Herschel measurements of the D/H and ¹⁶O/¹⁸O ratios in water in the Oort-cloud comet C/2009 P1 (Garradd)^⋆,⋆⋆

¹ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
e-mail: dominique.bockelee@obspm.fr
² Dept. of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
³ RAL Space, Science & Technology Facilities Council, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
⁴ Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
⁵ California Institute of Technology, Pasadena, CA 91125, USA
⁶ Space Research Centre, Polish Academy of Science, Ul. Bartycka 18A, 00-716 Warszawa, Poland
⁷ European Space Astronomy Centre, Villanueva de la Cañada, 28692 Madrid, Spain
⁸ Joint Astronomy Centre, 660 N. Aohoku Place, Hilo, HI 96720, USA
⁹ Institute for Space Imaging Science, Department of Physics and Astronomy, University of Lethbridge, Lethbridge, Alberta, T1K 3M4, Canada

Received: 4 June 2012
Accepted: 24 July 2012

Abstract

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, H₂O, and 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 H₂¹⁶O 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 ¹⁶O/¹⁸O ratio of 523 ± 32 is, however, consistent with the terrestrial value.