Pluto’s companion, Charon, is almost too large to be a moon (Image: Keck Telescope)
Pluto is an alien world in almost all respects, but in one way it resembles Earth: it has a large, close companion. In 1978, James Christy at the US Naval Observatory in Washington DC realised that a bump on some images of Pluto wasn’t a defect in observations, as had been assumed, but a giant moon. This was the largest moon to be discovered since 1846, when Neptune’s moon Triton was found a couple of weeks after its planet. As Pluto is god of the underworld, Christy named it Charon, after the ferryman who carries souls to the underworld across the river Styx.
Rather than Pluto’s chemical tutti-frutti, Charon’s surface is mainly water ice. But it also has a dash of ammonia, which produces a distinctive dip in its infrared spectrum. Since the 1970s planetary scientists have thought ammonia might act as an antifreeze, enabling chilly moons such as Saturn’s Titan to have subsurface oceans, and explaining features on Jupiter’s moon Europa and elsewhere that look like frozen flows.
Advertisement
Ammonia has been found in the atmospheres of giant planets and in the plumes spouting from Saturn’s moon Enceladus, “but the only solid surface where we have identified ammonia in the entire solar system is Charon”, says Bill McKinnon of Washington University in St Louis, Missouri. New Horizons will find out whether there is enough to inject some activity into Charon’s landscape, perhaps lubricating geysers. “I want to see if there are flows of semi-solid ammonia water ice, or eruptions,” says McKinnon.
Charon probably formed in a similar way to Earth’s moon, when something huge collided with the proto-Pluto and blasted out debris into a surrounding disc. Simulations show that Charon and the other moons, Nix, Hydra, Kerberos and Styx, could have grown from the disc. But in the simulations, Charon moves outwards after it forms and this destabilises the orbits of the small moons, says McKinnon. And the latest Hubble images suggest Kerberos is black as coal, while the other moons are significantly paler, hinting that Kerberos may have a different origin (91av, 6 June, p 16).
New Horizons should nail down the orbits, sizes and compositions of these moons, and perhaps discover others. “Given that we keep finding these things we’re likely to find more,” says McKinnon. With a full picture of the satellite system, models can be refined. If it turns out that the collision couldn’t have created the small moons after all, then they might instead have been captured from the surrounding Kuiper belt – suggesting that other dwarf planets out there are likely to have multi-moon systems too.
Read more: “Fly by Pluto with the New Horizons probe“
This article appeared in print under the headline “Charon’s secrets”
