Hiʻiaka (moon)
Hiiaka, formal designation Haumea I, is the larger, outer moon of the trans-Neptunian dwarf planet Haumea. Discovered by Michael E. Brown and the Keck Observatory adaptive optics team on 26 January 2005, it is named after Hiiaka, the patron goddess of the Big Island of Hawaii and one of the daughters of Haumea. The moon follows a slightly elliptical orbit around Haumea every 49.5 days, at a distance of.
Hiiaka is an elongated and irregularly shaped body with a mean diameter of, making it the sixth-largest known moon of a trans-Neptunian object. It has a very low bulk density of, which indicates it is mostly made of loosely-packed water ice and rock. Telescope observations have shown that Hiiaka has a highly reflective surface made of crystalline water ice, much like Haumea itself. Hiiaka rotates about its axis every 9.68 hours. Like its smaller sibling moon Namaka, Hiiaka is believed to be a fragment of Haumea that was ejected in the aftermath of a giant impact 4.4 billion years ago.
Discovery
Hiiaka was the first satellite discovered around Haumea. It was discovered on 26 January 2005 by Michael E. Brown and the W. M. Keck Observatory adaptive optics team at Mauna Kea, Hawaii. The discovery of Haumea had not been made public at the time, so the discovery of Hiiaka was announced later on 29 July 2005. When Hiiaka was announced, it given the temporary provisional designation, which indicates it is the first moon of Haumea discovered in 2005. At the time, Brown had been nicknaming Haumea "Santa," so he nicknamed the Hiiaka "Rudolph," after one of Santa Claus's reindeer.Haumea, Hiiaka, and Namaka were all officially named after Hawaiian deities by the International Astronomical Union on 17 September 2008. In Hawaiian mythology, Hiiaka is the patron goddess of hula and is the daughter of the fertility goddess Haumea. These names were proposed to the IAU by Brown's team in September 2006, who wanted to pay tribute to the location where they discovered the moons of Haumea.
Physical characteristics
Size, mass, and density
Stellar occultations by Hiiaka on 6 and 16 April 2021 reveal that the moon is an elongated object resembling an ellipsoid with dimensions of. These correspond to a volume-equivalent diameter of. Hiiaka is the sixth-largest known moon of a trans-Neptunian object, after Charon, Dysnomia, Vanth, Ilmarë, and Actaea. Despite its relatively large size, Hiiaka is not in hydrostatic equilibrium because its elongated shape is inconsistent with that expected for its current rotation period. Hiiaka's lack of hydrostatic equilibrium is most likely due to high material strength.Hubble Space Telescope measurements of gravitational perturbations in Hiiaka's orbital path show that the moon has a mass of. A simplified assumption of Haumea's oblateness suggests that Hiiaka has a mass of. The latter mass estimate points to a very low density of, which indicates Hiiaka has a highly porous and icy interior. Hiiaka is too small for its interior to undergo differentiation, so it lacks a substantial core. Hiiaka's highly porous interior supports the hypothesis that the moon accumulated from icy fragments flung off by Haumea's rapid rotation.
Rotation
Hiiaka rotates about its axis in 9.68 hours. The moon's rotation is not tidally locked to Haumea because it likely formed far from Haumea, where the dwarf planet's tidal forces are weak enough to have little effect on rotation. Hiiaka's rotation period was first measured in a 2016 study using 2009–2010 observations from the Magellan and Hubble Space Telescope, which showed that Hiiaka's brightness periodically varies by 19% as it rotates. Plotting Hiiaka's light curve shows a sawtooth waveform, which indicates irregularites and angular features in the moon's shape. Observations found no change in Hiiaka's rotational brightness variations over 15 years, indicating that the moon's rotation is aligned with Haumea's rotation—having an axial tilt or obliquity close to 0° with respect to Haumea. The orientation of Hiiaka's shape seen in stellar occultations adds further evidence to Hiiaka's low obliquity.Simulations show that gravitational peturbations by Haumea should cause Hiiaka's spin axis to precess on a timescale of decades. The axial precession rate of Hiiaka depends on its obliquity with respect to its orbit around Haumea; if Hiiaka has a larger obliquity, then its precession period would be longer. The axial precession of Hiiaka may be determined by monitoring the gradual change in its light curve amplitude over several years.
Surface and composition
Like Haumea, the surface of Hiiaka is dominated by water ice in composition. Hiiaka's similar composition to Haumea is a major piece of evidence to the theory that it originated from material ejected from Haumea. The abundance of water ice on Hiiaka's surface causes deep absorption features in Hiiaka's near-infrared spectrum, particularly at wavelengths of and. An additional absorption feature at indicates that the water ice on Hiiaka's surface is primarily in crystalline form. It is unclear why Hiiaka's crystalline water ice has not completely turned into amorphous form as would be expected for constant irradiation by cosmic rays; a resurfacing mechanism besides impact cratering remains yet to be seen. Cryovolcanism is unlikely to occur on Hiiaka due to its small size and lack of tidal heating.Hiiaka has a very high geometric albedo of 0.74, as measured by optical and occultation observations. Hiiaka's albedo is even higher than Haumea's, which is unusual considering that the moon is made of the same material as Haumea. Near-infrared spectroscopy has shown that Hiiaka exhibits deeper water ice absorption features than Haumea, indicating that the water ice on Hiiaka's surface is either fresher or purer than that of Haumea, or is made of particle sizes larger than those on Haumea's surface. The latter possibility could explain Hiiaka's higher albedo if its surface contains water ice grains between in size, similar to those seen in Saturn's bright icy moons Enceladus and Tethys.
Origin
Namaka and Hiiaka are widely believed to be fragments of Haumea that were ejected in the aftermath of a giant impact 4.4 billion years ago, when Neptune was migrating outward and gravitationally scattering objects in the Kuiper belt. This impact event is hypothesized to involve two large Kuiper belt objects of similar size, which obliquely collided with each other and merged into a single, rapidly rotating body that eventually deformed into an ellipsoidal body, becoming Haumea today. While this hypothesis explains Haumea's rapid rotation and high bulk density, it fails to explain the existence of Haumea's moons and family of icy KBOs on similar orbits, because such an energetic impact would have ejected fragments at speeds several times Haumea's escape velocity.Rather than having formed directly from a giant impact, Haumea's family and moons are instead believed to have been ejected via rotational fissioning of Haumea roughly 80 million years after the impact. A 2022 study led by Jessica Noviello and collaborators proposed that Haumea continued differentiating and growing its rocky core after the giant impact, which led to a gradual speed-up of Haumea's rotation rate as a consequence of angular momentum conservation. Centrifugal forces on Haumea's equator eventually grew so great that icy surface material began ejecting into orbit around Haumea, forming a disk of material that eventually coalesced into moons. About 3% of Haumea's initial mass and 14% of its initial angular momentum were lost via rotational fissioning.