The moon and Earth were very hot shortly after the giant impact that formed the moon. The moon, being much smaller than Earth, cooled more quickly. Since the moon and Earth were tidally locked early on, the still-hot Earth — more than 4,530 degrees Fahrenheit (2,500 degrees Celsius) — would have cooked the near side of the moon, keeping it molten. On the other hand, the far side of the moon would have cooled, albeit slowly.
The difference in temperature between the moon's halves influenced the formation of its crust. The lunar crust possesses high concentrations of aluminum and calcium, elements that are very hard to vaporize.
"When rock vapor starts to cool, the very first elements that snow out are aluminum and calcium," study co-author Steinn Sigurdsson of Penn State said in a statement.
Aluminum and calcium would have more easily condensed in the atmosphere on the colder far side of the moon. Eventually, these elements combined with silicates in the mantle of the moon to form minerals known as plagioclase feldspars, making the crust of the far side about twice as thick as that of the near side.
"Earthshine, the heat of Earth soon after the giant impact, was a really important factor shaping the moon," Roy said.
When collisions from asteroids or comets blasted the moon's surface, they could punch through the near side's crust to generate maria. In contrast, impacts on the far side's thicker crust failed to penetrate deeply enough to cause lava to well up, instead leaving the far side of the moon with a surface of valleys, craters and highlands, but almost no maria.
"It's really cool that our understanding of exoplanets is affecting our understanding of the solar system," Roy said.
Future research could generate detailed 3D models testing this idea, Roy suggested. The authors detailed their findings June 9 in the Astrophysical Journal Letters.
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