CAMBRIDGE, MASSACHUSETTS — Billions of years ago, the ancient moon had a powerful dynamo at its core that produced a strong global magnetic field.

Scientists at the Massachusetts Institute of Technology say that they have pinned down when that dynamo stopped, leading to the field's dissipation.

In a study published in Science Advances, researchers say that the Apollo mission collected lunar rocks that preserve a record of the moon's ancient magnetic field.

However, most of these rocks dated to between 3 and 4 billion years, or too long ago for studying the end of the moon's internal dynamo.

To research the moon's later magnetic history, the scientists tested two samples that were 1 billion years old and found them to record a weak lunar magnetic field.

As the rocks were left by a meteor impact, scientists reheated the rocks and obtained the same readings to make sure the impact's heat did not interfere with the rocks' magnetic records.

According to the MIT news release, the moon used to be much closer to earth, and the gravitational effect caused the liquid lunar core to wobble, which created the magnetic field.

The weak magnetism recorded by the two rocks suggests the gravitational effects had begun to seize up one billion years ago.