First.....Lets start with the basics. What the heck is Terraforming anyway? In a nutshell, Terraforming or terraformation (literally, "Earth-shaping") is the hypothetical process of deliberately modifying the atmosphere, temperature, surface topography or ecology of a planet, moon, or other body to be similar to the environment of Earth to make it habitable by Earth-like life.
The concept of terraforming developed from both science fiction and actual science. These days, "actual science" is what is driving this renewed interest. Indeed, Carl Sagan, an astronomer, proposed the planetary engineering of Venus in 1961, which is considered one of the first accounts of the concept. The term was coined by Jack Williamson in a science-fiction short story ("Collision Orbit") published in 1942 in Astounding Science Fiction, although terraforming in popular culture may predate this work.
While Mercury, Venus, Earth, Mars, and even the Moon have been studied in relation to the subject, Mars is usually considered to be the most likely candidate for terraforming. Much study has been done concerning the possibility of heating the planet and altering its atmosphere, and NASA has even hosted debates on the subject. Several potential methods for the terraforming of Mars may be within humanity's technological capabilities, but at present, the economic resources required to do so are far beyond that which any government or society is willing to allocate to it.
Lets consider Mars first...
Terraforming Mars would entail two major interlaced changes: building the atmosphere and heating it. A thicker atmosphere of greenhouse gases such as carbon dioxide would trap incoming solar radiation. Because the raised temperature would add greenhouse gases to the atmosphere, the two processes would augment each other. Carbon dioxide alone would not suffice to sustain a temperature above the freezing point of water, so a mixture of specialized greenhouse molecules might be manufactured.
Terraforming Venus requires two major changes: removing most of the planet's dense 9 MPa (1,300 psi) carbon dioxide atmosphere, and reducing the planet's 450 °C (842 °F) surface temperature. These goals are closely interrelated because Venus's extreme temperature may result from the greenhouse effect caused by its dense atmosphere.
Although usually disregarded as being too hot, Mercury may in fact be one of the easiest bodies in the solar system to terraform. Mercury's magnetic field is only 1.1% that of Earth's but it is thought that Mercury's magnetic field should be much stronger, up to 30% of Earth's, if it weren't being suppressed by certain solar wind effects.
It is thought that Mercury's magnetic field was suppressed after "stalling" at some point in the past (possibly caused by the Caloris basin impact) and, if given a temporary "helping hand" by shielding Mercury from solar wind by placing an artificial magnetic shield at Mercury-Sun L1 (similar to the proposal for Mars), then Mercury's magnetic field would "inflate" and grow in intensity 30 times stronger at which point Mercury's magnetic field would be self sustaining provided the field wasn't made to "stall" by another celestial event.
Despite being much smaller than Mars, Mercury has a gravity nearly identical in strength to Mars due to its increased density and could, with a now augmented magnetosphere, hold a nitrogen/oxygen atmosphere for millions of years.
To provide this atmosphere, 3.5×1017 kilograms of water could be delivered by a similar process as proposed for Venus by launching a stream of kinetic impactors at Hyperion (the moon of Saturn) causing it to be ejected and flung into the inner solar system. Once this water has been delivered, Mercury could be covered in a thin layer of doped titanium dioxide photo-catalyst dust which would split the water into its constituent oxygen and hydrogen molecules, with the hydrogen rapidly being lost to space and a 0.2-0.3 bar atmosphere of pure oxygen being left behind in less than 70 years (assuming an efficiency of 30-40%). At this point the atmosphere would be breathable and nitrogen may be added as required to allow for plant growth in the presence of nitrates.
Temperature management may not be required, despite an equilibrium average temperature of ~159 Celsius. Millions of square kilometers at the poles have an average temperature of 0-50 Celsius, or 32-122 Fahrenheit (an area the size of Mexico at each pole with habitable temperatures). The total habitable area is likely to be even larger given that the previously mentioned photo-catalyst dust would raise the albedo from 0.12 to ~0.6, lowering the global average temperature to tens of degrees and potentially increasing the habitable area. The temperature could be further managed with the usage of solar shades.
Mercury has the potential to be the fastest celestial body to terraform at least partially, giving it a thin but breathable atmosphere with human-survivable pressures, a strong magnetic field, with at least a small percentage of its land at survivable temperatures at closer to the north and south poles provided water content could be constrained to avoid a runaway greenhouse effect.
Artist's conception of the Moon terraformed as seen from Earth
Although the gravity on Earth's moon is too low to hold an atmosphere for geological spans of time, if given one, it would retain it for spans of time that are long compared to human lifespans.
Landis and others have thus proposed that it could be feasible to terraform the moon, although not all agree with that proposal. Landis estimates that a 1 PSI atmosphere of pure oxygen on the moon would require on the order of two hundred trillion tons of oxygen, and suggests it could be produced by reducing the oxygen from an amount of lunar rock equivalent to a cube about fifty kilometers on an edge. Alternatively, he suggests that the water content of "fifty to a hundred comets" the size of Halley's comet would do the job, "assuming that the water doesn't splash away when the comets hit the moon." Likewise, Benford calculates that terraforming the moon would require "about 100 comets the size of Halley's."
Space Tourism Magazine has begun to research and propose some additional, new cutting edge theories to terraform Mars in a much shorter time frame. Stay tuned to learn more about those in the upcoming months.
Thanks for reading.