The moon’s magnetic field lasted at least a billion years longer than previously thought, researchers have revealed, shedding light on an enduring lunar mystery and expanding the possibilities in the hunt for habitable worlds beyond Earth.

Nowadays, the moon has no global magnetic field, but that was not always the case; between 4.25bn and 3.56bn years ago, the lunar magnetic field was similar to that of the Earth. The field is thought to have been generated by the churning movement of fluids within the moon’s molten core – a sort of lunar dynamo.

But scientists have long puzzled over when the magnetic field disappeared, with previous research unable to tell whether the field had disappeared completely by 3.19bn years ago or had lingered on in a weaker form.

“One the question that we were trying to answer was really when did the magnetic field cease, [so] we wanted to study younger lunar samples – rocks that are younger than 3.56bn years old,” said Sonia Tikoo, a planetary scientist and co-author of the research from Rutgers University.

Writing in the journal Science Advances, Tikoo and colleagues from the University of California, Berkeley and Massachusetts Institute of Technology, describe how they set about unpicking the conundrum by analysing a lunar rock brought back by the Apollo 15 mission in 1971.

The sample contains fragments of basalt that had broken off larger rocks. According to a dating technique based on the ratio of different isotopes of argon, the basalt formed from lava flows about 3.3bn years ago.

These fragments are bound together in the sample by a glassy material, which the team say probably formed when some of the basalt melted following a meteorite impact. The researchers dated the formation of the glassy material to between 1bn and 2.5bn years ago.

Crucially, the impact also melted iron-containing grains within the basalt. These crystallised again within the glassy material as it quickly cooled, capturing a record of the magnetic field of the moon at that time.

The lunar rock sample from the Apollo 15 mission. The rock consists of basalt fragments welded together by a dark glassy matrix produced by melting caused by a meteorite impact. Photograph: Nasa

After a series of experiments at different temperatures, the team found the grains formed when the moon had a magnetic field about a tenth as strong as that of the Earth, at five microtesla.

Such a field is 1,000 times stronger than that measured at the moon’s Apollo 15 landing site by astronauts, and far stronger than than would be expected from the influence of the Earth’s magnetic field.

“This is not contamination from the Earth’s field, it is not from the sun’s field, it is not from the galaxy’s field – we can rule things out,” said Tikoo.

The upshot, says Tikoo, is that the lunar dynamo was still going until somewhere between one billion and 2.5bn years ago.

But questions remain. “What we don’t have a good grasp on yet is what generated the lunar magnetic field,” said Tikoo.

Tikoo says the longevity of the field rules out the dynamo arising as a result of large impacts – a process that would have only yielded a temporary magnetic field. Impacts large enough to cause even a temporary field tailed off after about 3.7bn years ago. Instead, she says, the new findings suggest multiple mechanisms might have been at play.

The early, strong, magnetic field, she says, is likely to have been generated by the influence of the Earth’s gravitational pull on the lunar mantle, with the wobbling of the mantle churning up the moon’s liquid core. However, as the moon spiralled away from Earth, and the gravitational pull became weaker, another mechanism could have taken hold, generating a weaker field.



One possibility, she says, is that this weaker field was generated as the moon’s core cooled, with energy being released as the iron solidified and lighter elements, such as carbon and sulphur, stirred the core as they buoyantly rose up.

“The combination of these effects can help generate a dynamo that lasts for quite a long time – and this is the mechanism we think is operating the Earth’s dynamo right now,” said Tikoo.

The findings, she adds, could also cause a stir when it comes to the hunt for extraterrestrial worlds that might host life, suggesting that small planetary bodies should no longer be written off as unlikely to have long-lived magnetic fields, which are important for retaining an atmosphere and water.

Richard Harrison, professor of Earth and planetary materials at the University of Cambridge said the study dramatically expands the timescale for which the moon was generating a magnetic field, helping to shed light on processes deep within it.

“Despite the fact the moon is the one place we have actually visited in person in the solar system … there are still many unanswered questions,” he said.

