The composition of the Earth-Moon system indicates that the Moon probably formed from a collision between the proto-Earth and a Mars-sized body. That collision was incredibly violent, and left the Earth hot enough that its atmosphere would primarily consist of vaporized silicate rock. Once it solidified, those conditions would have left the planet very dry, with our current water largely delivered by smaller bodies that have impacted the Earth since. So far, only a single type of meteorite has been found to have hydrogen and oxygen isotopes that matched those found in the oceans. But researchers have now checked a comet derived from the Kuiper belt, and showed that it also is a good match for the Earth's oceans.

Most hydrogen comes in a form in which its nucelus consists of a single proton, but there's also an istope called deuterium that contains both a proton and a neutron. In the Earth's oceans, only about 1.6 in every 5,000 water molecules contain deuterium, so if we're looking for sources for our planet's water, we need to find bodies that have a similar ratio. We've looked at six comets that originate in the Oort cloud (the distant-most bodies associated with the Sun), and they have ratios about double that found on Earth. That left enstatite chondrites, a type of meteorite, as the best match for Earth's water.

Now, using the ESA's Herschel observatory, researchers have gotten a good reading on the comet 103P/Hartley 2, which orbits near Jupiter but probably got its start in the Kuiper belt, just outside the orbit of Neptune. And it turns out that the deuterium/hydrogen ratio is nearly an exact match for that in Earth's oceans. That means a large population of comets have just become candidates for seeding our planet with water.

That's the good part of the results, but there's a confusing part as well. The models of the dynamics of the early solar system indicate that we should see higher D:H ratios as we get further from the Sun, but 103P/Hartley 2 has a ratio that looks similar to that of the inner planets. The authors suggst that the best way to explain this is through a model in which material in the disk around the young Sun was more thoroughly mixed than we thought. We'll have to wait and see if the people who model the formation of the solar system agree.

Nature, 2011. DOI: 10.1038/nature10519 (About DOIs).