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We've gone back to the beginning of geologic time; indeed, before the beginning. Welcome to the Hadean Eon, the time before land: the Hadean includes all of Earth's history before the formation of the oldest surviving rocks, and it lasts until 3.8 billion years ago. The name Hadean derives from the Greek name for Hell, and if ever Earth was a Hell-world, it's now.

It's about 4.6 billion years ago. The Earth has formed as rock and dust in the solar nebula join together through collisions and gravitational interactions. The young planet is a fluffy, homogenous melange of metals and silicates, still chilled from the cold of space; but now it sets about the important business of compression and differentiation. Gravity pulls the rock together, increasing the density and shrinking the diameter, the incredible pressure generating heat; enough heat that, combined with the energy of still frequent meteorite collisions, and the heat of short-life radioactive isotopes decaying, the rock begins to melt. The heavy metals, iron and nickel, sink through the rock to gather at the center; the lighter rocks, mostly mafic silicates, float to the surface, still largely molten. We begin to see a metallic core and a separate mantle, and the currents of molten metal begin to generate a planetary magnetic field.

By 4.4 billion years ago, the crust, still semi-molten, is an ocean of dark iron and magnesium silicates. Light felsic rocks float above the pyroxenes and basalts. An atmosphere begins to fizz out of the rock, outgassing from vents and volcanoes: carbon dioxide, water vapor, nitrogen, carbon monoxide, methane, ammonia, hydrochloric and sulfuric acid. Acidic water condenses out of the sky to form scalding, caustic oceans, heavy with dissolved ions and strangely combined molecules.

Is there life in this ocean Acheron? Maybe. All the necessary elements are there; water; energy; and the amino acids and complex, self-replicating organics that form easily and naturally in such an environment. It indeed would be more a miracle if there isn't life here than if there is, spontaneously creating itself again and again.

But whatever life may be here will not be our ancestors. The solar system is far from quiet, and giant meteorites still strike the earth regularly. These are not wussy Chicxulub-scale collisions; when a rock the size of Texas hits a planet, it brings with it enough energy to liquefy the entire crust. Thus the reason no rocks have survived from the Hadean to our present.

And when a Mars-sized rock bangs into the planet, it rips and shatters and melts the Earth's crust and mantle on a huge scale. A new body coalesces from the debris: Earth gets a baby sister, our Moon, and the two begin to orbit around a common center. Could any life last through that?

Slowly, though, Earth's orbit clears of debris, much as the arena floor clears in a demolition derby. Sometime before 3.6 billion years ago (this number fluctuates wildly as geologists argue over dating techniques) the last major collision occurs. Rocks begin to form which will still be here for present-day geologists to analyze: we have left the Hadean, and must move on to the Archean Eon.

Sources: Levin, Harold. The Earth Through Time. John Wiley and Sons, Inc.: 2003
Notes from Dr. Holtz's Geol120 lecture
Displays at the National Museum of Natural History

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