 In this photo taken by theHubble Space Telescope, Hiʻiaka is the brighter spot near the top, directly on topHaumea (center). | |
| Discovery[1][2] | |
|---|---|
| Discovered by | |
| Discovery site | W. M. Keck Obs.,Mauna Kea |
| Discovery date | 26 January 2005 |
| Designations | |
Designation | (136108) Haumea I[3][4] |
| Pronunciation | /hiːʔiˈɑːkə/ Hawaiian:[ˈhiʔiˈjɐkə] |
Named after | Hiʻiaka |
| S/2005 (2003 EL61) 1[2] S/2005 (136108) 1[3] | |
| Orbital characteristics[a] | |
| Epoch 28 May 2008 12:00UT (JD 2454615.0) | |
| 49371±45 km | |
| Eccentricity | 0.0542±0.0012 |
| 49.462±0.083 d[6]: 4770 | |
| 154.53°+2.05° −2.00° | |
| Inclination |
|
| 13.110°+0.030° −0.031° (toecliptic) | |
| 98.34°+2.02° −2.06° | |
| Satellite of | Haumea |
| Physical characteristics | |
| Dimensions |
|
| 370±20 km (volume equivalent)[7] | |
| Mass | (1.6±0.2)×1019 kg (2025)[7] 1.213+0.322 −0.311×1019 kg (2024)[5]: 6 |
Meandensity | 0.640±0.080 g/cm3[7][b] |
| 9.68±0.02 h[7] | |
| ≈ 0° wrt Haumea[7]: 2, 4 | |
| Albedo | 0.74±0.15[7][c] |
| Temperature | ≈ 40 K (same as Haumea)[9]: 8 |
| ~ 20[d] | |
| 3.21±0.12 (average)[7]: 8 | |
Hiʻiaka, formal designation(136108) Haumea I, is the larger, outer moon of thetrans-Neptuniandwarf planetHaumea. Discovered byMichael E. Brown and theKeck Observatoryadaptive optics team on 26 January 2005, it is named afterHiʻiaka, thepatron goddess of theBig Island of Hawaii and one of the daughters ofHaumea. The moon follows a slightlyelliptical orbit around Haumea every 49.5 days, at a distance of 49,400 km (30,700 mi).
Hiʻiaka is an elongated and irregularly shaped body with amean diameter of 370 km (230 mi), making it the sixth-largest known moon of a trans-Neptunian object. It has a very lowbulk density of0.64 g/cm3, which indicates it is mostly made of loosely-packedwater ice and rock. Telescope observations have shown that Hiʻiaka has a highly reflective surface made ofcrystalline water ice, much like Haumea itself. Hiʻiaka rotates about its axis every 9.68 hours. Like its smaller sibling moonNamaka, Hiʻiaka is believed to be a fragment of Haumea that was ejected in the aftermath of a giant impact 4.4 billion years ago.
Hiʻiaka was the firstsatellite discovered around Haumea. It was discovered on 26 January 2005 byMichael E. Brown and theW. M. Keck Observatoryadaptive optics team atMauna Kea, Hawaii.[2][1] The discovery of Haumea had not been made public at the time,[11][12] so the discovery of Hiʻiaka was announced later on 29 July 2005.[2] When Hiʻiaka was announced, it given the temporaryprovisional designationS/2005 (2003 EL61) 1, which indicates it is the first moon of Haumea (then known as2003 EL61) discovered in 2005.[2] At the time, Brown had been nicknaming Haumea "Santa," so he nicknamed the Hiʻiaka "Rudolph," after one ofSanta Claus's reindeer.[11][12]
Haumea, Hiʻiaka, and Namaka were all officially named afterHawaiian deities by theInternational Astronomical Union (IAU) on 17 September 2008.[13] InHawaiian mythology,Hiʻiaka is the patron goddess ofhula and is the daughter of thefertility goddessHaumea.[13] 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.[14]
Stellar occultations by Hiʻiaka on 6 and 16 April 2021 reveal that the moon is an elongated object resembling anellipsoid with dimensions of 480 km × 360 km × 286 km (298 mi × 224 mi × 178 mi).[7] These correspond to a volume-equivalent diameter of 370 km (230 mi).[7] Hiʻiaka is the sixth-largest known moon of a trans-Neptunian object, afterCharon (1212 km),Dysnomia (615 km),Vanth (443 km),Ilmarë (403 km), andActaea (393 km).[e] Despite its relatively large size, Hiʻiaka is not inhydrostatic equilibrium because its elongated shape is inconsistent with that expected for its current rotation period.[7]: 4 [10]: 164 Hiʻiaka's lack of hydrostatic equilibrium is most likely due to highmaterial strength.[7]: 4
Hubble Space Telescope measurements of gravitationalperturbations in Hiʻiaka's orbital path show that the moon has a mass of1.213+0.322
−0.311×1019 kg.[5]: 6 A simplified assumption ofHaumea's oblateness suggests that Hiʻiaka has a mass of(1.6±0.2)×1019 kg.[7]: 3 The latter mass estimate points to a very low density of0.64 g/cm3, which indicates Hiʻiaka has a highlyporous and icy interior.[7]: 3–4 Hiʻiaka is too small for its interior to undergodifferentiation, so it lacks a substantial core.[7]: 4 Hiʻiaka's highly porous interior supports the hypothesis that the moon accumulated from icy fragments flung off by Haumea's rapid rotation.[7]: 3–4
Hiʻiaka rotates about its axis in 9.68 hours.[7] The moon's rotation is nottidally locked to Haumea because it likely formed far from Haumea, where the dwarf planet'stidal forces are weak enough to have little effect on rotation.[15]: 2 Hiʻiaka's rotation period was first measured in a 2016 study using 2009–2010 observations from theMagellan andHubble Space Telescope, which showed that Hiʻiaka's brightness periodically varies by 19% (0.23magnitudes[5]: 11 ) as it rotates.[15] Plotting Hiʻiaka'slight curve (brightness over time) shows asawtooth waveform, which indicates irregularites and angular features in the moon's shape.[15]: 3, 5 Observations found no change in Hiʻiaka's rotational brightness variations over 15 years, indicating that the moon's rotation is aligned with Haumea's rotation—having an axial tilt orobliquity close to 0° with respect to Haumea.[7]: 2, 4 The orientation of Hiʻiaka's shape seen in stellar occultations adds further evidence to Hiʻiaka's low obliquity.[7]: 2
Simulations show that gravitational peturbations by Haumea should cause Hiʻiaka's spin axis toprecess on a timescale of decades.[15]: 5 The axial precession rate of Hiʻiaka depends on its obliquity with respect to its orbit around Haumea; if Hiʻiaka has a larger obliquity, then its precession period would be longer.[15]: 5 The axial precession of Hiʻiaka may be determined by monitoring the gradual change in its light curveamplitude over several years.[15]: 5 [5]: 11
Like Haumea, the surface of Hiʻiaka is dominated by water ice in composition. Hiʻiaka's similar composition to Haumea is a major piece of evidence to the theory that it originated from material ejected from Haumea.[16][7]: 3 The abundance of water ice on Hiʻiaka's surface causes deepabsorption features in Hiʻiaka's near-infrared spectrum, particularly at wavelengths of1.5 μm and2.0 μm.[16] An additional absorption feature at1.65 μm indicates that the water ice on Hiʻiaka's surface is primarily incrystalline form.[17] It is unclear why Hiʻiaka's crystalline water ice has not completely turned intoamorphous form as would be expected for constantirradiation bycosmic rays;[17] a resurfacing mechanism besidesimpact cratering remains yet to be seen.[7]: 3 Cryovolcanism is unlikely to occur on Hiʻiaka due to its small size and lack oftidal heating.[7]: 3
Hiʻiaka has a very highgeometric albedo of 0.74, as measured by optical and occultation observations.[7] Hiʻiaka's albedo is even higher than Haumea's (0.51), which is unusual considering that the moon is made of the same material as Haumea.[7]: 3 Near-infraredspectroscopy has shown that Hiʻiaka exhibits deeper water ice absorption features than Haumea,[16][18]: L2 [7]: 3 indicating that the water ice on Hiʻiaka's surface is either fresher or purer than that of Haumea, or is made of particle sizes larger than those on Haumea's surface.[7]: 3 The latter possibility could explain Hiʻiaka's higher albedo if its surface contains water ice grains between50 and 100 μm in size, similar to those seen inSaturn's bright icy moonsEnceladus andTethys.[7]: 3
Namaka and Hiʻiaka are widely believed to be fragments of Haumea that were ejected in the aftermath of a giant impact 4.4 billion years ago (77–82 million years after theformation of the Solar System), whenNeptune wasmigrating outward and gravitationally scattering objects in theKuiper belt.[21]: 1–2, 14 Thisimpact 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 anellipsoidal body, becoming Haumea today.[21]: 2 While this hypothesis explains Haumea's rapid rotation and high bulk density, it fails to explain the existence of Haumea's moons andfamily of icy KBOs on similar orbits, because such an energetic impact would have ejected fragments at speeds several times Haumea'sescape velocity.[21]: 2
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 (147–162 million years after Solar System's formation).[5]: 15 [21]: 1, 14 A 2022 study led by Jessica Noviello and collaborators proposed that Haumea continueddifferentiating and growing its rocky core after the giant impact, which led to a gradual speed-up of Haumea's rotation rate as a consequence ofangular momentum conservation.[21] 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.[21]: 2–3 About 3% of Haumea's initial mass and 14% of its initialangular momentum were lost via rotational fissioning.[21]: 1
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| Ranked by size | ||
