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The hit parade from NASA's New Horizons mission to Pluto continued on Friday with views of an alien-looking plain in the dwarf planet's "heart," dark streaks on the surface that look like windswept trails, and mountainous terrain named after Everest climber Tenzing Norgay.
The focus of Friday's news conference, conducted at NASA Headquarters in Washington, was the frozen plain in Pluto's light-toned, heart-shaped region. The "heart" is informally called Tombaugh Regio — named after the late astronomer Clyde Tombaugh, who discovered Pluto in 1930. New Horizons' scientists call the plain "Sputnik Planum," or Sputnik Plain, after the Soviet satellite whose launch opened the Space Age in 1957.
The plain is dominated by polygon-shaped hills that are roughly 12 miles (20 kilometers) across and divided by what appear to be shallow troughs. Some of the troughs are filled with darker material. Others are traced by bumpy hills. And some of the polygons have pitted surfaces. In New Horizons' view, which spans about 300 miles (500 kilometers), there's nary a crater to be seen.
"This terrain is not easy to explain," said Jeff Moore, a planetary scientist from NASA's Ames Research Center who leads New Horizons' geology, geophysics and imaging team.
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The pits may have been formed by sublimation, in which ice turns directly from a solid into a gas. In Pluto's case, the surface is covered with a layer of frozen nitrogen, methane and carbon monoxide. A preliminary analysis of spectral data revealed that there's a high concentration of carbon monoxide ice on the left side of Tombaugh Regio.
As for the polygons themselves, "the surface that is most reminiscent of the surface you're looking at is the high latitudes of the northern hemisphere of Mars," Moore said. The polygons in Mars' north polar region were formed by thermal contraction. That's similar to what happens when a patch of muddy ground dries on Earth.
On Pluto, the polygons may be the result of a similar process, or they may have been formed by convection — like blobs of wax in a lava lamp, or blobs of oatmeal in a cooking pot. "It's just really too early to say," he said.
The bottom line is that even these early pictures show that Pluto is much more than a frozen iceball. "Pluto is every bit as geologically active as any other place we've seen in the solar system," Moore said.
Dark streaks on the ice
Some of the images from New Horizons show miles-long streaks of dark material on Pluto's icy plains. Moore said the streaks look similar to the dark wind streaks documented by Mars orbiters.
Moore didn't rule out the possibility that they might be plumes of material ejected from the surface, similar to the geyser plumes spotted by NASA's Voyager probe on Triton, Neptune's largest moon. However, Moore cautioned that the team couldn't confirm the discovery of Plutonian plumes — which was the subject of speculation before the flyby.
He said the "least crazy idea" was that the dark stains were produced when methane frost settles onto the surface, undergoes irradiation and turns into a darker type of hydrocarbon. Winds in the thin Plutonian atmosphere might blow the material around.
Flying over mountains and plains
A couple of days ago, New Horizons' scientists were wowed by the sight of 11,000-foot-high mountains of ice on Pluto's surface. During Friday's briefing, the team unveiled a flyover video focusing on a mountainous region nicknamed Norgay Montes as well as on Sputnik Planum.
Friday's flyover was simulated using stereo imagery of the region that was captured by New Horizons' Long Range Reconnaissance Imager from a distance of 48,000 miles (77,000 kilometers). The video shows features with a resolution as small as a half-mile (1 kilometer).
The name given to Norgay Montes pays tribute to Tenzing Norgay, the Nepalese Sherpa mountaineer who made the first ascent to the summit of Mount Everest with British climber Edmund Hillary in 1953. Moore said that Norgay Montes would be the first feature on another planetary body named after a citizen of Nepal. Like the other names given to Plutonian features, Norgay Montes is a provisional name that technically still requires the International Astronomical Union's approval.
Pluto's 'tail' in space
Preliminary data from New Horizons suggests that the solar wind is stripping away Pluto's predominantly nitrogen atmosphere to create an "ion tail" that trails behind the dwarf planet, said University of Colorado planetary scientist Fran Bagenal, one of the mission's co-investigators.
"This is just a first tantalizing look at Pluto's plasma environment," Bagenal said in a NASA news release. Researchers currently estimate the atmospheric loss at 500 tons per hour, but Bagenal said much more detailed observations are due to be sent back in August
Smiley face on Nix?
The New Horizons team also released a picture of one of Pluto's smaller moons, Nix, taken from a distance of about 360,000 miles (590,000 kilometers). The 12-pixel-wide picture doesn't provide much detail, but it does suggest that there's surface variation — in fact, if you squint your eyes, you can almost make out a smiley face.
Previous observations have suggested that Nix is elongated, but in Friday's image, the 25-mile-wide (40-kilometer-wide) moon presents a more circular profile. It's not like the crumpled, potato-like shape of another Plutonian moon, Hydra, as seen in a New Horizons image released earlier this week.
Stay tuned for more
The images were the latest product of a mission that blasted off nine and a half years ago and reached its climax this week. The piano-sized New Horizons probe zoomed past Pluto and its moons on Tuesday at a speed of more than 30,000 mph (50,000 kilometers per hour), coming within 7,750 miles (12,500 kilometers) of the dwarf planet.
The spacecraft is now almost 2.5 million miles (3.9 million kilometers) beyond Pluto and 3 billion miles (5 billion kilometers) from Earth. That's so far away that it takes four and a half hours for signals to reach Earth. And because of the vast distance and the trade-offs involved in designing the $728 million mission, those signals can be sent only at a rate of 1,000 to 2,000 bits per second.
As a result, it will take 16 months to transmit all the data from the flyby. The next round of imagery is due to be unveiled in a week, NASA said.
After New Horizons is done with Pluto, the team intends to point the probe toward one of two smaller objects a billion miles farther out in the Kuiper Belt, the broad ring of icy material beyond the orbit of Neptune. If NASA gives the go-ahead for that extended mission, such a flyby would take place around 2019.