Rosetta’s comet is rich in deuterium

Planetary scientists have long suspected that comets and asteroids delivered water and organic compounds to Earth during an epoch known as the Late Heavy Bombardment, hundreds of millions of years after the planet formed. But the cometary contributions and their provenance are under debate. Comparing the deuterium-to-hydrogen ratio (D/H) in seawater to that found in different populations of comets is a reliable way to distinguish among the possibilities. In the dozen or so orbiting comets probed to date, observed D/H ratios are thought to represent the local values where and when the comets’ building blocks condensed. The latest isotopic measurement comes from the European Space Agency’s Rosetta spacecraft, which is now orbiting the 4-km-wide comet 67P/Churyumov–Gerasimenko shown here. Using Rosetta’s mass spectrometer, Kathrin Altwegg of the University of Bern in Switzerland and her colleagues measured the D/H ratio of the comet’s tenuous atmosphere and found it to be three times Earth’s. The high value is a strike against the theory that Kuiper belt comets delivered Earth’s water, which had been strengthened three years ago when the Herschel space telescope spotted a comet (103P/Hartley 2) whose D/H ratio matched Earth’s. Like Hartley 2, comet Churyumov–Gerasimenko is thought to have originated in the Kuiper belt, just beyond Neptune’s orbit. Altwegg and colleagues speculate that the inconsistent values might reflect diverse origins, despite the comets now being part of the same family. The new measurement points to chondritic asteroids, whose D/H ratios are much more consistently Earthlike, as the more likely source of our oceans. (K. Altwegg et al., Science, in press.)