Humankind is taking one step closer into unveiling the mysteries at the edge of the solar system. 

NASA's New Horizons spacecraft revealed four mysterious dark spots that are evenly spaced along Pluto's equator.

NASA scientists told The LA Times that each of the spots is about 300 miles in diameter with a surface area similar in size to the state of Mississippi.

"New Horizons" principal investigator Alan Stern of the Southwest Research Institute in Colorado said that they don't know what the spots are, but they are dying to find out, as it serves as a real puzzle.

An image printed by ABC showed the mysterious spots with consistent even spacing along the dwarf planet's equator. Each spot is circular at about 480 kilometers in diameter.

The new black-and-white images indicated that Pluto has two different faces.

It has taken the New Horizons spacecraft more than nine years to traverse 3 billion miles of space between Earth and Pluto, but it is finally homing in on its ultimate destination. It is now more than 9.5 miles from the Pluto system. By July 14, the spacecraft will make its closest approach to the dwarf planet at 7,700 miles of the surface.

From that distance, New Horizons will be able to determine what Pluto is made of, create temperature maps of its multicolored surface and look for auroras in its thin atmosphere.

If a scientist believes that Pluto and Charon are the products of collision between two primitive bodies in the early Solar System, one could expect them to look similar, and this is what the flyby data of New Horizons will answer, said BBC. 

Compared to the sullen gray color of Charon, its largest moon, Pluto's surface has a much lighter tan. This color difference is also a mystery that researchers are trying to solve.