Magnetic Mars

NASA's Global Surveyor has detected a magnetic field around Mars. While it's unlikely that the geologically inactive planet is generating a field the way Earth does, the discovery may lend support to the contention that Mars once had a field nearly as strong as Earth's that has since faded. If so, that early, strong field might have fended off cosmic rays deleterious to life.

The discovery came Monday when the recently arrived Mars Global Surveyor made its first observations during a pass 250 kilometers above the planet. "It looks like strong evidence for a planetary magnetic field," says space physicist Mario Acuña of NASA's Goddard Space Flight Center in Greenbelt, Maryland, who is the principal investigator for Surveyor's magnetometer. Magnetometers sent to Mars on previous missions did not pick up the field because they were not sensitive enough or, like the 1989 Russian Phobos spacecraft, did not pass close enough to the planet, he says.

"It's a somewhat surprising result," says planetary physicist David Stevenson of the California Institute of Technology, "because it's not that easy to get as large a field as the spacecraft has found." Theoreticians assume that Mars could not have long retained the internal heat that in Earth churns the molten-iron core to produce electrical currents and thus the magnetic field. If Mars ever had an Earth-like geodynamo, it's likely it has turned off, says Stevenson.

The field could be a remnant, imprinted on crustal rock before the geodynamo wound down, he adds. Such remnant magnetism has been reported in meteorites from Mars, including ALH84001 with its putative evidence of ancient life. Although twice as large as expected, the field is about 1/800 that of Earth.