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 fallback. 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)%.