Tuesday, March 15, 2016

Periods of Prehistory: Hadean



 No book featuring time-travel into prehistory is complete without a basic understanding of how the Earth has changed over its 4.5 billion year lifespan. Today, we’re going to talk about the earliest epoch in Earth’s history—the Hadean!


In case the name didn’t give it away, the Hadean was not a particularly fun place to be. Long before life, and after the Earth formed out of a swirling mass of rock and stardust in the milky way, the surface was formed of a mix of molten rock and magma as volcanism was constant.  The earth was forming, so heavier elements, like iron, sank down and became the Earth’s core, while lighter ones like silica began to form the crust. Gases, like hydrogen and methane, escaped into space, because there wasn’t much of an atmosphere.

And of course, the Earth was being constantly bombarded by meteorites and debris. Getting hit by huge comets is thankfully rare nowadays, but back then it was common enough to contribute to the Earth’s very structure. Water ferried along by comets boiled as it hit the ultra-hot Earth, and eventually this formed an atmosphere made up of steam.

This was all around 4.5 billon years ago, when another very important comet hit Earth. Only this wasn’t a comet—it was a planetoid the size of Mars, called Theia. The resulting strike looked a little like this:



The result? The moon!

Indeed, the moon is thought to be formed out of a combination of bits of Earth ripped off the early forming Earth and the remnants of the planetoid that struck it. The moon is closer to home than we think.

The Hadean closed out when the constant impacts began to slow and the volcanism cooled down. With the cooling came the falling of the steam atmosphere as rain, forming the oceans. Plate tectonics, or the movement of the continents, began, and the Earth’s crust finished forming, with the earliest rocks like granite and quartz. And according to a few researchers, a few proteins began to coalesce, forming the precursors to life. 

No one from the Sixth Event visits the Hadean--that would be a very short trip. But knowing a little about the earliest Earth is still fascinating. 

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