Re: Why is planet Mercury's core considered to be iron?

Hi Matt,

The Earth's core is about 54% of its diameter, as compared with 75% for Mercury, so much smaller by comparison. You're right that a larger core should produce more internal heat (from the radioactive decay of elements like uranium and thorium, rather than compression). However, Mercury's small size (its radius is only about 40% that of the Earth) means that it cools much more efficiently than the Earth. Thermal models of Merucry indicate that the iron core should have cooled and solidified long ago. This is something of a problem, as Mercury has a magnetic field, which requires at least part of the core to be liquid iron.

Iron is proposed for the core of the innermost, terrestrial planets because it is the most abundant of the heavy elements - plutonium, mercury and gold are all much more rare, by many orders of magnitude. For example, iron atoms are approximately 20-30 million times more abundant than mercury.

Models of the formation of Mercury do have problems account for the large observed fraction of iron required to explain its density. We expect that the warm, inner regions of the solar system will be rich in elements which have high condensation temperatures - iron, silicon, magnesium, aluminium, calcium and nickel. Lighter elements, with lower condensation temperatures, would not have condensed into planets close to the Sun. Chemical equilibrium models, in which the fraction of elements present are determined by their condensation temperatures, do give higher iron abundances closer to the Sun. However, they don't quite explain the very high density that Mercury has. I think that the most successful model for explaining Mercury's density, at the moment, is probably the impact theory, which postulates that Mercury was stripped of much of its crust by a giant impact, early in the history of the Solar System.

For more information, you could try looking up the "Encyclopedia of the Solar System" ed. Paul Weissman, Lucy-Ann McFadden, Torrence V. Johnson, Academic Press, 1999. This is an excellent reference about current research topics in the Solar System, and includes more detail about unsolved questions surrounding Mercury (of which there are many!) Some information and references can also be found at Views of the Solar System.

Jim O'Donnell