The rocky planets in the inner part of the solar system have a common structure: a core, a mantle, and a crust. Astronomers obviously have to draw conclusions about planetary cores (even the earth’s) from indirect means. For example, a planet’s observed magnetic field can constitute a big clue.
Mercury, scientists have long known, is a special case with a very large core. Its core is 85% of the planet’s mass.
Scientists have long believed that this came about because much of Mercury’s original mass, the mass of its crust and much of its mantle has been blasted out into space by hit-and-run collisions. The core wasn’t always unusually large. It is simply the core of what is left. That, at least, was long the dominant view.
Strange New Worlds:
There is a new view, though, contesting this. It derives from new elaborations of a very old theory (the Kant-Laplace theory) of how the solar system was created. During the early formation of our solar system, at a time when it consisted of a sun and huge disk-like swirling cloud of dust and gas, the planets closer to the sun would include more iron in their core than those further away.
It may be solar magnetism, not fortuitous collisions, then, that ends up determining the size and composition of each of the inner planets’ cores.