No. 77: Sep-Oct 1991
Mercury, largely hidden in the sun's glare, also conceals beneath its baked, cratered surface: (1) far more iron than solar-system theory allows; and (perhaps) (2) a dynamo that should not exist.
Let us take the excess-iron problem first. Mercury's density is 5.44 (compared to earth's 5.52), so that it very likely contains much iron. Our moon, which resembles Mercury in size and external appearance, only has a density of 3.34, implying an altogether different origin. In the currently accepted theory of solar-system formation, all of the planets and their satellites condensed from a primordial disk of dust sur rounding the just-formed sun. The planets closer to the solar inferno lost more of their easily vaporized constituents due to the sun's heat. The cooler, outer planets were able to retain large amounts of ices. In this scenario, we would expect Mercury to be rich in iron and rocks. This seems to be the case, but it has too iron to fit the theory. Astronomers have tried to save the theory by supposing that a large asteroid sideswiped Mercury tearing off part of its outer layer of lighter rocks, leaving the heavier iron core untouched. The theory doesn't say what happened to the debris from this colossal collision.
As for Mercury's magnetic field, it is small, only 1% that of the earth. But where does it originate? All of the other planets with magnetic fields (earth, Jupiter, Saturn, Neptune, and Uranus) rotate rapidly and are believed to have molten interiors, allowing fluid dynamos to form. Mercury, in contrast, spins very slowly and seems solid throughout. Therefore, the magnetic dynamos that supposedly create the fields of other planets cannot exist inside Mercury. (Crosswell, Ken; "Mercury -- the Impossible Planet," New Scientist, p. 26, June 1, 1991.)
Comment. Could Mercury be a permanent magnet? M. Stock suggests this in a letter in the July 13 issue of New Scientist.)
Reference. Facts about Mercury's anomalous magnetic field may be found in AHZ1 and AHZ2 in our catalog: The Moon and the Planets. Details here.