Washington: Nasa’s New Horizons spacecraft has discovered a vast, craterless plain in Pluto’s ‘heart’ that appears to be no more than 100 million years old, and is possibly still being shaped by geologic processes.

This frozen region is north of Pluto’s icy mountains, in the centre-left of the heart feature informally named Tombaugh Region. “This terrain is not easy to explain," said Jeff Moore, leader of the New Horizons Geology, Geophysics and Imaging Team (GGI) at Nasa’s Ames Research Centre in California.

“The discovery of vast, craterless, very young plains on Pluto exceeds all pre-flyby expectations," said Moore. This fascinating icy plains region—resembling frozen mud cracks on Earth—has been informally named “Sputnik Planum" (Sputnik Plain) after the Earth’s first artificial satellite.

It has a broken surface of irregularly-shaped segments, roughly 20km across, bordered by what appear to be shallow troughs, Nasa said. Some of these troughs have darker material within them, while others are traced by clumps of hills that appear to rise above the surrounding terrain.

Elsewhere, the surface appears to be etched by fields of small pits that may have formed by a process called sublimation, in which ice turns directly from solid to gas, just as dry ice does on Earth. The irregular shapes may be the result of the contraction of surface materials, similar to what happens when mud dries.

Alternatively, they may be a product of convection, similar to wax rising in a lava lamp, researchers said. On Pluto, convection would occur within a surface layer of frozen carbon monoxide, methane and nitrogen, driven by the scant warmth of Pluto’s interior.

Pluto’s icy plains also display dark streaks that are a few miles long. These streaks appear to be aligned in the same direction and may have been produced by winds blowing across the frozen surface. The “heart of the heart" image was taken when New Horizons was 77,000km from Pluto, and shows features as small as 1km across.

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.