Ice flows, haze offer more clues to Pluto’s geology

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Exotic ices flow across Pluto’s surface, and its reddish color appears to come from a thin haze, the latest report from the New Horizons spacecraft reveals. The new finds have set scientists scrambling to construct the story of how Pluto’s climate and weather work.

A stunning image of the dwarf planet in silhouette, released July 24, reveals a layer of haze extending at least 130 kilometers above Pluto’s surface. That’s five times farther than predicted, Michael Summers, a New Horizons scientist at George Mason University in Fairfax, Va., said at a news conference. Still, Pluto’s surface pressure has been sliced in half over the last two years, suggesting that half its atmosphere has frozen and fallen to the dwarf planet’s surface.

Seeing a haze shows how the atmosphere of Pluto and its surface are connected, Summers said. The atmosphere has methane gas. When ultraviolet light from the sun interacts with methane in the upper atmosphere, it transforms the gas into more complex hydrocarbon gases, such as ethylene and acetylene. These gases fall to lower, colder parts of Pluto’s atmosphere and condense into ice particles. Ultraviolet sunlight converts these hazes to hydrocarbons called tholins, which fall to the ground and give the planet its dark patches. “We think this is how Pluto’s surface got its reddish hue,” Summers said.

With the latest data, researchers can tie the colors they see to the composition of the dwarf planet, New Horizons deputy project scientist Cathy Olkin, of the Southwest Research Institute in Boulder, Colo., explained. Images taken as the spacecraft approached Pluto reveal banding patterns of dark and light spots that give hints of weather and seasonal patterns, which the team still doesn’t fully understand. That banding pattern, Olkin noted, is abruptly interrupted by Pluto’s bright heart-shaped landscape, which has its own story to tell.

This landscape is right in the belly of the dwarf planet. Scientists call it the beating heart of Pluto, probably because it appears to be a vast reservoir that acts as the supply depot for Pluto’s entire atmosphere and geology, said William McKinnon, a New Horizons coinvestigator at Washington University in St. Louis. Based on more detailed images of this heart region, mission scientists now think that nitrogen snow may be moving from the western to eastern regions of the landscape. There’s also evidence of flowing ices, which appear to creep around elevated islands at the edges of the heart.

“To see geologic activity is a dream come true,” McKinnon said. Flowing ice supports the idea that Pluto may have some kind of ocean deep below its surface. Analysis of further New Horizons data could confirm this prediction, McKinnon said.

Currently the team has only 4 to 5 percent of the data that the New Horizons probe collected during its flyby of Pluto. Additional images and information will be released in a few weeks. “Starting in September, that’s when the spigot opens again,” said mission leader Alan Stern, of the Southwest Research Institute. With Pluto’s unfolding level of complexity, its atmosphere, possible ocean and moon system, it’s hard not to call this world a planet, he noted.