of aluminium) would likely have to be obtained either from Mars, scavenged asteroids, or Mars' moons (themselves captured asteroids). The energy required to manufacture these mirrors would be roughly 120 MW-years (for comparison, world energy consumption was ~18TW-years in 2013). This would probably be manageable with the creation of many orbital nuclear power plants, but this, in turn, would lead to the problem of cooling the power plants without a supply of water.
Alternatively, very potent greenhouse gases could be used, such as perfluorocarbons. Chlorofluorocarbons could also be used, but are to be avoided (they deplete the already tiny Martian ozone layer, worsening the problem of radiation exposure. Luckily, perfluorocarbons could be manufactured on Mars, due to the availability of both fluorine and carbon. This would also be less energy-intensive than building orbital mirrors. Eventually, Mars' climate could be made much warmer, and slightly more moister, due to the melting of the ice caps. The air outside would not be breathable, but humans could remain inside domes with breathable air, while plants are introduced to the outside. These plants would then begin the process of converting the carbon dioxide-rich atmosphere into a more oxygen-dominated atmosphere, allowing humans to breathe eventually. Some of the oxygen would also be converted to ozone by the incoming radiation, further shielding the surface from solar radiation. While the decreasing carbon dioxide % would also be accompanied by decreasing temperatures, this shortfall could be made up by the addition of more perfluorocarbons into the atmosphere. However, an inert, non-toxic gas would still be needed to make up most of the atmosphere, such that it becomes breathable.
Ammonia could also be used for the initial heating of the atmosphere. This could be achieved by redirecting an ammonia-rich asteroid from the Kuiper belt (it's actually easier to use one of these, as they orbit slower, so require a smaller impulse to change their orbit shape). This could be achieved by constructing an array of nuclear power stations behind it, which would vaporise the ammonia, causing it to expand outward and leading to a reaction force pushing the asteroid to Mars. If a gravity assist from Uranus was also used, the asteroid could be propelled towards Mars in a quicker and cheaper manner. Unfortunately, asteroids would need to be continually impacted into Mars, due to the loss of atmosphere, so the asteroids, which are unimaginably damaging and hard to aim, may be incompatible existing Martian colonies. As such, this method could only be used before large number of people become present on Mars. This method could also be used to add nitrogen to the atmosphere, which is essential if humans want to breathe outside of their habitats.
Any plants that we tried to grow on the Martian regolith would suffer stunted growth and potentially die. This is because the Martian soil is composed of 0.5% perchlorate ions by mass, which is highly toxic to both plants and vertebrates. Unfortunately, even the few plants capable of surviving the perchlorate would concentrate perchlorate ions in their leaves, making them toxic to any animals which tried to eat them, and ruining the chance of using untreated soil for farming or for creating a Martian ecosystem similar to Earth's.
Unfortunately, we can't use bacteria to try and remove perchlorate, at least until we create a magnetic shield for Mars. The UV radiation which reaches the Martian surface splits the perchlorate ions into oxygen and other ions, such as hypochlorite and chlorite ions. Unfortunately, these ions are highly toxic to almost all bacteria (sodium hypochlorite is more commonly known as bleach). Therefore, a magnetic shield is essential for removing perchlorate ions from the Martian regolith.
Overall, Martian terraforming is feasible, but only at such a time when humanity has vast resources and has significantly advanced technologically so that this can be undertaken. Personally, I think that if sufficient resources were allocated, this could be possible by means of first establishing a magnetic shield at the L1 Lagrange point, then redirecting several ammonia-rich asteroids at Mars to add nitrogen, before pumping large quantities of perfluorocarbons into the atmosphere, and introducing perchlorate-feeding bacteria and plants. To add water, additionally in the order of several hundred reasonably sized comets would need to be redirected, a huge undertaking, but worth it to create a much more stable and Earth-like climate.
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