History Of The World: The Prequel

by Christopher Paul on February 27, 2011


Earth – For Physicists is a fantastic piece on how the Earth and Moon were formed billions of years ago. Don’t let the title fool you, though, its a pretty easy read even if you aren’t a science geek. Included are some computer simulations like the one above and some artist renderings that help you visualize what might have happened.

But the whole story of how the Earth and Moon came to be is so intriguing, I started surfing Wikipedia and read up on some of the subjects introduced in this article. Some of my most favorite parts are:

On the formation of the moon:

…the giant impact theory seems to fit the data best. People take it so seriously that the hypothetical doomed planet that hit Earth even has a name: Theia. In Greek mythology, Theia was a titan who gave birth to the Moon.

In 2004, the astrophysicist Robin Canup, at the Southwest Research Institute in Texas, published some remarkable computer simulations of the Big Splat. To get a moon like ours to form — instead of one too rich in iron, or too small, or wrong in other respects — she had to choose the right initial conditions. She found it best to assume Theia is slightly more massive than Mars: between 10% and 15% of the Earth’s mass. It should also start out moving slowly towards the Earth, and strike the Earth at a glancing angle.

The result is a very bad day. Theia hits the Earth and shears off a large chunk, forming a trail of shattered, molten or vaporized rock that arcs off into space. Within an hour, half the Earth’s surface is red-hot, and the trail of debris stretches almost 4 Earth radii into space. After 3 to 5 hours, the iron core of Theia and most of the the debris comes crashing back down. The Earth’s entire crust and outer mantle melts. At this point, a quarter of Theia has actually vaporized!

After a day, the material that has not fallen back down has formed a ring of debris orbiting the Earth. But such a ring would not be stable: within a century, it would collect to form the Moon we know and love. Meanwhile, Theia’s iron core would sink down to the center of the Earth.

And on how the Moon’s craters were formed:

During the Late Heavy Bombardment, the Moon was hit by 1700 meteors that made craters over 100 kilometers across. The Earth could easily have received 10 times as many impacts of this size, with some being much larger. To get a sense of the intensity of this pummeling, recall the meteor impact that may have killed off the dinosaurs at the end of the Cretaceous period 65 million years ago. This left a crater 180 kilometers across. Impacts of this size would have been routine during the Late Heavy Bombardment.

Later, it takes about how the Earth’s atmosphere was born and how oxygen was released into it. Finally the article talks about the Earth’s glacial cycles and gives hints as to what spawned multicellular organisms.

If you’re interested in the origins of the Earth, Moon, and how life came to be, definitely read this article.

The Earth – For Physicists – John Baez, UC Riverside via Kottke

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