Icy mountain ranges can be seen rising from Pluto's surface, according to the first close-up images released on Wednesday from NASA's New Horizons' spacecraft after its historic of flyby of the dwarf planet.
The mountains' elevation reaches 3400 metres, the US space agency said.
Scientists were also stunned to see a close-up section of Pluto that showed no sign of craters, despite its home in the Kuiper Belt, the region beyond Neptune where cosmic debris is constantly pelting Pluto and its five moons.
NASA said the findings suggested that Pluto was geologically active and contained parts that were youthful in astronomical terms - perhaps less than 100 million years old, a small fraction of the 4.5 billion year age of the solar system.
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"It might be active right now," said project scientist John Spencer.
Scientists first saw hints of a geologically active phenomenon on Triton, a moon of Neptune that was glimpsed by the Voyager 2 space mission in the 1980s. It also had virtually no impact craters.
"Now we have settled the fact that these very small planets can be very active after a long time and I think it is going to send a lot of geophysicists back to the drawing boards to try and understand how exactly you do that," said principal investigator Alan Stern.
"The bedrock that makes those mountains must be made of H20, of water-ice."
Other kinds of ice that are abundant on the surface of Pluto are made of nitrogen, methane and carbon monoxide.
"You can't make mountains out of that stuff," Spencer told reporters. "It is just too soft."
New Horizons, a $US700 million ($950 million) nuclear-powered spacecraft, spent much of Tuesday snapping pictures and collecting data as it zoomed by Pluto.
The piano-sized spacecraft passed 12,400 kilometres from Pluto's surface.
The information the spacecraft has gathered is only beginning to reach Earth, after a journey of nearly 10 years and 4.8 billion kilometres.
And the data is going to keep coming in for the next 16 months, according to Stern.
Pluto, long considered the farthest planet from the Sun before it was reclassified as a dwarf planet in 2006, has never before been explored.
