Pluto’s icy landscape comes into view

New Horizons reveals varied terrain, evidence of active geology

surface of Pluto

Mountains of water ice roughly as tall as the Rockies tower over a young landscape to the south of Pluto’s heart-shaped region. Snows of nitrogen and methane blanket the peaks. On the dwarf planet, water ice probably behaves like bedrock on Earth.

NASA, JHUAPL, SWRI

LAUREL, Md. — Alan Stern, head of the New Horizons mission to Pluto, had just three words for the team of scientists and engineers assembled with him on July 14: “We did it.”

At 8:52:37 p.m. Eastern time, a radio antenna near Madrid received the first signal from the spacecraft since it buzzed the dwarf planet. After decades of planning and a 9.5-year journey across nearly 5 billion kilometers of interplanetary space (SN: 6/27/15, p. 16), the New Horizons probe reported that it was in good health and that the mission was a success. The spacecraft flew within 12,500 kilometers of Pluto, right on schedule.

“Our spacecraft did exactly what it was supposed to do,” said mission operations manager Alice Bowman. “Just like we planned it, just like we practiced.”

From now on, Pluto and its five moons will no longer be abstract points of light, but real worlds with many stories to tell. And even with just a handful of observations retrieved from the hundreds still stored on New Horizons, Pluto and its largest moon, Charon, are already reshaping what astronomers thought they knew about the frigid backwaters of the solar system.

Young surface, ancient world Scalloped icy terrain (bottom) in the center of Pluto’s “heart” (lighter area, top) is probably no more than 100 million years old. The troughs and hills hint at active geology. A nearby area within the heart shows unusual concentrations of carbon monoxide (green, top), higher than anywhere else on Pluto. NASA, JHUAPL, SWRI
“I expected it would be complicated and amazing,” says Will Grundy, a planetary scientist at Lowell Observatory in Flagstaff, Ariz. “But I had no idea it would be this complicated and amazing.”

Pluto and Charon are vibrant, diverse worlds. Mountains, canyons, icy bedrock and nitrogen snow are some of the features that shape these worlds, which are unlike any other orbiting the sun. Some terrains have been freshly resurfaced, shaped by internal forces that are not at all understood. Others have stood the test of time, serving as witnesses to most of the solar system’s history.

“We’re not saying Pluto looks like this world or that world,” says planetary scientist Cathy Olkin of the Southwest Research Institute in Boulder, Colo. “There’s nothing like it.”

Ask any scientist on or off the team about how a 4.6-billion-year-old solitary ice world is still actively reworking itself, and the most common response is a long pause followed by a shrug.

“We’re like geologists that are just starting out on learning geology,” Grundy says. Out there, where temperatures top out at −223° Celsius and bedrock is forged from hardened water ice, planetary scientists are on unfamiliar ground.

Not that the mission scientists have had much time to fully process what they’ve seen. They had to squeeze in cursory analyses while navigating a circuslike atmosphere at the Johns Hopkins Applied Physics Laboratory in Laurel, Md., home of the mission operations center. Among the spacecraft-shaped hats, Pluto trading cards (collect all nine!), flag-waving and standing ovations, mission control at times felt like a political rally.

But with the celebration behind them, the team can now turn to the fun part: figuring out what Pluto and Charon have to say.

Both worlds already show incredible diversity. Pluto displays a prominent heart-shaped landscape, informally named Tombaugh Regio in honor of the dwarf planet’s discoverer. This territory is devoid of craters, which means it’s likely young. To the north, the heart gives way to older, pockmarked ground. The heart’s “right ventricle” blends into a hummocky, undulating terrain, while the other half runs up against a dark belt that wraps around the equator.

The bright regions appear to be coated in nitrogen, methane and carbon monoxide ice. Darker lands are probably stained by hydrocarbons tarnished by long exposure to the sun’s ultraviolet radiation.

To the south of Tombaugh Regio lies what might be the biggest surprise of all: mountains of solid water ice roughly 3,000 meters high. Cover them with pine trees and they wouldn’t look out of place in the Rockies. Mission scientists estimate that the surface here is no more than 100 million years old.

Pluto’s largest moon, Charon, has a dark patch on its north pole, a feature not seen elsewhere in the solar system. Craters dot some of Charon’s older terrain (closeup view, far right), including one that appears to house a mountain (arrow). NASA, JHUAPL, SWRI
“This is a recurring story,” says David Jewitt, a planetary scientist at the University of California, Los Angeles who is not part of the New Horizons team. “We’ve been to a whole bunch of small places that no one expected to be active.”

The frozen moons of the giant planets show similar diversity and activity. Water geysers erupt on Saturn’s moon Enceladus; the Uranian moon Miranda exhibits towering ice cliffs; and the surface of Triton, Neptune’s largest moon, has been resurfaced.

Moons are influenced by the planets they orbit. A planet’s gravity stretches and relaxes its moons, which generates heat that can power geologic activity.

But absent a large planet, what’s heating Pluto? Planetary scientists have a couple of ideas. Radioactive elements such as thorium and uranium trapped in a rocky core might be heating the surface. Or perhaps a subsurface ocean is freezing, releasing enough heat to power geysers and cryovolcanoes.

Charon, meanwhile, is quite different from Pluto, a mystery given that the duo have lived next to each other for over 4 billion years. Charon’s surface is darker, with more craters (though not as many as expected) and less ice. Cliffs 1,000 kilometers long cut across the landscape, while canyons 10 kilometers deep carve notches out of the moon’s visible edge.

“Charon looks like an amalgam of some Saturnian and Uranian satellites,” says Paul Schenk of the Lunar and Planetary Institute in Houston, “but its dark pole is one of a kind.”

Blackened splotches on the moon’s north pole are not found anywhere else in the solar system — and Pluto is probably to blame. As Pluto’s atmosphere leaks into space, it may carry aloft flecks of darkened hydrocarbons, some of which land on Charon. The pole, which is coming out of a decades-long winter, might be cold enough for a thin veneer of Pluto’s effluent to stick to the surface.

Planetary scientists clearly have their work cut out for them. Any conclusions the team comes to now could very well be proved wrong in a month. Only 1 percent of the data are being transmitted right away. The rest of the observations won’t finish downloading until late 2016.

As the data trickle in, researchers will study the smaller moons, look for rings encircling Pluto, and try to understand how its atmosphere is escaping and whether it shares that atmosphere with Charon, which would be another solar system first.

At this point, the scientists are struggling to simply identify what they’re seeing. “It’s like piles of stuff with grooves on it,” says planetary scientist John Spencer of the Southwest Research Institute while describing land near the ice mountains. Not very technical, perhaps, but this is an alien environment in an unexplored realm of the solar system.

“It’s baffling,” Spencer says, “but it’s baffling in a very interesting and wonderful way.”

Christopher Crockett is a freelance science writer and editor based in Arlington, Va. He has a Ph.D. in astronomy from the University of California, Los Angeles.

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