Rocks returned from the Moon by Apollo astronauts continue to continue to give up secrets about our planet’s partner in space.

The Moon may have been more active than thought. Photo: Paul Sutherland

A chunk of lunar basalt from the Sea of Tranquility supports the theory that the Moon once had a strong magnetic field of its own, generated by a molten metal core. What’s more, it suggests this force was still present 3.7 billion years ago – 500 million years later than previously thought.

The first rocks were brought back from the Moon by Apollo 11 astronauts Neil Armstrong and Buzz Aldrin in July 1969. Geologists were surprised to find that many of them were magnetized, contradicting the idea that the Moon had always been a cold, inactive world. It is a puzzle they have been working to solve for more than 40 years.

A vital clue came from a piece of basalt from the return samples labelled 10020. It is thought this was ejected from deep inside the Moon after a collision with an asteroid around 100 million years ago.

Benjamin Weiss, associate professor of planetary science at the Massachusetts Institute of Technology, has led the forensic team that worked to establish why this rock has a magnetic personality. They conclude that the rock must have had that property 3.7 billion years ago when it cooled from magma. Their detective work ruled out the possibility that it might have been magnetized at any time since.

Weiss, who studied the rock with graduate student Erin Shea, said: “It’s basically been in cold storage for 3.7 billion years, essentially undisturbed. It retains a beautiful magnetization record.”

Weiss collaborated with researchers at the University of California at Berkeley and the Berkeley Geochronology Center, who determined the rock’s age using radiometric dating. Their work has been published in the US journal Science.

The scientists are still debating how the lunar dynamo was powered. It may have self-sustaining, as Earth’s has been as our planet slowly cooled. But previous models have told us that this model would only have sustained a dynamo in the Moon for a few hundred million years after it formed, meaning that it would have disappeared 4.2 billion years ago at the latest.

The fact it was still around 500 million years later suggests that something other than cooling may have driven the dynamo. Weiss says: “The moon has this protracted history that’s surprising. This provides evidence of a fundamentally new way of making a magnetic field in a planet a new power source.”

The answer may be that the dynamo was powered by the Earth’s gravitational pull. The Moon was much closer to us billions of years ago and so this force may have continued to stir the liquid metal at the Moon’s core long after our natural satellite would otherwise have cooled.

It is an effect that we see further out in the Solar System today where the powerful tidal pull exerted by Jupiter causes extreme volcanic activity on satellite Io.