Hiʻiaka (moon)

• 1. Introduction

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  Hiʻiaka is the larger of two known moons circling the dwarf planet Haumea, and it briefly went by another name: Rudolph. Its discoverer had already nicknamed the world it orbits Santa, so the reindeer name got passed down to the moon that followed. That playful aside hides a genuinely strange body. Hiʻiaka is elongated rather than round, and its icy surface reflects an unusual amount of sunlight. Its rotation, meanwhile, seems to answer to Haumea's own spin. What kind of event leaves a moon with a shape like that? Why hasn't billions of years of cosmic ray bombardment changed its ice the way scientists would expect? And what does its orbit and its spin reveal about how it, and a smaller sibling moon, actually came into existence?

• 2. Discovery

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  Michael E. Brown and the Keck Observatory's adaptive optics team found Hiʻiaka on the 26th of January 2005, observing from Mauna Kea, Hawaii. It was the first satellite ever detected orbiting Haumea. Haumea's own discovery had not yet been made public, so word of Hiʻiaka waited until the 29th of July 2005. When the announcement finally came, the moon carried a temporary provisional designation marking it as the first moon of Haumea found that year.

  On the 17th of September 2008, the International Astronomical Union formally named Haumea, Hiʻiaka, and Namaka after figures from Hawaiian mythology. In that tradition, Hiʻiaka is the patron goddess of hula, one of the daughters of the fertility goddess Haumea. Brown's team had proposed the names to the IAU back in September 2006, wanting to honor the Hawaiian location where they made the discovery. That 2008 vote also gave Namaka, Hiʻiaka's smaller sibling, a name from the same mythology, but said nothing about how large either moon actually was.

• 3. Size, Mass, and Shape

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  Every 49.5 days, Hiʻiaka completes a slightly elliptical orbit around Haumea, passing by at a distance of about 49400 kilometers. The moon itself is elongated and irregular rather than round. Stellar occultations observed on the 6th and the 16th of April 2021 measured a volume-equivalent diameter of 370 kilometers. That makes Hiʻiaka the sixth-largest known moon of any trans-Neptunian object. Only five moons rank larger: Charon at 1212, Dysnomia at 615, Vanth at 443, Ilmarë at 403, and Actaea at 393 kilometers.

  At 370 kilometers across, Hiʻiaka is still not in hydrostatic equilibrium. Its elongated shape doesn't match what its current rotation period would normally produce. Researchers think this is most likely due to unusually high material strength holding that shape in place. Gravitational tugs on Hiʻiaka's orbit, measured by the Hubble Space Telescope, point toward a very low bulk density. A separate estimate based on Haumea's oblateness arrives at a similarly low figure, around 0.64. That density suggests Hiʻiaka is built from loosely packed water ice and rock rather than solid material. The moon is too small for its interior to separate into distinct layers, so it lacks any substantial rocky core. That porous, icy structure supports the idea that Hiʻiaka accumulated from fragments flung off of Haumea by the dwarf planet's own rapid spin.

• 4. Rotation

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  Hiʻiaka rotates once every 9.68 hours, but that spin is not tidally locked to Haumea. It most likely formed far enough from Haumea that the dwarf planet's tidal pull had little effect on shaping its rotation. A 2016 study, using 2009-2010 observations from the Magellan and Hubble Space Telescopes, first measured this rotation period. That research also found Hiʻiaka's brightness swinging by 19 percent, or 0.23 magnitudes, over the course of each rotation. Plotted over time, that brightness produces a sawtooth-shaped light curve, a pattern pointing to sharp, irregular, angular features in the moon's shape.

  Over 15 years of observation, scientists found no change in how Hiʻiaka's brightness varies as it spins. That stability points to an axial tilt close to zero degrees relative to Haumea's own rotation. The orientation of Hiʻiaka's shape, captured through occultation observations, lines up with that same low-obliquity picture. Simulations suggest Haumea's gravity should still cause Hiʻiaka's spin axis to precess, slowly shifting direction over a span of decades. A larger obliquity would stretch that precession out even longer, while a smaller one would speed it up. Scientists expect to pin down that precession rate by tracking subtle changes in Hiʻiaka's light curve over the coming years.

• 5. Surface and Composition

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  Water ice dominates Hiʻiaka's surface, just as it does on Haumea itself. That shared composition is one of the strongest clues that Hiʻiaka was once part of Haumea. Near-infrared observations reveal deep absorption features caused by that water ice, concentrated in specific parts of the spectrum. An additional, narrower absorption feature shows that the ice on Hiʻiaka's surface sits mainly in crystalline form. It remains unclear why constant bombardment by cosmic rays hasn't converted that crystalline ice into amorphous ice, as physics would predict. Some resurfacing process other than impact cratering seems to be at work, though scientists haven't identified it. Cryovolcanism is considered unlikely on Hiʻiaka, given its small size and its lack of tidal heating.

  Hiʻiaka has a very high geometric albedo of 0.74, reflecting far more sunlight than a typical icy body. That figure is even higher than Haumea's own albedo of 0.51, despite the two objects sharing the same basic material. Near-infrared spectroscopy shows Hiʻiaka's water ice absorption features running deeper than Haumea's own. That difference suggests Hiʻiaka's ice is fresher, purer, or made up of larger particles than the ice covering Haumea. Larger grains could explain the higher albedo on their own, in a pattern similar to Saturn's bright icy moons Enceladus and Tethys.

• 6. Origin

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  About 4.4 billion years ago, a giant impact is thought to have created both Hiʻiaka and Namaka as ejected fragments of Haumea. That impact struck early, within the solar system's first 77-82 million years. At the time, Neptune was migrating outward, gravitationally scattering objects throughout the Kuiper belt. The leading hypothesis holds that two large Kuiper belt objects of similar size collided at an angle and merged into one fast-spinning body. That body then deformed into the ellipsoidal shape Haumea has today. This impact hypothesis explains Haumea's rapid spin and its unusually high density. It fails, however, to explain the existence of Haumea's moons or its wider family of icy Kuiper belt objects sharing similar orbits. An impact violent enough to match the evidence would have flung fragments out several times faster than Haumea's escape velocity.

  Rather than forming directly in the giant impact, Haumea's moons and its icy family are instead believed to have formed through rotational fissioning. That fissioning occurred roughly 80 million years after the impact, or 147-162 million years after the solar system itself formed. A 2022 study led by Jessica Noviello and collaborators proposed that Haumea kept differentiating after the impact, growing its rocky core over time. As that core grew, conservation of angular momentum gradually sped up Haumea's rotation. Eventually, centrifugal force at Haumea's equator grew strong enough to fling icy surface material into orbit, forming a disk that later coalesced into moons. That fissioning cost Haumea about 3 percent of its original mass and 14 percent of its original angular momentum. What remained in orbit, spun off from Haumea's own restless rotation, became the icy, elongated moon known today as Hiʻiaka.