Ocean-like water in the Jupiter-family comet 103P/Hartley 2

A drop in the ocean Earth's bulk composition is similar to that of a group of oxygen-poor meteorites called enstatite chondrites, thought to have formed in the early solar nebula. This leads to the suggestion that proto-Earth was dry, and that volatiles including water were delivered by asteroid and comet impacts. The deuterium-to-hydrogen (D/H) ratios measured in six Oort cloud comets are much higher than on Earth, however, apparently ruling out a dominant role for such bodies. Now the Herschel Space Telescope has been used to determine the D/H ratio in the Kuiper belt comet 103P/Hartley 2. The ratio is Earth-like, suggesting that this population of comets may have contributed to Earth's ocean waters. For decades, the source of Earth's volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558 ± 0.001) × 10 −4 , has been a subject of debate. The similarity of Earth’s bulk composition to that of meteorites known as enstatite chondrites 1 suggests a dry proto-Earth 2 with subsequent delivery of volatiles 3 by local accretion 4 or impacts of asteroids or comets 5 , 6 . Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96 ± 0.25) × 10 −4 . The D/H value in carbonaceous chondrites, (1.4 ± 0.1) × 10 −4 , together with dynamical simulations, led to models in which asteroids were the main source of Earth's water 7 , with ≤10 per cent being delivered by comets. Here we report that the D/H ratio in the Jupiter-family comet 103P/Hartley 2, which originated in the Kuiper belt, is (1.61 ± 0.24) × 10 −4 . This result substantially expands the reservoir of Earth ocean-like water to include some comets, and is consistent with the emerging picture of a complex dynamical evolution of the early Solar System 8 , 9 .