A sample brought back to Earth by the Apollo 15 mission in 1971 indicates that the interior of the Moon may have been active for longer than previously believed. According to researchers at MIT and Rutgers University, the molten core of the ancient Moon was an active dynamo that drew power from the Earth's gravitational pull to keep its magnetic field going for a billion years longer than previously thought.

Today, the Moon is largely inert, but in its early days it was much more active and had a magnetic field that was thought to die out about a billion years after the satellite was formed. The question is, did the magnetic field vanish completely, or did some other process power the lunar dynamo and allow it to linger on in some weaker form for another billion years?

To find the answer, the MIT/Rutgers team examined a rock sample, numbered 15498, collected by the crew of Apollo 15 from the southern rim of Dune Crater near Hadley Rille on August 1, 1971. The sample is a conglomerate of minerals and rock samples that welded together into a glassy matrix that has excellent magnetic recording properties and retained the magnetic properties of when it was formed.

According to the team, the sample is relatively young – only about one to 2.5 billion years old, whereas previously studied samples were about 3.2 billion years old. This was important because most rocks on the Moon date back to when it cooled from a molten mass after the great bombardment that took place during the formation of the Solar System ended. This means that any magnetic field recorded would be from the first billion years of the lunar field's existence.

Sample 15498 is different in that it's a product by later asteroid strikes that melted local rocks, which then reformed and recorded the magnetic field of that time.

The Apollo 15 moon rock sample that was analyzed by MIT and Rutgers University researchers(Credit: MIT)

A very sensitive magnetometer was used to make an initial measurement of the sample to determine its natural magnetism. It was then exposed to heat close to the extreme temperatures in which it was formed in a special oxygen-free furnace intended to simulate lunar conditions, and its magnetism was measured again. By comparing the two, it's possible to determine what the original magnetic strength was.

Previous research indicated that four billion years ago the Moon had a magnetic field with a strength of about 100 microteslas. The strength of this field dropped off about three billion years ago, but the question remained whether it vanished or if it became much weaker. From the evidence of 15498, around 1 to 2.5 billion years ago the field dropped to around five microteslas, which is 10 times weaker than the Earth's magnetic field, before it died out about two billion years ago.

The team says that this indicates that two mechanisms were at work in producing the ancient lunar magnetic field. Planetary magnetic fields are generated by the churning molten core of a planet, which acts as a dynamo, and the researchers suggest that the gravitational pull of the Earth may have powered this process in the Moon.

"The concept of a planetary magnetic field produced by moving liquid metal is an idea that is really only a few decades old," says Benjamin Weiss, professor of planetary sciences in MIT's Department of Earth, Atmospheric and Planetary Sciences. "What powers this motion on Earth and other bodies, particularly on the Moon, is not well-understood. We can figure this out by knowing the lifetime of the lunar dynamo."

The team says that the earlier, stronger field was at a time when the Moon orbited much closer to the Earth. This means that the tidal forces affecting the Moon were much stronger, causing the liquid interior of the young satellite to be pulled and rotated like the armature of a dynamo.

But as the Moon pulled away from the Earth, the tidal forces became weaker and the field became weaker by orders of magnitude. After this, the Moon's core continued to churn as it slowly cooled and solidified.

"As the Moon cools, its core acts like a lava lamp – low-density stuff rises because it's hot or because its composition is different from that of the surrounding fluid," says Weiss. "That's how we think the Earth's dynamo works, and that's what we suggest the late lunar dynamo was doing as well."

The research was published in Science Advances.

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