Weird water found on comet 67P (Image: ESA/Rosetta/NAVCAM)

IF EARTH got its water from space, it probably wasn’t delivered by comet. That’s according to the latest data from the European Space Agency’s Rosetta spacecraft, which has been analysing the water content of comet 67P/Churyumov-Gerasimenko and found it doesn’t match the water on Earth.

The question of where Earth got its water – whether from asteroids, comets or in some other way – is a subject of ongoing debate. So analysing comet 67P’s water was one of Rosetta’s main goals. 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 (Science, DOI: 10.1126/science.1261952).


“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

“In the end, Earth’s oceans are probably a mix of many things,” says Altwegg.

The data collected so far may not be the last word on 67P’s water. It is possible that there are pockets of water with a different deuterium-to-hydrogen ratio that could be released as the comet nears the sun and heats up. The two halves of the duck-shaped 67P might even have different hydrogen signatures, which would suggest they were once distinct bodies that formed in different parts of the solar system before colliding. “That would be a very interesting result,” says Altwegg.

ROSINA’s job has been made harder by the loss of Philae, the probe that landed on 67P in November but was unable to survive due to a lack of solar power. That could make signals of heavier molecules, particularly the amino acids necessary for life, more ambiguous.

“The identification of molecules is certainly more difficult,” says Altwegg, who had hoped to compare ROSINA’s data with Philae’s Ptolemy instrument, which was designed to measure molecules in a different way. “If you have two different instruments you can resolve it, and this we cannot do, so it’s all on ROSINA more or less.”

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This article appeared in print under the headline “Comet water is not like Earth’s”