The Earth and the Moon once shared a magnetic shield, protecting their atmospheres
Four and a half billion years ago, the Earth’s surface was a menacing, hot mess. Long before life began, temperatures were scorching and the air was toxic. Plus, as a young child, the Sun bombarded our planet with violent bursts of radiation called eruptions and coronal mass ejections. Streams of charged particles, called solar winds, threatened our atmosphere. In short, our planet was uninhabitable.
But a neighboring shield may have helped our planet maintain its atmosphere and eventually continue to develop life and living conditions. That shield was the moon, says a NASA-after a study in the journal Scientific progress.
“The moon appears to have provided a significant protective barrier against the solar wind for the Earth, which was critical to the Earth’s ability to maintain its atmosphere during that time,” said Jim Green, NASA’s chief scientist and lead author of the new study. “We look forward to following these findings when NASA sends astronauts to the moon through the Artemis program, which will return critical samples from the lunar South Pole.”
A brief history of the moon
The moon formed 4.5 billion years ago when a Marsan object called Thea crashed into proto-Earth when our planet was less than 100 million years old, according to leading theories. The debris from the collision merged into the moon, while other remains rejoined the Earth. Due to gravity, the presence of the Moon stabilizes the Earth’s rotating axis. At that time, our planet was spinning much faster, lasting only 5 hours a day.
And in the early days, the moon was also much closer. As the moon’s gravity attracts our oceans, the water heats up slightly and this energy is dissipated. This causes the Moon to move away from the Earth at a rate of 1.5 inches per year or about the width of two neighboring pennies. 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, compared to the current 238,000 miles. At one point, the moon also “locked in the tide,” meaning that the Earth sees only one side of it.
Scientists once thought that the moon never had a long-lasting global magnetic field because it had such a small nucleus. The magnetic field causes electric charges to move in invisible lines that bend toward the moon at the poles. Scientists have long known about the Earth’s magnetic field, which creates beautifully colored rays in the Arctic and Antarctic.
The magnetic field serves as a shield, causing electric charges to move along its invisible lines. Scientists have long known about the Earth’s magnetic field, which causes beautifully colored glows in the Arctic and Antarctic. The movement of liquid iron and nickel deep into the Earth, still flowing due to the heat left by the Earth’s formation, generates magnetic fields that form a protective bubble surrounding the Earth, the magnetosphere.
But thanks to studies of lunar surface samples from the Apollo missions, scientists have learned that the moon once had a magnetosphere. Evidence continues to be collected from samples that have been sealed for decades and recently analyzed with modern technology.
Like Earth, the heat from the moon’s formation would keep the iron flowing 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’s still cooling,” Green said. “The larger the mass, the longer it takes to cool.”
The new study simulates how the magnetic fields of the Earth and the Moon behaved about 4 billion years ago. Scientists have created a computer model to examine the behavior of magnetic fields in two positions in their respective orbits.
At times, the Moon’s magnetosphere would serve as a barrier to the harsh solar radiation that falls on the Earth-Moon system, 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. The important thing for the evolution of the Earth is that the high-energy particles of the solar wind cannot completely penetrate the bound magnetic field and take away the atmosphere.
But there was also an exchange of atmosphere. Extreme ultraviolet light from the Sun would remove electrons from neutral particles in the Earth’s upper atmosphere, which would make these particles charged and allow them to travel to the Moon along the lines of the lunar magnetic field. This may have contributed to the moon maintaining a thin atmosphere at this time. The discovery of nitrogen in lunar rock samples 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 estimate that this general magnetic field situation, with the Earth and Moon magnetospheres attached, could persist 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 also how the lunar interior evolves,” said David Draper, deputy chief scientist and co-author of the study. “It tells us what the core of the moon could be – probably a combination of both liquid and hard metal at some point in its history – and it’s a very important part of the puzzle of how the moon works from the inside out.”
Over time, as the interior of the moon cools, our nearest neighbor loses its magnetosphere and eventually its atmosphere. The field must have shrunk significantly 3.2 billion years ago and disappeared about 1.5 billion years ago. Without a magnetic field, the solar wind took away the atmosphere. That’s why Mars lost its atmosphere: solar radiation took it away.
If our Moon played a role in protecting our planet from harmful radiation during critical early times, then similarly there may be other moons around terrestrial exoplanets in the galaxy that help preserve the atmosphere for their host planets and even contribute to habitability conditions, scientists say. This would be of interest to the field of astrobiology – the study of the origin of life and the search for life beyond Earth.
Human research 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 early atmosphere. The mysterious and complex processes are difficult to unravel, 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 many opportunities to test these ideas. When astronauts return the first samples from the lunar South Pole, where the Earth’s and Moon’s magnetic fields are most closely linked, scientists can look for chemical signatures on the Earth’s ancient atmosphere, as well as volatile substances such as water from meteors and asteroids. Scientists are particularly interested in areas of the lunar South Pole that have not seen any sunlight for billions of years – “permanently shaded regions” – because raw solar particles would not remove volatile matter.
For example, nitrogen and oxygen may have traveled from Earth to the Moon along magnetic field lines and been trapped in these rocks.
“Significant samples from these permanently shaded regions will be crucial for us to unravel this early evolution of volatile matter on Earth by testing our model assumptions,” Green said.
Reference: “When the Moon had a magnetosphere” by James Green, David Draper, Scott Boardsen and Chuanfei Dong, October 14, 2020, Scientific progress.
DOI: 10.1126 / sciadv.abc0865
Other co-authors of the article are Scott Bordson of the University of Maryland, Baltimore County; and Chuanfei Dong from Princeton University in New Jersey.