No. 71: Sep-Oct 1990
Radio telescopes can give planetary astronomers a rough idea of the temperature existing several feet below the surface of a distant planet. Scrutinizing Mercury with their big electronic "ears," they have found two spots on the plamet where the temperatures are several hundred degrees higher than in the surrounding areas. Actually, these hot spots are easy to understand; because, to a Mercurian, the sun comes to a stop in the sky over one of these points and then moves backwards to the other point 180° away. As the sun tarries over these two spots, it heats them preferentially. The strange apparent motion of the sun is due to the 3:2 ratio between Mercury's period of revolution around the sun (88 days) and its axial spin period (59.6 days). What is surprising is that the energy detected radiating from the two hot spots is all reradiated solar energy; that is, there seems to be no contribution at all from Mercury's core! If no heat is leaking out of Mercury's core, the core itself is very likely solid. If it is solid, it cannot establish convection cells and thus generate a magnetic field through dynamo action. But back in 1975, the Mariner 10 spacecraft radioed back that Mercury actually does possess a magnetic field, and a surprisingly large one at that. (Wilford, John Noble; "Theory of Mercury's Hot Poles Is Shown to Be a Fact," New York Times, June 13, 1990. Cr. J. Covey.)
Comment. Something is clearly awry. This inconsistency could mean that the dynamo theory presumed to be responsible for planetary magnetic fields is incorrect.
Reference. Mercury's anomalous magnetic field is cataloged in section AHZ in our catalog: The Moon and the Planets. To order, visit: here.