Mercury's Magnetism

The tiny craft had been in space for 16 months and was nearly out of steering fuel. Yet flight controllers at Pasadena's Jet Propulsion Laboratory last week managed to keep Mariner 10 alive and performing well through its third --and closest--encounter with the solar system's innermost planet. As it passed only 200 miles above Mercury's scorched surface, the half-ton robot swooped over the planet's north polar region, sent back some 300 closeup pictures and confirmed a puzzling fact-that Mercury has an innate magnetic field

Close Look. The four-hour flyby was an unexpected bonus at the end of an already successful $100 million mission. Three months after its launch in November 1973, Mariner 10 passed Venus and took the first closeup pictures of the cloud-shrouded planet. Then slowed by Venusian gravity, it plunged toward the sun, approaching Mercury in March and again in September 1974. On those flybys, Mariner got the first close look at the planet and detected a weak magnetic field that some scientists thought might be caused by Mercury's interaction with the solar wind, a stream of charged particles from the sun.

Last week Mariner established beyond doubt that Mercury's field was distinctly its own. Scientists believe that the earth's magnetism is generated by a dynamo-like motion within its liquid outer core caused by the earth's rotation. But whether Mercury also has a liquid core is a subject of debate. Even if it does the planet probably rotates too slowly (once every 58 2/3 earth days) for the dynamo effect to occur. Thus, as Mariner fell silent in its eternal orbit of the sun, it left behind a major mystery: How did Mercury acquire its magnetic field?

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