Scientists working on the Mars Global Surveyor project announced last week that new satellite evidence indicates that the magnetic field around Mars is most likely of crustal rather than dynamo origin. They also say that the field is much stronger than previously anticipated and that it could greatly vary in strength in different locations on the planet.
“Based on evidence collected over several orbits, we now have strong evidence that the field is of crustal origin,” says Mario Acuna, an astrophysicist at NASA’s Goddard Space Flight Center who is the principal investigator on the magnetometer and electron reflectometer instruments on the satellite. The new findings follow the satellite’s dramatic detection on September 15 of the outer-most boundary of Mars’ magnetic field — or bow shock — shortly after the Surveyor began in elliptical orbit around the planet.
Scientists have long debated whether Mars has a magnetic field. The findings have implications for understanding the history, geology, and thermal evolution of Mars and for reexamining theories of magnetic fields. A magnetic field shields a planet from fast-moving, electrically charged solar particles that could affect and erode its atmosphere and surface. The field also can protect against some cosmic rays, which are an impediment to life.
Crustal origin of Mars’ magnetic field indicates that that the ferromagnetic rock on the surface “remembers” and is preserving an older, internal magnetic field that may no longer exist. The crustal remembrance, says Acuna, is similar to a nail that remains magnetized after being brought into contact with a magnet. Crustal origin also could indicate that, even though the dynamo inside the planet’s core once was strong enough to leave behind its signature in the crustal rocks, the dynamo now may be frozen and no longer spinning, and the planet’s internal heat source may be gone. However, Acuna says he does not rule out the possibility that a weak magnetic field originating in the planet’s core could be found at a later date.
The finding of a crustal magnetic field may not necessarily imply that Mars is geologically dead, says Janet Luhmann, a research scientist at the Space Science Laboratory at the University of California at Berkeley. “It doesn’t mean that there isn’t another kind of geology going on,” she says. “It’s just that the combination of convection and rotation required to create a dynamo is not present.” Mars’ magnetic field could be very complex and likely varies in intensity from region to region depending on surface rock and whether an area is facing the solar wind, says Acuna. Determining the history of the planet, he says, is in some ways detective work. “We have to construct the geology and composition that explains the signature observed and which is consistent with other things we know about Mars,” he says, adding, “My suspicion is that Mars will occupy a special place as being unique in magnetic properties. It may not be a clearly magnetic or nonmagnetic planet.”
The strength of the Martian magnetic field has surprised scientists. While early speculation suggested the field would be just 30-60 nT strong, recent evidence suggests the field is 400 nT strong — almost as strong as Mercury’s field. Acuna says that when measurements are taken closer to the Martian surface, they could reveal an even stronger field. Earth’s magnetic field, by comparison, is 3000 nT.
The new findings also indicate that the
planet’s magnetic field is stronger than that of meteorites
suspected of having originated from Mars, Acuna says. One hindrance
to understanding magnetic fields is that different planetary fields
do not fit into a known, neat general theory. “We do not
have a general theory of magnetic fields that will apply with
equal validity to all Earth-like planets,” says Sean Solomon,
director of the Department of Terrestrial Magnetism at the Carnegie
Institution of Washington, and president of AGU. Solomon adds
“Every time we’ve gone to a new planet, we’ve been
surprised." - Randy Showstack