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

S. Lorek; B. Gundlach; P. Lacerda; J. Blum

doi:10.1051/0004-6361/201526565

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arXiv · 1601.05726

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

S. Lorek·B. Gundlach·P. Lacerda·J. Blum

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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)

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S. Lorek, B. Gundlach, P. Lacerda, J. Blum. 2016-01-21. Comet formation in collapsing pebble clouds. What cometary bulk density implies for the cloud mass and dust-to-ice ratio. https://doi.org/10.1051/0004-6361%2F201526565

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