Making Sense of the Moon's Magnetism

Lunar Prospector Experiment Seeks Out Faint
Traces From the Primordial Era

by Robert Sanders

The launch Jan. 6 of the first NASA moon mission in 25 years-the Lunar Prospector-has carried into lunar orbit a Berkeley experiment measuring the patchwork of weak magnetic fields on the moon's surface.

The moon lacks a strong magnetic field like the Earth's, but exhibits a much weaker surface field that varies from place to place-from 100 to 10,000 times fainter than Earth's field. Berkeley astrophysicists hope that a complete mapping of the faint fields on the surface-last attempted 25 years ago by Apollo 15 and 16-will reveal the history of the lunar surface, including whether it once had a global magnetic field like the Earth.

"The moon may well have had a global magnetic field 3.8 billion years ago, but that died away," said Robert Lin, a professor of physics and one of the principal investigators for the magnetic mapping project. "These magnetic anomalies may be left over from that period, yielding information about that primordial era.

"We hope that by mapping the magnetic field of the entire moon and comparing that with surface features and the results of other mapping onboard Lunar Prospector, we can solve this puzzle and find out what the old field was."

Lin and project engineer David Curtis at the Space Sciences Laboratory built the instrument, called an electron reflectometer, that will fly aboard the Lunar Prospector.

Prospector was originally scheduled for launch from Cape Canaveral Jan. 5, but the launch was postponed until Jan. 6. It is one of NASA's much-touted "faster, better, cheaper" missions and the third to be launched in its Discovery Program. Its predecessors were the recent successful Mars Pathfinder mission and the Near Earth Asteroid Rendezvous (NEAR) mission launched in 1996. Prospector is a joint project of NASA Ames Research Center and Lockheed Martin Missiles and Space.

The faint magnetic field on the moon's surface was first detected by magnetometers placed by Apollo astronauts on the surface, and then remotely mapped by a Berkeley experiment aboard Apollo 15 in 1971 and Apollo 16 in 1972, using instruments built by Lin and Kinsey Anderson, now professor emeritus of physics. Those instruments obtained only a crude measurement of the fields, and over only about 10 percent of the lunar surface, in a band around the equator.

Now, 25 years later, Lin and Anderson are collaborating again to send an updated instrument aboard Lunar Prospector to map the entire surface of the moon with 10 times the resolution, down to 20 to 30 kilometers (12 to 20 miles). A complete map of the surface should be finished within about six months, Lin said, at which point the instrument will remap the areas of high magnetic field in even greater detail, down to a few kilometers resolution-a scale of about a mile.

The origin of the faint fields is a mystery, though last November similar but somewhat stronger surface fields were detected on the planet Mars by Mars Global Surveyor, by an instrument also built by Lin and Curtis in collaboration with Goddard Space Flight Center and Centre d'Etude Spatiale des Rayonnements, Toulouse.

Some lunar magnetic hot spots, such as a strip called Rima Sirsalis, may date from about 3.8 billion years ago, a time when the moon may have had a global magnetic field as large as that of the Earth today.

Other hot spots may carry hints of the moon's past magnetic field. They may have been created when asteroids struck the surface, melted and during cooling captured the magnetic field at the site of impact.

Still others may be the result of more recent asteroid or comet impacts. One puzzling finding from the earlier mappings by the Apollo subsatellites was that regions of high magnetic field seem to be exactly opposite large impact craters-that is, on the side of the moon directly opposite a large impact crater. New information from Lunar Prospector will show whether or not that is a coincidence and may suggest an explanation.

Two other areas of strong magnetic field seem to be associated with a surface feature called an albedo swirl-a region of contrasting light and dark, reminiscent of cream stirred into coffee. More detailed mapping will show whether this, too, is mere coincidence or a real association, perhaps caused by a juxtaposition of irregular magnetic fields.

Magnetic field information also could provide constraints on the physical processes undergone by the moon in its evolution, such as how the core formed, the thermal evolution of the crust, tectonic processes and erosion.

The electron reflectometer is one of five scientific instruments aboard Lunar Prospector, which will fly in a polar orbit at an altitude of about 100 kilometers (63 miles) above the surface. Three of these-a neutron spectrometer, a gamma ray spectrometer and an alpha particle spectrometer-are designed to "prospect" for minerals, water ice and gases on the lunar surface. In particular, the neutron spectrometer will seek to confirm the detection of ice in craters at the poles, as reported earlier based on radar measurements by the Defense Department's Clementine probe during a lunar fly-by in 1994.

A combined magnetometer/electron reflectometer will measure the moon's magnetic field above the surface and at the surface, respectively, from an orbit 100 kilometers above the surface. Finally, a Doppler gravity experiment will provide the best measurements to date of the moon's gravitational field, allowing future moon missions to use fuel more efficiently.

The electron reflectometer, which measures the energy and incoming direction of electrons, is a very sensitive detector of surface magnetic fields, as serendipitously discovered by Lin and Anderson during the Apollo 15 and 16 missions to the moon. While flying identical instruments on these two missions, originally designed to look at charged particles around the moon, they found that many electrons were coming from the surface.

It turned out that these were being reflected by surface magnetic fields acting as magnetic mirrors. Charged electrons from the solar wind corkscrew around the magnetic fields as they approach the surface. As the magnetic field increases, they spiral tighter and tighter until, if the field is strong enough or the angle of approach shallow enough, they reverse direction and corkscrew back into space.

These electrons are measured by the reflectometer, which looks in all directions as the satellite rotates through one revolution.

"By looking at the number and angle at which they come in, we get a measure of the strength of the magnetic field at the surface," Lin said.

The data is shipped directly to Berkeley for analysis via the Internet from NASA Ames Research Center, which controls the satellite. Prospector was launched by an inexpensive Lockheed Martin launch vehicle, Athena II, taking its maiden voyage. A three-stage rocket, it put the satellite in a parking orbit around Earth while its systems were deployed and checked. Then the satellite was kicked into a lunar orbit. The electron reflectometer and related magnetometer-provided by Goddard Space Flight Center to measure magnetic fields at the position of the satellite-will be deployed on one of three booms projecting from the body of the spacecraft.


Copyright 1998, The Regents of the University of California.
Produced and maintained by the Office of Public Affairs at UC Berkeley.
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