NASA's latest Mars rover, Curiosity, is currently its way to Mars, on a mission to explore whether life could exist there. If we're going to colonize Mars — and some scientists say we must — it's likely that we'll start by terraforming. Terraforming, or planetary engineering, is the process of altering the climate of a planet to be more hospitable to life and human exploration. Of all the bodies in the solar system, Mars is by far the best candidate. Here's how that would work.

Once A Wet, Warm Planet
To understand how Mars might have a habitable future, it's important to understand a bit about Mars' past.
Mars' geological history is divided into three ages, which are from oldest to youngest the Noachian, the Hesperian and the Amazonian. The Noachian epoch, ranging from about 4.1 to about 3.7 billion years ago, is characterized by heavy asteroid bombardment and abundant surface water. This is the so-called "warm, wet" period. The Hesperian, ranging from 3.7 to somewhere between 1.7 and 3.0 billion years ago, is characterized by heavy volcanic activity and massive water flow. The Hesperian was an intermediate age between the warm wet Noachian and the the cold, dry Amazonian, which is the Mars we know today as being not the kind of place to raise a kid.
It is this unique history that makes Mars such an attractive candidate for terraforming. Unlike the other bodies in the solar system, Mars has a history (ancient though it may be) of being far more Earthlike than anywhere else in the solar system short of Earth itself. The proposals for terraforming the other bodies in the solar system are all highly theoretical, involving massive energy flux and near magical technology like changing planetary orbits, speeding up planetary rotation, sequestering hundreds of bars of atmosphere (1 bar = atmospheric pressure at sea level on Earth) or constructing planet-wrapping Dyson spheres to prevent atmospheric escape.
By contrast, Mars, with its 24 hr 37 minute day, relatively abundant water ice and history of warmer wetter conditions seems more within the grasp of near future human civilization to terraform. Add to this the fact that not-too-crazily-optimistic estimates of Mars' current conditions suggest that increasing the atmospheric pressure and temperature to more life friendly conditions may be more a matter of a nudge than a shove and it's easy to see why this world holds such fascination for would-be planetary engineers.
Why Did Mars Die?
So, what happened on Mars? Why did this once warm and inviting place, with enough atmospheric pressure and a high enough temperature to support liquid water, become a frozen rock with an atmosphere too thin to support even water ice? The answer boils down to...or rather, freezes out to...volcanism and bombardment.
In the Noachian period, Mars experienced a high frequency of asteroid impacts. The energy and volatile gas introduced into the Martian environment by the Noachian bombardment helped maintain the warm, wet climate and the thick atmosphere.

The other key to Mars' warm wet past are its rather impressive range of volcanoes. Olympus Mons, located in a highly volcanic region of Mars known as the Tharsis Bulge, is the largest mountain in the solar system, being some three times larger than Mount Everest and twice as large as Mauna Kea (which is the largest mountain on Earth.) As on Earth, volcanoes on ancient Mars played a vital role in atmospheric recycling. Atmospheric carbon dioxide and nitrogen bind readily with minerals on the surface, especially in the presence of water, to form carbonates and nitrates. Volcanoes recycle these volatile elements by cooking nitrogen and carbon dioxide out of minerals and reintroducing them into the atmosphere. Volcanoes were also useful for replacing gas lost from the Martian atmosphere into space.
When volcanic activity halted on Mars, atmospheric recycling stopped and Mars absorbed its atmosphere like a sponge. As the core cooled, any magnetic field Mars may have had would also have decreased, making Mars' upper atmosphere vulnerable to stripping by the electrically charged solar winds. As the atmosphere thinned, the planet became colder. Eventually, the temperature dropped low enough for carbon dioxide ice to form and draw more gas out of the atmosphere.
The Building Blocks of a New Environment
So the good news is that much of the material we need to give Mars a thicker, warmer atmosphere are still present on its surface and buried in its regolith. Despite these promising circumstances, however, it's clear that one does not simply walk into terraforming Mars. In his definitive text, Terraforming: Engineering Planetary Environments, Martyn Fogg laid out five critical challenges:
1. The surface temperature must be raised
2. The atmospheric pressure must be increased
3. The chemical composition of the atmosphere must be changed
4. The surface must be made wet
5. The surface flux of UV radiation must be reduced
Fogg suggests that the engineering of the Martian environment will proceed through ecopoisis, a term coined by Robert Haynes for the process of making a planet more hospitable for primitive microbial life, to something approaching full terraforming, in which the climate of Mars will more closely resemble that of Earth's.
The most promising approach to dealing with the first two items is to reverse the runaway freezeout of the Martian atmosphere by initiating a runaway greenhouse effect. Current atmospheric pressure on Mars is between 6 and 7 millibars at low elevations. That's less than 1% of Earth's pressure at sea level. The inventory of frozen carbon dioxide remaining on the Martian surface is estimated to be between one hundred and one thousand millibars, with a good deal of it existing frozen on the surface at the poles and the rest underground in the regolithic permafrost. Increasing atmospheric pressure and temperature is a matter of warming the poles to the point where they sublimate into the atmosphere. Carbon dioxide, being a greenhouse gas, will retain more of the sun's heat and promote the melting of yet more carbon dioxide out of the planetary regolith, which will retain more heat and promote further degassing. This concept of creating a runaway greenhouse effect to release Mars' reserves of frozen carbon dioxide has become known as "the standard paradigm" of Martian ecopoiesis.
Image by Dane Spangler
Jumpstarting a Greenhouse Effect
Okay, so how do we warm up the Martian poles? Several approaches have been suggested, from spreading dark material on the poles to lower their albedo, to industrial ice farming to good old fashioned thermonuclear detonations. In Technological Requirements For Terraforming Mars, Chris McKay and Robert Zubrin suggest a more elegant scheme: orbital mirrors. Constructed in high orbit above Mars, the mirrors would reflect sunlight back onto Martian surface. In McKay and Zubrin's model, the mirrors would not exactly orbit Mars. Rather, they would reside directly above Mars' night side, held in place by a balance of forces between Mars' gravity and the solar light pressure. The orbital mirror plan has the advantage of continually introducing extra heat into the Martian climate long after the poles have sublimated. Even in the later stages of terraforming, Mars' distance from the sun will make the increased insolation from the orbital mirrors desirable.
Another key to stabilizing Mars atmosphere is the activation of its hydrosphere. Water promotes ecopoiesis not only by providing a vital element for life, but also stabilizing the climate. Water retains heat and reduces the drastic swings in temperature over the diurnal cycle and water vapor is a potent greenhouse gas which will help hold thermal energy in the atmosphere.
Current models suggest that there are large quantities of water stored in permafrost aquifers. Release of this water will require a good deal more energy than will be required for the release of carbon dioxide. Nuclear mining, even with high-yield devices, would produce far too much fallout. Another approach to releasing Martian water is controlled asteroid impact, simulating the hydrosphere-promoting bombardment of the Noachian epoch. This would require a great deal of energy, however, and would be very difficult to control with any precision. Zubrin and McKay suggest that the orbital mirrors used to melt the poles could be refocused on smaller areas of the permafrost. Water melted out of the southern highland permafrost would be directed into the northern lowlands and into the Hellas basin in the south to create shallow planetary seas.
Image by Stephan Martiniere
Making the Air Breathable
These alterations to the Martian climate would go a far way to making Mars more habitable for microbial life and more easily explorable by humans, but the remaining challenges of reducing UV flux and making the atmosphere breathable will require considerably more time and effort. Mars' thick atmosphere of carbon dioxide would block a good deal of the incoming UV radiation, but carbon dioxide does not significantly block UV radiation in the 190 nm to 300 nm range. Current UV flux on Mars is about 6 Watts per square meter, which would be enough to kill most organisms. The plan here would be to introduce highly UV resistant lifeforms, such as lichen, directly on the surface or to grow cyanobacteria in soil which would protect the organisms from UV, and in mats on the newly formed seas, with layers of dead cells protecting the living cells beneath. These organisms would release oxygen which would slowly build to breathable levels and would form ozone in the upper atmosphere, which would reduce the harmful 190-300 nm UV flux. These organisms would also provide nutrients to help build the Martian soil up to the point where it could support more complex plants.
The difficulty with this biogenic approach to ozone formation is that Mars simply doesn't have enough nitrogen to support large scale life. Atmospheric nitrogen is at trace levels. Contrast this with Earth where 78% of the atmosphere is nitrogen. Nitrogen is an essential element for life and its scarcity on Mars presents a serious challenge to ecopoiesis. Unlike carbon dioxide, which disappeared both into carbonates and frozen carbon dioxide ice, Mars' nitrogen is pretty much all stored in mineral form as nitrates in the regolith, which means that the energy required to free Mars' reserves of nitrogen will be massive. It may be possible to introduce significant atmospheric nitrogen from extraplanetary sources, such as ammonia rich asteroids. One especially fun idea would be to introduce large quantities of nitrous oxide (yup, WhipIt good!), which is a powerful greenhouse gas and would help warm the planet. Unfortunately, N2O photodisassociates rapidly in the presence of UV. But once we get that ozone up and running, it's party time on Mars!
Another problem with making the Martian atmosphere breathable is that even with adequate levels of oxygen, atmospheric concentrations of carbon dioxide above 5% are lethal to humans. If the inventory of CO2 turns out to be on the low end of the estimated range, Fogg suggests a more modest approach to CO2 release than the standard paradigm coupled with the rapid introduction of nitrogen. This process would be slower in generating initial ecopoiesis but would leave Mars with an atmosphere that would be more conducive to full terraforming.
Finally, if we cannot restart Mars' volcanoes or otherwise promote geological demineralization of bound volatiles, a terraformed Mars will have to be maintained with constant re-introduction of volatile elements and restoration of the atmosphere lost to the solar wind. But since the loss of atmosphere to space and to mineralization would take place over centuries, we might have time for some more radical planetary engineering, such at the construction of deep moholes to release gas trapped in the Martian crust and even the construction of an artificial moon to provide tidal force to reactivate Mars' geological process.
It may, however, be ultimately impossible to fine tune Mars' climate to support human life as we know it today. In the centuries it takes for us to get to this point, it might just be easier to engineer humans to tolerate the conditions that we can produce on Mars. As Kim Stanley Robinson pointed out in his Mars Trilogy, humans do not just terraform Mars, Mars aeroforms us.