Thermophysical evolution of planetesimals in the primordial disc

Abstract

The Primordial Disk of small icy planetesimals, once located at 15–30 AU from the Sun, was disrupted by giant planet migration in the early Solar System. The Primordial Disk thereby became the source region of objects in the current–day Kuiper Belt, Scattered Disk, and Oort Cloud. I present the thermophysics code ‘Numerical Icy Minor Body evolUtion Simulator’, or NIMBUS, and use it to study the thermophysical evolution of planetesimals in the Primordial Disk prior to its disruption. Such modelling is mandatory in order to understand the behaviour of dynamically new comets from the Oort Cloud, as well as the activity of Centaurs and short–period comets from the Scattered Disk, that return pre–processed to the vicinity of the Sun. I find that bodies in the midst of the Primordial Disk with diameters ranging 4–200 km lost all their CO ice on time-scales of order 0.1–10 Myr depending on size, through a combination of protosolar and long–lived radionuclide heating. CO and other hypervolatiles therefore require a less volatile host for their storage. I consider two possible hosts: amorphous water ice and CO2 ice. Because of the high luminosity of the protosun, some Primordial Disk bodies may have sustained significant crystallisation, CO: CO2 segregation, and CO2 sublimation in the uppermost few tens of meters. I discuss how this may affect coma abundance ratios and distant activity in dynamically new comets.

The Asteroid-Comet Continuum

“…Figure 22). Moreover, thermal evolution models suggest that pure CO cannot survive long-term in comets (Davidsson 2021). Explanations attempting to match activity to the sublimation of less volatile, less abundant materials (such as H 2 S) struggle to provide a convincing match to the 8 to 10 au critical distance (Poston et al 2018).…”

“…Establishing proof of the existence of amorphous ice in JFCs and Centaurs would be of great scientific value. It would imply that ice in the precursor Kuiper belt objects has also survived in the amorphous state, significantly limiting the post-accretion thermal processing of these bodies (see Davidsson 2021 and the chapter by Prialnik and Jewitt).…”

Thermal processing of Jupiter Family Comets during their chaotic orbital evolution

“…Indeed, Guilbert-Lepoutre (2012) found that crystallization of amorphous water ice can be a source of activity, efficient at heliocentric distances up to 10-12 au, sustained though for a limited time (typically hundreds to thousands of years). This implies that active Centaurs should have suffered a recent orbital change Davidsson (2021). nuanced this result, by showing that crystallization would be efficient only up to 8-10 au, while the sublimation of CO 2 would be the only source of activity in the 10-12 au region.…”

“…Considerable modelling efforts have examined the possibility of thermal processing and activity even from the earlier stages of evolution. Such studies propose that alterations start as early as the formation of comet nuclei, and before their displacement in outer solar system reservoirs (e.g Prialnik & Merk 2008;Raymond et al 2020;Davidsson 2021), but also during the prolonged storage phase, where surface temperatures between 30 and 50 K can be attained (e.g. Capria et al 2000;De Sanctis et al 2000, 2001Choi et al 2002).…”

Are there any pristine comets? Constraints from pebble structure

“…Combined, these two effects are known to regulate the near-surface temperature and therefore the activity of comets. In recent years, various studies have indeed looked at different aspects related to the surface of comets, that are associated with these thermal modifications (Hu et al 2019;Gundlach et al 2020;Davidsson 2021). However, much less is known about the role of pebble structure on the internal, long-term physical and thermal evolution of comets.…”