Pluto鈥檚 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鈥檛 a defect in observations, as had been assumed, but a giant moon. This was the largest moon to be discovered since 1846, when Neptune鈥檚 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鈥檚 chemical tutti-frutti, Charon鈥檚 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鈥檚 Titan to have subsurface oceans, and explaining features on Jupiter鈥檚 moon Europa and elsewhere that look like frozen flows.
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Ammonia has been found in the atmospheres of giant planets and in the plumes spouting from Saturn鈥檚 moon Enceladus, 鈥渂ut the only solid surface where we have identified ammonia in the entire solar system is Charon鈥�, 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鈥檚 landscape, perhaps lubricating geysers. 鈥淚 want to see if there are flows of semi-solid ammonia water ice, or eruptions,鈥� says McKinnon.
Charon probably formed in a similar way to Earth鈥檚 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 (快猫短视频, 6 June, p 16).
New Horizons should nail down the orbits, sizes and compositions of these moons, and perhaps discover others. 鈥淕iven that we keep finding these things we鈥檙e likely to find more,鈥� says McKinnon. With a full picture of the satellite system, models can be refined. If it turns out that the collision couldn鈥檛 have created the small moons after all, then they might instead have been captured from the surrounding Kuiper belt 鈥� suggesting that other dwarf planets out there are likely to have multi-moon systems too.
Read more: 鈥�Fly by Pluto with the New Horizons probe鈥�
This article appeared in print under the headline 鈥淐haron鈥檚 secrets鈥�
