Excerpt from slate.com

WE CALL Earth a water , and that’s pretty fair: Our planet’s surface is 70 per cent covered in it, it makes up a percentage of our air, and there’s even a substantial amount of it mixed in to the planet’s mantle, deep underground. 
But where the heck did it come from?

This is no idle . We have a lot of water here, and it must have come from somewhere. There are two obvious source — it formed here along with the Earth, or it was brought to Earth from space. Which is the dominant source has been a topic of long and heated debate among astronomers.

The first big science results have just been announced by the European science team working with the Rosetta probe, and, in my opinion, they throw more gasoline on the fire. Measurements made by the probe show that comets like 67P/Churyumov — Gerasimenko — the one Rosetta is orbiting — couldn’t have been the source of our water.

But that hardly helps the underlying question! Why not? Ah, the details …

When the Earth formed 4.55 billion years ago (give or take), there was a lot of water in the disk of material around the Sun. Close in to the Sun, where it was , that water was a gas, and farther out it formed ice. We see that latter part echoed down through time now in the form of icy moons around the outer planets.

You’d expect water collected on Earth along with everything else (metals, silicates, and so on). When the Earth cooled, a lot of that water bubbled up from the interior or was outgassed by volcanism.

Where does water come from?
Where does water come from? Source: Getty Images

But we have another big source, too: comets. These are dirty snowballs, rock and dust held together by water frozen as ice. They formed farther out in the , where ice was more plentiful. Long ago, just a few hundred million years after Earth formed and started to cool, there was a tremendous flood of comets sent down into the inner , disturbed by the gravitational dance of the outer planets as they settled down into their orbits. This Late Heavy Bombardment, as it’s called, could have supplied all of Earth’s water.

How to tell? Well, it turns out that in this one case, hipsters are right: Locally sourced is measurably different than stuff trucked in.
Water is made up of one oxygen atom and two hydrogen atoms. Hydrogen atoms, it so happens, come in two flavours: The normal kind that has single proton in its nucleus, and a heavier kind called deuterium that has a proton and a neutron (there’s also tritium, with two neutrons, but that’s exceedingly ). Deuterium is far more than the normal kind of hydrogen, but how depends on what you look at. The ratio of deuterium to hydrogen in Earth’s water can be different than, say, water in comets, or on Mars.
Note I said, “can be”. We know the ratio differs across the solar system. But suppose we find the same ratio in comets as we do on Earth. That would be powerful evidence that water here began out there. Astronomers have looked at a lot of comets trying to pin down the ratio, and what they’ve found is maddening: Some comets have a ratio very different from Earth’s, and only one (103P/Hartley 2) has a ratio similar to ours.

Jets of material — including water — emanate from comet 67P/Churyumov — Gerasimenko.
Jets of material — including water — emanate from 67P/Churyumov — Gerasimenko. Source: AP

Now that’s interesting: 103/P is a Jupiter-family comet, meaning it used to orbit the Sun far out, but dropped into the inner solar system, got its orbit modified by Jupiter, and now has a much shorter path that keeps it in the inner solar system.
Rosetta’s comet, 67/P, is also a Jupiter-family comet. You’d expect them to have roughly similar deuterium/hydrogen ratios.

They don’t. 67/P, according to Rosetta, has three times the deuterium per hydrogen atom as Earth (and 103/P).
What does that mean? It’s not clear, which is why this is maddening. It could be simply that not all Jupiter-family comets have the same ratio; they may all have different origins (born scattered across the solar system, so with different D/H ratios), but now belong to the same family. Or it could mean that 67/P is an oddball, with a much higher ratio than most other comets like it. That would seem unlikely, though, since we’ve studied so few you wouldn’t expect an oddball to be found so easily.

Making things more complicated, some asteroids in the main belt between Mars and Jupiter have water on them, and it appears to have an Earth-like D/H ratio. But we think they have so little water that it would take a lot more of them impacting the early Earth to give us our water than it would comets. That’s possible, but we know of comets hit us back then, so it’s still weird that the D/H ratios don’t seem to work out. Still, it’s nice that there could be another potential source to study, and this Rosetta result does lend credence to the idea that asteroids did the wet work.

So what do comets have to do with it?
So what do comets have to do with it? Source: Getty Images

So if you ask where Earth’s water come from, the answer is: We still don’t know…