First observations of H₂O and CO₂ vapor in comet 67P/Churyumov-Gerasimenko made by VIRTIS onboard Rosetta

¹ LESIA, Observatoire de Paris, LESIA/CNRS, UPMC, Université Paris-Diderot, 92195 Meudon, France
e-mail: dominique.bockelee@obspm.fr
² INAF–IAPS, Istituto di Astrofisica e Planetologia Spaziali, via del fosso del Cavaliere 100, 00133 Rome, Italy
³ Space Physics Research Laboratory, University of Michigan, Ann Arbor, MI 48109, USA
⁴ Institute for Planetary Research, Deutsches Zentrum für Luft- und Raumfahrt (DLR), 12489 Berlin, Germany
⁵ Université Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble, 38026 Grenoble, France
⁶ Department of Physics, Oxford University, Oxford OX1 3PU, UK
⁷ Lunar Planetary Laboratory, University of Arizona, Tucson, A2 85721-0092, USA
⁸ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁹ National Central University, 320 Taipei, Taiwan
¹⁰ Space Research Centre, Polish Academy of Sciences, Bartycka 18a, 00-716 Warsaw, Poland

Received: 13 April 2015
Accepted: 23 July 2015

Abstract

Context. Outgassing from cometary nuclei involves complex surface and subsurface processes that need to be understood to investigate the composition of cometary ices from coma observations.

Aims. We investigate the production of water, carbon dioxide, and carbon monoxide from the nucleus of comet 67P/Churyumov-Gerasimenko (67P). These species have different volatility and are key species of cometary ices.

Methods. Using the high spectral-resolution channel of the Visible InfraRed Thermal Imaging Spectrometer (VIRTIS-H), we observed the ν₃ vibrational bands of H₂O and CO₂ at 2.67 and 4.27 μm, respectively, from 24 November 2014 to 24 January 2015, when comet 67P was between 2.91 and 2.47 AU from the Sun. Observations were undertaken in limb-viewing geometry at distances from the surface of 0 to 1.5 km and with various line-of-sight (LOS) orientations in the body-fixed frame. A geometry tool was used to characterize the position of the LOS with respect to geomorphologic regions and the illumination properties of these regions.

Results. The water production of 67P did not increase much from 2.9 to 2.5 AU. High water column densities are observed for LOS above the neck regions, suggesting they are the most productive in water vapor. While water production is weak in regions with low solar illumination, CO₂ is outgassing from both illuminated and non-illuminated regions, which indicates that CO₂ sublimates at a depth that is below the diurnal skin depth. The CO₂/H₂O column density ratio varies from 2 to 60%. For regions that are in sunlight, mean values between 2 and 7% are measured. The lower bound value is likely representative of the CO₂/H₂O production rate ratio from the neck regions. For carbon monoxide, we derive column density ratios CO/H₂O < 1.9% and CO/CO₂< 80%. An illumination-driven model, with a uniformly active surface releasing water at a mean rate of 8 × 10²⁵ s^-1, provides an overall agreement to VIRTIS-H data, although some mismatches show local surface inhomogeneities in water production. Rotational temperatures of 90–100 K are derived from H₂O and CO₂ averaged spectra.