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Lunar shielding was critical to Earth’s ability to maintain its atmosphere

Earth and Moon Galileo Composite

The Earth and the Moon, shown here in a composition of two images from the Galileo mission from the 1990s, have a long shared history. Billions of years ago, they connected magnetic fields. Credit: NASA / JPL / USGS

The Earth and the Moon once shared a magnetic shield, protecting their atmospheres

Four and a half billion years ago, the surface of the Earth was a threatening, hot mess. Long before the emergence of life, temperatures were scorching and the air was toxic. Furthermore, as a child, the Sun bombarded our planet with violent bursts of radiation called rockets and coronal mass ejections. Streams of charged particles called the solar wind threatened our atmosphere. Our planet was, in short, uninhabitable.

But a nearby shield may have helped our planet maintain its atmosphere and ultimately develop life and habitable conditions. That shield was the Moon, he tells NASAstudy in the journal Advances in science.

Earth's magnetic field lines

This illustration shows the magnetic field lines that the Earth generates today. The Moon no longer has a magnetic field. Credit: NASA

“The Moon appears to have presented a substantial protective barrier against the solar wind for Earth, which was critical to the Earth’s ability to maintain its atmosphere during this period,” said Jim Green, NASA’s chief scientist and lead author of the new. study. “We look forward to following up on these findings when NASA sends astronauts to the moon through the Artemis program, which will return critical samples of the lunar South Pole.”

A Brief History of the Moon

The Moon formed 4.5 billion years ago when a MarsA similar-sized object called Theia crashed into proto-Earth when our planet was less than 100 million years old, according to major theories. The debris from the collision merged into the Moon, while other remnants reincorporated into the Earth. Due to gravity, the presence of the Moon has stabilized the rotation axis of the Earth. At that time, our planet was spinning much faster, with one day only lasting 5 hours.

And in the early days, the Moon was also much closer. When the Moon’s gravity draws on our oceans, the water is heated slightly and that energy is dissipated. This results in the Moon moving away from Earth at a rate of 1.5 inches per year, or about the width of two adjacent cents. Over time, this really adds up. 4 billion years ago, the Moon was three times closer to Earth than it is today – about 80,000 miles away, up from 238,000 miles today. At some point, the Moon also became “stuck in the sea,” which means that the Earth only sees one side of it.

Lunar magnetic field

When the Moon had a magnetic field, it would be shielded from the oncoming solar wind, as shown in this illustration. Credit: NASA

Scientists once thought that the Moon never had a long-lasting global magnetic field because it has such a small core. A magnetic field causes electrical charges to move along invisible lines, which bow at the poles towards the Moon. Scientists have long known about the Earth’s magnetic field, which creates the beautiful colored auroras in the Arctic and Antarctic regions.

A magnetic field acts as a shield that causes electrical charges to move along its invisible lines. Scientists have long known about the Earth’s magnetic field, which causes the beautiful colored auroras in the Arctic and Antarctic regions. The movement of liquid iron and nickel in the depths of the Earth, which still flows due to the heat left by the Earth’s formation, generates the magnetic fields that form a protective bubble that surrounds the Earth, the magnetosphere.

But thanks to studying samples of the lunar surface of the Apollo missions, scientists discovered that the Moon also had a magnetosphere. Evidence continues to collect from samples that have been sealed for decades and recently analyzed with modern technology.

Like the Earth, the heat from the Moon’s formation would have kept the iron flowing deep within, though not that long due to its size.

“It’s like baking a cake – you take it out of the oven and it’s still cooling down,” Green said. “The bigger the mass, the longer it takes to cool down.”

A magnetic shield

The new study simulates the behavior of the magnetic fields of the Earth and the Moon some 4 billion years ago. Scientists created a computer model to examine the behavior of magnetic fields at two positions in their respective orbits.

Magnetic fields of the Earth and the Moon

This illustration shows how Earth and its Moon both had magnetic fields connected billions of years ago, helping to protect their atmospheres from streams of damaging solar particles, according to new research. Credit: NASA

At certain times, the Moon’s magnetosphere would serve as a barrier to the harsh solar radiation raining down on the Earth-Moon system, the scientists write. This is because, according to the model, the magnetospheres of the Moon and Earth would be magnetically connected in the polar regions of each object. Importantly, due to the evolution of the Earth, the high-energy solar wind particles failed to fully penetrate the coupled magnetic field and strip the atmosphere.

But there was also an atmospheric exchange. The Sun’s extreme ultraviolet light would have stripped electrons from neutral particles in Earth’s upper atmosphere, charging those particles and allowing them to travel to the Moon along the lines of the lunar magnetic field. This, too, may have contributed to the maintenance of the Moon in a subtle atmosphere at the time. The discovery of nitrogen in samples of lunar rocks supports the idea that the Earth’s atmosphere, which is dominated by nitrogen, contributed to the ancient atmosphere of the moon and its crust.

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

“Understanding the history of the lunar magnetic field helps us understand not only the possible early atmospheres, but also how the lunar interior evolved,” said David Draper, deputy chief scientist and co-author of the study. “It tells us what the Moon’s core might have looked like – probably a combination of liquid and solid metal at some point in its history – and that’s a very important piece of the puzzle for how the Moon works inside it.”

Over time, as the interior of the Moon cooled, our closest neighbor lost its magnetosphere and eventually its atmosphere. The field must have decreased significantly 3.2 billion years ago and disappeared about 1.5 billion years ago. Without a magnetic field, the solar wind stripped the atmosphere. This is also why Mars has lost its atmosphere: solar radiation has ripped it away.

If our Moon played a role in shielding our planet from harmful radiation during an early critical period, then similarly, there may be other moons around terrestrial exoplanets in the galaxy that help preserve atmospheres for their host planets, and they even contribute to habitability conditions, scientists say. This would be interesting for the field of astrobiology: the study of the origins of life and the search for life beyond Earth.

Human exploration can tell us more

This modeling study presents ideas on how the ancient histories of the Earth and the Moon contributed to the preservation of the Earth’s primeval atmosphere. The mysterious and complex processes are difficult to understand, but new samples from the lunar surface will provide clues to the mysteries.

As NASA plans to establish a sustainable human presence on the Moon through the Artemis program, there may be more opportunities to test these ideas. When astronauts return the first samples from the lunar South Pole, where the magnetic fields of the Earth and the Moon have connected most strongly, scientists can look for chemical traces of Earth’s ancient atmosphere, as well as volatile substances such as water that were released by the impact of meteors and asteroids. Scientists are particularly interested in areas of the lunar South Pole that have not seen any sunlight in billions of years – the “permanently shaded regions” – because hard solar particles would not have removed the volatiles.

Nitrogen and oxygen, for example, may have traveled from the Earth to the Moon along magnetic field lines and become trapped in those rocks.

“Significant samples from these permanently shaded regions will be critical for us to be able to untangle this early evolution of Earth’s volatiles by testing our model assumptions,” Green said.

Reference: “When the Moon had a magnetosphere” by James Green, David Draper, Scott Boardsen and Chuanfei Dong, 14 October 2020, Advances in science.
DOI: 10.1126 / sciadv.abc0865

The other co-authors of the paper are Scott Boardsen of the University of Maryland, Baltimore County; and Chuanfei Dong from Princeton University in New Jersey.

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