Astroboffins remain baffled by four mysterious dark spots captured in images buzzed back to Earth from NASA's New Horizons probe as it dashes towards its fateful rendezvous with dwarf planet Pluto.
In particular, scientists have been fascinated with the size and spacing of the spots, which face Pluto's largest moon, Charon, and are understood to be linked to a dark belt circling its equatorial region.
The project's program bod Curt Niebur, who is based at NASA's headquarters in Washington, said: "It's weird that they're spaced so regularly."
The New Horizons 'craft makes it closest flyby to the remote freezeworld less than 48 hours from now on 14 July at 7:49AM EDT (11:49AM GMT).
However, the probe has already snapped what will be the final "best look that anyone will have of Pluto's far side for decades to come," said principal prober Alan Stern of the Southwest Research Institute in Boulder, Colorado.
But those pesky dark spots – estimated to be 300 miles (480 kilometres) across – are proving big head-scratchers for the US space agency's boffins.
NASA's Jeff Moore said:
We can’t tell whether they’re plateaus or plains, or whether they’re brightness variations on a completely smooth surface.
Moore added that scientists may be able to piece the puzzle together once NASA combines "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."
As for showtime on 14 July, New Horizons will hurtle at 30,800 miles (49,600 kilometres) per hour as it zips past the dwarf planet roughly 7,800 miles (12,500 kilometres) from the "encounter hemisphere" side of Pluto's face, which has whale and heart-shaped features. ®