Negative ions in the coma of comet Halley

Abstract

IN March 1986, the Giotto spacecraft encountered comet Halley, approaching to within ∼600 km of the nucleus. Results from this encounter have shown that the inner coma contains a mixture of cometary neutral gas and dust, thermal ions and electrons, fast cometary pick-up ions, and decelerated solar-wind ions and electrons, as well as fast neutrals¹produced by charge exchange between pick-up ions and cold neutrals. Here we report the detection of a new component of the inner coma of comet Halley: negatively charged cometary ions. These ions are observed in three broad mass peaks at 7–19, 22–65 and 85–110 AMU, with densities reaching ≳1, ∼5x10^-2 and ∼4x10^-2cm^-3, respectively, at a distance of ∼ 2,300 km from the nucleus. The ion species thought to be present include O^-, OH^-, C^-, CH^-, CN^- and heavier complex CHO molecular ions. As negative ions are easily destroyed by solar radiation at ∼1 AU (ref. 2) an efficient production mechanism, so far unidentified, is required to account for the observed densities. The detection of negative ions in the coma near 1 AU implies that in similar neutral gas and dust environments farther away from the Sun (in Jupiter's or Saturn's magnetospheres³, for example), negative ions should also be present. If the negative-ion densities are large enough, they could play an important part in physical processes such as radiative transfer or charge exchange.