22nd June: Charon

On this date in 1978, Charon, Pluto's satellite, was discovered. Here are ten facts about Pluto's moon.

1. It was discovered by United States Naval Observatory astronomer James Christy in Washington, D.C., using photographic plates of Pluto. He noticed that every so often, a slight elongation appeared on the side of Pluto. Further investigation showed it to be a separate body.
2. Christy named the new object "Charon" after his wife, Charlene, whose nickname was "Char". His colleagues objected at first and wanted to call it Persephone in keeping with the naming of moons after mythological figures. However, Christy discovered that Charon was a mythological figure - the ferryman who ferries souls across the river Styx - so the name stuck.
3. The name Charon means “of keen gaze”, either fierce, flashing eyes, or to eyes which are a bluish-gray colour.
4. Pluto has four other moons - Nix, Hydra, Kerberos and Styx, but Charon is by far the largest. It has half the diameter of Pluto and an eighth of the mass, which is pretty large in comparison to its parent body. Some scientists suggest it might not be a moon at all, but that Pluto and Charon form a binary system. The barycentre, or centre of orbit, is outside of Pluto.
5. Charon is tidally locked with Pluto which means anyone standing on Pluto would always see the same side of Charon - which is the case with our own moon, too.
6. Charon and Pluto orbit each other every 6.387 days.
7. Photos from the New Horizons spacecraft show that Charon is darker at the poles. Hence the geeks at NASA have nicknamed the area "Mordor". They think the variation in colour is due to seasonal changes, where some substances escape when the area is heated by the sun, leaving behind substances called tholins, which are reddish in colour. After a few million years the icy crust is covered by the tholins.
8. Tholins are organic macromolecules which could be the building blocks for life.
9. Charon has an unusual surface feature that looks like a mountain in a moat. Scientists don't know yet how this came about.
10. Charon may have been formed by a collision with another object around 4.5 billion years ago. The other object disintegrated and blasted off a big chunk of Pluto. The debris coalesced to form Charon. However, Charon and Pluto are sufficiently different from each other in terms of how icy and rocky they are, suggesting they may have been two separate objects which collided but didn't destroy each other, and were then caught in each other's gravity. If that happened, there was quite possible an exchange of gases between the two - methane, nitrogen and related gases would have been released from the atmosphere of Pluto and travelled the 19,000 km (12,000 mi) or so to Charon.