“Solar rainstorm” filled the first oceans

Astrogeologists have been a-scratchin their egg-shapes for a good few years now about the origin of the oceans. Where did all that wet stuff come from?

This is how they are workin it out: they look for somewhere else in the solar system that has water with a similar make up and assume ours came from the same place. The thing to look for, say them astroeggs, is the isotopic make up of the water: the ratio of hydrogen to deuterium.

It turns out there are three sources of water in the solar system. Comets carry water which has about twice the ratio of hydrogen to deuterium. Asteroids of the carbonaceous chondrite variety also contain water which has the same ratio as Earth’s oceans. And the solar nebula, the disk of gas and dust from which the solar system formed, has water in which the ratio is 7 times less than on Earth.

On that basis, the answer is obvious: the oceans musta come from chunks of carbonaceous chondrites. The thinkin’ is that these chunks clumped together as the Earth formed, generating heat that evaporated any water they contained. When everything cooled down, this condensed to form the oceans.

Not so fast, cowboy, says Hidenori “Hiddena” Genda at the Tokyo Institute of Technology. He and a buddy have dashed off some impressive analysis of the Earth’s early atmosphere which is thought to have been rich in hydrogen.

Now hydrogen escapes into space more readily than its heavier isotopic cousins which woulda left the atmosphere rich in deuterium. When that happened, deuterium musta been pumped into the oceans, dramatically increasing the ratio of hydrogen to deuterium.

But by how much? Hidden Agenda says that if the oceans came from the solar nebula, this process would have increased the ratio of hydrogen to deuterium in sea water to just what we see today.

This ain’t conclusive but it is compelling. If correct, it means the oceans are the result of a monstrous solar rainstorm that musta left Earth a-rinsed and awash at least 3.8 billion years ago after it passed through a giant cloud of the wet stuff. Kinda biblical, doncha think?

Ref: arxiv.org/abs/0709.2025: Origin of the Ocean on the Earth: Early Evolution of Water D/H in a Hydrogen-rich Atmosphere

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