Recent images taken by NASA's New Horizons spacecraft reveal four mysterious dark spots that appear to be evenly spaced along Pluto's equator.
Each of the spots is about 300 miles in diameter and has a surface area similar in size to the state of Mississippi, NASA scientists report.
"It's a real puzzle -- we don't know what the spots are, and we can't wait to find out," said New Horizons principal investigator Alan Stern of the Southwest Research Institute, Boulder, Colo., in a statement.
The images show just how much lighter the mottled tan surface of Pluto is compared to the moody gray of its largest moon, Charon. The discrepancy in their color is yet another mystery that researchers hope to resolve soon.
It has taken the New Horizons spacecraft more than nine years to traverse 3 billion miles of space between the Earth and Pluto. But it is finally homing in on its ultimate destination.
New Horizons is now less than 9.5 million miles from the Pluto system.
On July 14 the spacecraft will make its closest approach to the dwarf planet, flying within 7,700 miles of the surface.
From that distance, New Horizons will be able to determine what Pluto made of, create temperature maps of its multicolored surface and look for auroras in its thin atmosphere.
The spacecraft will also take pictures of the planet's surface at a high enough resolution that if they were pictures of Los Angeles, they would show individual runways at Los Angeles International Airport.
The recently released images pictured above are a combination of black-and-white images taken by the spacecraft's Long-Range Reconnaissance Imager and lower-resolution color pictures from the spacecraft's Ralph instrument.
The pictures accurately depict what you might see if you were riding on the New Horizons spacecraft yourself, according to a NASA news release.