The spacecraft's magnetometer, which began making measurements of Mars' magnetic field after its capture into orbit on September 11, 1997, detected the magnetic field on September 15.
The existence of a planetary magnetic field has important implications for the geological history of Mars and for the possible development and continued existence of life on Mars.
"Preliminary evidence of a stronger than expected magnetic field of planetary origin was collected and is now under detailed study," said Dr. Mario H. Acuna, principal investigator for the magnetometer/electron reflectrometer instrument at NASA's Goddard Space Flight Center, Greenbelt, MD.
"Much more additional data will be collected in upcoming orbits during the aerobraking phase of the mission to further characterize the strength and geometry of the field. The current observations suggest a field with a polarity similar to that of Earth's, with a maximum strength not exceeding 1/800ths of the magnetic field at the Earth's surface."
The presence of a magnetic field, together with clear evidence of ancient volcanic activity on Mars, strongly suggests that the planet once had a molten metallic core much like that of the earth. Evidently the planet's core has cooled greatly since the time when vast amounts of lava poured across the Martian landscape, building the largest volcanic mountains known in the solar system.
Mars today might be geologically dead, or nearly so. But long ago, its magnetic field might have been much stronger than today.
"A magnetic field shields a planet from fast-moving, electrically charged particles from the Sun which may affect its atmosphere, as well as from cosmic rays, which are an impediment to life," Acuna said. "If Mars had a more active dynamo in its past, as we suspect from the existence of ancient volcanoes there, then it may have had a thicker atmosphere and liquid water on its surface."
Though no definitive proof of life has yet been found on Mars, there is mounting evidence suggesting the possibility that life once existed there. The presence of a magnetic field further increases the chances that life might have flourished on Mars in ancient times.
Additional information about the magnetic field discovery and the Mars Global Surveyor mission is available on the web at http://www.jpl.nasa.gov
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