As closely as Mariner 10's measurements could determine, Mercury's magnetic field, though only 1 percent as strong as Earth's, resembles Earth's field in being roughly dipolar and oriented along the planet's axis of rotation. While the existence of the field might conceivably have some other explanation—such as, for example, remanent magnetism, the retained imprint of an ancient magnetic field frozen into the rocks during crustal cooling—most researchers became convinced that it is produced, like Earth's field, by a mag-netohydrodynamic dynamo mechanism (involving motions within an electrically conducting fluid) in the outer portions of Mercury's iron core. Measurements by Messenger's magnetometer, made during the spacecraft's first flyby in January 2008, confirm that Mercury's magnetic field is basically dipolar. They fail to reveal any crustal contributions that might be expected from remanent magnetism, so it seems clear that Mercury's dynamo is currently operating.
Mercury's magnetic field holds off the solar wind with a teardrop-shaped bubble, or magnetosphere, whose rounded end extends outward toward the Sun about one planetary radius from the surface. This is only about 5 percent of the sunward extent of Earth's magnetosphere. The planet's atmosphere is so thin that no equivalent to Earth's ionosphere exists at Mercury. Indeed, calculations suggest that on occasion the solar wind is strong enough to push the sunward boundary (magnetopause) of the magnetosphere beneath Mercury's surface. Under these conditions solar wind ions would impinge directly on those portions of Mercury's surface immediately beneath the Sun. Even infrequent occurrences of such an event could dramatically alter the atomic composition of surface constituents. Preliminary measurements by Messenger suggest that Mercury's magnetosphere may have an unusual configuration, a kind of double magnetopause, perhaps due to the abundance of heavy ions, primarily sodium, that come from Mercury's surface and atmosphere.
Mercury's magnetospheric processes are of interest to geophysicists and space scientists, who hope one day to test their conception of Earth's magnetosphere through examination of an Earthlike field with a very different scale and in a different solar wind environment. For example, Mariner 10 instruments recorded rapidly varying energetic particles in the planet's magnetotail, the elongated portion of the magnetosphere downstream from the planet's nightside. This activity was much like the geomagnetic substorms on Earth that are associated with auroral displays. The origin of such events on Earth may be more directly understood from comprehensive global data that will be gathered by Messenger once it enters orbit around Mercury in 2011.
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