The ice content of Kuiper belt objects

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).

Table 1 The volume abundances of sulfides (c₁), silicates (c₂) and hydrocarbons (c₃); the ice volume abundances c_4P, c_4C and c_4T; and the non-volatiles-to-ices mass ratios δ_P, δ_C and δ_T, in Pluto, Charon and Triton, respectively, for the end-cases of CI-chondritic and solar composition, and for compact and porous ices in the pebbles which accreted into KBOs

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

$$\begin{array}{c}{\rho }_{\text{KBO}}=c_1{\rho }_1+c_2{\rho }_2+c_3{\rho }_3+c_4{\rho }_4\\ =c_5{\rho }_5+\left(1‒c_1/c_5\right){\rho }_4\end{array}$$

(1)

where ρ₅ = ρ₁ + (c₂/c₁)ρ₂ + (c₃/c₁)ρ₃ and c₅ = 1 / (1 + c₂/c₁ + c₃/c₁) allow us to relate ρ_KBO to the ratios c₂/c₁ and c₃/c₁ only. Equation 1 provides c₁, and the constraint c₁ < c₅ provides the maximum possible values ρ_max = c₅ρ₅ of ρ_KBO (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 layers^2,8,10 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.