Comet nuclei composition and evolution

Abstract

Thanks to Rosetta orbiter's and Philae lander's data our knowledge of cometary nuclei composition has experienced a great advancement. The properties of 67P/Churyumov-Gerasimenko nucleus are discussed and compared with other comets explored in the past by space missions. Cometary nuclei are made by a collection of ices, minerals, organic matter, and salts resulting in very dark and red-colored surfaces. When relatively far from the Sun (> 3AU), exposed water and carbon dioxide ices are found only in few locations of 67P/CG where the exposure of pristine subsurface layers or the recondensation of volatile species driven by the solar heating and local terrain morphology can sustain their temporary presence on the surface. The nucleus surface appears covered by a dust layer of variable thickness caused by the back-fall flux of coma grains. Depending on local morphology, some regions are less influenced by the back-fall and show consolidated terrains. Dust grains appear mostly dehydrated and are made by an assemblage of minerals, organic matter, and salts mixed together. Spectral analysis shows that the mineral phase is dominated by silicates and fine-grained opaques (possibly including Fe-sulfides such as troilite or pyrrhotite), and ammoniated salts. Aliphatic and aromatic groups, with the presence of the strong hydroxyl group, are identified within the organic matter. The surface composition and physical properties of cometary nuclei evolve with heliocentric distance and seasonal cycling: approaching perihelion the increase of the solar flux boost the activity through the sublimation of volatile species which in turn causes the erosion of surface layers, the exposure of ices, the activity in cliffs and pits, the collapse of overhangs and walls, and the mobilization and redistribution of dust. The evolution of color, composition, and texture changes occurring across different morphological and geographical regions of the nucleus are correlated with these processes. In this chapter we discuss 67P/CG nucleus composition and evolutionary processes as observed by Rosetta mission in the context of other comets previously explored by space missions or observed from Earth.

The Galactic Interstellar Object Population: A Framework for Prediction and Inference

“…While in reality, stars will each produce a distribution of ISOs that formed at different positions in their protoplanetary disk and thus have a range of compositions, this simplification of only modeling planetesimals that form exterior to the water ice line is justified by the proportionally greater reservoir of snowline-exterior planetesimals, and the higher efficiencies of formation mechanisms dynamically stripping them into the interstellar population (Fitzsimmons et al 2023). In our solar system, the vast majority of Oort Cloud objects are ice rich (Meech et al 2016); therefore both these and the majority of ISOs produced by the solar system must have formed outside of the water ice line (Filacchione et al 2022). Additionally, planetesimals beyond the water ice line are more loosely bound to their parent stars, so will be more easily ejected (e.g., Moro-Martín 2018).…”