Research Stories

by Robert Burnham

Charon is a very cold place. Frigid geysers spew material up through cracks in the crust of Pluto's companion world. Charon just might be the equivalent of an outer solar system ice machine, according to Jason Cook.

Cook is a doctoral student in the School of Earth and Space Exploration at Arizona State University. He led a team of planetary scientists studying the surface of Charon.

"There are a number of mechanisms that could explain the presence of crystalline water ice on the surface of Charon," Cook says. His team gathered spectral evidence that points consistently to cryovolcanism, the eruption of liquids and gases in an ultra-cold environment.

"Cryovolcanism brings liquid water to the surface where it freezes into ice crystals," Cook explains. "That implies that Charon's interior possesses liquid water."

Cook and his colleagues studied a number of other mechanisms that could explain the presence of water ice crystals on Charon. The only mechanism that explained the data was cryovolcanism.

Scientists know that cryovolcanism in the outer solar system is a fairly common occurrence. Enceladus is a moon of Saturn. Europa orbits Jupiter. Both show evidence of water ice oozing or spewing out from beneath the surfaces.

Evidence for the ice deposits came from high-resolution spectra. To gather the data, the scientists needed several high-tech systems. They used the Hawaii-based Gemini Observatory's Adaptive Optics system (ALTAIR) coupled with the near-infrared instrument (NIRI). They made observations with the Frederick C. Gillett Gemini North telescope on Hawaii's Mauna Kea. Their findings show the fingerprints of ammonia hydrates and water crystals spread in patches across Charon.

Cook says the discovery could have profound implications for other similar-type worlds in the Kuiper Belt. This region of the solar system extends out far beyond the orbit of Neptune. The Kuiper Belt contains many small bodies, the largest of which include Pluto and Charon.

Cook's team was looking to find evidence of methane, carbon dioxide, ammonia, and a form of ammonia called ammonia hydrate on the surface of Charon. Ammonia hydrate helps keep liquid water from freezing solid.

"Ammonia hydrate makes it easier for water to escape from the inside before it turns to ice," explains Steven Desch, an ASU assistant professor and Cook's thesis advisor. "It is literally an antifreeze. And like the antifreeze we're familiar with here on Earth, it depresses the melting point of water."

For more information, the published results appear in The Astrophysical Journal, volume 663.