The moon lost its magnetic fields one billion years ago: Study

MIT scientists have determined when the moon's Dynamo stopped, leading to the dissipation of its magnetic field. Using lunar rocks collected by the Apollo mission, they found that most rocks were too old to study the end of the Dynamo. However, two samples from 1 billion years ago showed a weak magnetic field, suggesting the moon's gravitational effects were waning. The moon was once closer to Earth, causing its liquid core to wobble and create a magnetic field. The findings indicate that these gravitational effects began to diminish 1 billion years ago.

What method did scientists use to ensure that the meteor impact heat did not interfere with the rocks' magnetic records?

What was the significance of the weak magnetism recorded by the two rocks?