Mercury is half a Mars. It’s 2/3s iron-alloy core and has an uncompressed density of 5.3 (Earth is just 4.08), which makes it the densest planet. But it is so close to the Sun that it is also the fleetest, thus not showing any signs of being overly leaden. At a mass of 0.0553 Earths (Mars being 0.10745) it very nearly is half a Mars, but packed into a volume of 37.3% of Mars. Thus it is Mars missing its upper mantle. Like Mars it has polar caps, revealed by RADAR in the early 1990s. Its sidereal rotation period (‘day’) is 59 days, while its year is 88 days – a ratio of 2-to-3. Thus its solar day, or sol, is exactly 2 years or 3 ‘days’ long. Its eccentricity is 0.205630, so its orbit (a = 0.387098 AU) varies from 0.466697 AU to 0.307499 AU, and its insolation from 4.59 Earths to 10.58 Earths, thus making its surface temperature range from 558 K to 688 K at its subsolar point. However its rotational axis is almost perpendicular to its orbital plane – thus it has no seasons, and its polar regions stay much cooler on average. Near the Poles it only gets as hot as Earth’s Moon, and the vast shadows of its polar craters remain cold enough for ice to accumulate, apparently lofted there as water vapour by its very thin atmosphere.
Mercury also has a dipole magnetic field akin to Earth’s but weaker. Thus its surface is protected from the raw solar wind, though its arctic regions must encounter a lot of ions, perhaps combining the protons with surface oxides to make water. The recent visit of “Messenger” (first flyby of three before orbital insertion) also spotted several volcanoes, indicating occasional eruptions of volatiles from within – most likely sulfur compounds and water – which will migrate to the poles, perhaps before being snatched away by the solar wind. Thus the ice-caps might be an acidic mixture, with benefits for any colonization efforts. Life can not live on plain water alone.
Because Mercury’s core is relatively accessible will that make it a desirable object for mining efforts? That seems reasonable because Mercury has large amounts of solar energy too, to power mineral extraction, refining and export. Yes, export. A next-to-nonexistent atmosphere and lots of sun means Mercury is perfect for gigantic mag-lev launchers. Also its proximity to the Sun means that Hohmann transfer windows are relatively frequent to ALL the other planets. Here’s some transfer times for Hohmann, Elliptical and Parabolic orbits…
…times in Earth days, distance is to the Sun in millions of km. Orbital transfers are computed from Mercury’s average distance to the Sun, to the target planet’s average, thus it varies a bit depending on actual position. A Hohmann orbit is the minimum energy transfer – exactly half an orbital ellipse from one planet to the other. The elliptical is a segment of a transfer ellipse, in this case a quarter of the ellipse (i.e the target planet’s radius is equal to the transfer orbit’s semi-major axis.) And the parabolic is a Solar escape orbit. As you can see the transit times are pretty rapid for the inner planets, as orbits go. Venus is mere weeks away and even a trip to Jupiter is under a year for a parabolic orbit. As we’re talking bulk cargo this probably isn’t odious with sufficient planning. Faster trips, for personnel, will need much higher energies.
So, in theory, Mercury could supply metals to all the inner planets and the near asteroids. You might wonder: why couldn’t the metal asteroids supply the rocky asteroids more quickly? Surely they’re closer?
Problem is that asteroids aren’t continually in convenient positions for a minimum energy transfer. Take Ceres (rock/ice) and Vesta (rock/metal), some 2.767 AU and 2.362 AU from the Sun respectively. Ceres takes 4.6 years and Vesta takes 3.63 years to orbit the Sun. Between transfer windows is, on average, over 17 years because their orbital periods are so close together. Yet Mercury’s windows to Ceres open up every 0.254 years. Thus it’s easy to see the advantage. Of course things are a bit complicated by the orbital eccentricity of both – Ceres’ is about ~0.08 – but the principle remains the same. That and sunlight that’s 37 times stronger on average.