6. Mercury: a battered world
An iron planet
The terrestrial bodies - the Moon, Mars, Venus and Earth - exhibit a fairly linear relationship between size and mean mass density, in which bigger objects have greater density. But Mercury does not conform to this relationship. Although it is less than half the size of the Earth and not much bigger than our Moon, the bulk mass density of Mercury, at 5,430 kilograms per cubic meter, is typical of a far larger planet. The most natural explanation of Mercury's high mass density is that it contains an unusual amount of iron, which is cosmically the most abundant heavy element.
In order to account for the planet's large mass density, the apparent dearth of iron on the surface has to be balanced by a large iron core. Relative to its size, Mercury would have the largest metallic core of all the terrestrial planets. The dense iron core takes up 75 percent of the planet's radius, so Mercury is mostly iron core surrounded by a relatively thin silicate mantle. In this respect, it is the opposite of the Moon that has a thick rocky mantle and a relatively small iron core.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University