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Physicist Rob Sheldon on Earth, comets, and water

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Study hints that ancient Earth made its own water -- geologically
mantle circulation delivers new rock, possibly water, to the crust via mid-ocean ridges/Byrd Polar and Climate Research Center

From Phys.org:

A new study is helping to answer a longstanding question that has recently moved to the forefront of earth science: Did our planet make its own water through geologic processes, or did water come to us via icy comets from the far reaches of the solar system?

The answer is likely “both,” according to researchers at The Ohio State University— and the same amount of water that currently fills the Pacific Ocean could be buried deep inside the planet right now.

This follows a recent story: Rosetta finding: Earth’s water not from comets (More likely, authors say, from asteroids)

Some background thoughts from physicist Rob Sheldon:

It is thought that the Oort Cloud, a trillion or so comets out beyond the orbit of Pluto, has a combined mass of two or so Earths. And of course, the oceans are the top 0.01% of the earths volume, so they don’t come close to being an appreciable component of the Earth’s mass.

In the standard model of the Big Bang, which I’m not happy with, some 90% of the matter is hydrogen, 10% helium, less than 1% lithium, and the rest, including oxygen, is too small to matter. Water, as you are aware, is 2 H and an O. So to get water, stars have to burn the hydrogen to helium, then helium to carbon, then carbon to nitrogen and oxygen, at which point they go unstable and explode. Some stars go through smaller explosions that shed the outer layers without disintegrating. But one way or the other, stars send their ash out as dust and large dust clouds intermix with molecular H clouds, which presumably is where the water comes from.

As water goes through the hydrologic cycle on earth, the D/H ratio does change. That’s how the Germans were making heavy water over at a Norwegian hydroelectric plant before commandos destroyed it in 1943–by electrolysing the water. That’s why early theories about the higher D/H ratio of ocean water than the sun theorized that the Earth’s oceans had been half boiled away.

The point about comets, is that they have never melted, never electrolyzed, never gone through the phase changes that are sensitive to weight. That’s what a Science article last summer was about–primordial D/H ratios observed in icy comets surrounding a distant star that was building its own solar system. The technique argued that the comets were about 100K, about the temperature of liquid nitrogen, and had never melted.

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