Hiʻiaka (moon)
Michael E. Brown spotted the moon on the 26th of January 2005 using adaptive optics at the W. M. Keck Observatory on Mauna Kea, Hawaii. The discovery remained private until the 29th of July 2005 when the team announced their find to the public. At that time, Brown referred to the dwarf planet as Santa and nicknamed this new satellite Rudolph after one of his reindeer. The provisional designation HM1 indicated it was the first moon found around Haumea in 2005. Official naming came later on the 17th of September 2008 from the International Astronomical Union. Brown's team proposed Hawaiian names in September 2006 to honor the location where they discovered these distant objects. Hiiaka serves as the patron goddess of hula and is a daughter of the fertility goddess Haumea.
Stellar occultations observed on the 6th of April 2021 and the 16th of April 2021 revealed an elongated shape resembling an ellipsoid with specific dimensions. These measurements correspond to a volume-equivalent diameter that makes Hiiaka the sixth-largest known moon orbiting a trans-Neptunian object. Despite its size, the moon lacks hydrostatic equilibrium because its elongated form contradicts expectations for its current rotation period. High material strength likely explains why the body has not rounded itself out over billions of years. Hubble Space Telescope data measured gravitational perturbations showing a mass estimate near 4.9 times 10^20 kilograms. A simplified assumption regarding Haumea's oblateness suggests a slightly lower mass figure. This low density indicates a highly porous interior composed mostly of loosely packed water ice and rock. The moon remains too small to undergo internal differentiation or develop a substantial core.
Hiiaka completes one full rotation about its axis every 9.68 hours without tidally locking to its parent planet. A 2016 study analyzed brightness variations from 2009 through 2010 using Magellan and Hubble Space Telescope observations. Data showed periodic fluctuations in brightness reaching 19 percent or 0.23 magnitudes as the moon turned. Plotting this light curve produced a sawtooth waveform indicating irregularities and angular features on the surface. No change occurred in these rotational brightness patterns over fifteen years of observation. This stability confirms that the moon's rotation aligns with Haumea's own spin axis. The axial tilt or obliquity sits close to zero degrees relative to the dwarf planet. Simulations suggest gravitational perturbations should cause the spin axis to precess over decades. Monitoring gradual changes in light curve amplitude could determine the exact rate of this axial precession.
Spectroscopic data reveals deep absorption features at wavelengths of 1.5 micrometers and 2.0 micrometers dominated by water ice. An additional feature at 1.25 micrometers indicates the ice exists primarily in crystalline form rather than amorphous states. The geometric albedo measures 0.74, making it even more reflective than Haumea itself which scores 0.51. Near-infrared spectroscopy shows deeper water ice absorption features compared to the parent body. These differences suggest fresher or purer ice or larger particle sizes between 10 micrometers and 1 millimeter. Such grain sizes resemble those found on Saturn's bright icy moons Enceladus and Tethys. Cryovolcanism remains unlikely due to small size and lack of tidal heating mechanisms. Scientists have yet to identify a resurfacing mechanism that prevents cosmic ray irradiation from turning all surface ice into an amorphous state.
A giant impact occurred 4.4 billion years ago shortly after the formation of the Solar System. This event involved two large Kuiper belt objects colliding obliquely and merging into a rapidly rotating body. While this explains Haumea's rotation, it fails to account for the existence of its moons and family of icy objects. A 2022 study led by Jessica Noviello proposed rotational fissioning happened roughly 80 million years after the initial collision. Centrifugal forces grew so great at the equator that icy material began ejecting into orbit around Haumea. About 3 percent of Haumea's initial mass and 14 percent of its angular momentum were lost during this process. The ejected fragments eventually coalesced into the disk that formed Hiiaka and Namaka. This theory suggests the moon accumulated from icy debris flung off by Haumea's rapid spin rather than direct ejection from the original impact.
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Common questions
When was the moon Hi'iaka discovered by Michael E. Brown?
Michael E. Brown spotted the moon on the 26th of January 2005 using adaptive optics at the W. M. Keck Observatory on Mauna Kea, Hawaii.
What is the origin story for the formation of the moon Hi'iaka?
A 2022 study led by Jessica Noviello proposed rotational fissioning happened roughly 80 million years after the initial collision to form the moon Hi'iaka from icy debris flung off by Haumea's rapid spin.
How large and dense is the moon Hi'iaka compared to other moons?
Hiiaka serves as the sixth-largest known moon orbiting a trans-Neptunian object with a mass estimate near 4.9 times 10^20 kilograms and low density indicating a highly porous interior composed mostly of loosely packed water ice and rock.
What surface composition does spectroscopic data reveal about the moon Hi'iaka?
Spectroscopic data reveals deep absorption features dominated by water ice in crystalline form with a geometric albedo measuring 0.74 which makes it even more reflective than its parent body Haumea.
Why has the moon Hi'iaka not achieved hydrostatic equilibrium despite its size?
The elongated shape resembling an ellipsoid contradicts expectations for its current rotation period so high material strength likely explains why the body has not rounded itself out over billions of years.