MERCURY: A LARGE IRON-RICH CORE
The values of its mass and radius imply that Mercury must contain a large fraction of iron, the only heavy element sufficiently abundant to account for the planet's high mean density. The iron and nickel content may be as much as 65 percent of the total mass of Mercury. By analogy with terrestrial, lunar, and meteoritic chemical abundances, we presume that silicates and oxides of iron are also prevalent.
An unexpected discovery by the Mariner 10 mission was that Mercury has a shock front like the wave surrounding the bow of a ship plowing through water. It is caused by the onrushing solar wind particles colliding with the planet's magnetic field. Thus it is apparent that Mercury has a magnetic field, which is about 1 percent as strong as that of the earth. The magnetic axis of its field almost coincides with Mercury's axis of rotation.
The magnetic field is intrinsic to the planet and is most likely the result of an internal mechanism that continuously generates the field in much the same way as the earth's does. This is additional evidence for a large iron core. Chemical differentiation appears to have occurred very early in the planet's history, probably the first half billion years. Since then, the surface has been largely undisturbed by thermal and tectonic processes.
The model that has been derived for Mercury is one with a crust overlying a silicate mantle, which in turn surrounds a molten (or partially molten) iron-rich core. The core radius may be as much as 75 percent of the planetary radius, a percentage that is considerably greater than that for any of the other terrestrial planets, including the earth. Such a core should be adequate to generate the magnetic field observed by Mariner 10.
