Hiʻiaka (moon)

Hiʻiaka, also known as (136108) Haumea I, is the larger moon that orbits the dwarf planet Haumea. It was discovered in 2005 by Michael E. Brown and the team at the Keck Observatory using special adaptive optics technology. This moon is named after Hiʻiaka, the patron goddess of the Big Island of Hawaii, who is one of the daughters of the goddess Haumea.

Hiʻiaka travels in a slightly stretched path around Haumea, completing one orbit every 49.5 days at a distance of about 49,400 kilometers. It is not a perfect circle but has an interesting shape, measuring about 370 kilometers across on average. Scientists think Hiʻiaka is made mostly of water ice mixed with rock, which makes it very light for its size.

Observations show that Hiʻiaka’s surface shines brightly because it is covered in crystalline water ice, similar to Haumea. The moon spins around its own axis roughly every 9.68 hours. Along with its smaller companion moon Namaka, Hiʻiaka is believed to have broken away from Haumea after a huge collision that happened about 4.4 billion years ago.

Discovery

Hiʻiaka was the first moon found 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. At the time, the discovery of Haumea itself had not been made public, so the discovery of Hiʻiaka was announced later on 29 July 2005. When Hiʻiaka was announced, it was given the temporary provisional designation S/2005 (2003 EL₆₁) 1.

Later, Haumea, Hiʻiaka, and another moon named Namaka were officially named after Hawaiian deities by the International Astronomical Union on 17 September 2008. In Hawaiian mythology, Hiʻiaka is the patron goddess of hula and is the daughter of the fertility goddess Haumea.

Physical characteristics

Stellar occultations by Hiʻiaka in April 2021 showed that the moon is stretched out, with dimensions of about 480 km × 360 km × 286 km (298 mi × 224 mi × 178 mi). This gives it a volume-equivalent diameter of 370 km (230 mi). Hiʻiaka is the sixth-largest known moon of a trans-Neptunian object, after Charon, Dysnomia, Vanth, Ilmarë, and Actaea.

Hiʻiaka rotates about its axis once every 9.68 hours. It is not tidally locked to Haumea because it likely formed far enough away for Haumea's gravitational effects to be weak. Observations from the Hubble Space Telescope measured Hiʻiaka's mass to be about 1.213 × 10¹⁹ kg. Its low density of 0.64 g/cm³ suggests a porous, icy interior, supporting the idea that it formed from icy fragments thrown off by Haumea's rapid rotation.

The surface of Hiʻiaka, like Haumea, is made mostly of water ice. This similarity supports the theory that Hiʻiaka came from material ejected during Haumea's formation. Hiʻiaka's surface has very deep absorption features at certain wavelengths, showing that its water ice is mostly in a crystalline form. Hiʻiaka has a very high reflectivity (geometric albedo) of 0.74, higher than Haumea's, possibly because its water ice grains are larger or purer.

Origin

Namaka and Hiʻiaka are thought to be pieces of Haumea that broke off after a huge crash 4.4 billion years ago. This happened when Neptune was moving outward and pushing objects in the Kuiper belt.

Scientists believe that Haumea and its moons formed when the planet spun so fast that pieces of it flew off into space. This happened about 80 million years after the big crash. As Haumea kept spinning, bits of ice and rock broke away and eventually came together to form its moons.