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

Solar system formation models predict that the building blocks of planetesimals were mm- to 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 per cent. We measure the average dust bulk density rho D = 795(-65)(+840) kg m(-3) that, coupled to the nucleus bulk density, provides the average dust-to-ices mass ratio delta = 8.5. We find that the measured densities of the 67P pebbles are consistent with a mixture of (15 +/- 6) per cent of ices, (5 +/- 2) per cent of Fe-sulphides, (28 +/- 5) per cent of silicates, and (52 +/- 12) per cent of hydrocarbons, in average volume abundances. This composition matches both the solar and CI-chondritic 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 versus crystalline Mg, Fe-olivines and pyroxenes. We find that the pebbles have a microporosity of (52 +/- 8) per cent (internal volume filling factor phi P = 0.48 +/- 0.08), implying an average porosity for the 67P nucleus of (71 +/- 8) per cent, lower than previously estimated.