Our planet's lopsided moon may have swallowed up its smaller sibling after a collision, which may have resulted in its asymmetric terrain, scientists propose.

Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, and his colleague Martin Jutzi recently wrote an article about the creation of the moon, which is published in the journal Nature.

The scientists believe a second moon once orbited the Earth, and during its youth, slowly collided with its bigger sibling, leaving small traces that forever shaped our current moon.

It is known that the visible side of the moon is comprised of low-lying plains, and that its farside is high and mountainous. But the difference in our planet's moon goes beyond its outer layer, for the crust on the farside is about 50 kilometers thicker than that on the nearside.

Planetary scientists call this collision the "big impact" and believe that it resulted in debris pancaking the bigger moon. The smaller moon was the lightweight when compared to its sibling that was three times wider and 25 times heavier with a strong gravity.

"They're destined to collide. There's no way out. ... This big splat is a low-velocity collision," Asphaug told CBS.

Asphaug said this slow crash happened at more than 5,000 miles per hour, and that the smaller moon was more than 600 miles wide. It took hours for the rocks and crust from the smaller moon to spread over and around the bigger moon without creating a crater, and a faster crash would've caused the opposite.

"By definition, a big collision occurs only on one side," Asphaug told Nature, "and unless it globally melts the planet, it creates an asymmetry."

Many theories on how the asymmetry happened have been offered, but no solid conclusion has been made.

Planetary scientist Alan Stern, former NASA associate administrator for science, told CBS that the theory is a "very clever new idea," but one that is not easily tested to learn whether it is right.

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An artist's conception of two moons that could have collided to form into one moon. IBT
Four snapshots from the computer simulation of a collision between the moon and a smaller companion moon
Four snapshots from the computer simulation of a collision between the moon and a smaller companion moon show most of the companion moon is accreted as a pancake-shaped layer, forming a mountainous region on one side of the moon. M. Jutzi and E. Asphaug, Natur
Earth039s Moon
During its mission, the Galileo spacecraft returned a number of images of Earth's only natural satellite. Galileo surveyed the moon on Dec. 7, 1992, on its way to explore the Jupiter system in 1995-1997.This color mosaic was assembled from 18 images taken by Galileo's imaging system through a green filter. On the upperleft is the dark, lava-filled Mare Imbrium, Mare Serenitatis (middle left), Mare Tranquillitatis (lower left), and Mare Crisium, the dark circular feature toward the bottom of the mosaic. Also visible in this view are the dark lava plains of the Marginis and Smythii Basins at the lower right. The Humboldtianum Basin, a 400-mile impact structure partly filled with dark volcanic deposits, is seen at the center of the image. Image Credit:
South Pole -- Aitken Basin
This LOLA image centers on the South Pole-Aitken (SPA) basin, the largest impact basin on the Moon (diameter = 2600 km), and one of the largest impact basins in the Solar System. The distance from its depths to the tops of the highest surrounding peaks is over 15 km, almost twice the height of Mount Everest on Earth. Image credit: NASA/Goddard
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