Plutos companion Charon could contain liquid water

Gemini Observatorys ALTAIR optics system has revealed evidence of icy deposits of ammonia hydrates and water crystals on Plutos companion Charon. Scientists say this is the best described evidence yet for the existence of these compounds on worlds such as Charon.

Washington, July 18 : Gemini Observatory's ALTAIR optics system has revealed evidence of icy deposits of ammonia hydrates and water crystals on Pluto's companion Charon. Scientists say this is the best described evidence yet for the existence of these compounds on worlds such as Charon.

According to the observations, frigid geysers spew icy material up through cracks in the crust of Charon and recoat parts of its surface in ice crystals, transforming the rocky body into the equivalent of an outer solar system ice machine.

Scientists say this action could be occurring on timescales as short as a few hours or days and at levels that would coat Charon to a depth of one millimetre every 100 000 years.

Researchers say this discovery could have profound implications for other similar-type worlds in the Kuiper Belt, which is the region of the solar system that extends out beyond the orbit of Neptune and contains a number of small bodies, the largest of which include Pluto and Charon.

"There are a number of mechanisms that could explain the presence of crystalline water ice on the surface of Charon," said Jason Cook, the PhD student at Arizona State University who led the team of planetary scientists studying the surface of Charon.

"Our spectra point consistently to cryovolcanism, which brings liquid water to the surface, where it freezes into ice crystals. That implies that Charon's interior possesses liquid water," he said.

Cook further said the presence of water ice crystals on Charon are not likely to be "primordial ice" made at the time the solar system formed, because such ice would become amorphous (that is, it would lose its crystalline appearance) in a few tens of thousands of years, due to solar ultraviolet radiation and cosmic ray bombardment.

"Processes that create fresh, icy patches on other worlds, such as impact "gardening" by meteorites and convection of subsurface materials to the surface, are not supported by the chemical fingerprints of the water and ammonia hydrates on Charon's surface. The only mechanism that explained the data was cryovolcanism, the eruption of liquids and gases in an ultra-cold environment," he said.

Cook said the key to understanding cryovolcanism on Charon is to look at its physical makeup.

"Charon's surface is almost entirely water ice. So it must have a vast amount of water under the surface, and much of that should be frozen as well. Only deep inside Charon could water be a liquid. Yet, there is fresh ice on the surface, meaning that some liquid water must somehow reach the surface. The ammonia sitting on the surface provides the clue. It's the ammonia that helps keep some material liquid. It makes it all feasible. Without ammonia the water could not get out there," he said.

"In the case of Charon, it is thought that heat from internal radioactivity creates a pool of melted water mixed with ammonia inside the ice shell. As some of the subsurface water cools and approaches the freezing point, it expands into the cracks in the ice shell above it. Due to the expansion, even a small vertical crack of a half a kilometre at the base of the ice shell will allow material to propagate to the outer surface of Charon in a matter of hours, making that the conduit for the water.

"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. I half expect that if we ever get to actually see a plume going off on Charon, we'd be seeing the process that makes tiger stripes similar to what we see on Enceladus and other frozen worlds," he said.

The study "Near-infrared Spectroscopy of Charon: Possible Evidence for Cryovolcanism on Kuiper Belt Objects" appears in the Astrophysical Journal.

ANI