A new model for generating a global magnetic field in the ancient moon could help solve a 40-year-old mystery.

The Earth’s magnetic field exists because it has a spinning solid core surrounded by hot metallic liquid, which churns around lava-lamp style and generates magnetism. But the moon is too small and cool to possess such a molten interior and therefore lacks a global magnetic field.

Yet when Apollo astronauts brought the first samples back from the moon’s surface, scientists discovered many of the rocks were magnetized.

“At first people said, ‘What are you talking about?’ since this was com...