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Lunar Prospector Data Show How Meteor Impacts Shaped the Moon's Magnetic Field

By Robert Sanders, Public Affairs
posted September 9, 1998

The first four months of data from the Lunar Prospector, a satellite that has orbited the moon since January, has yielded a wealth of new information about the moon's magnetic fields and the possible geologic history of the lunar surface.

In particular, magnetic field measurements by an instrument built at Berkeley's Space Sciences Laboratory strongly support the theory that giant meteor impacts billions of years ago created areas of strong magnetic field diametrically opposite the impact site on the lunar surface.

"We have analyzed data from most of two impact basins on the lunar surface -- Mare Imbrium and the Sea of Serenity -- and remarkably, the correlation that we first glimpsed on the Apollo missions 25 years ago still holds," said physics professor Robert Lin, one of the principal investigators for the magnetic mapping project.

"The fact that regions of strong magnetic field cover whole basins antipodal to the point of impact makes the hypothesis that the magnetism has something to do with these large impacts seem much firmer."

These regions of strong magnetic field also create their own miniature magnetospheres several hundred kilometers across, akin to the much larger magnetospheres that surround planets like Earth and block the solar wind.

"These mini-magnetospheres are close to the minimum size you can get in the solar system, and are the smallest ever observed," said Lin, who serves as director of the Space Sciences Laboratory.

The findings were reported in a special section of last week's issue of the journal Science devoted to the first scientific findings from the five instruments aboard the Lunar Prospector. The satellite was was launched Jan. 6, 1998, and is the first NASA moon mission in 25 years.

The moon has no global magnetic field like the Earth because it no longer has an internal dynamo. So it was a surprise when magnetometers placed by astronauts on the surface in the 1970s detected a faint magnetic field. Lin and now professor emeritus of physics Kinsey Anderson crudely mapped these fields using an electron reflectometer they built and flew aboard Apollo 15 in 1971 and Apollo 16 in 1972.

Those measurements indicated a correlation between meteor impact basins -- dark, roughly circular features on the face of the moon -- and strong magnetic fields on the diametrically opposite side of the moon.

Using an improved instrument aboard Lunar Prospector, they already have mapped the magnetic fields of nearly the entire area opposite the impact basins Mare Imbrium and Mare Serenitatis, or Sea of Serenity, and have confirmed the correlation.

Theorists believe that when large meteors collided with the moon between 3.6 and 3.85 billion years ago -- a time when the moon had a global magnetic field -- they created a huge cloud of gas and dust that converged on the opposite side of the moon. The ionized gas or plasma would have compressed the moon's magnetic field and, when the debris fell to the surface, magnetized it in a process known as shock magnetization.

Surprisingly, the magnetic field in these antipodal regions was coherent over an area of a couple hundred kilometers -- about 100 miles -- rather than being a jumble of randomly oriented regions, which is typical of most of the lunar surface. When this happens, the area can screen out the solar wind that normally impinges on the lunar surface, just as the Earth's magnetic field screens out the high-energy particles in the solar wind.

A complete map of the moon's surface should be completed within several months, Lin said, at which point the instrument will remap, in even greater detail, the areas of high magnetic field.

Working with Lin at the Space Sciences Laboratory are research physicist David Mitchell, project engineer David Curtis, physicist Charles Carlson and J. McFadden.

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