During its early history, the Earth was struck by enough material falling from space to have boiled off its oceans—and that's ignoring the fact that the entire surface of the planet was vaporized by the collision that formed the Moon. This means that Earth wasn't in any condition to look like the blue marble we see today, with abundant oceans and an atmosphere.

So where did all the water come from? Once the pace of collisions slackened, each impact was able to deliver material to the Earth that wasn't immediately boiled off by the next one. Over time, objects returned the Earth to its watery state. But scientists have been debating over which objects for quite some time, with the pendulum swinging back and forth between asteroids and comets. Now, thanks to Rosetta's visit to comet 67P/Churyumov-Gerasimenko, asteroids are looking like a better bet.

Tracing the origin of water isn't as simple as running the numbers on a license plate. But the Solar System provides a helpful clue in the form of deuterium, a heavier isotope of hydrogen. Closer to the Sun, processes seem to have led to an exchange between deuterium and hydrogen, leaving those regions with a relatively low abundance of the heavier isotope. More distant bodies, which received less radiation from the Sun, have much more deuterium than bodies in the inner Solar System. In general, it appears that there was a nearly linear increase in deuterium as distance from the Sun increased.

The amount of deuterium in Earth's oceans seemed to fit with this simple trend, which suggests it got its water from bodies that had formed nearby. This pointed the finger at asteroids, as comets only formed in the outer regions of the Solar System, where the amount of deuterium is likely to be much higher. Measurements of material from the tail of Halley's comet seemed to support this.

But measurements of two comets that orbit near Jupiter (103P/Hartley and 45P/Honda–Mrkos–Pajdušáková) showed deuterium levels that are similar to those of Earth's oceans, which caused some scientists to revisit the idea that comets could have delivered the Earth's water.

This is where Rosetta, and more specifically its ROSINA instrument, comes in. Rosina is a mass spectrometer (that is, it separates chemicals based on their mass) sensitive enough to detect the difference between the isotopes of hydrogen, as well as different oxygen isotopes, in water molecules. When Rosetta first emerged from hibernation, ROSINA was able to detect low levels of water still evaporating off the spacecraft 10 years after it left Earth, along with a bit of propellant and some of the grease used on the hardware.

Once Rosetta got within 100 kilometers of the comet, however, material from 67P/Churyumov-Gerasimenko began to dominate. As it did, the amount of deuterium went up, reaching levels three times that of Earth's oceans—making it hard to see how the oceans could have been created by a body like this.

This leaves us in a rather confused position. It's possible that the population of comets contains bodies with different sites of origin within the Solar System and thus different amounts of deuterium. Alternatively, one or more of the comets we've sampled could be oddballs and not representative of the population as a whole. Or it could be time to revisit the idea that asteroids acted as waterboys for the early Earth.

The one thing that's clear is that Rosetta's instruments started delivering from almost the moment they were switched on. And that's before any of it made it to the surface of 67P/Churyumov-Gerasimenko.

Science, 2014. DOI: 10.1126/science.1261952 (About DOIs).