As the New Horizons spacecraft approaches Pluto, the dwarf planet become ever more fascinating — and now it's tantalizing scientists with a strange string of dark spots across its surface.

The four spots are all about 300 miles wide, and why they would all be around the same size and why so regularly spaced along the planet's equator, is unclear, those scientists say.

Unfortunately, NASA researchers say, the string of dark patches are on the hemisphere New Horizons will not be looking at during its closest flyby of the planet on July 14.

However, they say, the space probe will have plenty of chances to examine them in the days leading up to that close encounter.

Nothing like it has been seen on any other planet in our solar system, they note.

 "It's a real puzzle — we don't know what the spots are, and we can't wait to find out," says mission principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colorado.

NASA has released true color images of Pluto and its largest moon, Charon.

That's presented another mystery, Stern says — the dramatic difference in the color and appearance of Pluto when compared with the much darker and grayer Charon.

Many scientists have suggested the dwarf planet and its biggest moon are the result of a collision of two primitive bodies in the early era of the solar system — but in that case, it would seem likely they would be similar in color and appearance.

Researchers are hoping New Horizons' close flyby will answer this and many other questions.

The spacecraft will pass Pluto at a distance of about 7,700 miles, traveling at about 8.5 miles per second; much too fast to go into orbit.

Still, as it hurtles by, it will capture as much data as possible on Pluto and its five moons, sending information and photos back to Earth.

One thing the scientists will be looking for in New Horizons' closer looks at the planet is clouds.

The same two instruments that created the images showing the mysterious spots — the Long-Range Reconnaissance Imager (LORRI) and the Ralph digital imager — will be used to look for clouds across the dwarf planet as it both approaches and departs from the planet.

"We're looking for clouds in our images using a number of techniques," says science team postdoctoral researcher Kelsi Singer at the Boulder institute. "If we find clouds, their presence will allow us to track the speeds and directions of Pluto's winds."