Since the New Horizons spacecraft began taking close-up pictures of Pluto 10 days ago, eyes around the world have been seduced by the bright icy “heart” that sits on the dwarf planet’s belly. But the heart does not sit still. “We describe it, poetically, as the beating heart,” says William McKinnon, a mission co-investigator from Washington University in St. Louis, Missouri.
Today, New Horizons team members revealed the latest discoveries from the $720 million mission in a press conference at NASA headquarters in Washington, D.C. They include images that show nitrogen ice flowing out from the edge of the heart, informally called Tombaugh Regio after Pluto’s discoverer. The flows appear to curl around obstacles and move into old craters, filling them up. At Pluto temperatures, ices of nitrogen, methane and carbon monoxide are soft and malleable enough to flow. “We interpret them to be just like glacial flows on Earth,” says McKinnon, who says the flows occurred as recently as within the past few tens of millions of years—a blink of an eye, geologically speaking.
The flow images are “breathtaking,” says Alexander Hayes, a planetary scientist at Cornell University. “We’re seeing activity on the surface. That means it’s powered by something.” Debate has centered on two mechanisms: bottom-up, in which icy materials would burble up through Pluto’s crust and out onto its surface, driven by residual heat in the dwarf planet’s interior; and top-down, in which frosts from Pluto’s atmosphere eventually accumulate into thick glaciers. Hayes favors the bottom-up mechanism, because he finds it difficult to envision frosts accumulating into the hundreds-of-meters-thick layers needed for them to flow like glaciers. “My gut reaction upon first seeing is that that might be tough to do,” he says.
But McKinnon says there are other places around the fringe of Tombaugh Regio where ice appears to be flowing into the heart. It could be that, as Pluto’s extreme seasons push reservoirs of ice around the globe, it just ends up in Tombaugh Regio. The heart might be Pluto’s wellspring; it might also be Pluto’s bathtub.
The other big revelation from the press conference was an image New Horizons snapped when it passed behind Pluto as it blocked the sun. It showed a ring of haze, created in the thin atmosphere as ultraviolet light strikes molecules of methane and nitrogen and turns them into small smoglike particles. Michael Summers, a mission co-investigator at George Mason University in Fairfax, Virginia, says haze particles, which can float buoyantly above the atmosphere, were found as much as 130 kilometers above Pluto’s surface, far higher than expected.
Curled, whorling patterns at the edge of Tombaugh Regio suggest glacierlike flows of nitrogen ice.
The team also found that atmospheric pressure was plummeting—a sign that the atmosphere itself is shrinking, as molecules escape to space or condense out onto the surface. Pluto is now moving farther away from the sun in its 248-year orbit; it reached its closest point in 1989. There were always some worries that Pluto’s atmosphere would eventually “freeze out” and collapse completely—and proponents for a flyby mission wanted to get there before it did. Mission principal investigator Alan Stern says that, after years of remote measurements showing a growing atmosphere, these are the first data that indicate it could be heading for a collapse. “It would be an amazing coincidence,” Stern says. “There are some on our team that would say, ‘I told you so.’”
The team also presented a new high-resolution mosaic, taken when New Horizons was 450,000 kilometers away from Pluto. It shows features as small as 2.2 kilometers across—about twice the resolution of the full-frame global view captured on 13 July and revealed on 14 July, the morning of closest approach, when the spacecraft flew within 12,500 kilometers of the surface. But the data transmissions are rolling back slowly, over the course of 16 months, because of the vast distances and the slow download speeds of the radio connection. Only about 5% to 6% of the probe’s pictures and instrument readings been retrieved so far.
Image downloads from New Horizons will now pause for nearly 2 months while the team focuses on gathering data from its particle and plasma instruments. In August, the team plans to choose between two candidate Kuiper belt objects—far smaller than Pluto—and then will steer the spacecraft to an encounter with the winner in 2019. In mid-September, New Horizons will begin focusing on sending home image data again. “Starting in September, the spigot opens again,” Stern says.
Check out Science’s full coverage of NASA’s New Horizons Pluto flyby.