Composition measurements and the history of cometary matter

Composition data show that production from many nucleosynthetic sites and a sequence of condensation processes characterize the history of matter found in the smaller objects of the solar system. Estimates are presented for elemental abundances in the material emitted by Halley's comet. They are based on in situ measurements on the gas and dust in the coma. It appears that Halley's nucleus is not depleted in the volatile elements C and O, whereas N is deficient. The abundance data show that a large fraction of the material in Halley's nucleus condensed at very low temperature. Comparison of the results published so far by the Giotto and Vega groups on molecular, elemental and isotopic abundances with data from meteorites, meteors, and the interstellar gas confirm that comets are regular members of the solar system which have preserved the original characteristics of the condensed and accreted matter better than other bodies in this system.