Skip to main content

In the northwestern region of Pluto’s Sputnik Planum, swirl-shaped patterns of light and dark suggest that a surface layer of exotic ices has flowed around obstacles and into depressions, much like glaciers on Earth.

Ten days after NASA's New Horizons probe whizzed by Pluto, scientists say they are closer to understanding the remote world's most distinctive and intriguing feature – the heart-shaped patch named after Pluto's discoverer, Clyde Tombaugh.

Armed with a hoard of new images and data that was transmitted back to Earth this week, mission team members say they can confirm that Tombaugh Regio is dominated by a blend of frozen nitrogen, methane and carbon monoxide, some of which appears to be spilling over into adjacent areas.

The new details support the emerging consensus that Pluto is more active internally than many scientists had expected, and it may even harbour flowing water deep below its icy surface.

"Our picture of Pluto as a complex active world … basically increases the theoretical likelihood that there might still be an ocean down there," said Bill McKinnon, a co-investigator on the mission during a news briefing Friday in Washington, D.C.

New Horizons is now 12 million kilometres away from Pluto and receding rapidly after its historic July 14 encounter with the distant orb. As images from the probe's closest approach continue to stream back to Earth, scientists are gradually assembling a detailed mosaic of Pluto's surprisingly varied and complex surface.

The latest images include dramatic views of the boundaries of Tombaugh Regio, where what appear to be tongues of nitrogen ice can be seen creeping, glacier-like, over and around geographic barriers such as cliffs and craters.

"We interpret them to be just like glacier flow on the Earth," Dr. McKinnon said.

The difference is that in the extreme cold of Pluto, where the surface temperature is -235 C, nitrogen can form a malleable solid rather than a gas. The rugged and eroded terrain it flows over is made of frozen water, which on Pluto is as immobile and brittle as rock.

The centre of Tombaugh Regio, a smooth white plain named Sputnik, has already been observed to be divided into many cell-like blobs with polygonal boundaries.

Dr. McKinnon said that they might be due to convection in the ice, which could be dissipating heat from below. The theory is plausible, he said, as long as the ice is at least 800 metres thick.

Harder to explain is where Pluto's internal heat is coming from, including the energy that has created imposing mountain ranges and what look to be global-scale tectonic features.

Data received this week also include the revelation that Pluto's atmospheric pressure is half of what it was just two years ago, based on an earlier measurement made when Pluto crossed in front of a distant star. The rapid thinning of the atmosphere is likely a function of Pluto moving further away from the sun in its 248-year orbit. It was last nearest the sun in 1989. As it moves away, scientists expect the atmosphere to start freezing and settling onto Pluto's surface, something New Horizons may have captured in progress.

A final parting shot, made as New Horizons swivelled around to look at Pluto after closest approach, shows the atmosphere looking like a bright ring, backlit by the sun. A detailed analysis of the image reveals the atmosphere to be quite hazy.

Researchers say they have received only about 5 per cent of all the data the spacecraft gathered during its closest approach. It will take more than a year to send the rest back to Earth because the probe's distance and weak radio signal limits the rate of transmission, with new discoveries likely to be spread out over that time.

"We are facing the Christmas that keeps on giving," deputy project scientist Kim Ennico told reporters last week.