Cold message from Mercury's 'hot poles.' (planet)

Cold message from Mercury's 'hot poles'

A day on Mercury lasts about 57 Earth days, and a Mercury year lasts but 1-1/2 Mercury days. This astronomical oddity results from a gravitational "resonance" that causes Mercury to turn three times on its axis for every two trips it makes around the sun. As a result of gravitational resonance and the planet's elliptical orbit, the sun preferentially heats two particular surface areas. Some scientists refer to these areas as Mercury's "hot poles," even though they lie at the planet's equator rather than at the ends of its rotation axis. "The only reason we call them 'poles' is that they're 180[degrees] apart," says Jack O. Burns of New Mexico State University in Las Cruces.

Burns, Michael J. Ledlow of the University of New Mexico in Albuquerque and their colleagues have now used radio emissions -- solar energy reradiated from just below Mercury's surface -- to produce what they call the first radiofrequency images of the planet. The resulting maps, presented this week at the American Astronomical Society meeting in Albuquerque, clearly show the hot poles, but they also leave planetary scientists with a longstanding enigma.

The puzzlement first arose when the Mariner 10 spacecraft observed Mercury from a sun-circling orbit in March and September of 1974 and March of 1975. Measurements from the first and third encounters indicated the planet has an "intrinsic" magnetic field -- one produced in Mercury's interior rather than merely "induced" by the impacts of charged particles from the solar wind.

Many planetary scientists have believed that an intrinsic magnetic field requires a "dynamo" effect, in which internal heat creates convection currents within a rotating planet. Mercury is small enough, however, that some theorists have argued that its core would have cooled and shut off the dynamo long ago. The dynamo idea is still an oft-invoked element in studies of planetary magnetospheres, so Mercury's intrinsic field has represented a quandary ever since its discovery.

Ledlow's team used the Very Large Array -- a group of antennas near Socorro, N.M. -- to study Mercury's radio emissions at wavelengths of 2 and 6 centimeters. "There does not appear to be any excess heat arising from the core of Mercury," they report. "The temperature map can be explained by reradiation of solar energy."

If Mercury were adding any more than about 1 kelvin to the heat coming from the sun, the data would have revealed that contribution, Burns says. "This indicates that Mercury does not have a large, hot molten core that many believe is needed to produce the strong magnetic field via the dynamo model," the researchers conclude.

Their results, combined with Mercury's slow rotation on its axis, indicate the planet "doesn't match the dynamo model at all," says Burns. The model remains in evolution, however, and study coauthor Galen R. Gisler of Los Alamos (N.M.) National Laboratory asserts, "I think you can still squeeze some kind of a dynamo in."