Today's APOD is an Extra Detail Magnification (or "EDM", for short) of a detail obtained from a picture taken by the NASA - New Horizons Spacecraft which is a fantastic High-Resolution View of Charon just before the closest approach, which occurred on July 14, 2015. The Original Image - which here has been re-processed in Absolute Natural Colors - combined a series of blue, red and InfraRed images that were all taken by the Ralph/Multispectral Visual Imaging Camera (or "MVIC", for short), located onboard the NASA - New Horizons Spacecraft.
The most striking Surface Feature visible in this EDM is the gray/brown North Polar Region of Charon, for now just informally named Mordor Macula, which, if you pay attention, seems to be a large - and relatively flat-floored - Depression, surrounded by a Wall, several Impact Craters of various dimensions and large Hills and Mountains, which covers the entire North Pole of Charon.
Its origin, as well as its truly peculiar color, are far away for being understood; however, and in our humble opinion, as IPF, the possibility that what we are looking at is the remnant of a powerful Impact - which must have occurred eons and eons ago -, or the Remains of a simply gigantic Volcanic Edifice, could - at least until we shall get more information (if ever) about this unbelievable Surface Feature - be taken into some consideration.
For your information, Charon is roughly 754 miles (such as approx. 1213,44236 Km) across; this image resolves Surface Details as small as about 1,8 miles across (such as approx. 2,896812 Km) per picture element (---> pixel). Remember that the NASA - New Horizons Spacecraft traveled more than three billion miles over nine-and-a-half years to reach the Plutonian System.
The image (which is a crop obtained from an Original NASA - New Horizons Spacecraft's enhanced color and NON Map-Projected View published on the NASA - Planetary Phojournal with the ID n. 19968)