Ice and dust of 67P/Churyumov-Gerasimenko tell an unexpected story

We may have to rewrite the story of the evolution of the protosolar nebula. It is the conclusion reached by Marco Fulle and coauthors on the basis of the data collected by the Rosetta mission on the comet 67P/Churyumov-Gerasimenko. The important result is based both on the ratio between ices and dust of the comet and on the distribution of pebbles and fractals.

Which is the structure of a comet nucleus ? And how comets were born ? Rosetta is providing answers quite far from what was suggested before, linking the formation of our Solar System to that of extrasolar ones recently observed by ALMA. First of all, comets appear much drier than supposed so far, actually drier than carbonaceous chondrites, the most primitive meteorites collected on Earth. This is consistent both with important transfers of minerals from the inner solar nebula to the outermost regions, thus making the asteroidal belt the iciest area in the protoplanetary disc; and with the bulk densities of Triton, Pluto and Charon, which require an ice content of 20% only of their volume, to match the mineral composition observed by Rosetta in its target comet, 67P/Churyumov-Gerasimenko.

All these Trans-Neptunian Objects were probably formed by the gravitational collapse of a swarm of pebbles of similar sizes. Comets, thanks to their small mass, still preserve these pebbles, actually observed by the CIVA cameras on board Philae, with a preferential size of half cm. The space among the pebbles is not empty. Computations show that when 67P was formed, at a disc age of about 1.5 Myr, the most primitive dust in the disc, i.e. extremely fluffy mm-sized fractal aggregates of sub-micron interstellar grains, was still more abundant than the pebbles, growing thanks to the compaction of bouncing collisions among fractals. The pristine fractal dust was thus entrapped among the pebbles, and preserved intact until Rosetta discovered it in 67P for the first time. This is a proof that comets are time-capsules providing us the intact pristine material which formed ourselves: comets did never suffer any catastrophic collision, as confirmed by the lacking small craters observed by New Horizon on Pluto's small satellite Nix.

Details in the two papers led by M. Fulle, INAF-OATS, in the MNRAS Rosetta Special Issue

Figure: the Lander Philae on the surface of the 67P nucleus, where it discovered the pebbles that
formed (and are still forming) all planets (Credit: ESA, OSIRIS Team)