Modeling the Contact Surfaces Formed by Pebble Collisions: Application to Formation of Comet 67P/Churyumov--Gerasimenko

TL;DR

The paper addresses how contact surfaces between colliding pebbles during gravitational collapse influence the internal structure of comets, focusing on 67P/Churyumov--Gerasimenko. It introduces a contact-surface model tied to the compressive strength of dust aggregates and validates it with numerical simulations of pebble collisions. By applying the model to 67P’s measured tensile strength and bulk density, it derives constraints on formation conditions, suggesting low-velocity pebble collisions (below $\sim10\ \mathrm{cm\ s^{-1}}$) and a microscopic filling factor $<0.6$, and estimates pebble radii of $130\ \mu\mathrm{m}$ or smaller when the bouncing-sticking transition sets the collision velocity. This work links microscopic pebble properties to macroscopic cometary observables and supports pebble-cloud gravitational-collapse as a plausible pathway for 67P's formation.

Abstract

Modeling the contact surfaces formed by pebble collisions is crucial to understanding the formation process of comets, which are thought to be composed of pebbles. In this paper, we develop a new model to estimate the contact surface radius and the number of contact points as functions of collision velocity, and examine the formation process of comet 67P/Churyumov--Gerasimenko. Our model is based on the compressive strength of dust aggregates obtained from numerical simulations and assumes that all the impact energy of the pebbles is used for their mutual compression. We compare our model with numerical simulations of pebble collisions, in which we prepare the initial pebbles in the form of compressed dust aggregate spheres and measure the contact surface and pebble radii using two- and three-dimensional characteristic radii, respectively. We also apply our model to the formation scenario of comet 67P, whose tensile strength and bulk density have already been estimated in the literature. We find that its low tensile strength points to formation via pebble collisions at velocities below $\sim10\mathrm{\ cm\ s^{-1}}$ when a microscopic filling factor of pebbles is lower than 0.6, suggesting that inelastic bouncing collisions played a role in damping the collision velocities. By assuming that the pebble collision velocity is determined by the transition velocity between bouncing and sticking, we estimate the pebble radius inside comet 67P to be 130 $\mathrm{μm}$ or smaller.