Moon Shielding Was Critical to Earth’s Ability to Maintain Its Atmosphere

The earth and moon once shared a magnetic shield to protect their atmosphere

Four and a half billion years ago the surface of the earth was a threatening, hot mess. Long before life came, temperatures dropped and the air was toxic. As a young child, the sun bombarded our planet with violent bursts of radiation known as flares and coronal mass ejections. Currents of charged particles called solar winds threatened our atmosphere. In short, our planet was uninhabitable.

But a nearby shield may have helped our planet maintain its atmosphere and eventually develop life and habitable conditions. That shield was the moon, says a NASA-led study in the journal Advances in science.

“The moon appears to have provided Earth with a significant protective barrier against solar wind, which was critical to Earth’s ability to maintain its atmosphere during this time,” said Jim Green, NASA chief scientist and lead author of the new study. “We look forward to pursuing these findings as NASA, through the Artemis program, sends astronauts to the moon who return critical samples from the lunar south pole.”

A brief history of the moon

The moon was formed 4.5 billion years ago when a MarsAn object called Theia hit Proto-Earth when our planet was less than 100 million years old, according to leading theories. The debris from the collision merged with the moon, while other remnants built themselves back into the earth. Due to gravity, the presence of the moon stabilized the earth’s axis of rotation. At that time our planet was spinning much faster and a day only lasted 5 hours.

And in the early days the moon was much closer too. As the moon’s gravity pulls on our oceans, the water is heated slightly and this energy is dissipated. This causes the moon to move away from Earth at a rate of 1.5 inches per year, or about the width of two neighboring pennies. Over time, that really adds up. Four billion years ago, the moon was three times closer to Earth than it is today – about 80,000 miles away compared to the current 238,000 miles. At some point, the moon was also “tidal-locked”, which means that the earth only sees one side of it.

Scientists once thought that the moon never had a long lasting global magnetic field because it has such a small nucleus. A magnetic field causes electrical charges to move along invisible lines that bend toward the moon at the poles. Scientists have long known the Earth’s magnetic field that creates the beautifully colored aurors in the Arctic and Antarctic.

A magnetic field acts as a shield, causing electrical charges to move along its invisible lines. Scientists have long known the Earth’s magnetic field that causes the beautifully colored aurors in the Arctic and Antarctic. The movement of liquid iron and nickel deep in the earth, still flowing due to the heat left over from the earth’s formation, creates the magnetic fields that form a protective bubble that surrounds the earth, called the magnetosphere.

Thanks to studies of samples of the lunar surface from the Apollo missions, scientists found that the moon once had a magnetosphere as well. There is still evidence of samples that have been sealed for decades and recently analyzed using modern technology.

Like Earth, the heat of the lunar formation would have made the iron flow deep inside, though not nearly as long because of its size.

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

A magnetic shield

The new study simulates how the magnetic fields of the earth and moon behaved about 4 billion years ago. Scientists created a computer model to study the behavior of the magnetic fields at two positions in their respective orbits.

At certain times, the moon’s magnetosphere would have served as a barrier against the harsh solar radiation that rained down on the earth-moon system, scientists write. This is because, according to the model, the magnetospheres of the moon and earth would have been magnetically connected in the polar regions of each object. It is important for the development of the earth that the energy-rich solar wind particles could not completely penetrate the coupled magnetic field and strip off the atmosphere.

But there was also an atmospheric exchange. The extreme ultraviolet light of the sun would have withdrawn electrons from the neutral particles in the uppermost atmosphere, charged these particles and enabled them to move along the lunar magnetic field lines to the moon. This may have helped the moon to maintain a thin atmosphere at this point as well. The discovery of nitrogen in lunar rock samples supports the idea that the Earth’s atmosphere, dominated by nitrogen, contributed to the ancient lunar atmosphere and its crust.

Scientists reckon that this common magnetic field situation with the magnetospheres of the earth and moon could have existed 4.1 to 3.5 billion years ago.

“Understanding the history of the lunar magnetic field helps us understand not only possible early atmospheres, but the evolution of the lunar interior,” said David Draper, NASA’s assistant chief scientist and co-author. “It tells us what the core of the moon could 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.”

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 has stripped the atmosphere. This is also the reason why Mars has lost its atmosphere: solar radiation has removed it.

Similarly, if our moon played a role in protecting our planet from harmful radiation at a critical early time, there may be other moons around terrestrial exoplanets in the galaxy that help preserve the atmosphere for their host planets and even make it habitable conditions contribute, say scientists. This would be of interest to 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 model study presents ideas on how the ancient stories of the Earth and the Moon helped preserve the Earth’s early atmosphere. The mysterious and complex processes are difficult to figure out, but new samples from the lunar surface provide clues about the mysteries.

With NASA planning to establish a sustainable human presence on the moon through the Artemis program, there may be several ways to test these ideas. When astronauts return the first samples from the moon’s south pole, where the magnetic fields of the Earth and Moon are most closely linked, scientists can look for chemical signatures of Earth’s ancient atmosphere, as well as volatile substances like water given off by impacting meteors and asteroids. Scientists are particularly interested in areas of the lunar south pole that have not seen any sunlight for billions of years – the “permanently shaded regions” – because the hard solar particles would not have removed any volatile components.

For example, nitrogen and oxygen may have traveled from the earth to the moon along the magnetic field lines and become trapped in these rocks.

“Significant samples from these permanently shadowed regions will be of vital importance for us to unravel this early evolution of Earth’s volatiles and test our model assumptions,” said Green.

Reference: “When the moon had a magnetosphere” by James Green, David Draper, Scott Boardsen and Chuanfei Dong, October 14, 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 Princeton University in New Jersey.