The high temperature origin of cometary silicates

Laboratory analysis of particles returned from active comet Wild 2 directly determine detailed properties of silicates that accreted near the edge of the solar system. The particles were ejected from the comet’s ice-rich subsurface and probably stored below ~100K. Analyses of well-preserved silicates provide chemical, isotopic and mineralogical data at levels of detail that cannot be obtained by astronomical or even in-situ methods. This unique data relates to initial protoplanetary disk grains and their transport. The return mission was called Stardust because the sampled comet was thought to be composed of interstellar particles, but none of its >micron silicates have isotopic signatures of pre-solar grains. They clearly formed in the solar system. Isotopically anomalous pre-solar grains are found but only at 0.1% abundance and only for submicron particles. Analysis of hundreds of well-preserved 1-60 µm silicates prove that they formed in high temperature environments that must have proceeded the comet’s ices and organics. Most of these have igneous origins, just like chondrules in primitive meteorites, with formation temperatures >1800 K. Many of these have distinctive igneous textures and are composed of olivine, pyroxene, metal, chromite and glass. The excellent preservation of glass and other silicates show that they were never exposed to P/T conditions that could melt ice to form aqueous alteration phases abundantly found in all carbon-rich meteorites. It is striking to compare the anhydrous comet solids with those from primitive asteroid Ryugu, returned by Hayabusa 2. Due to ice melting, nearly all Ryugu components formed inside the asteroid while Wild 2 solids formed in nebular gas. The broad range of oxygen isotope and minor element compositions of specific silicates indicate origin in numerous disk environments that differed in isotopic composition, T and O/H gas ratios enhanced by evaporation of solids. The comet mix does not match any meteorite type. Wild 2 is an assembly of pristine nebular solids that formed in diverse widely separated regions of the nebular disk. Components include condensates and Calcium Aluminum Inclusions (CAIs) that have ¹⁶O-rich compositions like the Sun.