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Experimentalists can now generate terapascal pressures in the laboratory, conditions sufficient to alter the structure of atoms and the nature of interatomic bonding ( 1 ). These are the pressures of planets' interiors and origins—7 TPa at Jupiter's center, 4 TPa in the middle of Saturn, 0.36 TPa for Earth's inner core—and planet growth involves impacts that generate pressures into the terapascal range ( 2 ). Understanding materials and their properties at such conditions provides key insights into how planetary bodies form and then evolve over billions of years. On page 1063 of this issue, Fratanduono et al. ( 3 ) establish a new calibration for such experiments, and their pressure-volume relations for gold (Au) and platinum (Pt) can now serve as reliable standards to >1 TPa. 

Impact-induced mixing between bolide and target is fundamental to the geochemical evolution of a growing planet, yet aside from local mixing due to jetting – associated with large angles of incidence between impacting surfaces – mixing during planetary impacts is poorly understood. Here we describe a dynamic instability of the surface between impacting materials, showing that a region of mixing grows between two media having even minimal initial topography. This additional cause of impact-induced mixing is related to Richtmyer-Meshkov instability (RMI), and results from pressure perturbations amplified by shock-wave refraction through the corrugated interface between impactor and target. However, unlike RMI, this new impact-induced instability appears even if the bodies are made of the same material. Hydrocode simulations illustrate the growth of this mixing zone for planetary impacts, and predict results suitable for experimental validation in the laboratory. This form of impact mixing may be relevant to the formation of stony-iron and other meteorites.