A new study by scientists at the University of California Santa Cruz suggests that the Earth may have had more than one moon over four billion years ago. According to the study, a collision between the moon we know today and a smaller satellite occurred, leaving the far side of the moon a rocky, mountainous region. Researchers used computer simulations to study the “giant-impact” theory that is used to explain the creation of the moon.
The study, co-authored by planetary scientists Erik Asphaug and Martin Jutzi and published in the journal Nature, posits that the rugged and mountainous dark side of the present-day moon was formed by a collision with a smaller moon. Previous theories concerning the moon’s formation and existence have suggested that upon the moon’s creation, it either absorbed other celestial bodies or ejected them into interstellar space. The study states that the second moon would have been formed in the same way the present-day moon was formed- by debris that was ejected after a protoplanet collided with the Earth.
The topographical differences between the near and far sides of the moon have been a mystery to scientists for years. The near side of the moon, which is visible to the earth, is relatively flat, while the far side of the moon is much more mountainous, with a crust fifty kilometers thicker than the near side. The study posits that the existence of a second moon is accountable for the far side of the moon’s geographical landscape.
Using a computer model, scientists have determined that the second moon orbiting the earth would have been approximately 750 miles wide, with a mass one-thirtieth of the moon we know today. The theory is that the smaller moon crashed into the bigger moon, causing the dark side of the moon to have a thicker crust with more mountains.
Scientists believe that the impact between the moons would have been a somewhat slow collision, due to the fact that they were in the same orbit, at approximately 5,000 miles per hour. Such a slow collision would mean that instead of an impact crater, the lunar material from the smaller moon would have covered the dark side of the bigger moon. In the hours after the collision, the smaller moon would have been reduced by gravity to a thin layer on the moon’s existing crust.
Asphaug and Jutzi suggest that tidal forces from Earth could have played a part in orchestrating the collision. The research suggests that the tidal forces would have caused the moons to migrate outwards over the course of tens of millions of years. Once they got to a certain point, the gravity of the sun would begin to influence each moon’s orbit, eventually causing a collision.
To study the details of the collision, scientists used a computer to simulate the impact of both moons. The simulation also allowed the researchers to study the aftermath of the collision, including the remaining lunar material. The researchers have come to the conclusion that the material from the second moon, upon impact, created a new layer of solid crust, which formed a mountainous region on the moon.
Using the same computer model, scientists have been able to further study the variations in the composition of the moon’s crust. The crust on the near side of the moon contains mostly potassium (K), rare-earth elements (ree) and phosphorous (P), which are known together as KREEP. The computer-model research suggests that the elements would have remained concentrated together in lunar magma as it crystallized before the aoon cooled. The researchers suggest that the most likely cause of this is an impact between the moon and another celestial body.
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