Moon's magnetic field lasted longer than thought

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Researchers have been studying the very, very ancient history of the moon's magnetic field by analyzing rocks, collected by astronauts, that are billions of years old, the Massachusetts Institute of Technology said Wednesday.

Magnetic fields are generated from the cores of moons and planets, and rocks can record fields to which they are exposed.

In a paper published today in Science Advances, researchers from MIT and Rutgers University report that a lunar rock collected by NASA's Apollo 15 mission exhibits signs that it formed 1 to 2.5 billion years ago in the presence of a relatively weak magnetic field of about 5 microtesla.

In their latest study, researchers focused on a rock that was between 1 and 2.5 billion years old.

Since the Moon is smaller and less dense than our planet, it's no mystery as to why the magnetic field eventually vanished. Roughly 3.56 billion years after the collision, the Moon's magnetic field was at peak strength before it started to fade to about 4 microteslas about 400 million years later.

At that point, the moon's magnetic field was the same strength as Earth's is today - about 50 microtesla.

"What powers this motion on Earth and other bodies, particularly on the Moon, is not well-understood", he said. "We can figure this out by knowing the lifetime of the lunar dynamo".

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"Whenever we look at exoplanets or the moons of exoplanets that could be in the habitable zone, we can consider the magnetic field as an important player in habitability", says Tikoo. After this, the Moon's core continued to churn as it slowly cooled and solidified. "So there were no young samples we could measure to see if there was a field after 3 billion years". The small, young rock-partially coated with melted glass-likely formed during a meteor impact on the lunar surface.

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. This means that any magnetic field recorded would be from the first billion years of the lunar field's existence. By comparing the two, it's possible to determine what the original magnetic strength was. They measured how the rock's magnetization changed as they increased the surrounding temperature. In collaboration with Grove, Suavet built a customized, oxygen-deprived oven in which to heat the rocks, preventing them from rusting while at the same time simulating the oxygen-free environment in which the rocks were originally magnetized.

Dr Sonia Tikoo, lead author of the study, said: 'The Earth's magnetic field is a shield that protects us from unsafe solar wind particles and ionizing radiation, so magnetic fields play a key role in the habitability of planets and, possibly, moons. Such a dramatic dip suggests to Weiss and his colleagues that the moon's dynamo may have been driven by two distinct mechanisms.

The researchers have proposed the dynamo is a result of the gravitational pull between the Earth and the Moon.

While the moon no longer has a magnetic field, a new study has revealed that it once had a field very similar to Earth's. It was still active as of 2ish billion years ago.

Over time, as the Moon chilled, its field weakened. A different mechanism may have then kicked in to sustain this weakened field. 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. "That's how we think the Earth's dynamo works, and that's what we suggest the late lunar dynamo was doing as well".

Part of the problem is we know don't quite know when the Moon's dynamo ground to a complete halt, largely because there aren't a lot of fresh rock samplings melting and solidifying after 3 billion years ago.

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