CAPE CANAVERAL: Vast frozen plains exist next door to Pluto's big, rugged mountains sculpted of ice, scientists said Friday, three days after humanity's first-ever flyby of the dwarf planet.

The New Horizons spacecraft team revealed close-up photos of those plains, which they're already unofficially calling Sputnik Planum after the world's first man-made satellite.

"Have a look at the icy frozen plains of Pluto," principal scientist Alan Stern said during a briefing at NASA headquarters. "Who would have expected this kind of complexity?"

"I'm still having to remind myself to take deep breaths," added Jeff Moore, head of the New Horizons geology team at NASA's Ames Research Center in California. "I mean, the landscape is just astoundingly amazing."

Spanning hundreds of miles, the plains are located in the prominent, bright, heart-shaped area of Pluto. Like the mountains unveiled Wednesday, the plains look to be a relatively young 100 million years old - at the most. Scientists speculate internal heating - perhaps from icy volcanoes or geysers- might still be shaping these crater-free regions.

"This could be only a week old for all we know," Moore said. He stressed that scientists have no hard evidence of erupting, geyser-like plumes on Pluto - yet.

Stern described the pictures coming down from 3 billion miles away as "beautiful eye candy."

Another possibility could be that the terrain, like frozen mud cracks on Earth, formed as a result of contraction of the surface.

The plains - which include clusters of smooth hills and fields of small pits - are covered with irregular-shaped, or polygon, sections that look to be separated by troughs. Each section is roughly 12 miles (20 kilometers) across.

The height of the hills is not yet known, nor their origin. It could be the hills were pushed up from below, or are knobs surrounded by eroded terrain, according to Moore. The fields of pits resemble glacial fields on Earth.

New Horizons' science team promised Friday that the data will allow them to produce elevation maps of both Pluto and its big moon Charon.

It will take 16 months to transmit to Earth all the data collected during the close encounter. The Johns Hopkins University Applied Physics Laboratory is managing the $720 million mission, which began with a launch from Cape Canaveral, Florida, in 2006 - months before Pluto was demoted from a full-fledged planet.

Stay tuned, meanwhile, for NASA's next Pluto update - next Friday. The pictures should keep getting better and better.

"This is just a taste of what I'm sure is in the unsent data" yet to come, Moore said.

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