Spectral evolution of ROSETTA target comet 46P/Wirtanen

The compositional evolution of the coma of ROSETTA target comet 46P/Wirtanen was studied along its pre-perihelion orbit. At 2.34 AU the first gaseous species, CN and C-3, were detected. C-2 and NH2 arose at 2.0 AU. The production rate curves of gas and dust show a steep increase between 1.8 AU and 1.6 AU indicating a distinct change of the outgassing conditions. The Ca production increases more rapidly with decreasing solar distance than CN. As one consequence the C-2/CN ratio strongly varies with heliocentric distance. Due to this variation a classification of 46P/Wirtanen by the taxonomy introduced by A'Hearn et al. (1995) distinguishing comets with typical abundance ratios from carbon-chain depleted comets, is impossible. It would be classified as depleted beyond about 1.6 AU whereas it would be designated typical at smaller distances. The knowledge of the unexpected changes in the activity curves of 46P/Wirtanen will help to optimize the ROSETTA mission scenario in particular with regard to science operations.