Lunar scientists have known for 50 years that the Moon once had a magnetic field, but its cause, and why it faded has been a mystery. New evidence has dated the field's disappearance to 1-1.5 billion years ago, which, while an epic amount of time, is far more recent than previous estimates. If correct, the finding sheds light on what caused the field in the first place and could help find life around nearby stars.
One of the major discoveries of the Apollo missions was that some Moon rocks returned by the astronauts were magnetized. The missions confirmed the geologically dead satellite has no magnetic field today to induce this effect. Planetary scientists concluded that at one time, before its core cooled, the Moon had a lunar dynamo that induced a magnetic field, which left its legacy in rocks formed at the time.
Professor Benjamin Weiss of MIT has sought an answer to the question of when the field died out. There are three major theories on what caused the field in the first place – thermal convection, mantle precession, and core crystallization. Models suggest the first of these would have lasted only around a billion years from the satellite's formation, and the second no more than 2 billion years. However, core crystallization driving a convection dynamo could have lasted until close to the present day.
Resolving which of these was responsible would not only tell us a lot about the Moon's development, but would inform models of the lifespan of such fields on other worlds. Since a magnetic field is probably an essential requirement for biology, the question is of utmost importance to anyone interested in life beyond the Solar System.
In Science Advances Weiss provides evidence that at the times when the molten rocks formed by two large impacts cooled, the Moon's magnetic field was less than one ten-millionth of a tesla. For comparison, the Earth's field is more than 250 times that, and it is thought the Moon's was once even stronger.
Weiss's formations cooled 440 and 910 million years ago, but the paper points to other samples that indicate a substantial field persisted until at least 1.9 billion years ago, too recent for models of mantle precession, which would have diminished as the Earth-Moon distance grew.
Core crystallization is the only known explanation for such long-lasting magnetism, but it may not be the whole story. Weiss thinks mantle precession may have originally been the dominant cause of a magnetic field, with crystallization taking over as it faded.
Magnetic fields protect their planet, or moon, from stellar winds or explosions that would otherwise strip away the atmosphere. Without them life is unlikely, particularly around red dwarf stars, where proximity heightens the dangers on potentially habitable worlds such as Proxima b and the members of the Trappist-1 system. For life to get beyond the single-celled stage such worlds would need both very strong, and very long-lived magnetic fields, and understanding what drove the Moon's might help us work out where these occur.
