The New Horizons spacecraft has revealed an unexpected dark spot on the surface of Charon, the largest moon of Pluto. The dark area appears over a polar region of the distant body.

Charon is the largest satellite in that system, but astronomers were never able to see detail on the surface of this body until now. Many astronomers believed the moon would be nearly entirely covered in ice, due to its great distance from the sun.

"This system is just amazing. The science team is just ecstatic with what we see on Pluto's close approach hemisphere: Every terrain type we see on the planet — including both the brightest and darkest surface areas — are represented there, it's a wonderland!" said Alan Stern, the New Horizons Principal Investigator for the Southwest Research Institute.

The dark pole of Charon is being referred to as an anti-polar cap, just the opposite of the ones seen on our home planet.

Charon was discovered in June 1978, by James Christy, an astronomer working for the U.S. Naval Observatory. The researcher was examining highly-magnified images of Pluto when he found the body, then thought of as the ninth planet in the Solar System, and noticed it did not appear to be round. What first looked like a bump on the side of Pluto turned out to be the largest satellite in Pluto's system of moons.

Charon has a diameter of approximately 750 miles, roughly half that of Pluto. From the surface of Pluto, Charon would appear seven times as wide as our own moon as seen from Earth. Despite its significant volume relative to its partner, the satellite has a mass just 12 percent that of the dwarf planet. Astronomers believe this likely suggests Charon is composed of a nearly 50-50 composite of rock and ice. Pluto, on the other hand, is believed to be around 70 percent rock.

"Even though Pluto and Charon are partners, they are known to be quite different in appearance and composition. As New Horizons reveals them in far greater detail than ever before possible, we hope to find out why that's so," Stern said.

Astronomers already know of five moons orbiting Pluto, including Charon, although more may be discovered as New Horizons closes in on Pluto. The closest approach will take place on July 14, 2015. Because of the relative sizes of Pluto and Charon, the two bodies are often classified as a double planet, although neither are true planets.

The New Horizons team just learned they will benefit from fortunate planetary mechanics. When the spacecraft passes over the dwarf planet, it will pass over the hemisphere that appears to have the most varied and interesting landscape.