The origin of complex organic ions in the coma of Comet Halley

Abstract

Heavy ion mass spectra were obtained in the inner coma of Comet Halley by the Giotto-PICCA instrument. Outside the cometary ionopause, where ≳2000 K ion temperatures limited the effective mass resolution to 3 amu, the spectra exhibit a series of distinct mass groups that are separated by a constant ≈ 15 amu and extend to at least 120 amu. We show that this pattern is characteristic of molecules composed of carbon, hydrogen, oxygen, and nitrogen (CHON molecules), and thus does not imply the presence of a polymer, such as polyoxymethylene. Higher resolution (0.8-1.3 amu) 35-70 amu spectra, obtained inside the cometary ionopause, where the ion temperature was ≈ 300 K, show a dominance of odd mass number ions. This effect may be explained if the nitrogen abundance is less than 7% relative to carbon and oxygen and if ion-molecule reactions have depleted the reactive radical ions. We find that neither gas-phase chemistry in the coma nor irradiation of the nucleus in its current orbit by ultraviolet photons, solar wind particles, and cosmic rays can produce enough heavy molecules to account for the abundances observed by PICCA. Even after 4.5 Gyr in the Oort cloud, cosmic rays could have produced a sufficient supply of heavy molecules only of the efficiency for the process is ≫ 10 ^-3 complex molecules per eV of cosmic ray energy. Alternatively, ultraviolet and cosmic ray irradiation of icy precometary grains within interstellar molecular clouds and/or the protosolar nebula could supply the required abundance of heavy molecules.