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

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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.

At a glance


  1. Submillimetre water emission lines from comet 103P/Hartley 2.
    Figure 1: Submillimetre water emission lines from comet 103P/Hartley 2.

    The time of the observations was 20days after perihelion, when the comet was 1.095au from the Sun and 0.212au from Herschel. Because the H2O ground state rotational lines in comets are optically thick29, 30, observations of the rare oxygen isotopic counterpart, H218O, provide a more reliable reference for the D/H determination. The spectra of the 110–101 lines of HDO (a) and H218O (b) at 509.292 and 547.676GHz, respectively, were obtained with the Heterodyne Instrument for the Far Infrared (HIFI) High Resolution Spectrometer (HRS) between 17.28 and 17.64 November 2010 ut. The line intensities, expressed in the main-beam brightness temperature scale, are 0.011±0.001 and 0.117±0.002Kkms−1, for HDO and H218O respectively, averaging the two instrument polarizations. The velocity scale is given relative to the velocity of the comet’s nucleus. The spectral resolution is 141 and 132ms−1 for the HDO and H218O spectra, respectively. For details of the observational sequence and basic parameters of the data analysis, see Supplementary information.

  2. D/H ratios in the Solar System.
    Figure 2: D/H ratios in the Solar System.

    Orange squares, values measured for water in the Oort-cloud comets 1P/Halley, C/1996 B2 (Hyakutake), C/1995 O1 (Hale–Bopp), C/2002 T7 (LINEAR) and 8P/Tuttle. Arrow (for 153P/Ikeya–Zhang), upper limit. Purple square, present measurement in the water of 103P/Hartley 2. Black symbols, D/H ratio in H2 in the atmosphere of the giant planets—Jupiter (J), Saturn (S), Uranus (U) and Neptune (N). Light blue and green symbols, D/H values for water in the plume of Saturn's moon Enceladus and in CI carbonaceous chondrites, respectively. Error bars, 1σ. The D/H determinations in comets originating from the Oort cloud are twice the value for the Earth’s ocean (blue line) and about a factor of ten larger than the protosolar value in H2 (broad yellow line), the latter being comparable to the value in atomic hydrogen found in the local interstellar medium (ISM, red horizontal line). The D/H ratio in the Jupiter-family comet 103P/Hartley 2 is the same as the Earth’s ocean value and the chondritic CI value. Uranus and Neptune have been enriched in deuterium by the mixing of their atmospheres with D-rich protoplanetary ices. For further details, see Supplementary Table 1.


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Author information


  1. Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany

    • Paul Hartogh,
    • Miguel de Val-Borro &
    • Miriam Rengel
  2. California Institute of Technology, Pasadena, California 91125, USA

    • Dariusz C. Lis,
    • Martin Emprechtinger &
    • Geoffrey A. Blake
  3. LESIA-Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France

    • Dominique Bockelée-Morvan,
    • Nicolas Biver,
    • Jacques Crovisier &
    • Raphael Moreno
  4. Rosetta Science Operations Centre, European Space Astronomy Centre, 28691 Villanueva de la Cañada, Madrid, Spain

    • Michael Küppers
  5. Astronomy Department, University of Michigan, Ann Arbor, Michigan 48109, USA

    • Edwin A. Bergin
  6. Space Research Centre, Polish Academy of Sciences, 00-716 Warsaw, Poland

    • Slawomira Szutowicz


This paper represents the combined work of the HssO (the Herschel guaranteed time key programme “Water and related chemistry in the solar system”) team members listed as authors. P.H. is the coordinator of this programme. All authors contributed to this work, including observation planning, data analysis and writing of the manuscript. N.B., D.B.-M., M.R., R.M, M.d.V.-B. and M.E. carried out the data reduction and contributed to the modelling efforts. All authors were collectively involved in the discussion and interpretation of the results.

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Supplementary information

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  1. Supplementary Information (214K)

    The file contains Supplementary Text, Supplementary References and Supplementary Table 1.

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