Science Focus
original post »During its early history, the Earth was struck by enough material falling from space to have boiled off its oceans—and that's ignoring the fact that the entire surface of the planet was vaporized by the collision that formed the Moon. This means that Earth wasn't in any condition to look like the blue marble we see today, with abundant oceans and an atmosphere.
So where did all the water come from? Once the pace of collisions slackened, each impact was able to deliver material to the Earth that wasn't immediately boiled off by the next one. Over time, objects returned the Earth to its watery state. But scientists have been debating over which objects for quite some time, with the pendulum swinging back and forth between asteroids and comets. Now, thanks to Rosetta's visit to comet 67P/Churyumov-Gerasimenko, asteroids are looking like a better bet.
Tracing the origin of water isn't as simple as running the numbers on a license plate. But the Solar System provides a helpful clue in the form of deuterium, a heavier isotope of hydrogen. Closer to the Sun, processes seem to have led to an exchange between deuterium and hydrogen, leaving those regions with a relatively low abundance of the heavier isotope. More distant bodies, which received less radiation from the Sun, have much more deuterium than bodies in the inner Solar System. In general, it appears that there was a nearly linear increase in deuterium as distance from the Sun increased.
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