Space.com:
When rocks form from cooling lava, they create a record of the
magnetic field they are experiencing. By comparing rocks of
different ages, a history of a planet or a moon's magnetic
field can be established. An examination of one of the rocks
brought back from Earth's moon by the
Apollo 11 astronauts has shown that the Moon's
magnetic field lasted at least 160 million years longer than
previously thought. Initial measurements of lunar rocks
suggested that the Moon lost its magnetic field less than 3.77
billion years ago, but the new measurement shows that the
magnetic field was still strong up to at most 3.56 billion
years ago. Like Earth's magnetic field, the Moon's was likely
generated by a
dynamo
created by the
motion of the
core inside the mantle. However, the Moon cooled enough
that the dynamo effect ended, though why the magnetic field
lasted well after the cooling is unknown. One theory is that
major asteroid impacts could have contributed to the
continuance of the dynamo effect by reheating large amounts of
rock. However, the new measurement comes from rocks formed well
after the last major impacts. The only other widely accepted
theory is that the Moon's core spins around a slightly
different axis than the mantle, causing wobbles that would
drive the dynamo.
