How old is the Moon? Astronaut-gathered samples from the lunar surface put its age at about 4.35 billion years. However, other evidence suggests it’s much older, around 4.53 billion years old. A team of scientists published a recent paper that suggests the Moon’s surface age was “reset” in a melting event.
The generally accepted theory about the Moon’s formation goes like this: about 4.5 billion years ago and about 200 million years after the solar system began to form, something happened. A single Mars-sized object named Theia (or possibly a series of objects) collided with or was somehow captured by infant Earth. That tumultuous crash sent a lot of molten rock and debris into space. Eventually, most of it coalesced to form the infant Moon, which settled into orbit around Earth. Debris from the inner solar system bombarded the pair for millions of years thereafter.
Fast-forward to the present day, where we can study rocks collected from the lunar surface during the Apollo missions. Scientists analyzed those samples and found them to be about 4.35 billion years old. That suggests the Moon is NOT 4.53 billion years old. Which is true? It turns out there’s strong evidence for an older Moon. It exists in some zircon minerals on the lunar surface. They’re at least 4.51 billion years old. thermal models and simulations also suggest a lunar age somewhere between 4.43 and 4.53 billion years. So, why are lunar surface rocks almost 200 million years younger?
Dating the Moon
As it turns out, both numbers could be true. The Moon could have formed very early, but it experienced something that changed its geological clock. According to UC Santa Cruz professor Francis Nimmo and a team of researchers, the Moon likely did form 4.51 million years ago in that catastrophic collision with baby Earth. But, 180 million years later, it may have experienced a “remelting”. That reset the ages of lunar rocks to around 4.35 billion years. That’s why the surface samples collected by the Apollo astronauts show a younger age.
“We predict that there shouldn’t be any lunar rocks that are older than 4.35 billion years because they should have experienced the same resetting,” said Nimmo. “Because this heating event was global, you shouldn’t find rocks anywhere on the Moon that are significantly older than that.”
Nimmo and his colleagues suggest that a global remelt of lunar rocks could account for the existence of younger surface rocks. The Apollo rocks suggest something happened, and the return of rocks from China’s Chang’e 6 mission could offer more evidence for that theory. For their paper, the authors used modeling to show that the Moon may have experienced sufficient tidal heating to cause this remelting approximately 4.35 billion years ago, which could “reset” the apparent formation age of these lunar samples.
Modeling a Lunar Surface Reset
What could cause a global melting strong enough to reset the age of the Moon’s rocks? Nimmo suggest that the Moon experienced tidal heating due to the evolution of its orbit around Earth. This happened because the Moon was closer to Earth, and the orbit was pretty unstable during certain epochs. Thanks to the immense tidal pull from Earth, the Moon could have been heated, which led to the alteration of its geology and the “age reset” of its rocks.
It turns out that the Moon isn’t the only place in the solar system where this could happen. The volcanic moon Io in orbit around Jupiter experiences the same type of tidal attraction as it orbits. That helps explain Io’s extensive volcanic activity and surface “paving” by the frequent eruptions from its volcanic features. It also explains why we don’t see widespread craters on Io.
If the same thing happened to the infant Moon after its original formation, cooldown, and subsequent bombardment, we wouldn’t see any of its original craters. They’d have been covered by subsequent eruption and melting when the Moon’s orbit was stabilizing.
Why is the Lunar Surface Age Important?
The formation and evolution of the solar system and its many different bodies is still a hot area of study. Among other things, scientists want to understand the timing of events that shaped solar system objects. For that, they need a better understanding of the geology of each object. More data leads to better models of every aspect of solar system formation—from the first “push” in the protosolar nebula to such events as collisions, tidal heating, orbital dynamics, and surface evolution of different worlds. That’s where planetary science missions come in handy. They provide “in situ” data about each world (or object, in the case of asteroids, moons, comets, and rings), and they fill in gaps in the history of each place.
“As more data becomes available—particularly from ongoing and future lunar missions—the understanding of the Moon’s past will continue to evolve,” Nimmo said. “We hope that our findings will spark further discussion and exploration, ultimately leading to a clearer picture of the Moon’s place in the broader history of our solar system.”
For More Information
A “Remelting” of Lunar Surface Adds a Wrinkle to Mystery of Moon’s True Age
Tidally Driven Remelting Around 4.35 billion Years Ago Indicates the Moon is Old
Moon Formation