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 chondrites1 suggests a dry proto-Earth2 with subsequent delivery of volatiles3 by local accretion4 or impacts of asteroids or comets5, 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 water7, 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 System8, 9.
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- Supplementary Information (214K)
The file contains Supplementary Text, Supplementary References and Supplementary Table 1.