Variations in cometary dust composition from Giotto to Rosetta, clues to their formation mechanisms

Abstract : This paper reviews the current knowledge on the composition of cometary dust (ice, minerals and organics) in order to constrain their origin and formation mechanisms. Comets have been investigated by astronomical observations, space missions (Giotto to Rosetta), and by the analysis of cometary dust particles collected on Earth, chondritic porous interplanetary dust particles (CP-IDPs) and ultracarbonaceous Antarctic micrometeorites (UCAMMs). Most ices detected in the dense phases of the interstellar medium (ISM) have been identified in cometary volatiles. However, differences also suggest that cometary ices cannot be completely inherited from the ISM. Cometary minerals are dominated by crystalline Mg-rich silicates, Fe sulphides and glassy phases including GEMS (glass with embedded metals and sulphides). The crystalline nature and refractory composition of a significant fraction of the minerals in comets imply a high temperature formation/processing close to the proto-Sun, resetting a possible presolar signature of these phases. These minerals were further transported up to the external regions of the disc and incorporated in comet nuclei. Cometary matter contains a low abundance of isotopically anomalous minerals directly inherited from the presolar cloud. At least two different kinds of organic matter are found in dust of cometary origin, with low or high nitrogen content. N-poor organic matter is also observed in primitive interplanetary materials (like carbonaceous chondrites) and its origin is debated. The N-rich organic matter is only observed in CP-IDPs and UCAMMs and can be formed by Galactic cosmic ray irradiation of N2- and CH4-rich icy surface at large heliocentric distance beyond a ‘nitrogen snow line’.