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See NASA's 'last, best look' at Pluto's mysterious dark spots

NASA's New Horizons probe gets one last glimpse at four strange dark spots along Pluto's equator as scientists puzzle over their origin.

Pluto's dark spots
The dark spots are visible at the lower end of this Pluto image. NASA/JHUAPL/SWRI

Dwarf-planet Ceres isn't the only heavenly body littered with strange spots. The asteroid-belt-dwelling object has some competition from dwarf planet Pluto, the focus of NASA's New Horizons spacecraft. A series of four unusual dark spots first came into view in early July as the probe closed in on the target of its nearly 10-year mission.

The early images make the spots look circular and uniform, but a new image reveals some fascinating details that weren't visible from so far away. The picture was taken Saturday at a distance of around 2.5 million miles from Pluto. On Tuesday, New Horizons will buzz the dwarf planet from just 7,800 miles away -- the closest it will come on the mission.

The boundaries are irregular and the spots themselves very complex, looking more like splashes of ink than circles. They are around 300 miles across.

"We can't tell whether they're plateaus or plains, or whether they're brightness variations on a completely smooth surface," Jeff Moore, of NASA's Ames Research Center, said in a statement.

The spots are located near Pluto's equator on the side that faces Charon, the dwarf planet's largest moon. New Horizons is moving closer in, but it won't have a good view of this side of Pluto. New Horizons principal investigator Alan Stern said this picture is "the last, best look that anyone will have of Pluto's far side for decades to come."

The spacecraft launched in early 2006 with a mission to study Pluto and its moons. Scientists hope to learn more about the dwarf planet's geologic makeup and surface features as the probe delivers clearer images and gathers data from close up.