Mordor Macula

Mordor Macula is the informal name for a large dark area about $375 km$ in diameter near the north pole of Charon, Pluto's largest moon. It is named after the black land called Mordor in J.R.R. Tolkien's The Lord of the Rings.

Geography
Mordor Macula is located at Charon's north pole. Mordor Macula's dark "core" is roughly 375 kilometers in diameter, though a surrounding lighter halo extends further into the highlands of Oz Terra, beginning to appear north of 45°N and trending darker with increasing latitude.

The boundary of Mordor Macula, though ill-defined, is roughly circular; two major disruptions to this circular boundary are observed, however. One, named McCaffrey Dorsum, is a curvilinear ridge 1-2 kilometers high and stretching between ~150° and ~0°W between 72° and 77°N. McCaffrey Dorsum appears to act as a barrier to the spread or distribution of dark material. The pole-facing slopes of the ridge contain some of the darkest terrain observed on Charon, whilst the equator-facing slopes show little deposition of the dark material. The second disruption is Dorothy, the largest-known impact basin on Charon. The floor of Dorothy is significantly lighter, appearing to carve an arc into the dark core of Mordor Macula. An unnamed ~80 km wide impact crater within Mordor Macula also has a bright floor. Broadly, the distribution of Mordor Macula's dark material seems only loosely influenced by local topography.

Geology
The terrain Mordor Macula occupies appears to be rather dramatic, with numerous scarps and irregular depressions. In the direction of the 90°E meridian, Mordor Macula is roughly at the same altitude as Oz Terra. In the direction of the 90°W meridian by contrast, Mordor Macula's terrain gently slopes downwards to ~2 km below Charon's mean elevation before encountering the McCaffrey Dorsum. Beyond the ridge is a massive deep canyon, Caleuche Chasma. Due to Charon's current geological inactivity, Mordor Macula has been relatively unchanged since its formation.

The dark material is reddish-brown in color and has been likened to the dark equatorial belt of Pluto. Pluto's dark equatorial terrain is covered in deposits of tholins, a generic term for a tar-like mixture of radiation-processed organic compounds. As such, the dark reddish-brown coloration of Mordor Macula is likely caused by tholins as well. However, spectral observations of Charon's north pole is still dominated by signatures of water ice, so the dark deposits must be thin and relatively sparse. This is supported by the inconsistent correlation between the distribution of dark material and local topography, indicating that the material is not a thick and discrete deposit.

Origin
The origin of Mordor Macula is not completely understood. It may be a deposit of frozen gases captured from Pluto's escaping atmosphere. A leading hypothesis is that nitrogen and methane escape from Pluto's atmosphere and are then deposited into the cold poles of Charon, where scattered ultraviolet light then transforms the molecules into tholins, a generic term for a tar-like organic slurry. Despite the fact that during Pluto winter the Northern hemisphere goes without sunlight for more than 100 years, enough radiation makes it to Charon's surface to form the red tholins. The extreme orbital eccentricity and axial tilt of Pluto and Charon's orbits drive a large sublimation event during Plutonian spring, where methane and other gases from Pluto's surface may escape the planet's gravity and condense onto Charon's surface, an effect called "cold trapping". This hypothesis implies that a similar red spot should exist on Charon's south pole as well—and indirect evidence suggests this is true.

It is also possible that the Mordor Macula has a cryovolcanic origin. Observations of other Kuiper belt objects (KBOs) show similar features as Mordor Macula, making the atmospheric transfer model less likely. A second hypothesis of Mordor Macula's origin states that cryovolcanism in Vulcan Planitia pumped enough methane into the paleoatmosphere from Charon's subsurface to create Mordor Macula. Methane is first released into the atmosphere via cryovolcanism, where most of the gas migrates to Charon's north and south poles, freezing and becoming trapped there as ice. Some of the gas will reach Charon's escape velocity and be lost to space. After a long period of time, solar radiation irradiated the trapped methane ice and produced tholins, creating the dark deposits observed today.