On the surface, our Moon and Mercury, the closest planet to the Sun, look like cousins. They both go through the same range of phases and occasionally eclipse part of the Sun from our view. The Moon is tidally locked in synchronous rotation; Mercury is in a 3:2 spin–orbit resonance (it rotates three times for every two orbits), both have extremely tenuous exospheres, they are both geologically inactive but show clear signs of ancient volcanism. And as a result of their lack of atmospheric or geological activity, the most conspicuous feature of both objects is the billions of years’ worth of impact craters they’ve endured, particularly early on. They both have surface temperatures that range by many hundreds of Kelvin over the course of very long days (from 100 K to 400 K over the course of 29.5 Earth days in the case of the Moon; from 100 K to 700 K over the course of 176 Earth days in Mercury’s case). They’re both small. Both host substantial water-ice deposits in permanently shadowed polar craters.
Yet despite all these similarities, there are important differences as well. The Moon has a very low density, contains very little iron, and has a negligible magnetic field. Mercury is very dense, with an iron core that is about 85% of its radius, and despite barely rotating it has a detectable magnetic field. And then there is the subjective difference: while the Moon is by far the best studied object in the Solar System, it has been only very recently that our knowledge of Mercury has come to the same level as that of the other terrestrial planets—Venus, Earth, and Mars. We will study these three more active planets in the next module, exploring the many factors which make Earth particularly well-suited to support life—a phenomenon which, it seems, could not possibly have emerged naturally under the harsh conditions of the Moon and Mercury, but which Venus and Mars were both almost capable of.
cosmiCave.org 