Comet 67P/Churyumov-Gerasimenko preserved the pebbles that formed planetesimals

Solar System formation models predict that the building-blocks of planetesimals were mmto cm-sized pebbles, aggregates of ices and non-volatile materials, consistent with the compact particles ejected by comet 67P/Churyumov-Gerasimenko (67P hereafter) and detected by GIADA (Grain Impact Analyzer and Dust Accumulator) on-board the Rosetta spacecraft. Planetesimals were formed by the gentle gravitational accretion of pebbles, so that they have an internal macroporosity of 40%. We measure the average dust bulk density ρD = 795 +840 −65 kg m −3 that, coupled to the 67P nucleus bulk density, provides the average dust-to-ices mass ratio δ = 8.5. We find that the measured densities of the 67P pebbles are consistent with a mixture of (15 ± 6)% of ices, (5 ± 2)% of Fe-sulfides, (28 ± 5)% of silicates, and (52 ± 12)% of hydrocarbons, in average volume abundances. This composition matches both the solar and CIchondritic chemical abundances, thus showing that GIADA has sampled the typical non-volatile composition of the pebbles that formed all planetesimals. The GIADA data do not constrain the abundance of amorphous silicates vs. crystalline Mg,Feolivines and pyroxenes. We find that the pebbles have a microporosity of (52 ± 8)% (internal volume filling factor φP = 0.48± 0.08), implying an average porosity for the 67P nucleus of (71± 8)%, lower than previously estimated.