The facts about Mercury

Mercury is the nearby planet to the Sun and the eighth biggest. Mercury is somewhat smaller in diameter than the moons Ganymede and Titan but more than twice as enormous.

Mercury's orbit is extremely eccentric; at perihelion it is just 46 million km from the Sun but at aphelion it is 70 million. The position of the perihelion processes about the Sun at a very slow rate. 19th century astronomers made extremely careful observations of Mercury's orbital parameters but could not sufficiently explain those using Newtonian mechanics. The tiny differences between the observed and predicted values were a slight but nagging problem for many decades. It was thought that one more planet (sometimes called Vulcan) slightly nearer to the Sun than Mercury might account for the discrepancy. But in spite of much effort, no such planet was found. The real reply turned out to be much more dramatic: Einstein's General Theory of Relativity! Its right prediction of the motions of Mercury was a main factor in the early acceptance of the theory.

Mercury is greatly denser than the Moon (5.43 gm/cm3 vs. 3.34). Mercury is the second densest most important body in the solar system, after Earth. Actually Earth's density is due in part to gravitational density; if not for this, Mercury would be denser than Earth. This signifies that Mercury's dense iron core is comparatively larger than Earth, probably comprises the greater part of the planet. Mercury therefore has only a comparatively thin silicate mantle and crust.

Mercury's inner is dominated by a big iron core whose radius is 1800 to 1900 km. The silicate outer shell (analogous to Earth's mantle and crust) is just 500 to 600 km thick. At least some of the core is perhaps molten. Mercury truly has an extremely thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly flee into space. Thus on the contrary, to the Earth and Venus whose atmospheres are stable, Mercury's atmosphere is always being replenished.