The Moon never had a magnetic field
New analyses of lunar rock collected in the 1970s show that the Moon has never had a protective magnetic field. And that is good news for future lunar missions.
Nearly 50 years after the last astronaut left the Moon, new analyses of samples collected during NASA’s Apollo missions are changing our idea of the Moon’s geological history.
The Moon today does not, like Earth, have a magnetic field which protects against cosmic radiation. But in the 1970s, when astrogeologists analysed the lunar rock which Apollo astronauts had collected, they concluded that some 3.7 billion years ago the satellite did have a very powerful magnetic field.
The theory was based on the fact that scientists had found clear indications of magnetisation in rock from the Moon. Based on this, they concluded that the Moon must have had a geodynamo, with melted electrically-conductive substances deep inside the satellite that, like an electromagnet, could keep up a permanent magnetic field. But the new analyses show that this theory was wrong. We now know that the Moon’s core is probably too small and compact for a geo-dynamo, and there is a much more likely external explanation of magnetic evidence in lunar rock up to 4 billion years – most likely that it was magnetised by asteroid impacts.
NASA plans to return to the Moon in 2024, and the new discovery is interesting for future missions. If the Moon was not sheltered by a sustained magnetosphere, then the loose lunar soil (regolith) should hold buried 3He, water and rare surface mineralss acquired from solar winds and Earth’s magnetosphere over at least those 4 billion years. The Moon may have had an initial magnetic field for some 100 million years after it formed, before it cooled and stabilised.