Earth's water came from asteroids not comets, Rosetta mission probe results suggest

One of the most important goals of the European Space Agency’s Rosetta mission at the comet 67P/Churyumov-Gerasimenko is to help planetary scientists work out where the Earth’s oceans came from.

When the Earth coalesced out of the dust and gases surrounding the baby Sun 4.5 billion years ago, it already contained a lot of water. Most of that evaporated away, however, as our planet came together.

Scientists think that the water we see on Earth today was delivered around 500 million years after the planet formed, when all of the inner worlds of the Solar System were pummelled by asteroids and comets, a time known as the Late Heavy Bombardment.

Comets are largely made of water ice, whereas asteroids are mainly rocky (though they also contain water). Some theories suggest that it was comets that brought water to Earth in the Late Heavy Bombardment; others think it was asteroids.

To test that, the Rosetta spacecraft used an on-board spectrometer called Rosina to analyse the composition of the comet's water and to compare it to the Earth's.

Rosina measured the amount of heavy water in water released by the comet. Heavy water is made from a version of hydrogen that contains a neutron as well as a proton in its nucleus. The ratio of normal to heavy water on a planetary body is something like an identifying signature.

Kathrin Altwegg from the University of Bern led the latest research and her team used Rosina data to show that that there was a higher percentage of heavy water on the comet (some three times more) than there was on the Earth.

Because the values for Earth and 67P do not match, that means the water on our planet was more likely to have come from asteroids than comets such as 67P/Churyumov-Gerasimenko. Their results are published today in the journal Science.

We’ll get more data on the question of the Earth’s water when the results from Philae start to come through in a few months. Though it didn't all go quite to plan when it landed on the comet, the tiny probe still managed to do most of its primary science experiments before its batteries ran out, two and a half days after getting to the surface.

Those measurements are still being analysed and will tell scientists what other molecules are present on the comet - not only the types of water but whether or not there are complex organic molecules, the kinds of things that might have brought the building blocks of life to the Earth billions of years ago.

Comets like 67P/Churyumov-Gerasimenko are frozen time capsules of the earliest years of our solar system, left-overs from when the planets were still forming. Bit by bit, Rosetta and Philae are starting to uncover their secrets.