The moon may be older than scientists previously thought. New research sheds light on its history

Physicists, chemists, and mathematicians walked into the bar. It sounds like the beginning of a bad joke, but in my case it was the beginning of an idea that could change the way scientists think about the history of the moon.

All three of us were interested in the moon, but from different perspectives. As a geophysicist, I thought about the interior of the moon. Torsten Kleine studied its chemistry. Alessandro Morbidelli then wanted to find out what the moon’s formation could tell us about how the planet was assembled 4.5 billion years ago.

When we met to discuss the actual age of the moon, we found it important to have these multiple perspectives.

How was the moon formed?

At a conference held in Hawaii in the late 1980s, a group of scientists solved the problem of how the moon formed. Their research suggested that a Mars-sized object crashed into the early Earth, ejecting molten material into space. The glowing matter coalesced into the body we now call the moon.

This story explains a lot. First, because life began on the moon by melting, there is almost no material that easily evaporates, such as water. Most of the Earth was formed from the outside of the Earth, which contains very little iron, so it only has a small iron core. It has a buoyant white crust made of minerals that floated to the surface as the molten moon solidified.

The newly formed, bright moon was initially very close to Earth, approximately at the distance of a television satellite. The early Moon would have caused huge tides on the early Earth, but the Earth itself would have mostly melted and rotated rapidly.

These currents took energy from the Earth’s rotation and transferred some of it into the Moon’s orbit, slowly pushing the Moon away from the Earth and slowing the Earth’s rotation in the process. This movement continues today, with the moon still moving away from Earth about 2 inches per year.

As the moon receded, it passed through certain points where its orbit was temporarily disturbed. These orbital perturbations are important elements of its history and are an important part of our hypothesis.

When was the moon formed?

When the moon actually forms and moves away from Earth is a thorny question.

Thanks to the Apollo astronauts, scientists now have a collection of moon rocks that can be used to determine their age. The oldest rocks are all about 4.35 billion years old, which is about 200 million years after the birth of the solar system.

Many geochemists, like my colleague Torsten Kleine, suggest (not unreasonably) that these rocks are as old as the Moon.

But people like Alessandro Morbidelli, who studies planet formation, didn’t really like this answer. In their model, planets wiped out most of the material floating around the early solar system long before 200 million years had passed. It seemed much less likely that a giant moon-forming impact would occur as late as the rock samples suggested.

What did we propose?

This is where Kleine, Morbidelli, and I come in. We followed up on suggestions from a 2016 study that found that the moon can sometimes experience extreme heating events as it slowly moves away from Earth.

This heating occurs similarly to the heating on Jupiter’s active volcanic moon Io. The shape of the small body is crushed and stretched by the tide from the larger body. And just as a rubber ball gets warmer if you squeeze it hard enough, the rocks on Io and the Moon will get warmer as well.

All rocks contain tiny internal clocks. The decay of radioactive elements allows researchers to determine the age of the rock. But here’s the important point. When the moon warms up enough, the clock loses its memory and begins recording time only when the moon cools down again.

So in this photo, the pile of rocks that are about 4.35 billion years old doesn’t tell us when the moon formed, but rather when the moon was just passing through this tidal heating phenomenon. is. In other words, the formation of the moon must have happened much earlier.

While the early formation dates satisfy physicists who study planet formation, later dates recorded in rocks explain tidal reheating.

What’s next?

As often happens in science, two groups came up with similar ideas at the same time. While our group focused on tidal heating events that occurred when the Moon was far away from Earth, research by Steve Desch of Arizona State University focused on tidal heating events that occurred when the Moon was close to Earth. pointing out the phenomenon. It will take time to determine which of these two hypotheses is correct. It’s also possible that neither is correct.

More samples from the Moon will be needed to test these hypotheses. Fortunately, China’s Chang’e 6 mission just returned samples from the far side of the moon in June 2024. It would be consistent with our story if these samples also showed many rocks, all with an age of about 4.35 billion years ago. If you are much older, you will have to come up with a new story.

In earth and planetary science, geochemists and geophysicists often end up making different and contradictory hypotheses. This is partly because these fields use different types of measurements, but also because they use very different scientific languages. Overcoming this language barrier is difficult.

Our work is an example of how bridging the gap between language and science can sometimes benefit researchers on both sides.

This article is republished from The Conversation under a Creative Commons license. Read the original article.