 In this photo taken by theHubble Space Telescope, Hi'iaka 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 |
|
| 369±23 km[7]: 162 | |
| Mass | 1.213+0.322 −0.311×1019 kg[5]: 6 |
Meandensity | 0.461±0.149 g/cm3[b] |
| 9.68±0.03 h[7]: 160 | |
| ≈90° to Haumea's equator[8] | |
| Albedo | 0.68±0.05[7]: 162 |
| Temperature | ≈40 K (same as Haumea)[9]: 8 |
| ~20[c] | |
| 3.24±0.08[7]: 163 | |
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, the patron 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 369 km (229 mi), making it the sixth-largest known moon of a trans-Neptunian object. It has a very lowbulk density between0.46 g/cm3[b] and0.69 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, and appears to rotate sideways with respect to its orbit around Haumea. 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,[10][11] 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.[10][11]
Haumea, Hiʻiaka, and Namaka were all officially named afterHawaiian deities by theInternational Astronomical Union (IAU) on 17 September 2008.[12] InHawaiian mythology,Hiʻiaka is the patron goddess ofhula and is the daughter of thefertility goddessHaumea.[12] 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.[13]
Stellar occultations by Hiʻiaka on 6 and 16 April 2021 reveal that the moon is elongated, with estimated ellipsoid dimensions of 476 km × 370 km × 286 km (296 mi × 230 mi × 178 mi).[7]: 162 These correspond to a volume-equivalent diameter of 369 km (229 mi).[7]: 162 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).[d] In spite of its relatively large size, Hiʻiaka is not inhydrostatic equilibrium because its elongated shape is inconsistent with that expected for its current rotation period.[7]: 164
The mass of Hiʻiaka is estimated to be1.213+0.322
−0.311×1019 kg, using precise relativeastrometry from theHubble Space Telescope and taking perturbations into account.[5]: 6 Hiʻiaka's diameter and mass indicate it has a very low density between0.46 g/cm3[b] and0.69 g/cm3, which suggests Hiʻiaka's interior consists of highly porous water ice with a rock mass fraction between 50% and 70%.[7]: 163–164
Hiʻiaka is an elongated body that rotates about its axis in 9.68 hours.[7]: 160 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.[14]: 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.[14] Plotting Hiʻiaka'slight curve (brightness over time) shows asawtooth waveform, which indicates that the moon has an irregular and angular shape, rather than an ellipsoidal one.[14]: 3, 5
Simulations show that gravitational peturbations by Haumea should cause Hiʻiaka's spin axis toprecess on a timescale of decades.[14]: 5 The axial precession rate of Hiʻiaka depends on its axial tilt or obliquity with respect to its orbit around Haumea; if Hiʻiaka has a larger obliquity, then its precession period would be longer.[14]: 5 The axial precession of Hiʻiaka may be determined by monitoring the gradual change in its light curveamplitude over several years.[14]: 5 [5]: 11 A preliminary analysis from 2022 found that Hiʻiaka's light curve amplitude did not change between 2010 and 2021–2022, which suggests that Hiʻiaka's obliquity is close to 90° with respect to its orbit around Haumea—in other words, Hiʻiaka may be rotating sideways in its orbit.[8]
The near infrared spectrum of Hiʻiaka is dominated by water-ice absorption bands, which means that its surface is made mainly of water ice. The presence of the band centered at1.65 μm indicates that the surface water ice is primarily in thecrystalline form. Currently it is unclear why water ice on the surface has not turned intoamorphous form as would be expected due to its constant irradiation bycosmic rays.[15]
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.[18]: 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.[18]: 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.[18]: 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 [18]: 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.[18] 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.[18]: 2–3 About 3% of Haumea's initial mass and 14% of its initialangular momentum were lost via rotational fissioning.[18]: 1
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