NASA's New Horizons spacecraft has spotted what appears to be a strange dark patch at the pole of Pluto's biggest moon, Charon. The sighting is whetting researchers' appetites ahead of the probe's epic flyby of the dwarf planet system next month.

New Horizons has also detected a rich diversity of terrain types in Pluto's "close approach hemisphere" — the side of the planet New Horizons will zoom past at a distance of just 7,800 miles (12,500 kilometers) on July 14. The newly resolved features, which New Horizons captured in images taken from May 29 through June 19, are visible in a Pluto-Charon video that NASA released Tuesday.

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"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!" New Horizons principal investigator Alan Stern, of the Southwest Research Institute, said in a statement. "And about Charon — wow — I don’t think anyone expected Charon to reveal a mystery like dark terrains at its pole. Who ordered that?" [Destination Pluto: NASA's New Horizons Mission in Pictures]

During Tuesday's mission update, New Horizons team members reported that the spacecraft performed a course-adjusting engine burn on June 14 that will put the probe in an ideal position to observe Pluto and its moons during the upcoming close encounter.

This enhanced image of Pluto's largest moon, Charon, shows an odd dark patch near one of its poles. The picture was produced by taking imagery from the New Horizons probe's LORRI long-range camera and sharpening the view through a process known as deconvolution. NASA / JHUAPL / SwRI

Over the past week, New Horizons' path to Pluto was declared "all clear" of dust and debris, said Hal Weaver, a project scientist for the mission. Because the spacecraft is traveling at more than 30,000 mph (50,000 kilometers per hour), a collision with a relatively small rock could seriously damage the spacecraft. Weaver said the most recent debris search was the deepest yet and did not turn up any potentially dangerous objects.

In addition to looking for hazards, scientists also searched for new satellites around Pluto but found none.

This is a condensed version of a report from Space.com. Read the full report. Follow Calla Cofield on @Twitter. Follow Space.com on Twitter, Facebook and Google+.