On the origin and evolution of the material in 67P/Churyumov-Gerasimenko

International audiencePrimitive objects like comets hold important information on the material that formed our solar system. Several comets have been visited by spacecraft and many more have been observed through Earth- and space-based telescopes. Still our understanding remains limited. Molecular abundances in comets have been shown to be similar to interstellar ices and thus indicate that common processes and conditions were involved in their formation. The samples returned by the Stardust mission to comet Wild 2 showed that the bulk refractory material was processed by high temperatures in the vicinity of the early sun. The recent Rosetta mission acquired a wealth of new data on the composition of comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) and complemented earlier observations of other comets. The isotopic, elemental, and molecular abundances of the volatile, semi-volatile, and refractory phases brought many new insights into the origin and processing of the incorporated material. The emerging picture after Rosetta is that at least part of the volatile material was formed before the solar system and that cometary nuclei agglomerated over a wide range of heliocentric distances, different from where they are found today. Deviations from bulk solar system abundances indicate that the material was not fully homogenized at the location of comet formation, despite the radial mixing implied by the Stardust results. Post-formation evolution of the material might play an important role, which further complicates the picture. This paper discusses these major findings of the Rosetta mission with respect to the origin of the material and puts them in the context of what we know from other comets and solar system objects

Neutron and X-Ray diffraction study of internal stress in thermomechanically fatigued single-crystal superalloy

The relationship between internal stress and thermomechanical fatigue (TMF) in a Ni-based single-crystal superalloy is studied by neutron and X-ray diffraction. The extents of internal stress, deformation, lattice mismatch, and distortion during TMF are characterized by the determined deviatoric stress invariants and lattice parameters and compared with relevant microstructural information from scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that, in general, the macroscopic stress, plastic deformation, lattice mismatch, and distortion have all increased during TMF. The lattice mismatches of the TMF samples are at a high level, where the values along [100]/[010] are negative, but those along [001] are positive. The tetragonal lattice distortion of the γ matrix is slightly greater than that of the γ′ precipitates, where the c/a values of the γ matrix are smaller than 1, but that of the γ′ precipitate larger than 1. The γ matrix yields and becomes hardened at the initial TMF cycle and gradually loses most of its strength during the earlier TMF cycles, associated with stress relaxation and homogenous deformation. However, the γ′ precipitates yield and become hardened later, bearing the most stress up to the necking of the superalloy. This process is associated with a buildup of stress and significant concentrated and inhomogeneous distribution of deformation in the γ′ precipitate. The residual deformation states of the superalloy and its component phases at the earlier TMF are basically shearing, and only become stretched at a later stage of TMF. The microstructure of the TMF samples shows an initial stage of rafting, where the dislocations are accumulated at the γ/γ\ifmmode′\else$′$\fi interfaces of the γ matrix channels, but both dislocation networks and stacking faults are inhomogeneously distributed in the γ′ precipitates