Inside-out: Dwarf Planet Haumea's Internal Evolution and the Creation of the Haumean Collisional Family

The Kuiper Belt Object Haumea is the fastest-spinning large object in the Solar System, with a rotational period 3.915 hours [1], giving it a triaxial ellipsoid shape with mean radius ~780 km [2]. Its surface is covered by pure water ice [3]. Haumea has two moons [4] and a ring [5] and is the largest member of the only known collisional family (the Haumeans) in the Kuiper belt [6]. Haumea is thought to have suffered a giant impact [6], but the mass of the Haumeans (3% of Haumeas mass; [7]) is small, and their relative velocity (100s of m/s) is small compared to the ~1 km/s expected in a giant impact. How Haumea and its family members formed is one of many outstanding questions about the dwarf planet. We propose Haumea suffered a giant impact but that the Haumeans were ejected later in a fission process due to a geophysically-driven spin-up of Haumea. We propose Haumea had partially differentiated into a rocky core and ice mantle, with undifferentiated crust, at the time of the impact. The impact ejected crust and added angular momentum. Later, growth of Haumeas core decreased its moment of inertia (MOI), and Haumea spun up until icy material was flung off. Loss of 3% of its mass from the tip of the longest axis will carry away 9% of its angular momentum. Later still, the rocky core hydrated, increasing Haumeas MOI and slowing its rotation to modern values. To test this hypothesis, we seek self-consistent, hydrostatic equilibrium solutions of two-layer Haumeas in each stage, using the kyushu code [2]. Our calculations support the hypothesis that following a giant impact that left a partially differentiated Haumea in a high-spin state, further differentiation of the core (decreasing MOI) then hydration of the core (increasing MOI) led to a brief epoch during which Haumea could eject icy fragments from its long axis before evolving to its present state. References: [1] Rabinowitz, D. et al. 2006, ApJ 639, 1238. [2] Dunham, E. et al. 2019, ApJ 877, 41. [3] Pinilla-Alonso, N. et al. 2009, A&A 496, 547. [4] Ragozzine, D. and Brown, M. 2009, AJ 124, 1757. [5] Ortiz, J. et al. 2017, Nature 550, 219. [6] Brown, M. et al. 2007, Nature 446, 294. [7] Pike, R. et al. 2020 Nat Astro 4, 89.