In the 21 years since Charon was discovered, attempts to analyze its composition have always been complicated by the moon's proximity to Pluto. The two bodies are so close together (barely more than 12,000 miles, or 19,500 kilometers apart) and so far away from Earth (more than 30 times farther from the sun than Earth), that it has been difficult to make separate measurements of the two.
With the Keck observations, though, Brown was able to distinctly resolve the two and measure the spectral signature of Charon itself. He found that Charon -- which is composed predominantly of water ice at the surface -- is made of a form of ice that shouldn't exist at the frigid minus 370 degrees Fahrenheit (minus 225 Celsius) that is thought to be the hottest temperature the moon ever reaches.
The ice appears to be in a crystalline form: its molecules are neatly ordered and arranged into the hexagonal crystals that water naturally forms when it freezes. This is surprising because at cold temperatures in the depths of the solar system, ultraviolet radiation breaks down the neatly arranged crystalline structure, leaving ice in a random, broken down state that scientists call "amorphous."
At temperatures above minus 250 degrees Fahrenheit (minus 150 Celsius) or so, ice becomes crystalline on its own, but most scientists have thought Charon must have been so cold for so long, that its ice should all be amorphous.
The finding of crystalline ice described in a paper that Brown co-authored with Wendy Calvin, a professor of planetary geology at the University of Nevada-Reno. Brown and Calvin also argue that unique spectral features indicate that Charon's surface is made of about 25 percent ammonia and ammonia hydrate -- substances which might be evidence for volcanism on Charon. The paper is published in the Jan. 7 issue of Science.
Crystalline ice on the surface of Charon means that some event or condition has made the satellite much warmer than anyone had imagined possible, said Eliot Young, a Pluto expert at the Southwest Research Institute in Boulder, Colo.
"There's a couple of things that could cause it. Either something is heating it up, or there's been a recent event that deposited frost," Young said.
Such an event might be a huge cometary impact. Most scientists agree that at least one major projectile slammed into Pluto in its history. That is the popular explanation for why the planet seems to be tipped over. Pluto's axis is not perpendicular to its plane of orbit like that of most other planets. Instead, as seen from the plane of the solar system, the little globe lies on its side as it spins.
A huge cometary impact could have vaporized a great deal of water, which spread across Charon then condensed over its entire surface.
In their paper, Brown and Calvin suggest that the crystallized ice may be maintained by continuous bombardment from tiny meteorites. A steady shower of meteoritic particles may cause the vaporization and recrystallization of surface ice, they explain.
But there are other possibilities, Young said.
"There may be some heat source that we don't know about," Young said. "One possibility is that tidal stresses could dampen out energy and it would heat up." Another possibility could be that rocky material slowly sinking through a homogeneous mix of rock and ice could be releasing some measure of heat.
Whatever processes scientists decide on as candidates to explain the quizzical heating of Charon, the spectral analysis should help them determine the age of the moon's surface.
"Finding crystalline ice on Charon potentially, I think, puts a limit on how young Charon's surface is," Young said, "but we need better lab data to make that constraint a tight one."
Two big questions remain, he said. One is how fast does crystalline ice get turned into amorphous ice by ultraviolet radiation. The second question is how fast and at what temperature does amorphous ice return to crystalline form.
"Both of these are really long processes, so I think that extrapolating from lab data taken over the course of a few weeks to millions of years -- it's tough to know those rates accurately," Young said.
Volcanism in the Outer Solar System?
A second conclusion that Brown and Calvin draw from analysis of Charon's spectra is that the ice on the surface is about one-quarter ammonia hydrate and ammonia. While acknowledging that this argument is certain to be a very controversial, Brown said the spectral characteristics truly seem to indicate a mixture of water, ammonia and ammonia hydrate.
This is important because the presence of ammonia could show that Charon was at one time volcanically active, Brown said. "There is not that much ammonia in existence in the solar system, so it can't be that Charon is mostly made out of ammonia. If you see it, it must be a fairly thin surface veneer," he said.
Mixtures of ammonia and water melt at very low temperatures -- much lower than does pure water. Such mixtures can melt relatively easily in the interior of even cold planets, then erupt outward to flow across the surface, Brown said.
"It's just like seeing a volcanic flow on the surface of the Earth. It doesn't mean that the whole Earth is made out of this volcanic material. It just means that it has erupted and flows on the surface," he said, adding that there is really no other way to explain the high concentration of ammonia on the surface.
An ammonia-rich volcanic shell around the satellite would have profound implications for our understanding of how the outer planets and their moons have evolved, Brown said. "If there is volcanism on this small, cold body, then there has probably been volcanism of that sort on every icy satellite in the outer solar system, on everything out there."
But the existence of ammonia is far from proven, Young said. "It would be neat if there did turn out to be ammonia, but if I were setting the odds at Las Vegas, I would give you ten to one odds that it will turn out that there is ammonia on Charon," he said.
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