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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 .

The fossilized remnants of vaporized asteroids, called spherules, can be used to infer that the flux of asteroid impacts on Earth 3.5 billion years ago was much greater than it is now. Late show for Earth's bombardment The Late Heavy Bombardment was a period of time, generally put at about 4.1 billion to 3.8 billion years ago, when the inner planets of the Solar System were subjected to a high-frequency barrage of asteroids. This left its mark on the Moon, but on Earth the craters quickly disappeared owing to tectonic processes and erosion. In the first of two papers on the bombardment, Brandon Johnson and Jay Melosh determine the properties of the asteroids by looking at spherule beds: layers of debris ejected during the impacts. The thickness of spherule layers is expected to vary according to the size of the impactor and the speed at which it hit Earth. This historical record of impacts indicates that the number of projectiles colliding with Earth was substantially higher 3.5 billion years ago than it is today, with a gradual decline in the number of strikes after the Late Heavy Bombardment. Bottke et al . modelled the evolution of an asteroid belt that extended farther towards Mars than the present one. They find that most of the impactors traced by the spherule beds probably originated in this 'E-belt', which was disrupted during migrations of some of the giant planets. Impact craters are the most obvious indication of asteroid impacts, but craters on Earth are quickly obscured or destroyed by surface weathering and tectonic processes 1 . Earth’s impact history is inferred therefore either from estimates of the present-day impactor flux as determined by observations of near-Earth asteroids, or from the Moon’s incomplete impact chronology 2 , 3 , 4 . Asteroids hitting Earth typically vaporize a mass of target rock comparable to the projectile’s mass. As this vapour expands in a large plume or fireball, it cools and condenses into molten droplets called spherules 5 . For asteroids larger than about ten kilometres in diameter, these spherules are deposited in a global layer. Spherule layers preserved in the geologic record accordingly provide information about an impact even when the source crater cannot be found 1 . Here we report estimates of the sizes and impact velocities of the asteroids that created global spherule layers. The impact chronology from these spherule layers reveals that the impactor flux was significantly higher 3.5 billion years ago than it is now. This conclusion is consistent with a gradual decline of the impactor flux after the Late Heavy Bombardment.

This year, NASA's Dawn and New Horizons rendezvoused with Ceres and Pluto, respectively. These worlds, despite their modest sizes, have much to teach us about the accretion of the Solar System and its dynamical evolution.

ESA's Rosetta spacecraft has begun the next phase of its ambitious mission to land a probe on the nucleus of a comet, and ride with the comet towards the Sun.