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No. 77: Sep-Oct 1991

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Mercury: the impossible planet

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.

From Science Frontiers #77, SEP-OCT 1991. 1991-2000 William R. Corliss

Science Frontiers Sourcebook Project Reviewed in:


  • "A sourcebook of unexplained phenomena is therefore a valuable addition to a collection of scientific literature. William R. Corliss has provided this in the past with his source books of scientific anomalies in several subjects, and now he has provided it for astronomy. He has done an excellent job of collecting and editing a large amount of material, taken in part from scientific journals and in part from scientific reporting in the popular or semi-scientific press." -- "The Mysterious Universe: A Handbook of Astronomical Anomalies", reviwed by Thomas Gold, Cornell University, in Icarus, vol.41, 1980

  • "An interesting, systematic presentation of unusual weather [..] This book is recommended for a general audience" --"Corliss, William R., Tornados, Dark Days, Anomalous Precipitation, and Related Weather Phenomena, Sourcebook Project, 1983.", revieweed in Choice, September 1983
  • "..the science is necessarily somewhat speculative, but Corliss's symthesis is based on reputable sources." -- "Corliss, William R. (Compiler). Lightning, Auroras, Nocturnal Lights, and Related Luminous Phenomena" reviwed by Joseph M. Moran, Univ. of Wisconsin in Science Books and Films, Sep/Oct 1983

  • "Before opening the book, I set certain standards that a volume which treads into dangerous grounds grounds like this must meet. The author scrupulously met, or even exceeded those standards. Each phenomenon is exhaustively documented, with references to scientific journals [..] and extensive quotations" -- "Book Review: The moon and planets: a catalog of astronomical anomalies", The Sourcebook Project, 1985., Corliss, W. R., Journal of the Royal Astronomical Society of Canada, Vol. 81, no. 1 (1987), p. 24., 02/1987