Ice cubes in space

Researchers determine the composition and orbit of two moons at the fringes of the solar system

You’d need a mighty tall glass to hold two space objects that researchers have now identified as ice cubes at the fringes of the solar system. The larger of the icy bodies is about the width of Ohio, the smaller about twice the length of Rhode Island. Both bodies are moons of the dwarf planet Haumea. The trio, discovered in late 2004 and 2005, reside in the Kuiper Belt, a reservoir of objects beyond the orbit of Neptune whose most famous denizen is Pluto.

This diagram shows the motion of the larger, outer moon, Hi’iaka and the inner moon Namaka orbiting the dwarf planet Haumea (central blue dot). Motions are shown from 2005 (in red) to 2008 (in purple). D. Ragozzine

Spectra taken of the larger and outermost of the two moons, dubbed Hi’iaka, had indicated that its surface, unlike most Kuiper Belt objects, is made of nearly pure crystalline water-ice. Now, new spectra, taken with the Hubble Space Telescope, not only confirm the composition of Hi’iaka, but for the first time also show that the surface of the smaller moon, Namaka, has the same composition. Because both moons are too small to have undergone heating and cooling that would have caused heavier elements to sink to the cores, the icy surfaces are likely to be fair representations of the moons’ interiors.

“These things could be, essentially, ice cubes,” says Michael Brown of the California Institute of Technology in Pasadena, a codiscover of Haumea and its moons. Brown and Caltech colleague Wesley Fraser describe the new observations online ( and in the April 10 Astrophysical Journal Letters.

The frozen findings aren’t just a cosmic curiosity. Haumea, whose rapid spin is thought to have reshaped it into a squashed football, is glazed with water-ice. (The dwarf planet’s interior, in contrast, is made up of much denser material.) The similarity between the surface of Haumea and its moons strongly suggests that these satellites were not Kuiper Belt residents that happened to be captured by Haumea, but were chipped off the surface of the dwarf planet as a result of some cataclysmic event.

Indeed, Haumea is the only Kuiper Belt object known to have a collisional family — chunks created when a large impactor, perhaps 500 kilometers in diameter, struck the dwarf planet in the distant past.

In Hawaiian mythology, Hi’iaka and Namaka are both daughters of Haumea, the goddess of fertility, and the new findings provide fresh evidence that these moons are indeed offspring of the dwarf planet, Brown says.

“At face value, it looks like Haumea’s collisional family and the moons are one and the same — the product of some extraordinary event” early in the history of the solar system, comments Daniel Fabrycky of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

In a separate study, Brown and Caltech colleague Darin Ragozzine used both Hubble and the Keck Observatory atop Hawaii’s Mauna Kea to track the motions of the two moons relative to Haumea. This detailed look at the moons’ orbits reveals that, as seen from Earth, Namaka and Haumea began transiting, or passing in front of each other, two years ago. The researchers posted their findings online March 26 (, and the report will also appear in an upcoming Astronomical Journal.

Over the next few years, Namaka will journey across different sections of Haumea. The duration of each passage and the amount of light dimmed from Haumea will reveal the exact shape and size of the bodies, Ragozzine says.

A particularly rare and intriguing event will happen this July 2, he adds, when Namaka passes in front of Hi’iaka. Observations of this passage could reveal a wealth of new information about both moons.

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