U.S. space agency NASA's New Horizons mission team on Friday released a new close-up image of Pluto that reveals a mysterious icy-plains region in the center of the dwarf planet's "Heart."

The plains appear to be no more than 100 million years old, and are possibly still being shaped by geologic processes, NASA said in a statement.

"This terrain is not easy to explain," said Jeff Moore, leader of the New Horizons Geology, Geophysics and Imaging team. "The discovery of vast, craterless, very young plains on Pluto exceeds all pre-flyby expectations."

This image was acquired by the Long Range Reconnaissance Imager (LORRI) on Tuesday from a distance of 77,000 km. Features as small as a 1 km across are visible. New Horizons' Ralph instrument has revealed evidence of carbon monoxide ice.

This fascinating icy-plains region, informally named "Sputnik Planum" (Sputnik Plain) after the first artificial satellite, has a broken surface of irregularly shaped segments, about 20 km across, bordered by what appear to be shallow troughs.

This frozen region is in the center-left of the heart feature, north of Pluto's icy mountains, informally named "Tombaugh Regio" after Clyde Tombaugh, who discovered the dwarf planet in 1930.

An image of an equatorial region near the base of Pluto's bright heart-shaped feature shows mountains rising as high as 3,500 meters from the surrounding terrain.

Mission scientists will learn more about these mysterious terrains from higher-resolution and stereo images that New Horizons will pull from its digital recorders and send back to Earth during the next year.

The New Horizons spacecraft performed the first-ever flyby of Pluto on July 14, at about 12,472 km above the surface, and made the closest approach to the dwarf planet at 1149 GMT.