http://web.mit.edu/newsoffice/2009/moonrock-0115.html

Astronomers crack longstanding lunar mystery

Ancient rock's magnetic field shows that moon once had a dynamo in
its core

David Chandler, MIT News Office
January 15, 2009

The collection of rocks that the Apollo astronauts brought back from
the
moon carried with it a riddle that has puzzled scientists since the
early 1970s: What produced the magnetization found in many of those
rocks?

The longstanding puzzle has now been solved by researchers at MIT, who
carried out the most detailed analysis ever of the oldest pristine
rock
from the Apollo collection. Magnetic traces recorded in the rock
provide
strong evidence that 4.2 billion years ago the moon had a liquid core
with a dynamo, like Earth's core today, that produced a strong
magnetic
field.

The particular moon rock that produced the new evidence was long known
to be a very special one. It is the oldest of all the moon rocks that
have not been subjected to major shocks from later impacts --
something
that tends to erase all evidence of earlier magnetic fields. In fact,
it's older than any known rocks from Mars or even from the Earth
itself.

"Many people think that it's the most interesting lunar rock," said
Ben
Weiss, the Victor P. Starr Assistant Professor of Planetary Sciences
in
MIT's Department of Earth, Atmospheric and Planetary Sciences and
senior
author of a paper on the new finding being published in Science on
Jan.
16. The rock was collected during the last lunar landing mission,
Apollo
17, by Harrison "Jack" Schmidt, the only geologist ever to walk on the
moon.

"It is one of the oldest and most pristine samples known," said
graduate
student Ian Garrick-Bethell, who was the lead author of the Science
paper. "If that wasn't enough, it is also perhaps the most beautiful
lunar rock, displaying a mixture of bright green and milky white
crystals."

The team studied faint magnetic traces in a small sample of the rock
in
great detail. Using a commercial rock magnetometer that was specially
fitted with an automated robotic system to take many readings "allowed
us to make an order of magnitude more measurements than previous
studies
of lunar samples," Garrick-Bethell said. "This permitted us to study
the
magnetization of the rock in much greater detail than previously
possible."

And those data enabled them to rule out the other possible sources of
the magnetic traces, such as magnetic fields briefly generated by huge
impacts on the moon. Those magnetic fields are very short lived,
ranging
from just seconds for small impacts up to one day for the most massive
strikes. But the evidence written in the lunar rock showed it must
have
remained in a magnetic environment for a long period of time --
millions
of years -- and thus the field had to have come from a long-lasting
magnetic dynamo.

That's not a new idea, but it has been "one of the most controversial
issues in lunar science," Weiss said. Until the Apollo missions, many
prominent scientists were convinced that the moon was born cold and
stayed cold, never melting enough to form a liquid core. Apollo proved
that there had been massive flows of lava on the moon's surface, but
the
idea that it has, or ever had, a molten core remained controversial.
"People have been vociferously debating this for 30 years," Weiss
said.

The magnetic field necessary to have magnetized this rock would have
been about one-fiftieth as strong as Earth's is today, Weiss said.
"This
is consistent with dynamo theory," and also fits in with the
prevailing
theory that the moon was born when a Mars-sized body crashed into the
Earth and blasted much of its crust into space, where it clumped
together to form the moon.

The new finding underscores how much we still don't know about our
nearest neighbor in space, which will soon be visited by humans once
again under current NASA plans. "While humans have visited the moon
six
times, we have really only scratched the surface when it comes to our
understanding of this world," said Garick-Bethell.

The research, which also included MIT undergraduate student Jennifer
Buz
and David L. Shuster of the Berkeley Geochronology Center, was funded
by
the NASA Lunar Advanced Science and Exploration Research Program, as
well as the Charles E. Reed Faculty Initiatives Fund, the Victor P.
Starr Career Development Professorship, and the Ann and Gordon Getty
Foundation.
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