Cometary comae dust provides supporting evidence for a carbon gradient in solar system

Comets are primitive remnants of solar system formation. Cometary nuclei have remained cryogenically “preserved” in the outer solar system for the last 4.5 Gyr, minimally processed and retaining a record of the volatile and refractory material incorporated into their nuclei from the protoplanetary disk. The locations and duration of the formation of cometary nuclei spans conditions that encompass comets such as 26P/Grigg-Skjellerup with “ultraprimitive” particles with Mg-rich crystalline olivine and higher concentrations of isotopically-presolar materials and abundant organics as well as comet 81P/Wild 2 that has type II micro-chondrules, which include the terminal particle ”Iris’ that was found through isotope analyses to have formed 3 Myr after CAIs. Elemental C/Si ratios are derivable from the cometary dust thermal model parameters utilizing the ratios of the mass fractions of amorphous carbon and of silicates. Cometary elemental C/Si ratios agree with the sun and the ISM but are 20 times higher than even the most primitive carbonaceous chondrites. Comet 67P/ Churyomov-Gerismenko (Bardyn et al. 2017), and comets that include dynamically new comet C/2013 US₁₀ (Catalina) (Woodward et al. 2021) highlight the large elemental carbon reservoir in the outer disk as recorded in the carbonaceous component of the solid-state material that composes these particle populations of these comets. Cometary comae provide supporting evidence for a carbon gradient in the solar system.

We ask: What form is this carbon that is so prevalently observed to produce a very warm thermal emission in the 3–5 micron region for comets within a few AU of the Sun and that is successfully modeled using the optical constants of the highly absorbing material of amorphous carbon? Cometary samples reveal abundant aliphatic bonded and some aromatic bonded carbon, and rarely graphitic carbon and ”elemental carbon’. In Stardust, 2- and 3-ring PAHs are components of micron-sized organic particles (Clemett et al. 2010). However, incorporating solid state organics into thermal models for cometary comae is made challenging by the relative paucity of optical constants of aliphatic-rich/aromatic-poor and highly absorbing carbonaceous matter. Laboratory studies and optical constants are needed to predict emission spectra of solid-state organics in cometary comae for this new era of Webb’s spectral sensitivity.