It was thought that one source of the water that is found on Earth was that it arrived via comets. But the Rosetta probe that has been orbiting comet 67P since August (and whose lander Philae came to rest on the surface in November) had remote sensing devices and found signs of water there but scientists who studied the data say that this water seems to be from a different source than the water on Earth.

My first reaction to that news was how would they know this? After all, isn’t water just H₂O and the same everywhere?

The answer is, of course, that the symbols H and O for hydrogen and oxygen do not specify what the particular isotopes involved are. In the case of hydrogen, H is usually assumed to be a nucleus of just a proton but one has deuterium (D) atoms as well that consist of a nucleus of a proton and a neutron. On Earth, 0.0156% of hydrogen atoms contain deuterium atoms.

When the scientists studied the D/H ratio for the water on the comet, they found a value that is roughly three times that on Earth and they conclude that this precludes the possibility that it is Earth-like water. (You can read the paper here.)

Different comets seem to have different D/H ratios. The lead author Kathrin Altwegg, from the University of Bern in Switzerland, says:

“We have light water in some comets and very heavy water in other comets. We have to assume the mixture of all these comets is something that is heavier than what we have on Earth, so this probably rules out Kuiper Belt comets as the source of terrestrial water.”

Instead, she thinks that asteroids – dense, rocky objects that formed closer to the Sun than comets – seeded our oceans.

She said: “We know already something about the characteristic of asteroids by studying meteorites, which are pieces of asteroids – and the characteristics of asteroids are very much like our water.

“They are also much closer to the Earth, so it is more likely that they hit the Earth than the very distant comets, which are beyond Neptune.”

But there are other theories for our water’s origins than that they were deposited on the dry surface of the Earth by comets or asteroids. The water molecules could have been made deep underground from the time of the Earth’s formation and later slowly seeped to the surface.

Central to the study is the idea that rocks that appear dry to the human eye can actually contain water–in the form of hydrogen atoms trapped inside natural voids and crystal defects. Oxygen is plentiful in minerals, so when a mineral contains some hydrogen, certain chemical reactions can free the hydrogen to bond with the oxygen and make water.

Stray atoms of hydrogen could make up only a tiny fraction of mantle rock, the researchers explained. Given that the mantle is more than 80 percent of the planet’s total volume, however, those stray atoms add up to a lot of potential water.

One can distinguish the sources of water within the Earth by the presence of different types of minerals in it.

So water, seemingly so simple, is actually quite an intriguing molecule.