What Happened to Our Other Moon?

New Theory Sheds Light on Our Solar System’s History

by Alex Lee, age 14

    About 4.5 billion years ago, a Mars-sized asteroid collided into the newly molten Earth. The powerful impact threw scattered debris into space. Over millions of years this rubble formed a roughly-shaped sphere that started to orbit the Earth. Today, we call this sphere Earth’s moon.
   We know that the moon’s craters were created about half a billion years later (approximately four billion years ago) when a cluster of asteroids crashed into it. Explanations are still unclear, however, about how the moon became lopsided and why its rocky far side looks so much different from the smooth side that is visible from Earth.
While the moon might seem round to us, in fact, it bulges on one side. The side we can’t see is called the far side. And the far side of the moon is packed with high, jagged mountains.
   Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, thinks he has discovered why. He says that the Earth may have once had two moons. Asphaug theorizes that eventually these two moons crashed into each other, forming our moon’s current distinct physical characteristics.
   According to Asphaug, when that Mars-sized asteroid crashed into the Earth over four billion years ago, it also created a smaller lunar body called a moonlet. Asphaug and other researchers now think this smaller moon, or moonlet, was formed and existed in a gravity-neutral area known as a Lagrangian point.
   Lagrangian points are places where the gravity of two space bodies cancel each other out, in this case our Earth and our moon. In such areas, space debris can form lunar bodies that typically remain stable for a relatively short period of time. The sun’s gravity eventually exerts enough force to throw them from the region. Theoretically, space fragments can be attached around one of many of the moon’s Lagrangian points.
   At least, that is what current research suggests. To explain this hypothesis in more detail, Asphaug and colleague Martin Jutzi of the University of Bern in Switzerland simulated this process. They designed a model of a moonlet one-third of the moon’s width orbiting the Earth.
   In this simulation, the moonlet remained in a Lagrangian point for millions of years. As both moons continued to orbit the Earth, the moonlet was thrown from its Lagrangian point into the other moon’s path.
The collision between the moonlet and Earth’s moon would have happened at relatively slow speeds of about 4,500 and 6,700 miles per hour, according to the calculations of scientists. Therefore, the moonlet splattered like mud onto the larger moon, giving our moon its high, jagged, mountainous surface and lopsided shape.
   This new research reveals new evidence that might help explain one of the great mysteries of our solar system. It also provides a more detailed theory on how Earth’s moon was formed. But what many find most fascinating is that it just might explain the unusual and mysterious characteristics of our planet’s strangely shaped moon.


[Sources: Los Angeles Times; Associated Press; Atlas of the Universe]    


Fascinating article! I had no idea why the moon looks so strange. Nice work, Alex! – Mandy KroningerMadison (2012-01-17 08:13)
Great Article.! I Just Learned Something New About The Moon(: – Patricia Cazares (2012-01-17 16:20)
Wow I never knew Earth had two moons in the past! Great article, Alex! :) – Pallav RegmiMadison, WI (2012-01-17 16:24)
Who knew! I sure didn't! Thanks for sharing this article :) – VanessaMadison (2012-01-19 13:14)
Thanks for writing such an intriguing article, Alex! Keep up the good work! – Bri WilsonPiran, Slovenia (2012-01-26 04:08)
another great article, your reporting skills show that you really like to research and share information. 2012-01-28 – isabel hubbardmiddleton, wisc. (2012-01-28 17:22)
I never knew that the earth had a second moon, that would certainly make a lot of sense now! – SpenserMadison (2012-01-31 19:30)
I can't believe the Earth used to have two moons! How interesting!! – JordanMilwaukee, WI (2012-05-31 19:13)
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