Comet formation in collapsing pebble clouds. What cometary bulk density implies for the cloud mass and dust-to-ice ratio

TL;DR

This study investigates how the collisional compression of icy dust pebbles in collapsing clouds can explain the observed properties of comets, linking cloud mass and dust-ice ratios to comet density and porosity.

Contribution

It introduces a collision model for mixed ice/dust pebbles and constrains initial cloud parameters that produce comet-like nuclei consistent with observations.

Findings

Comets' high porosity and low density require cloud masses >1e18 g.

Low-mass clouds need initially compact pebbles to form comets.

Dust-to-ice ratio between 3 and 9 matches observed comet properties.

Abstract

Comets are remnants of the icy planetesimals that formed beyond the ice line in the Solar Nebula. Growing from micrometre-sized dust and ice particles to km-sized objects is, however, difficult because of growth barriers and time scale constraints. The gravitational collapse of pebble clouds that formed through the streaming instability may provide a suitable mechanism for comet formation. We study the collisional compression of cm-sized porous ice/dust-mixed pebbles in collapsing pebble clouds. For this, we developed a collision model for pebbles consisting of a mixture of ice and dust, characterised by their dust-to-ice mass ratio. Using the final compression of the pebbles, we constrain combinations of initial cloud mass, initial pepple porosity, and dust-to-ice ratio that lead to cometesimals which are consistent with observed bulk properties of cometary nuclei. We find that observed high porosity and low density of ~0.5 g/cc of comet nuclei can only be explained if comets formed in clouds with mass approximately M>1e18 g. Lower mass clouds would only work if the pebbles were initially very compact. Furthermore, the dust-to-ice ratio must be in the range of between 3 and 9 to match the observed bulk properties of comet nuclei. (abridged version)