Lunar Prospector Measurements Show How Meteor Impacts Have Shaped The Moon's Magnetic Field

[Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>]

University of California-Berkeley

Contacts:
Robert Sanders, UC Berkeley
(510) 643-6998, rls@pa.urel.berkeley.edu

Bill Steigerwald, NASA GSFC
(301) 286-5017, wsteiger@pop100.gsfc.nasa.gov

EMBARGOED FOR 4 P.M. E.D.T. THURSDAY (9/3/98) -- TO COINCIDE WITH
PUBLICATION IN THE JOURNAL SCIENCE

LUNAR PROSPECTOR MEASUREMENTS SHOW HOW METEOR IMPACTS HAVE SHAPED THE
MOON'S MAGNETIC FIELD

Berkeley -- 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 magnetic fields on the moon and the possible geologic
history of the lunar surface.

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.

[NOTE: Full text of the technical papers in SCIENCE are available for free
access at http://www.sciencemag.org/content/current/]


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