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Source of Moon's Magnetism Found

Moon rocksdelivered to Earth by Apollo astronauts held a mystery that has plaguedscientists since the 1970s: Why were the lunar rocks magnetic?

Earth'srotating, iron core produces the planet's magnetic field. But the moon does nothave such a setup.

Now,scientists at MIT think they have a solution. Some 4.2 billion years ago, themoon had a liquid core with a dynamo (like Earth'score today) that produced a strong magnetic field. The moon's magneticfield would have been about 1-50th as strong as Earth's is today, theresearchers say.

The MITteam found evidence for the molten-core theory by analyzing the oldest of allthe moonrocks 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 rockwas collected during the last lunar landingmission, Apollo 17, by Harrison "Jack" Schmitt, the onlygeologist ever to walk on the moon.

"Manypeople think that it's the most interesting lunar rock," said MIT's BenWeiss, who is senior author of a paper on the new finding being published in theJan. 16 issue of the journal Science.

Weiss andhis colleagues used a commercial rock magnetometer that was specially fittedwith an automated robotic system to study the rock's faint magnetic traces. Theresults helped them to rule out the other possible sources of the magnetictraces, such as magnetic fields briefly generated by huge impacts onthe moon. Those magnetic fields are very short lived, ranging from justseconds for small impacts up to one day for the most massive strikes.

Rather, therock readings showed it must have remained in a magnetic environment for a longperiod of time ? millions of years ? and thus the field must have come from a long-lastingmagnetic dynamo.

That's nota new idea, but it has been "one of the most controversial issues in lunarscience," Weiss said.

Until theApollo missions, many prominent scientists were convinced that the moon wasborn cold and stayed cold, never melting enough to form a liquid core. Apolloproved there had been massive flows of lava on the moon's surface, but the ideathat it has, or ever had, a molten core remained controversial.

Theirfindings fit in with the prevailing theory that themoon was born when a Mars-sized body crashed into the Earth and blastedmuch of its crust into space, where it clumped together to form the moon.

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