Mystery of Moon’s Magnetic Field Deepens
2013-05-21 0:00

By Charles Q. Choi | SPACE

The moon generated a surprisingly intense magnetic field until at least 3.56 billion years ago, 160 million years longer than previously thought, a new study reports.

These findings could shed light not just on the magnetic field of the moon, which is now extremely weak, but on that of asteroids and other distant worlds, investigators added.

Earth’s magnetic field is created by its internal dynamo, which itself is generated by the planet’s churning molten metal core. Research increasingly suggests that the moon once had a dynamo as well, with evidence of magnetism found in lunar rocks returned by Apollo astronauts.

Models of the moon’s core suggest its dynamo should have lasted only until about 4.1 billion years ago. However, last year, scientists revealed that the moon possessed a magnetic field for much longer than previously thought, with a powerful dynamo in its core from 4.2 billion years ago to at least 3.72 billion years ago.

Researchers have proposed two possibilities to explain why the moon’s dynamo lasted so long. One possible explanation is that giant cosmic impacts set the moon lurching enough to drive its dynamo. Another explanation has to do with how the moon’s core spins around a slightly different axis than its surrounding mantle layer, generating wobbles — known as precession — that could dramatically stir its core.

The cosmic-impact idea is supported by the fact that the moon experienced massive collisions until around 3.7 billion years ago, such as the one that created the 715-mile-wide (1,150 kilometers) Mare Imbrium, among other craters.

However, the dynamo generated by each impact would have lasted for a mere 10,000 years or so, scientists say. In contrast, if precession drove a lunar dynamo, the moon could have continuously possessed a magnetic field until as late as 1.8 billion years ago.

Now, a new analysis of the biggest lunar rock brought back to Earth by Apollo 11 astronauts in 1969 reveals the moon’s dynamo lasted about 160 million years longer than previously thought, well after the last of the largest crater-forming impacts hit the moon.


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