Rosetta mission reveals water source

In a report published on Wednesday, European researchers have concluded that water on Earth is likely to have come from asteroids which hit the planet billions of years ago.

Comets are the most primitive objects in the solar system and are rich in carbon. By smashing into the Earth, scientists had previously thought they brought elements that allowed life to exist.

Rosetta mission

The turn in scientific belief came as a result of data collected by the Rosetta spacecraft. The European Space Agency's Philae lander touched down on the duck-shaped 67P/Churyumov-Gerasimenko comet last month.

"We have to conclude … the terrestrial water was brought by asteroids more likely than comets," said Kathrin Altwegg, from the University of Bern, who is principal investigator for the ROSINA mass spectrometer that has been examining the chemical fingerprint of water and other gases in the comet.

Wrong water

The report, published in the US journal Science, drew together data gathered by the Rosetta mission which has been studying the type of comet which could have brought water to Earth four billion years ago. It found water, but the wrong kind. It was too heavy.

Scientists measured the ratio between deuterium, a hydrogen isotope, and hydrogen, which forms water when combined with oxygen.

The comet has shown "probably the highest level of deuterium to hydrogen ratio, the most heavy of any of the solar system's bodies," said Altwegg.

Such a high ratio of deuterium to hydrogen "probably means that it was formed at very low temperatures and that it could be ... most probably the original material from the very, very early time of our solar system, so it's a real treasure chest to explore how our solar system looks like 4.6 billion years ago," Altwegg said.

"On the other hand, water in asteroids has a lower deuterium/hydrogen ratio and is more similar to water on Earth," she added.

But it is not an entirely clear set of results, and scientists are expected to continue the discussion. While asteroids are likely carriers of water to Earth as they probably had more water on them four billion years ago than they do now, it may be that the Earth kept some of its original water in ice, or in its crust, at the poles.

Following a bouncy landing in November, the Rosetta mission, aims to explore the composition of comets, which are believed to be primordial clusters of ice and dust left from the building of the solar system 4.6 billion years ago.

ksb/jm (AFP, AP)