Astronomers have taken their first detailed look at Pluto's companion Charon, the most distant known moon in the solar system. This dark body appears to be covered with a mixture of water ice and ammonia ice, a chilly brew quite different from the frosts of methane, carbon monoxide, and nitrogen ices that sheath the surface of Pluto. The marked contrast is surprising, for many astronomers have regarded Pluto and Charon as a "double planet."
Pluto and Charon orbit each other every 6.4 days at a distance of just 20,000 kilometers--about 20 times less than the separation between Earth and its moon. This tight pirouette makes Charon exceedingly difficult to study and obscured its very presence until 1978. Images of the two objects usually blur together even in powerful telescopes. A rare night of spectacular atmospheric conditions at the W. M. Keck Observatory in Hawaii last May allowed planetary scientist Michael Brown of the California Institute of Technology in Pasadena to distinguish the light from Pluto and Charon more clearly than ever before.
Brown teamed with Wendy Calvin, a spectroscopy expert at the University of Nevada, Reno, to analyze the outer skin of Charon based on the spectrum of infrared light reflected from the moon. They saw a surface dominated by crystalline water ice, commonly seen on other moons in the outer solar system. But a curious dip in part of the spectrum demanded further explanation, the team reports in today's Science. Using models of the light reflected from a variety of ices and minerals, Brown and Calvin deduced that the most likely cause is ammonia ice, which until now astronomers had seen only in comets.
Early in the solar system's history, a large object probably struck Pluto; the debris later coalesced into Charon, astronomers believe. Volcanoes driven by the heat of the impact may have coated the moon's surface with a slushy ammonia-water mixture. Then, after the heat faded away, Charon would have frozen solid. Pluto, nearly five times more massive than Charon, retains a shroud of methane, carbon monoxide, and nitrogen in a thin atmosphere and as surface frosts, Brown says. In contrast, Charon's gravitational pull is too feeble to hold on to those volatile gases.
Planetary scientist Eliot Young of the Southwestern Research Institute in Boulder, Colorado, isn't yet sold on Charon's ammonia ice. "It's worth following up, but it's hard to check an exhaustive list of compounds when you are trying to match a small depression in a spectrum," he says. Even so, Young is excited by the prospect of learning more about Charon's composition. "If you're interested in what the early solar system was made of, Pluto, Charon, and comets are among your best bets."