Artist's conception of the terraformed Mercury
Mercury is the first planet from the Sun and the only one in the Solar System without a considerable atmosphere. It is the smallest terrestrial planet of the Solar System and despite being also smaller than the Solar System objects Ganymede and Titan, it is massive enough to have about the same surface gravity as the even larger planet Mars. 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[by whom?] that Mercury's magnetic field was suppressed after "stalling" at some point in the past and, if given a temporary "helping hand" by shielding Mercury from solar wind and placing an artificial magnetic shield at Mercury-Sun L1, 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.[citation needed]
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 baratmosphere of pure oxygen being left behind in less than 70 years (assuming an efficiency of 30-40%).[citation needed] At this point the atmosphere would be breathable and nitrogen 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.[citation needed]
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.
If terraformed, Mercury will face the problems of a Mountain Planet.