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What Was the Nature of Interstellar Material Incorporated into Comets?

Presentation #408.08 in the session Comets: Nucleus (Oral Presentation)

Published onOct 23, 2023
What Was the Nature of Interstellar Material Incorporated into Comets?

Comets are thought to be icy planetesimals left over from solar system formation and contain interstellar material from which our solar system was formed. Over the past decades, we have learned considerably about the properties and formation of comets and the early solar system by observation and modelling, and laboratory sample analysis. Yet a fundamental question remains unsettled concerning the origin of comets: What was the nature of the interstellar dust grains present in comets and, by extension, in the solar nebula? (Here interstellar grains refer to those formed and/or re-formed in the interstellar medium.)

I show progress in noninvasively identifying interstellar grains preserved in primitive or cometary interplanetary dust particles (IDPs) based on the revealed 3D grain morphology (shape, size, orientation), core-mantle structure, porosity, composition gradients, and nanomineralogy. The structures of whole aggregate IDPs, including intact grains and pores, are mapped down to 3D sub-10 nm detail, enabled by phase contrast X-ray nanotomography [Hu & Winarski, ApJL 923:L4 (8pp), 2021; MAPS 51, 1632-1642, 2016], from which the least altered IDP and pristine constituents are distinguished from altered particles and components. The identified primitive IDP, shaped like a quasi-ellipsoid with mean original porosity of ~ 50%, contains a population of elongated grains that tend to be aligned along the particle’s long axis. Those aligned elongated grains each contain at least a core of preferentially-oriented, twinned octahedral magnetite nanocrystals that are distinct, morphologically and texturally, from secondary magnetite found in primitive meteorites. The most pristine elongated grain identified yet is a nanodumbbell-shaped binary comprising a pair of axially aligned oblate, quasi-spheroidal grains with semiaxes of about 103×87×55 and 103×87×63 nm, respectively. Each member of the pair is primarily made up of the high-density magnetite core and a “dirty-silicate” mantle exhibiting rounded edges and pitted surfaces as well as composition gradients from core to surface. The findings yield a likely solution to the astronomical conundrum “the whereabouts and form of missing interstellar iron” and suggest that the composite interstellar grains containing oriented magnetite nanocrystals were incorporated into primordial planetesimals in the protoplanetary disk.

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