Moon’s magnetic field lasted longer than previously expected

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.