James W. Head, the Louis and Elizabeth Scherck Distinguished Professor of Geological Sciences, was part of a research team that has shown that the Moon had a surprisingly strong magnetic field 3.56 billion years ago. Samples of that age recovered during the Apollo 11 mission showed signs of having been magnetized by a stable field, the new research found. That suggests that the field persisted for at least 160 million years longer than researchers had previously thought. The new finding enables scientists to rule out one possible source for the field. It was thought that giant impacts might have caused molten material in the Moon’s interior to oscillate, creating a dynamo that in turn would generate a magnetic field. However, this work shows that the Moon’s field persisted for millions of years after a rash of giant impacts 3.7 million years ago, making those impacts an unlikely source. “The Moon was closer to the Earth in its earlier history” says Jim Head, “and the influence of the Earth’s gravitational pull on the lunar interior may have been a factor in the field lasting for so long.“ The findings are published in the Proceedings of the National Academy of Sciences.