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 suggests a dry proto-Earth with subsequent delivery of volatiles by local accretion or impacts of asteroids or comets. 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, 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.

The D/H ratio in cometary water is believed to be an important indicator of the conditions under which icy planetesimals formed and can provide clues to the contribution of comets to the delivery of water and other volatiles to Earth. Available measurements suggest that there is isotopic diversity in the comet population. The Herschel Space Observatory revealed an ocean-like ratio in the Jupiter-family comet 103P/Hartley 2, whereas most values measured in Oort-cloud comets are twice as high as the ocean D/H ratio. We present here a new measurement of the D/H ratio in the water of an Oort-cloud comet. HDO, H 2 O, and H 18 2 O lines were observed with high signal-to-noise ratio in comet C/2009 P1 (Garradd) using the Herschel HIFI instrument. Spectral maps of two water lines were obtained to constrain the water excitation. The D/H ratio derived from the measured H 2 16 O and HDO production rates is (2.06 ± 0.22) × 10 −4 . This result shows that the D/H in the water of Oort-cloud comets is not as high as previously thought, at least for a fraction of the population, hence the paradigm of a single, archetypal D/H ratio for all Oort-cloud comets is no longer tenable. Nevertheless, the value measured in C/2009 P1 (Garradd) is significantly higher than the Earth's ocean value of 1.558 × 10 −4 . The measured 16 O/ 18 O ratio of 523 ± 32 is, however, consistent with the terrestrial value.

Earth-and space-based observations provide synergistic information for space mission encounters by providing data over longer timescales, at different wavelengths and using techniques that are impossible with an in situ flyby. We report here such observations in support of the EPOXI spacecraft flyby of comet 103P/Hartley 2. The nucleus is small and dark, and exhibited a very rapidly changing rotation period. Prior to the onset of activity, the period

We report on the initial analysis of a Herschel-PACS full range spectrum of Neptune, covering the 51-220 μm range with a mean resolving power of ∼3000, and complemented by a dedicated observation of CH 4 at 120 μm. Numerous spectral features due to HD (R(0) and R(1)), H 2 O, CH 4 , and CO are present, but so far no new species have been found. Our results indicate that (i) Neptune's mean thermal profile is warmer by ∼3 K than inferred from the Voyager radio-occultation; (ii) the D/H mixing ratio is (4.5 ± 1) × 10 −5 , confirming the enrichment of Neptune in deuterium over the protosolar value (∼2.1 × 10 −5 ); (iii) the CH 4 mixing ratio in the mid stratosphere is (1.5 ± 0.2) × 10 −3 , and CH 4 appears to decrease in the lower stratosphere at a rate consistent with local saturation, in agreement with the scenario of CH 4 stratospheric injection from Neptune's warm south polar region; (iv) the H 2 O stratospheric column is (2.1 ± 0.5) × 10 14 cm −2 but its vertical distribution is still to be determined, so the H 2 O external flux remains uncertain by over an order of magnitude; and (v) the CO stratospheric abundance is about twice the tropospheric value, confirming the dual origin of CO suspected from ground-based millimeter/submillimeter observations.

We present Herschel observations of water isotopologues in the atmosphere of the Jupiter-family comet 45P/Honda-Mrkos-Pajdušáková. No HDO emission is detected, with a 3σ upper limit of 2.0 × 10 −4 for the D/H ratio. This value is consistent with the earlier Herschel measurement in the Jupiter-family comet 103P/Hartley 2. The canonical value of 3 × 10 −4 measured pre-Herschel in a sample of Oort-cloud comets can be excluded at a 4.5σ level. The observations presented here further confirm that a diversity of D/H ratios exists in the comet population and emphasize the need for additional measurements with future ground-based facilities, such as CCAT, in the post-Herschel era.