At first glance, one would consider the proposition of a base on Mercury, our Sun’s closest satellite, as ludicrous. With daytime temperatures reaching up to 700K — hot enough to melt lead — while the dark side of the planet experiences a temperature average of 110K — far colder than anywhere on Earth, combined with the lack of any substantial atmosphere, and being deep in the Sun’s gravitational potential well, conditions seem unfavorable.
First impressions can be misleading however, as it is well known that polar areas do not experience the extreme daily variation in temperature, with temperatures in a more habitable range (< 273 K (0 °C)) and it has been anticipated there may even be deposits of ice inside craters. http://nssdc.gsfc.nasa.gov/planetary/ice/ice_mercury.html
And is not just habitable temperature and ice-water in its polar regions that make Mercury an interesting candidate for an industrial base. There are a number of other factors making it more favourable than either a Looner or Martian base:
Mercury is the second densest planet in our solar system — being just slightly less dense than our Earth — and is rich in valuable resources, the highest concentrations of many valuable minerals of any surface in the Solar System, in highly concentrated ores. Also, being the closest planet to the Sun, Mercury has vast amounts of solar power available, and there are predictions that Mercury’s soil may contain large amounts of helium-3, which could become an important source of clean nuclear fusion energy on Earth and a driver for the future economy of the Solar System. Therefore it is a strong candidate for an industrial base.
Ticking other boxes — the gravity on the surface of Mercury is more than twice that of the Moon and very close to the surface gravity on Mars. Since there is evidence of human health problems associated with extended exposure to low gravity, from this point of view, Mercury might be more attractive for long-term human habitation than the Moon. Also, Mercury has the additional advantage of a magnetic field protecting it from cosmic rays and solar storms.
In fact, this idea is not a new one. Back in the 1980s, C.R. Pellegrino proposed covering Mercury with solar power farms, and transferring some of the resulting energy into a form useful for propulsion for interstellar travel. When one looks at the options we have available to us for first steps into space, we have another option available to us in Mercury.