GEMINI OBSERVATORY / AURA An artist's rendering depicts Pluto's moon Charon with Pluto in the background against the backdrop of the Milky Way. The plumes and brighter spots depicted at left on Charon are thought to be created as water with some ammonia hydrate mixed in "erupts" from deep beneath the surface. CLICK FOR LARGE

Pluto's moon has ice-spewing volcanoes

HILO » Astronomers using the Gemini telescope on top of Mauna Kea have discovered "ice volcanoes" on the little moon Charon circling the far distant dwarf planet Pluto, Gemini officials announced yesterday.

Liquid water from the interior of the moon bursts to the surface, where the minus-365 degree Fahrenheit temperature immediately turns it to ice crystals, Gemini officials said.

Gemini called it "an ice machine in the ultimate deep freeze."

The discovery was made by a team led by doctoral student Jason Cook at Arizona State University and published recently in the Astrophysical Journal.

Astronomers had known that Charon, discovered in 1978, had a surface covered with ice.

To learn that a dynamic process is coating the little moon, they needed the power of Gemini's 26-foot main mirror, optics that compensate for the ripple of the earth's atmosphere, and instruments that analyze infrared light.

They discovered that water remains in liquid form inside the moon because it has a natural antifreeze -- ammonia in the form of ammonia hydrate. The hydrate had been tentatively identified earlier.

"This clinches it," said Stephen Desch, Cook's thesis adviser at Arizona State University.

They also learned from spectral studies that the surface ice is in crystal form. If it had been there for any great length of time, cosmic rays would have broken down the crystals into an "amorphous" form.

Other distant moons -- Enceladus orbiting Saturn, Europa around Jupiter and possibly Ariel around Uranus -- all show signs of oozing or spewing water, Gemini officials said. But in those cases, the huge main planet sets up stresses that erupt the water.

Pluto, with a 1,413-mile diameter smaller than Earth's moon, is too small to cause such stresses in 728-mile diameter Charon.

Instead, water inside Charon is kept liquid by the ammonia "antifreeze" and heating from radioactivity, Gemini officials said.

"As some of the subsurface water cools and approaches the freezing point, it expands into the cracks in the ice shell above it," Cook said.

"Due to the expansion, even a small vertical crack of half a kilometer (about 1,600 feet) at the base of the ice shell will allow the material to propagate to the outer surface of Charon in a matter of hours, making that the conduit for the water," Cook said.

"As the water sprays out through the crack, it freezes and immediately 'snows' back down to the surface, creating bright ice patches that can be distinguished in near-infrared light," Gemini officials said.

These "ice volcanoes" are wispy. Even if there are repeated instances of them, they would need 100,000 years to coat the surface of Charon to a depth of 1 millimeter, less than 1/16 of an inch, Gemini said.