The Kuiper Belt Object Haumea’s Internal Evolution Drove its Family’s Formation

The large (average diameter ~1562 km), fast-spinning (rotation period 3.91 hours) Kuiper belt object Haumea in the Scattered Disk is the source of the only known dynamical family in the Kuiper belt. The Haumean family members are ice-rich, total 2-5% of Haumea’s mass, have small dispersion velocities (∆v < 160 m/s) relative to the dynamical centroid, and a shallow absolute magnitude/size distribution slope. These suggest ejection from a graze-and-merge (rather than catastrophic) collision, but a collision in the Scattered Disk is less likely than in the primordial Kuiper Belt. One plausible explanation for the Haumeans’ origins is that a proto-Haumea was hit by an object of similar size, merged into a single body, and only later (after being scattered) spun off the material that comprise the Haumean family members. We hypothesize the delay between the collision and ejection of the icy fragments was due to formation of Haumea’s core.

We evaluated this hypothesis using the geophysical code kyushu to characterize Haumea and quantify its rotation rate at various times in the evolution of Haumea’s interior, from immediately after the initial collision to the present. Haumea’s high angular momentum was likely caused by the early collision event; we find undifferentiated solutions with 3% more mass and 9% more angular momentum than modern-day Haumea. We show that as Haumea formed a rocky core and icy mantle, due to its reduced moment of inertia, Haumea would have spun up and experienced negative effective gravity along the longest axis, allowing ejection of ice. Ejection of 3% of its mass would have removed 9% of its angular momentum, and we find hydrostatic equilibrium solutions with these properties, and the same core as before the ejection. Finally, hydration of Haumea’s core would have increased its moment of inertia and decreased its spin rate, to modern-day Haumea. We also ran an independent interior thermal and compositional evolution code, IcyDwarf, to corroborate present-day density and size estimates of Haumea’s internal layers. These runs suggested that Haumea could have sustained a subsurface ocean early in its formation for 250 Myr, making it the most distant potential relict ocean world.