Haumea’s Shape, Composition, and Internal Structure

We have calculated the figure of equilibrium of a rapidly rotating, differentiated body to determine the shape, structure, and composition of the dwarf planet Haumea. Previous studies of Haumea’s light curve have suggested that Haumea is a uniform triaxial ellipsoid consistent with a Jacobi ellipsoid with axes ≈960 × 774 × 513 km and bulk density ≈2600 kg m^-3. In contrast, observations of a recent occultation of Haumea indicate that its axes are ≈1161 × 852 × 523 km and its bulk density is ≈1885 kg m^-3 these results suggest that Haumea cannot be a fluid in hydrostatic equilibrium and must be supported by interparticle forces. We have written a code to reconcile these contradictory results and to determine whether Haumea is in fact a fluid in hydrostatic equilibrium. The code calculates the equilibrium shape, density, and ice crust thickness of a differentiated Haumea after imposing axis lengths a and b. We find that Haumea is consistent with a differentiated triaxial ellipsoid fluid in hydrostatic equilibrium with axes of best fit a = 1050 km, b = 840 km, and c = 537 km. This solution for Haumea has ρ _avg = 2018 kg m^-3, ρ _core = 2680 kg m^-3, and core axes a _c = 883 km, b _c = 723 km, and c _c = 470 km, which equates to an ice mantle composing ∼17% of Haumea’s volume and ranging from 67 to 167 km in thickness. The thick ice crust we infer allows for Haumea’s collisional family to represent only a small fraction of Haumea’s pre-collisional ice crust. For a wide range of parameters, the core density we calculate for Haumea suggests that today the core is composed of hydrated silicates and likely underwent serpentinization in the past.