Abstract
THE bulk chemical composition of the dust grains in the coma of comet Halley has been determined1–11 by examination of data from the PUMA 1 and 2 and PIA mass spectrometers on the Vega and Giotto missions. Although the bulk elemental composition of rock-forming elements (that is, excluding C, H, N and O) seemed to be close to solar (that is, similar to CI carbonaceous chondrites), the ion ratios of some of these elements, such as Mg+/Si+ and Fe+/Si+, are rather different from those in CI chondrites. There has not been, however, a satisfactory investigation of the chemical composition of individual grains as a function of their mass. Here we re-evaluate the PUMA 1 and 2 data to perform such an analysis. We find that the compositions of heavy and light grains are very different, with light grains being magnesium-rich (silicon-deficient), whereas the mean Mg+/Si+ ratio in heavy grains is similar to CI chondritic. The marked difference in composition between light and heavy grains indicates that the origin of the two grain populations might be different.
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