The dust-to-ices ratio in comets and Kuiper belt objects

Comet 67P/Churyumov-Gerasimenko (67P hereinafter) is characterised by a dust transfer from the southern hemi-nucleus to the night-side northern dust deposits, which constrains the dust-to-ices mass ratio inside the nucleus to values a factor two larger than that provided by the lost mass of gas and non-volatiles. This applies to all comets, because the gas density in all night comae cannot prevent the dust fall-back. Taking into account GIADA data collected during the entire Rosetta mission, we update the average dust bulk density to ρD=785+520−115 kg m−3 that, coupled to the 67P nucleus bulk density, confirms an average dust-to-ices mass ratio δ = 7.5 inside 67P. The improved dust densities are consistent with a mixture of (20 ± 8)% of ices, (4 ± 1)% of Fe-sulphides, (22 ± 2)% of silicates, and (54 ± 5)% of hydrocarbons, on average volume abundances. These values correspond to solar chemical abundances, as suggested by the elemental C/Fe ratio observed in 67P. The ice content in 67P matches that inferred in Kuiper Belt Objects (KBOs hereinafter), (20 ± 12)% on average volume abundance, and suggests a water content in all Trans-Neptunian Objects (TNOs hereinafter) lower than in CI-chondrites. The 67P icy pebbles and the dust collected by GIADA have a microporosity of (49 ± 5)% and (59 ± 8)%, respectively.