According to the latest data from the European Space Agency’s Rosetta spacecraft  the water content of comet 67P/Churyumov-Gerasimenko doesn’t match the water on Earth. The spacecraft’s ROSINA instrument has been sniffing the vapour around the comet ever since reaching it in August.

Kathrin Altwegg of the University of Bern in Switzerland and her colleagues have now analysed 67P’s water, by looking at the amount of deuterium, a heavy isotope of hydrogen, and comparing it with the amount of regular hydrogen.

“As soon as we got water from the comet, the pattern changed immediately,” says Altwegg. The comet’s water has around three times as much deuterium as water on Earth, the researchers found. The ratio found on some other comets is much closer to that on Earth, suggesting a link between the icy space rocks and terrestrial water. The different composition of 67P’s water suggests a more complex picture.

“We know there is material out there that has the signature of Earth’s water in it, but is it the material that supplied it?” says Edwin Bergin of the University of Michigan, Ann Arbor, who previously found that comet Hartley 2 has similar water to that of Earth. Models of asteroid and comet motion within the solar system suggest asteroids were more likely to cross paths with Earth, but we don’t yet know if they had the right mix of water to create the oceans. “We need many more measurements of this type to get an understanding of the diversity within the population,” Bergin says