Moon rocks
delivered to Earth by Apollo astronauts held a mystery that has plagued
scientists since the 1970s: Why were the lunar rocks magnetic?
Earth's
rotating, iron core produces the planet's magnetic field. But the moon does not
have such a setup.
Now,
scientists at MIT think they have a solution. Some 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 moon's magnetic
field would have been about 1-50th as strong as Earth's is today, the
researchers say.
The MIT
team found evidence for the molten-core theory by analyzing 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.
The rock
was collected during the last lunar landing
mission, Apollo 17, by Harrison "Jack" Schmitt, the only
geologist ever to walk on the moon.
"Many
people think that it's the most interesting lunar rock," said MIT's Ben
Weiss, who is senior author of a paper on the new finding being published in the
Jan. 16 issue of the journal Science.
Weiss and
his colleagues used a commercial rock magnetometer that was specially fitted
with an automated robotic system to study the rock's faint magnetic traces. The
results helped 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.
Rather, the
rock readings showed it must have remained in a magnetic environment for a long
period of time — millions of years — and thus the field must 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 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.
Their
findings fit 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.
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