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),
CO+
, and CO+
2
(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.