John P. Millis, Ph.D. for redOrbit.com – Your Universe Online
Despite being the nearest astronomical object to Earth, and the only extraterrestrial site of human visitors, the Moon still contains mysteries that have puzzled scientists for the better part of a century. Perhaps the greatest question surrounding lunar history is its two-faced nature.
Due to the mutual gravitational interaction of the Earth-Moon system, the giant satellite is tidally locked with our planet, so we always see the same side. The familiar Earth-facing surface is rife with features known as “maria” – placid regions created when subsurface lava oozed through the crust to fill in craters left by ancient impacts.
However, when probes first imaged the far side of the Moon some 55 years ago, astronomers quickly noticed a distinct lack of these maria. The only conclusion is that the far side of the Moon somehow was formed and evolved differently than the near side. But how is that possible?
“I remember the first time I saw a globe of the moon as a boy, being struck by how different the farside looks,” said Jason Wright, assistant professor of astrophysics at Penn State University, in a statement. “It was all mountains and craters. Where were the maria? It turns out it’s been a mystery since the fifties.”
Previous studies suggested that perhaps the Earth once had two Moons, but that early on the two collided and formed into one – the differences in the sides resulting from the different histories of the two Moons prior to their merger. Now, Dr. Wright and his colleagues Steinn Sigurdsson (professor of astrophysics) and Arpita Roy (graduate student in astronomy and astrophysics) believe they have settled the problem once and for all.
The general consensus is that the Moon was formed when a Mars-sized planetoid collided with the Earth shortly after it formed. The glancing blow would have vaporized some of the Earth’s material, as well as that of the planetoid, but some of Earth’s outer structure would have been ejected into outer space. Our planet’s gravity, though, would have pulled much of it into orbit around us, eventually forming what we now know as the Moon.
But rather than looking like it does now, the Moon (as well as the Earth) would have been much hotter, and much closer together. In fact, the Moon would have been 10 to 20 times closer to Earth than it is now. “The moon and Earth loomed large in each others skies when they formed,” said Roy.
At this distance, the nearly 2,500 degree (Celsius) Earth would have warmed the side of the Moon – which would have immediately become tidally locked – closest to us. As the far side of the Moon cooled, the near side would have remained warm, heated by the Earth, which retained its temperature much longer due to its larger size.
As the Moon was subsequently impacted, the near side would have seen the crater sites filled in by the molten lava lurking beneath the still heated surface, while the far side the crust would have become too thick for the asteroids to penetrate. The result is a far side of the Moon covered in valleys, craters and highlands, but virtually no maria.
Results of this research are published in Astrophysical Journal Letters.
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