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Hiʻiaka (moon)
Larger moon of Haumea From Wikipedia, the free encyclopedia
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Hiʻiaka, formal designation (136108) 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 Hiʻiaka, 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 49,400 km (30,700 mi).
Hiʻiaka is an elongated and irregularly shaped body with a mean diameter of 369 km (229 mi), making it the fourth-largest known moon of a trans-Neptunian object. It has a very low bulk density between 0.46 g/cm3[b] and 0.69 g/cm3, which indicates it is mostly made of loosely-packed water ice and rock. Telescope observations have shown that Hiʻiaka has a highly reflective surface made of crystalline 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 moon Namaka, Hiʻiaka is believed to be a fragment of Haumea that was ejected in the aftermath of a giant impact 4.4 billion years ago.
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Discovery
Hiʻiaka 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.[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 temporary provisional designation S/2005 (2003 EL61) 1, which indicates it is the first moon of Haumea (then known as 2003 EL61) discovered in 2005.[2] At the time, Brown had been nicknaming Haumea "Santa," so he nicknamed the Hiʻiaka "Rudolph," after one of Santa Claus's reindeer.[10][11]
Haumea, Hiʻiaka, and Namaka were all officially named after Hawaiian deities by the International Astronomical Union (IAU) on 17 September 2008.[12] In Hawaiian mythology, Hiʻiaka is the patron goddess of hula and is the daughter of the fertility goddess Haumea.[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]
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Physical characteristics
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Size, mass, and density
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 To put this in perspective, if Hiʻiaka were in the asteroid belt, it would be larger than all but the three largest asteroids, after 1 Ceres, 2 Pallas, 4 Vesta, and 10 Hygiea. Hiʻiaka is the fourth-largest known moon of a trans-Neptunian object, after Charon (1212 km), Dysnomia (615 km), and Vanth (443 km).[d] In spite of its relatively large size, Hiʻiaka is not in hydrostatic equilibrium because its elongated shape is inconsistent with that expected for its current rotation period.[7]: 164
The mass of Hiʻiaka is estimated to be 1.213+0.322
−0.311×1019 kg, using precise relative astrometry from the Hubble Space Telescope and taking perturbations into account.[5]: 6 Hiʻiaka's diameter and mass indicate it has a very low density between 0.46 g/cm3[b] and 0.69 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
Shape and rotation
Hiʻiaka is an elongated body that rotates about its axis in 9.68 hours.[7]: 160 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.[14]: 2 Hiʻiaka's rotation period was first measured in a 2016 study using 2009–2010 observations from the Magellan and Hubble Space Telescope, which showed that Hiʻiaka's brightness periodically varies by 19% (0.23 magnitudes[5]: 11 ) as it rotates.[14] Plotting Hiʻiaka's light curve (brightness over time) shows a sawtooth 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 to precess 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 curve amplitude 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]
Spectrum and composition
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 at 1.65 μm indicates that the surface water ice is primarily in the crystalline form. Currently it is unclear why water ice on the surface has not turned into amorphous form as would be expected due to its constant irradiation by cosmic rays.[15]
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Origin
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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 the formation of the Solar System), when Neptune was migrating outward and gravitationally scattering objects in the Kuiper belt.[16]: 1–2, 14 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.[16]: 2 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.[16]: 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 [16]: 1, 14 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.[16] 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.[16]: 2–3 About 3% of Haumea's initial mass and 14% of its initial angular momentum were lost via rotational fissioning.[16]: 1
See also
- Namaka – the smaller moon of Haumea
- Haumea family – a population of water-ice rich Kuiper belt objects that were ejected from Haumea 4.4 billion years ago
- 2002 TX300 – one of the largest Haumea family KBOs, with diameter similar to Hiʻiaka
Notes
- The orbital elements listed in the infobox are time-averaged non-Keplerian orbital elements, which are derived from 2006–2015 Hubble Space Telescope (HST) observations.[5]: 5 These are listed as "HST-only fit" elements in Proudfoot et al. (2024),[5]: 3, 6 who found that the HST-only fit has the lowest systematic observational errors compared to the orbital elements derived from combined HST and Keck telescope observations.[5]: 5, 9
- The Hiʻiaka density of 0.685±0.134 g/cm3 reported by Vara Lubiano in 2023 uses the outdated Hiʻiaka mass of (1.8±0.1)×1019 kg from 2009.[7]: 163 Proudfoot et al. (2024) measured a lower mass for Hiʻiaka, which means the density of Hiʻiaka is must also be lower, if one uses Vara Lubiano's diameter measurement. Dividing the Proudfoot et al.'s (2024) mass by the volume calculated from Vara Lubiano's mean sphere-equivalent diameter gives a density of 461 ± 149 kg/m3 (0.461 ± 0.149 g/cm3) (uncertainties calculated via propagation of error).
- The average brightness difference between Hiʻiaka and Haumea in visible light is 2.81±0.08 magnitudes.[7]: 169 Observations in the Minor Planet Center's database give a visible light apparent magnitude of around 17 for Haumea;[4] adding Hiʻiaka's magnitude difference to Haumea's apparent magnitude gives an apparent magnitude of 19.8, rounded up to 20.
- see List of Solar System objects by size for a better comparison.
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References
External links
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