The data sent back to the National Aeronautics and Space Administration (NASA) center indicates a plain no more than 100 million years old. It is still possibly being shaped by geologic processes.

NASA scientists have informally named the plain Sputnik Planum after the Earth's first artificial satellite.

The plain lies north of Pluto's icy mountains and shows irregular shaped segments ringed by narrow troughs as well as features appearing to be groups of mounds as well as fields of small pits.

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

The images were taken by the Long Range Reconnaissance Imager (LORRI) on July 14 from a distance of 48,000 miles (77,000 kilometers).

Pluto's icy plains also display dark streaks that are a few miles long, aligned in the same direction. They may have been produced by winds blowing across the frozen surface.

Alan Stern, New Horizons principal investigator from the Southwest Research Institute said: "We've only scratched the surface of our Pluto exploration, but it already seems clear to me that in the initial reconnaissance of the solar system, the best was saved for last."

New Horizons is part of NASA's New Frontiers Program, managed by the agency's Marshall Space Flight Center in Huntsville, Alabama.

jm/kms (NASA, AFP)