September 3, 1998
In particular, magnetic field measurements by an instrument built at the University of California, Berkeley Space Sciences Laboratory give strong support to 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 Robert Lin, a professor of physics at UC Berkeley and 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 are reported in a special section of this week's issue of the journal Science devoted to the first scientific findings from Lunar Prospector, launched Jan. 6 of this year and the first NASA moon mission in 25 years. Prospector has been orbiting the moon at about 100 kilometers (63 miles) above the surface since its insertion into a lunar polar orbit in mid-January, telemetering data from five scientific instruments. Research papers discussing data from these other instruments also appear in the Sept. 4 issue of Science. 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, as large as hundreds of nanoteslas (the Earth's field is on the order of 30,000 nanoteslas). When Lin and now professor emeritus of physics Kinsey Anderson built an electron detector that flew aboard Apollo 15 in 1971 and Apollo 16 in 1972, they quickly realized they could use the instrument to remotely map the magnetic fields on the surface.
Though crude and covering only about 10 percent of the lunar surface, the measurements nevertheless 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.
"What was a fairly good hint in the Apollo measurements has turned into a strong correlation in the Lunar Prospector data," said David Mitchell, a research physicist at UC Berkeley's Space Sciences Laboratory. Lin and Anderson collaborated in building the current electron reflectometer aboard the Lunar Prospector in the first return mission since Apollo 16. Its polar orbit will allow the team to map the entire surface of the moon with ten times the resolution, down to 20-30 kilometers (12-20 miles). A complete map of the surface will be completed within several months, Lin said, at which point the instrument will remap in even greater detail the areas of high magnetic field, down to about four kilometers resolution -- a scale of about two miles. The first set of data, with resolution down to 50 kilometers (31 miles), included measurements of nearly the entire area opposite the impact basins called Mare Imbrium and Mare Serenitatis, or Sea of Serenity. Magnetic fields were as high as 40 nanoteslas, or about one one-thousandth that of the Earth.
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. The electron reflectometer observed a bow shock and magnetosheath, both created when the solar wind hits a magnetosphere, and Mitchell predicts that with more detailed measurements they are certain to detect the magnetosphere directly.
Since the solar wind is thought to darken the lunar soil, this may explain lighter areas of the moon, and in particular spiral swirls called Reiner Gamma swirls. These albedo swirls are regions of contrasting light and dark, reminiscent of cream stirred into coffee. Lin and his colleagues think the lighter areas may be areas screened from the solar wind by magnetic fields strong enough to generate a mini-magnetosphere.
"Our previous look at the magnetic moon was during the Apollo missions and it was very coarse," said Mario Acuna, a member of the team located at NASA's Goddard Space Flight Center in Greenbelt, Md. "The moon was previously interpreted as just a dead body with nothing interesting going on. With the new magnetic field data from Lunar Prospector, we are discovering that there is nothing dead about the moon -- the interaction with the solar wind is much more complex than it appeared. Using Lunar Prospector is like using a magnifying glass because it has much higher resolution and can make measurements with greater frequency. This is typical of science -- when you look closer, you see a lot more complexity."
Theorists came up with an explanation for magnetic fields antipodal to impact basins not long after the Apollo measurements hinted at a correlation. When a large meteor hits the moon, it and much of the lunar surface is vaporized and thrown into space, forming a cloud of debris and gas larger than the moon itself. Because of the heat released in the collision, much of the gas is ionized plasma in which the atoms are stripped of one or more electrons.
Such plasmas exclude magnetic fields, so as the cloud spread around the moon it pushed the moon's magnetic field in front of it. When the plasma cloud finally converged on the diametrically opposite side of the moon -- a mere five minutes after impact -- the squeezed magnetic field would be quite large, Lin said.
At the same time debris was falling back on the lunar surface, concentrated at the antipodal site also. If this debris crashed into the surface during the time when the magnetic field was high, it could have undergone shock magnetization. When rock is shocked, as when hit with a hammer, it can suddenly lose its own magnetic field and acquire that of the surrounding region.
If the moon today has no magnetic field, then where did the original magnetic field come from? Dating of Apollo moon rocks hints that during the period 3.6-3.85 billion years ago the moon did have a magnetic field, probably because its core was still liquid and spinning enough to generate a magnetic field comparable to that of the Earth. Mare Imbrium, Mare Serenitatis and two other impact basins that show evidence of strong antipodal magnetic fields, Mare Orientalis and Mare Crisium, all seem to have been created during this time period when the moon had a magnetic field.
"The data are still sparse and the interpretation is still a guess, but very soon I think we'll have proof that this is the story," Lin said. The electron reflectometer determines the surface magnetic field by measuring the energy and incoming direction of electrons reflected from magnetic fields on the lunar surface. Charged electrons from the solar wind corkscrew around the magnetic fields as they approach the surface, and 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. The energy and angle of approach of the reflected electrons thus indicate the strength of the magnetic field at the surface.
Collaborators on the electron reflectometer experiment include project engineer David Curtis, physicist Charles W. Carlson and J. McFadden at UC Berkeley's Space Sciences Laboratory; L.L. Hood of the Lunar and Planetary Laboratory at the University of Arizona, Tucson; and A. Binder at the Lunar Research Institute, Gilroy, Calif. The UC Berkeley research was supported by NASA.
Full text of the technical papers in SCIENCE are available for free access at http://www.sciencemag.org/content/current/