Science

Why does Earth have a Moon? New studies reveal its chaotic origin

Earth's only natural satellite may have been the result of a violent cosmic event.

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The Moon is responsible for the tides, our sleep cycles, and perhaps even the change in seasons. It’s hard to imagine what our planet would be without its celestial companion, but how the Moon came to be is still a mystery.

Two recent studies shed new light on the Moon’s origin story, suggesting that there’s more to the formation of Earth’s natural satellite. One study claims a messier origin to the Moon with multiple collisions that birthed the orbiting rock. In contrast, the second study claims that the Moon is much younger than previously believed.

The first study was published in The Planetary Science Journal, and the second study was published in the journal Nature.

What’s new — The most favored scenario of how the Moon formed suggests a single, violent impact that birthed Earth’s orbiter. But in one of the recent studies, scientists suggest that it took several “hit and run” collisions for the Moon to form rather than just one impact.

The authors behind the recent paper suggest that a small, Mars-sized protoplanet hit Earth and kept going, returned 1 million years later, hit Earth again, and kicked up material from both impacts is to form the Moon.

New studies shift the Moon’s origin story. NASA/Newsmakers

Meanwhile, a second paper argues that the Moon is younger than scientists had previously believed.

Previous estimates claimed that the Moon formed within the first 60 million years of the Solar System’s history. However, the new study claims that the Moon formed 142 million years after the birth of the Solar System.

The isotopic composition of the Moon compared with that of Earth’s suggest that while both bodies started with similar compositions, the Moon’s mantle — the layer between the core and the crust — suffered a shorter-lived period of decay compared to that of Earth’s, indicating its younger age.

How was the Moon created?

Scientists believe the Moon was created 4.5 billion years ago amidst the chaos of a young Solar System, with volatile material accreting to form planets.

The Earth and the Moon are bound together through gravity in the early chaos of the Solar System.Taro Hama @ e-kamakura/Moment/Getty Images

There are three main theories of how the Moon formed.

  • Giant impact theory — The Apollo astronauts brought back over 22 kilograms of rock and dust collected from the lunar surface. The samples revealed some striking similarities between Earth and the Moon, suggesting an almost identical chemical and isotopic composition.
  • Formed with Earth — In this scenario, as material bound together to form Earth, some material formed the Moon as well, and the smaller body wound up orbiting its larger companion
  • Capture theory — The Moon formed somewhere else beyond the Solar System. As it moved in, it was later captured by Earth’s gravity.

The giant-impact hypothesis is the most popular one among the three. The hypothesis suggests that the Moon formed from the ejected material of a major collision between a Mars-sized planetary body, known as Theia, and a young Earth, also known as the proto-Earth, right after our planet formed its initial crust. Both papers are variations on this scenario.

The debris left over from this impact collected in orbit around Earth, bound together through the force of gravity.

The authors behind the hit-and-run research argue that if the Moon was birthed from one giant impact, then it would have been composed mainly of the material from Theia. Instead, Earth and the Moon share a lot of the same composition.

"The standard model for the moon requires a very slow collision, relatively speaking," Erik Asphaug, a professor at the Lunar and Planetary Laboratory, said in a statement. "And it creates a moon that is composed mostly of the impacting planet, not the proto-Earth, which is a major problem since the moon has an isotopic chemistry almost identical to Earth."

But if a double impact would have occurred, then it would’ve mixed things up between the proto-Earth and Theia, resulting in the Moon’s composition.

The original idea behind Theia, before the hit-and-run scenario came out.

Why does Earth have a Moon?

Earth is not the only planet that formed or captured a Moon; moons are quite common throughout our star system.

Most planets in the Solar System have orbiting moons:

  • Mars has two small moons, Phobos and Deimos
  • Jupiter, the largest planet, has 79 moons
  • Saturn has a whopping 82 moons
  • Uranus and Neptune have 27 and 14 each, respectively
  • Several asteroids and dwarf planets also have moons. Pluto, for instance, has five.

Without the Moon, life on Earth would not have looked the same.

The Moon is in part responsible for Earth’s changing seasons. The impact that birthed the Moon may have also resulted in the planet’s tilt. This tilt gives Earth its changing seasons as parts of the planet tilts closer and further away from the Sun’s light and warmth.

The Moon still helps Earth maintain that tilt until today.

A 2018 study also suggests that 1.4 billion years ago, a day on Earth lasted for only 18 hours because the Moon was closer to our planet, affecting how Earth spun on its axis.

What is the Moon made of?

The surface of the Moon is scarred with impact craters from asteroids that crashed into it, ancient volcanoes, and dried up flows of lava.

Meanwhile, the crust of the Moon is made up of a rocky surface covered with regolith. The crust is about 60 miles thick, while the regolith is a thin layer of around 10 feet.

The analysis of the Apollo samples also suggested that volatile elements, such as carbon, no longer existed on the Moon, and that the Moon was ultimately dry. But the internal makeup of the Moon is still a bit of a mystery, and scientists believe it may be made of metallic iron, with some traces of sulfur and nickel.

What is the future of the Moon?

Earth and the Moon interact with one another, orbiting around a shared center of mass, but the Moon is moving away from Earth.

The reason why is due to the gravitational force the Moon exerts on Earth. The Moon’s gravity interacts with Earth’s oceans, causing the tides. As Earth rotates on its axis, the tidal bulge is slightly ahead of the Moon’s orbit around Earth. This causes some of the energy from Earth’s rotation to transfer to the Moon’s orbital motion, causing the Moon’s orbit to move slightly away from Earth.

The Moon is moving away from Earth at a rate of about 3.78 cm every year.

Humans are making their way back to the Moon soon with the Artemis mission, and this time, they plan to make the Moon a pit stop to further cosmic destinations. NASA plans to build a lunar base on the Moon’s surface, allowing astronauts to conduct research on the Moon and perhaps even use it as a launchpad for destinations like Mars.

But in the process, they’ll collect new materials that could unlock more insight into our lone natural satellite.

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