ROSINA/DFMS and IES observations of 67P: Ion-neutral chemistry in the coma of a weakly outgassing comet

¹ Space Science Division, Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas, 78228, USA
e-mail: sfuselier@swri.edu
² Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas TX 78249, USA
³ Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
⁴ LATMOS, 4 avenue de Neptune, 94100 Saint-Maur, France
⁵ Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, 2455 Hayward, Ann Arbor, MI 48109, USA
⁶ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace (LPC2E), UMR 6115 CNRS-Université d’Orléans, 45071 Orléans Cedex 2, France
⁷ Belgian Institute for Space Aeronomy (BIRA-IASB), Ringlaan 3, 1180 Brussels, Belgium
⁸ Institute of Computer and Network Engineering (IDA), TU Braunschweig, Hans-Sommer-Straße 66, 38106 Braunschweig, Germany
⁹ Space and Atmospheric Physics Group, Department of Physics, Imperial College London, Prince Consort Road, London, SW7 2AZ, UK
¹⁰ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
¹¹ Lockheed Martin Advanced Technology Center, Palo Alto, CA 94304, USA
¹² Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse, France
¹³ CNRS, IRAP, 9 avenue du colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
¹⁴ Laboratory of Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, CO 80303, USA
¹⁵ Swedish Institute of Space Physics, 75121 Uppsala, Sweden

Received: 28 March 2015
Accepted: 7 July 2015

Abstract

Context. The Rosetta encounter with comet 67P/Churyumov-Gerasimenko provides a unique opportunity for an in situ, up-close investigation of ion-neutral chemistry in the coma of a weakly outgassing comet far from the Sun.

Aims. Observations of primary and secondary ions and modeling are used to investigate the role of ion-neutral chemistry within the thin coma.

Methods. Observations from late October through mid-December 2014 show the continuous presence of the solar wind 30 km from the comet nucleus. These and other observations indicate that there is no contact surface and the solar wind has direct access to the nucleus. On several occasions during this time period, the Rosetta/ROSINA/Double Focusing Mass Spectrometer measured the low-energy ion composition in the coma. Organic volatiles and water group ions and their breakup products (masses 14 through 19), CO₂⁺ (masses 28 and 44) and other mass peaks (at masses 26, 27, and possibly 30) were observed. Secondary ions include H₃O⁺ and HCO⁺ (masses 19 and 29). These secondary ions indicate ion-neutral chemistry in the thin coma of the comet. A relatively simple model is constructed to account for the low H₃O⁺/H₂O⁺ and HCO⁺/CO⁺ ratios observed in a water dominated coma. Results from this simple model are compared with results from models that include a more detailed chemical reaction network.

Results. At low outgassing rates, predictions from the simple model agree with observations and with results from more complex models that include much more chemistry. At higher outgassing rates, the ion-neutral chemistry is still limited and high HCO⁺/CO⁺ ratios are predicted and observed. However, at higher outgassing rates, the model predicts high H₃O⁺/H₂O⁺ ratios and the observed ratios are often low. These low ratios may be the result of the highly heterogeneous nature of the coma, where CO and CO₂ number densities can exceed that of water.