NASA has released more images of Pluto after New Horizons made its historic flyby earlier in the week, and they show the spacecraft found vast frozen plains next to Pluto's big, rugged mountains of water ice.

Spanning about 400 kilometres across, the plains are located in the bright, heart-shaped area of Pluto. Like the mountains unveiled on Wednesday, the plains look to be a relatively young: 100 million years old.

Scientists speculate internal heating — perhaps from volcanoes or geysers — may have been responsible for these youthful-looking, crater-free regions. The plains appear to include smooth hills and fields of small pits.

Principal scientist Alan Stern says the pictures coming from five billion kilometres away are "beautiful eye candy."

Another photo shown at NASA's briefing on Friday is that of a carbon monoxide-rich area within the Tombaugh region, named for Pluto's heart-shaped feature.

Pluto

NASA displayed a graphic overlay on the Tombaugh region, showing a carbon monoxide-rich area during Friday's briefing. (NASA)

"There is no other carbon monoxide concentration [on Pluto] like this," said Stern. "It's a very special place on the planet."

NASA released other high-resolution images of the icy dwarf planet on Wednesday, a day after the flyby, showing mountains on Pluto and canyons on the largest of its five moons, Charon.

Some of the ice mountains are comparable in size to some of the smaller Rocky Mountains. Charon's canyons may be up to nine kilometres deep.

The lack of craters on both Pluto and Charon suggests the two are still geologically active.

Photos showing the crater-free Sputnik Planum adjacent to a set of mountain ranges illustrates the "stark contrasts on Pluto in terms of the geology," Stern told reporters. The region is named after the first artificial satellite to be launched from Earth into orbit.

The main component of Pluto's atmosphere at the highest altitudes is nitrogen, which escapes directly into space "because Pluto is so small," astrobiologist Randy Gladstone said. Lower down in the atmosphere are likely hydrocarbons and methane.

Scientists believe the nitrogen is escaping because gravity on Pluto is a lot weaker than it is on Earth and Mars.

Pluto

NASA's model depicts Pluto's atmosphere. It's believed that nitrogen, seen in blue, is ionized by the sun and carried off by solar winds. (NASA)

With files from CBC News