To the Editor — Rosetta has measured the bulk density of non-volatiles in a primitive Solar System object for the first time1. Models of Pluto and Charon2 assume rock densities ranging from 2,770 to 3,260 kg m−3 and a hydrocarbons-to-silicates mass ratio h/s = 0.2. However this value, first suggested for comet 1P/Halley3, is biased by a low dust-to-ices ratio that was later refined to larger values4, implying larger h/s ratios as well. Soft hydrogenated carbon alloys5 (the best terrestrial analogues of hydrocarbons in the protosolar nebula) have a bulk density similar to ices, so that the low bulk density of Kuiper belt objects (KBOs) can be due either to abundant ices or to hydrocarbons.
The composition of comet 67P/Churyumov–Gerasimenko (67P) was fixed1 by the volume abundances c1 of Fe-sulfides (bulk density ρ1 = 4,600 kg m−1), c2 of Mg and Fe olivines and pyroxenes (ρ2 = 3,200 kg m−3), c3 of hydrocarbons5 (ρ3 = 1,200 kg m−3), and c4 of ices (ρ4 = 917 kg m−3). The volume abundances depend on the ice porosity and on the pristine composition (between the solar and CI-chondritic end-cases1) in the pebbles forming comets and KBOs, and provide h/s = (c3ρ3)/(c2ρ2) >> 0.2 (Table 1).
We assume that the non-volatiles in comets and KBOs have a similar composition, which is fixed by the ratios c2/c1 and c3/c1 provided by Rosetta (Table 1). This is consistent with the elemental C/Fe ratio in 67P6 and in 1P/Halley3, with a possible origin of comets as fragments of KBOs, or with a probable common origin of comets and KBOs from similar pebbles7. Comets and KBOs differ instead in the abundance and composition of ices, which depend on their distance from the Sun during accretion and on their evolution.
Here we infer the ice abundance c4 in KBOs, where the lithostatic pressure eliminates all the voids among the pebbles following gravitational collapse7 and the subsequent evolution. Therefore the average KBO bulk density is
(1)
where ρ5 = ρ1 + (c2/c1)ρ2 + (c3/c1)ρ3 and c5 = 1 / (1 + c2/c1 + c3/c1) allow us to relate ρKBO to the ratios c2/c1 and c3/c1 only. Equation 1 provides c1, and the constraint c1 < c5 provides the maximum possible values ρmax = c5ρ5 of ρKBO (Table 1), which are close to the bulk densities of Pluto8 (1,860 kg m−3), Charon8 (1,700 kg m−3), and Triton9 (2,060 kg m−3). This fact shows that the ice content in these KBOs is necessarily low.
We validate this conclusion by computing the non-volatiles-to-ices mass ratios δ = (c1ρ5)/(c4ρ4) (Table 1), which are systematically larger than the value δ = 1.5 obtained by Pluto's models2, and imply a water content lower than in CI-chondrites2. The largest ice volume abundances are obtained for the CI-chondritic composition, and provide c4 = 24% on average in KBOs. The dominant frost observed on the surfaces of Pluto and Charon10 confirms a complete differentiation (also occurring during the largest impacts), storing all the ices in surface layers2,8,10 of thickness of about 95 and 80 km for Pluto and Charon, respectively, which envelope a thick mantle of hydrocarbons surrounding a smaller silicate nucleus. The icy surface layer of Triton is even thinner, 55 km at most. The C/Fe ratio in 67P6, close to the solar end-case, confirms a dominant abundance of hydrocarbons1 and suggests an ice content in Pluto even lower than in 67P.
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Acknowledgements
Rosetta is an ESA mission with contribution from its member states and NASA. Rosetta's Philae lander is provided by a consortium led by DLR, MPS, CNES and ASI. We thank the Rosetta Science Ground Segment at ESAC, the Rosetta Mission Operations Center at ESOC and the Rosetta Project at ESTEC for their outstanding work enabling the science return of the Rosetta Mission.
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Fulle, M. The ice content of Kuiper belt objects. Nat Astron 1, 0018 (2017). https://doi.org/10.1038/s41550-016-0018
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DOI: https://doi.org/10.1038/s41550-016-0018
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