“There Chairon stands, who rules the dreary coast –
A sordid god: down from his hairy chin
A length of beard descends, uncombed, unclean;
His eyes, like hollow furnaces on fire;
A girdle, foul with grease, binds his obscene attire”
— Virgil, “Aeneid”, Book 6, p. 298–301 (1st century BC)
Much like the mythological ferryman of Hades of the Greco-Roman pantheon, whose features and overall presence was unique and different from those of the master he served, Pluto’s moon Charon is a distinct world from the larger planet it orbits. And although our current knowledge of Pluto is considered to be scarce, that of Charon is even more limited still. Nevertheless, the latter is an extremely fascinating planetary world in its own right, whose contrast to Pluto can provide scientists with great insights about the larger population of Kuiper Belt objects that lie by the thousands beyond the orbit of Neptune.
As is the case with most of the discoveries that have characterised the history of astronomy, that of Charon was an unexpected one. The distant world was first spotted in 1978 by American astronomer James Christy at the U.S. Naval Observatory in Washington, D.C., while he was studying a series of photographic plates of Pluto. This fortuitous discovery proved to be a blessing for astronomers, which provided them with the means to better refine their measurements regarding the mass and size of Pluto itself while also allowing them to study a truly double-planet pair, a finding that was unique in the realm of the Solar System. Indeed, with a diameter of Charon of 1,206 km and a distance from Pluto of just 19,570 km, both celestial objects orbit their common center of mass, in essence forming a double-planet system.
It has long been theorised that because of this orbital configuration, Pluto and Charon formed together, from the accumulated orbital debris that surrounded the former after a giant impactor hit the latter during the early history of the Solar System. Yet the study of Charon has shown the mysterious moon is in many ways dissimilar to Pluto, casting some doubt to this hypothesis. For instance, a series of spectroscopic observations revealed Charon has a different chemical composition. Having a lower density than Pluto of 1.3 grams per cubic centimeter, Charon is dominated by water ice and is almost free of the nitrogen, carbon monoxide, and methane ices that are found on its planetary companion. Charon’s different chemical composition gives it a unique appearance as well, making it darker and much less reflective than the comparatively brighter Pluto. The predominance of water ices also means there is less rocky material on Charon’s interior, leaving open the possibility for the existence of underground reservoirs of liquid water. Detailed spectroscopic observations in 2007 uncovered the existence of crystalline water ice and hydrated ammonia in the form of patches across Charon’s surface. This finding is a significant one, because these chemical substances lose their crystalline structure due to solar irradiation at very short timescales of a few thousand years, meaning they must be constantly replenished by an unknown process on the frigid moon. After examining a series of possible mechanisms that could generate these chemical compounds, like solid-state convection and the movement of underground material from meteoroid impacts, scientists have concluded that cryovolcanism is the most likely source. The latter has been directly observed in various outer Solar System bodies like Saturn’s moon Enceladus and Neptune’s moon Triton, which are venting water and nitrogen ice respectively as well as other volatile elements into space through a series of geyser-like eruptions at their surfaces. “There are a number of mechanisms that could explain the presence of crystalline water ice on the surface of Charon,” says Jason Cook, a PhD student at the Arizona State University, who led the study that discovered the traces of crystalline water ice and hydrated ammonia on 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.”
At first glance, the current dynamical condition of the Pluto-Charon system disfavors such a possibility. Both Pluto and Charon are locked in a mutual synchronous rotation while having an almost circular, no-eccentric orbit with a period of 6.8 days, which makes the formation of any currently active geological processes on their surfaces unlikely. Nevertheless, the case for cryovolcanism and the potential existence of underground oceans on Charon have been bolstered by more recent studies, which have argued about the possibility for the existence of cracks on the surface of Pluto’s biggest moon Charon, not unlike those observed on the surface of Jupiter’s moon Europa.
A team of U.S. astronomers, led by Dr. Alyssa Rhoden, a planetary scientist at NASA’s Goddard Space Flight Center, in Greenbelt, Md., studied several formation and evolutionary models of the Pluto-Charon system, in order to check what specific geologic features each one would produce on the surface of Charon that could be possibly observed by the New Horizons spacecraft during its upcoming close flyby of the Pluto system on July 14. Since it is thought that both Pluto and Charon were initially much closer toward one another early in the history of the Solar System, they would have exerted much greater tidal stresses on each other during that time. By studying several models of Pluto’s and Charon’s dynamical evolution, scientists concluded these stresses would have caused severe tides on both celestial bodies, possibly leading to the formation of underground oceans of liquid water, similar to those that probably exist inside other outer Solar System moons as well, like Europa, Ganymede, and Enceladus, with some of it finding its way toward the surface through cracks on the upper crust. “As some of the subsurface water cools and approaches the freezing point, it expands into the cracks in the ice shell above it,” says Cook. “Due to the expansion, even a small vertical crack of a half a kilometer 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.”
Another great unknown surrounding Charon is the lack of any perceptible atmosphere. Previous observations have shown that contrary to its planetary companion, the faraway moon lacks the presence of an atmosphere. Yet newer theoretical studies have provided tantalising evidence that Charon may indeed be sharing the same atmosphere with Pluto! “There is a large body of [scientific] literature by atmospheric modelers going back to the late 1980’s which show that atmospheric molecules and atoms for that matter that are escaping Pluto’s atmosphere, have to pass the orbit of Charon,” commented on the subject Dr. Alan Stern, principal investigator for New Horizons mission at the Southwest Research Institute, in Boulder, Colo., during a recent NASA Google+ Hangout. “And much like a binary star system, Charon’s gravity can pull some of that atmospheric gas into orbit around itself and even onto the surface of Charon and create a secondary atmosphere [there] … It would be fantastic if we discovered something like this. Never in the history of planetary exploration have we seen two planetary bodies with a shared atmosphere, like what we may see in the Pluto system. It would be quite a wonderland!”
Much like its planetary companion, Charon is an extremely interesting and potentially active alien world, one that holds many well-kept secrets. Uncovering many of these secrets, which is only a matter of a few days, promises to completely rewrite our understanding of the entire outer Solar System, as well as our place in our planetary family at large.
One couldn’t ask for more exciting times to live through.
The last part of the article will appear tomorrow.
You can read Part 1 here.
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