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Apollo rocks put new spin on Moon's magnetic past

Apollo 11 astronauts on lunar surface

Samples of Moon rocks collected by the crew of Apollo 11 in 1969 are still unlocking new mysteries about the Earth's largest satellite 44 years later (Source: Neil Armstrong/NASA/Apollo 11)

Moon mystery A new study of Apollo 11 lunar rocks has revealed that the Moon's magnetic field lasted some 160 million years longer than previously thought.

The discovery reported in the journal of the Proceedings of the National Academy of Sciences, provides a new insight into the Moon's ancient geology and evolution.

Scientists including lead author Clement Suavet of the Massachusetts Institute of Technology, examined two mare basalt rocks collected by Neil Armstrong and Buzz Aldrin from the Sea of Tranquillity during the historic first manned mission to the lunar surface in 1969.

Suavet and colleagues subjected the 3.56 billion-year-old rocks to thermal cooling tests and AF ( alternating frequency) demagnetisation to precisely determine their properties.

They found both samples were magnetised in a stable and surprisingly intense magnetic field of at least 13 micro Tesla.

"This data indicates the magnetic field was continuously active well after the final large basin-forming impacts," they write.

Dynamo dynamics

A global magnetic field is generated by a moving liquid core called a dynamo.

Although the Moon does not have a global magnetic field today, computer studies indicate it had an active core until 4.1 billion years ago, while previous rock sample studies indicate a dynamo was still active 3.7 billion years ago.

There are a number of theories about what generated past lunar magnetism.

One theory is the possibility that it may have been powered by the movement of molten rocks in the core as different minerals crystallised at different temperatures and pressures during cooling -- a process known as a compositional convection dynamo.

Other ideas include tidal heating caused by either changes in the Moon's rotation due to large asteroid impact events, or slight wobbles in the Moon's spin, a process called precession.

"Large impacts have the potential to unlock the Moon from synchronous rotation [with Earth], such that the resulting differential motion between the librating mantle and core could generate a dynamo," they write.

However, when Suavent and colleagues searched for evidence of asteroids large enough to change the Moon's rotation they couldn't find any craters of the right size younger than around 3.8 billion years.

"This likely excludes impact-driven changes in rotation rate as the source of the dynamo at this time in lunar history," they write.

"Rather, our results require a persistent power source like precession of the lunar mantle or a compositional convection dynamo."

Unsolved questions

Ruling out one of the key models used to explain what caused lunar magnetism is a significant step forward, says Dr Simon O'Toole of the Australian Astronomical Observatory.

"We're narrowing down the possibilities," says O'Toole.

"What they've done is accrued more evidence that suggests that a major model, the impact driven changes in the rotation rate, has been ruled out and that's a pretty big thing."

"Trying to explain physical come up with a series of models and then you systematically rule out each one, based on the evidence," says O'Toole.

"That's really what science is all about."

Tags: physics, the-moon