The surface of the Earth was a violent, hot mess 4.5 billion years ago. When life was still a distant possibility, temperatures could melt eyelids, and the air killed in seconds. The sun, too, was young — and bombarded the baby Earth with unkind bursts of radiation called flares and coronal mass ejections — which shot streams of highly-charged particles toward a planet already hostile to life.
In short, the young Earth was not a very nice place to live.
However, the now-extinct magnetic shield of the moon may have helped our little blue planet keep its atmosphere and eventually support and develop life in a habitable climate, according to a NASA-led study published in the journal Science Advances.
Moon once protected Earth with shield, says NASA
"The moon seems to have presented a substantial protective barrier against the solar wind for the Earth, which was critical to Earth's ability to maintain its atmosphere during this time," said NASA's Chief Scientist and lead study author Jim Green. "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."
Predictably, the moon had a violent birth 4.5 billion years ago — when a Mars-sized proto-planet called Theia slammed maddeningly into the early Earth (which was less than 100 million years old), according to dominant theories.
The debris from this cataclysmic collision later coalesced into our moon, while the remaining material fell back into the Earth. The effect of the moon's gravity stabilized the Earth's spin axis, which at the time spun so fast a day lasted just five hours.
In those days, the moon was far closer to the Earth — and thus much bigger in the sky. While the moon's gravity pulls on our oceans, the water is heated — dissipating energy. This causes the moon to move away from the Earth at a rate clocked at 1.5 inches (3.81 cm) every year — roughly the width of two juxtaposed dimes.
Simulating the moon's magnetosphere
The new study — under NASA's leadership — simulated how the magnetic fields of the Earth and moon worked roughly 4 billion years ago. Scientists built a computer model to study the behavior of both objects' magnetic fields at two positions during their respective orbits, according to a blog post on NASA's website.
Sometimes the moon's magnetosphere would become a barrier to the harsh solar radiation slamming into the Earth-moon system, wrote the scientists. This is because the two magnetospheres were still connected in the polar regions of each object. Crucial for the Earth's evolution, the high-energy solar wind particles were unable to pierce through the superimposed magnetic fields — leaving a very strip-worthy atmosphere intact, and perhaps saving us from the fate of Mars.
Interestingly, the two atmospheres exchanged matter. Intense ultraviolet light from the sun likely stripped electrons from neutral particles high in the Earth's atmosphere, giving them the charge needed to travel to the moon along lunar magnetic field lines, said NASA.
This could have helped the moon maintain a thin atmosphere back then — which is a weird way to think about the moon. But since nitrogen was found in lunar rock samples, the idea of the Earth's atmosphere at one time sending material to become the moon's ancient atmosphere isn't so far-fetched — since the Earth's atmosphere was and still is dominated by nitrogen.
Studying the moon's magnetic fields helps us understand its interior
Scientists think this magnetic field overlap period between the moon and Earth probably happened in the time between 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 NASA's Deputy Chief Scientist and study co-author David Draper. "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."
Naturally, the moon's interior cooled down, lost its magnetosphere, and eventually its atmosphere. The magnetic field substantially diminished 3.2 billion years ago, and "died" roughly 1.5 billion years ago. Lacking a magnetic field, the solar wind stripped the early lunar atmosphere away — not unlike what happened to Mars.
It's extremely fascinating to imagine the air we breathe going to the moon and back, and new scientific discoveries like this are just the tip of a torrent of knowledge awaiting us once we return to our lunar satellite, which NASA plans to do during the Artemis program — slated to send the first woman and next man to the moon in 2024.