Modelling of metastable carbon atoms in comets: implications for ROSETTA

Abstract

The observations of CI 1931 Å emission in the ultraviolet spectra of comets show that a large fraction of carbon atoms in the cometary coma are produced in the metastable ¹D excited state. A coupled chemistry-transport model is used to study the chemistry of C(¹D) atom and the mechanisms of production of CI 1931 Å in the near nucleus environment. Dissociation of CO by solar UV photon and photoelectron impact are the dominant mechanisms of the C(¹D) production in the cometary coma. The CI 1931 Å emission is found to be governed dominantly by the density distribution of CO and C(¹D). The model predict height-integrated CI 1931 Å emission intensity of 20 R on comet 46P/Wirtanen, the target of the ROSETTA mission, and intensity averaged over HST FOC field-of-view of 6.7 R. However, the observed intensity could be higher since on comet Halley the model predicted intensity (Bhardwaj, 1999) was about a factor of 5 smaller than that observed by the IUE.