What drove the lunar dynamo?
By By Jennifer Chu, MIT News Office | 28 Jan 2012
New evidence from an ancient lunar rock suggests that the moon once harboured a long-lived dynamo - a molten, convecting core of liquid metal that generated a strong magnetic field 3.7 billion years ago. The findings, published today in Science, point to a dynamo that lasted much longer than scientists previously thought, and suggest that an alternative energy source may have powered the dynamo.
''The moon has this protracted history that's surprising,'' says co-author Benjamin Weiss, an associate professor of planetary science at MIT. ''This provides evidence of a fundamentally new way of making a magnetic field in a planet a new power source.''
The new paper is the latest piece in a puzzle that planetary scientists have been working out for decades. In 1969, the Apollo 11 mission brought the first lunar rocks back to Earth - souvenirs from Neil Armstrong and Buzz Aldrin's historic moonwalk.
Since then, scientists have probed the rocky remnants for clues to the moon's history. They soon discovered that many rocks were magnetised, which suggested that the moon was more than a cold, undifferentiated pile of space rubble. Instead, it may have harboured a convecting metallic core that produced a large magnetic field, recorded in the moon's rocks.
Exactly what powered the dynamo remains a mystery. One possibility is that the lunar dynamo was self-sustaining, like Earth's: As the planet has cooled, its liquid core has moved in response, sustaining the dynamo and the magnetic field it produces. In the absence of a long-lived heat supply, most planetary bodies will cool within hundreds of millions of years of formation.
A dynamo still exists within Earth because heat, produced by the radioactive decay of elements within the planet, maintains the core's convection. Models have shown that if a lunar dynamo were powered solely by cooling of the moon's interior, it would have been able to sustain itself only for a few hundred million years after the moon formed - dissipating by 4.2 billion years ago, at the very latest.
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