Had Pluto itself not proved to be so spectacular when NASA’s New Horizons probe flew past last year, there can be no doubt that its large moon Charon would have won more admirers.
The remarkable moon has a mysterious dark-red stain over its north pole, called “Mordor Macula” by the New Horizons team – where Macula means “dark spot” and Mordor refers to the “black land” in Tolkien’s The Lord of the Rings. While many bodies in the solar system have polar caps or hoods of some sort, these are typically bright, due to reflective ice or frost of some kind, rather than dark. So what’s going on at Charon? A new study, published in Nature, has proposed an answer.
One of Charon’s most interesting features is a vast chasm system, which cuts across the middle of Charon’s Pluto-facing hemisphere and reaches a maximum depth of 7.5km. This speaks of an era of upheaval when slabs of Charon’s icy crust were ruptured, tilted and then partially flooded by a type of lava produced by icy volcanism.
Charon’s surface is mostly dirty, grey water-ice, and the red stain around the north pole looks to be a thin film, coating but not burying the underlying topographic features such as craters.
Mordor Macula resembles the red-stained areas of Pluto. This led to early suggestions that they share a similar origin. However, Pluto’s red stuff is understood to be tarry molecules called tholins, which form in the atmosphere when sunlight strikes molecules and makes them sufficiently reactive to link together. This forms haze particles, which eventually settle to the ground. Unlike Pluto, however, Charon has no trace of an atmosphere, so how can tholins form there?
Molecules of Mordor Macula
The researchers behind the new study think they have the answer. The New Horizons probe detected two sorts of molecules leaking away from Pluto: methane and nitrogen. Although nitrogen is the most abundant gas in Pluto’s atmosphere, methane is lighter and is being lost about 500 times faster. Tholins and their precursors are far too heavy, and cannot escape Pluto in the same way, so Charon’s tholins must be somehow made from methane arriving from Pluto.
Charon orbits very close to Pluto, and quite a lot of Pluto’s escaped methane falls onto its surface. If a methane molecule strikes the night side of Charon it will stick, particularly near the winter pole where the temperature is lowest (less than 30 degrees above absolute zero). This is because at lower temperatures surface molecules vibrate more slowly, and atmospheric molecules travel more slowly, so collisions are gentle. However, if a methane molecule hits a sunlit part of the surface, it will tend to bounce off and fly away into space because of Charon’s weak gravity (which has long since allowed it to lose any atmosphere that it may once have possessed).
Ultraviolet sunlight shining on methane molecules coating Charon’s surface could stimulate photochemical reactions to link them into progressively longer molecular chains, to form tholins. However, the “Catch 22” is that a methane molecule on the cold night-side surface ought to be able to break free in the daytime, when the temperature rises to the giddy heights of -220°C – allowing it to float away into space and become permanently lost. And as there is no sunlight at night, methane molecules can’t link together (making them too massive to float away) before the dawn of a new day.
In a surprising result the new study shows that, even by night, Charon’s poles receive enough ultraviolet light to allow methane molecules to link together. This is because interplanetary dust in the region scatters sunlight in all directions, including onto the night side of Charon. Once a molecule is a chain of two or three methanes linked together (with or without the occasional nitrogen for good measure) it is probably heavy enough to stay bound to the surface even during the daytime. Normal daytime sunlight can then take over to complete the process of forming tholins.
But what about the south pole? Could Mordor Macula’s location at Charon’s north pole, where the temperature is low enough for cold-trapping of methane, be just a fluke? Apparently not. Although it was winter in Charon’s southern hemisphere when New Horizon made its flyby, keeping its south pole in darkness, the researchers were able to study the south pole by “Pluto shine” (sunlight reflected off Pluto). The spatial resolution of these data is poor, but it is sufficient to demonstrate that Charon’s south pole has similarly dark red material at its surface. So there is not just one Mordor on Charon, but two.