The first close-up photos of Pluto captured by the New Horizons spacecraft show the dwarf planet has mountains of ice as high as the Rocky Mountains on Earth, mission scientists say.
The images also show that Pluto and its largest moon Charon are geologically active, they say.
The dwarf planet was scanned by instruments aboard the space probe during a flyby of several hours, creating the images released Wednesday by NASA's New Horizons team.
"Home run!" said Alan Stern, mission principal investigator at the Southwest Research Institute in Boulder, Colorado. "New Horizons is returning amazing results already. The data look absolutely gorgeous, and Pluto and Charon are just mind-blowing."
One close-up image of the dwarf planet's equatorial region shows a range of mountains rising 11,000 feet above Pluto's icy surface. They are likely composed of the planet's bedrock layer of water ice, says mission scientist John Spencer.
"Water-ice at Pluto temperatures is strong enough to hold up big mountains," he said.
Possibly formed just 100 million years ago — a cosmic "yesterday" compared with the 4.56-billion-year age of the solar system — they suggest that area of Pluto may still be geologically active today, the scientists say.
Other images showed areas resurfaced by geological processes, possibly volcanism.
"This is one of the youngest surfaces we've ever seen in the solar system," said Jeff Moore at NASA's Ames Research Center in Moffett Field, California.
That was borne out by a lack of a single impact crater on the surface in the image, the scientist said.
New Horizons also captured images of Pluto's moon Charon, which also provided some surprises for the researchers, including a giant chasm four to six miles deep and regions displaying evidence of active resurfacing of the kind seen on Pluto.
"Originally I thought Charon might have an ancient terrain covered in craters ... it just blew our socks off when we had the new image," said New Horizons science team member Cathy Olkin.
The new images represent just a fraction of the data gathered by the spacecraft during its flyby, and New Horizons will continue to observe Pluto for the next 12 days as it moves away from it, NASA said.
The spacecraft also made observations of Pluto's four other moons: Nix, Hydra, Styx and Kerberos.