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CosmosUp | August 16, 2022

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Early Earth may Once have had Two Moons, Astronomers Say

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Early Earth may Once have had Two Moons, Astronomers Say

Earth may have once looked a little more like Mars: a planet with two moons. It’s not the first time the theory has been trotted out by Erik Asphaug, but it’s getting new life thanks to an upcoming conference about our natural satellite. Asphaug says the second moon would have been much tinier—about 1/30th the size of our moon. But after orbiting Earth for a few million years, it would have had quite the impact, literally, on its larger twin.

The astronomers came up with this scenario to explain why the moon’s far side is so much more hilly than the one that is always facing Earth. The theory, outlined in a research paper published Wednesday in the journal Nature, comes complete with computer model runs showing how it would happen and an illustration that looks like the bigger moon getting a pie in the face.

Outside experts said the idea makes sense, but they aren’t completely sold yet.

This all supposedly happened about 4.4 billion years ago, long before there was any life on Earth to gaze up and see the strange sight of dual moons.

The moons themselves were young, formed about 100 million years earlier when a giant planet smashed into Earth. They both orbited Earth and sort of rose in the sky together, the smaller one trailing a few steps behind like a little sister in tow.

Earth’s second moon The second moon around Earth would have been about 750 miles (1,200 kilometers) wide and could have formed from the same collision between the planet and a Mars-sized object that scientists suspect helped create the moon we see in the sky today, astronomers said.

To imagine where this other moon once was, picture the Earth and the moon as being two points in a triangle whose sides are equal in length.

The other point of such a triangle is known as a Trojan point, or a Lagrangian point, named after the mathematician who discovered them. At such a point, the gravitational attraction of the Earth and moon essentially balances out, meaning objects there can stay relatively stably. The Earth and moon have two Trojan points, one leading ahead of the moon, known as the L-4 point of the system, and one trailing behind, its L-5 point.

The researchers computed that this second moon could have stayed at a Trojan point for tens of millions of years. Eventually, however, this Trojan moon’s orbit would have destabilized once our moon’s orbit expanded far enough away from Earth.

The resulting collision would have been relatively slow at 4,500 to 6,700 miles per hour (7,200 to 10,800 kph), leading its matter to splatter itself across our moon as a thick extra layer of solid crust tens of miles thick instead of forming a crater.

“The physics is really surprisingly similar to a pie in the face,” Asphaug said.

And about a day later, everything was settled and the near and far sides of the moon looked different, Asphaug said.

Earth Has a Second Moon

Earth with two moons Beyond this “story”, some astronomers believe that Earth actually have 2 moons. Cornell University’s Mikael Granvik, Jeremie Vaubaillon and Robert Jedicke have calculated the population of “irregular natural satellites that are temporarily captured” by Earth.

They say these secret moons come and go quietly and without notice, but at least once we detected one. In fact, their calculations were confirmed by this observation: a mysterious titanium white object that was discovered rotating around Earth by the Catalina Sky Survey in Arizona, back in 2006.

That object was actually a small asteroid captured by Earth’s gravitational field. It rotated as a second moon until June 2007, when it left our home planet’s orbit. This study demonstrates that, even while they are not easily detected, these little moons come and go often, staying around for about ten months to about three spins around Earth and then wave goodbye.

A second moon isn’t just an astronomical matter. The moon plays a big role in literature and song. A poet Todd Davis, a professor of literature at Penn State University, said this idea of two moons will capture the literary imagination.

“I’ll probably be dreaming about it and trying to work on a poem,” Davis said.



  1. Gregg Weber

    Look at a tree from the top. First there are many small branches joining to form large branches. Finally there are just two which combine in the trunk.
    The moon could be like that or some of the smaller “branches” might have taken longer to crash into it.
    Yes, with 2 there are more chances of hitting what would become the moon, but with 2 each has less gravity than one big moon to attract.
    I wonder how many craters came from the collision and how many from elsewhere.

  2. Steve Stark

    Curious, did the other moon have water and oxygen…able to support life?

  3. Steve Stark

    If the other moon had supported life, interesting thought. A jump start on earth if it had…landing craters here? Could anything helpful have survived?

  4. bbrao basina

    The moon on the far side is much more hilly than the one facing the earth could be not due to collision. Some part of the earth got separated from earth when it was hot & semi solid form. This part got revolving around the earth in the same hot, semi solid form but somehow only one part facing the earth. In this process centrifugal forces played a crucial role to form hilly areas on the other side of the moon which was then in semi-solid state.
    Just as the earth has hot lavas in the core of the earth, there could be in the moon core too. But being much smaller than earth, the moon got cooled earlier than earth. Therefore no volcanoes on moon. But core could be hotter. The time gradients for all these steps could be in millions of years

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