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

Fulle, Marco; Della Corte, V.; Rotundi, A.; Rietmeijer, F. J. M.; Green, S.; Weissman, P.; Accolla, M.; Colangeli, L.; Ferrari, M.; Ivanovski, S.; Lopez-Moreno, J. J.; Mazzotta Epifani, E.; Morales, R.; Ortiz, J. L.; Palomba, E.; Palumbo, P.; Rodriguez, J.; Sordini, R. and Zakharov, V. (2016). Comet 67P/Churyumov-Gerasimenko preserved the pebbles that formed planetesimals. Monthly Notices of the Royal Astronomical Society, 462(Suppl_1) S132-S137.



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%. We measure the average dust bulk density ${\rho}D = 795 _{-65}^{+840} kg m^{-3}$ 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 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 vs. crystalline Mg,Fe-
olivines 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.

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