NEGATIVE-IONS IN THE COMA OF COMET HALLEY

IN March 1986, the Giotto spacecraft encountered comet Halley, approaching to within approximately 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 neutrals 1 produced 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 greater-than-or-similar-to 1, approximately 5 x 10(-2) and approximately 4 x 10(-2) cm(-3), respectively, at a distance of approximately 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 by solar radiation at approximately 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 magnetospheres 3, 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.