Washington: NASA's New Horizons spacecraft has captured its last and best image of the four mysterious dark spots on Pluto's far side, ahead of its historic flyby of the dwarf planet.
The spots appear on the side of Pluto that always faces its largest moon, Charon - the face that will be invisible to New Horizons when the spacecraft makes its close flyby on July 14, NASA said.
New Horizons principal investigator Alan Stern of the Southwest Research Institute, Boulder, Colorado, described the image as "the last, best look that anyone will have of Pluto's far side for decades to come."
The spots are connected to a dark belt that circles Pluto's equatorial region. What continues to pique the interest of scientists is their similar size and even spacing.
"It's weird that they're spaced so regularly," said New Horizons programme scientist Curt Niebur at NASA Headquarters in Washington.
"We can't tell whether they're plateaus or plains, or whether they're brightness variations on a completely smooth surface," said Jeff Moore of NASA's Ames Research Centre, California.
The large dark areas are now estimated to be 480 kilometres across, an area roughly the size of the state of Missouri.
In comparison with earlier images, we now see that the dark areas are more complex than they initially appeared, while the boundaries between the dark and bright terrains are irregular and sharply defined, NASA said.
In addition to solving the mystery of the spots, the New Horizons Geology, Geophysics and Imaging team is interested in identifying other surface features such as impact craters, formed when smaller objects struck the dwarf planet.
"When we combine images like this of the far side with composition and colour data the spacecraft has already acquired but not yet sent to Earth, we expect to be able to read the history of this face of Pluto," said Moore.
When New Horizons makes its closest approach to Pluto, it will focus on the opposing or "encounter hemisphere" of the dwarf planet.
On July 14, New Horizons will pass about 12,500 kilometres from the face with a large heart-shaped feature that has captured the imagination of people around the world.