Ancient Magnetosphere

Solar storms strip a planet’s atmosphere over time, and only a strong magnetosphere would be able to provide maximum protection. Lunar samples gathered by NASA’s Apollo missions recently revealed that the Moon had its own global magnetosphere, lasting from about 4.25 to 2.5 billion years ago. According to new research, the Earth-Moon coupled magnetospheres presented a previously unrecognized protective barrier against the solar wind for our home planet, reducing Earth’s atmospheric loss to space.

When the Moon had a magnetic field, it would have been shielded from incoming solar wind, as shown in this illustration. Image credit: NASA.

When the Moon had a magnetic field, it would have been shielded from incoming solar wind, as shown in this illustration. Image credit: NASA.

Planetary researchers once thought that the Moon never had a long-lasting global magnetic field because it has such a small core.

They have long known about Earth’s magnetic field, which causes the beautifully colored aurorae in the Arctic and Antarctic regions.

But thanks to recent studies of the lunar samples from the Apollo missions, they figured out that the Moon once had a magnetosphere, too.

Like Earth, the heat from the Moon’s formation would have kept iron flowing deep inside, although not for nearly as long because of its size.

“It’s like baking a cake: You take it out of the oven, and it’s still cooling off. The bigger the mass, the longer it takes to cool off,” said lead author Dr. James Green, chief scientist at NASA’s Headquarters in Washington, DC.

Dr. Green and colleagues created a computer model to look at the behavior of the magnetic fields of the Earth and Moon about 4 billion years ago.

At certain times, the Moon’s magnetosphere would have served as a barrier to the harsh solar radiation raining down on the Earth-Moon system.

That’s because, according to the model, the magnetospheres of the Moon and Earth would have been magnetically connected in the polar regions of each object.

Importantly for the evolution of Earth, the high-energy solar wind particles could not completely penetrate the coupled magnetic field and strip away the atmosphere.

But there was some atmospheric exchange, too. The extreme UV light from the Sun would have stripped electrons from neutral particles in Earth’s uppermost atmosphere, making those particles charged and enabling them to travel to the Moon along the lunar magnetic field lines. This may have contributed to the Moon maintaining a thin atmosphere at that time, too.

The discovery of nitrogen in lunar rock samples support the idea that Earth’s atmosphere, which is dominated by nitrogen, contributed to the Moon’s ancient atmosphere and its crust.

The scientists calculate that this shared magnetic field situation, with Earth and Moon’s magnetospheres joined, could have persisted from 4.1 to 3.5 billion years ago.

“Understanding the history of the Moon’s magnetic field helps us understand not only possible early atmospheres, but how the lunar interior evolved,” said co-author Dr. David Draper, deputy chief scientist at NASA’s Headquarters in Washington, DC.

“It tells us about what the Moon’s core could have been like — probably a combination of both liquid and solid metal at some point in its history — and that is a very important piece of the puzzle for how the Moon works on the inside.”

Over time, as the Moon’s interior cooled, our nearest neighbor lost its magnetosphere, and eventually its atmosphere.

The field must have diminished significantly 3.2 billion years ago, and vanished by about 1.5 billion years ago. Without a magnetic field, the solar wind stripped the atmosphere away.

“If our Moon played a role in shielding our planet from harmful radiation during a critical early time, then in a similar way, there may be other moons around terrestrial exoplanets in our Milky Way Galaxy that help preserve atmospheres for their host planets, and even contribute to habitable conditions,” the authors said.

“This would be of interest to the field of astrobiology — the study of the origins of life and the search for life beyond Earth.”

The study was published in the journal Science Advances.


James Green et al. 2020. When the Moon had a magnetosphere. Science Advances 6 (42): eabc0865; doi: 10.1126/sciadv.abc0865

This article is based on a press-release provided by the National Aeronautics and Space Administration.

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