Today, NASA’s New Horizons team unveiled the latest trove of geological goodies in close-up pictures of the surface of Pluto: hummocky hills that rise up above smooth plains of ice, patches of ice pocked by eroded pits, and troughs that form the boundaries of mysterious polygonal structures. Most tantalizing of all, the team has spotted streaks of material that may have blown downwind from dark spots. Although the team is not yet ready to declare that these spots are geysers shooting plumes above Pluto, scientists say the spots and streaks resemble actively spewing geysers on Neptune’s moon Triton that were discovered in 1989.
The evidence is accumulating that Pluto is an active world, and not only as a place shaped by top-down atmospheric processes of frost and wind and sublimating ice. There also appear to be processes working from the bottom up: forces that lift up water ice mountains the size of the Rocky Mountains and allow them to sit next to smooth plains of ice that, the team suspects, have been resurfaced as recently as within the past 100 million years—or even last week.
“Have a look at the icy frozen plains of Pluto,” said Alan Stern, the mission’s principal investigator at Southwest Research Institute (SwRI) in Boulder, Colorado, as he revealed a glimpse of a region named Sputnik Planum in a press conference today at NASA headquarters. “Who would have expected this kind of complexity?”
The team released the first results from measurements made as the spacecraft passed behind Pluto into its shadow. By measuring the way sunlight was eclipsed around the rim of Pluto, the team was able to analyze its atmosphere—and rule out models showing a turbulent atmosphere in favor of one that is more sluggish. Even with a more stagnant atmosphere, the part of it closest to the surface could still harbor winds blowing at a meter per second or two—enough to move tiny particles of ice around, says Randy Gladstone, a mission co-investigator at SwRI in San Antonio, Texas.
But the pictures, as usual, stole the show. Sputnik Planum is a region along the southern fringe of the left ventricle of the “heart,” now informally called Tombaugh Regio after Pluto’s discoverer. “I’m still having to remind myself to take deep breaths,” says Jeff Moore, a mission co-investigator at NASA’s Ames Research Center in Moffett Field, California. “The landscape is just astoundingly amazing.” To underscore the point, scientists used New Horizons’ terrain measurements to simulate a dramatic flyover video of the area and a nearby ice mountain range called Norgay Montes (see below).
Moore says that one of the few terrains that invites a confident diagnosis are the pitted regions, which form as ice sublimates into the atmosphere. He cannot say whether the hills are features that were pushed up above the surrounding plains, or whether they are composed of tougher materials that resisted erosion as the rest of the region wore down. “They can either be popping up or emerging from an erosion-lowering process,” he says. The polygonal troughs are also mysterious, he says. He doesn’t know whether they result from convection in the interior—the large-scale patterns of heat upwelling in Pluto’s mantle—or from contracting ice, analogously to the way mud cracks form on Earth.
Moore says it’s likely that the Sputnik Planum terrain—which also contains the geyserlike spots—extends all the way up into the left ventricle of the heart. Stern presented chemical evidence that this entire region is enriched in carbon monoxide ice. It could be either a pool of very thick layers of ice that welled up from below, or just a centimeter-thick veneer of carbon monoxide snow from above. Moore says the jury is still out on whether Tombaugh Regio was emplaced from below or shaped from above. Quite possibly, he says, both processes are in play: The terrain may have been deposited in a bout of activity a long time ago, and since been eroded. “It could be there’s a source region there,” Stern says. “It’s a very special place on the planet.”
New Horizons, a spacecraft the size of a baby grand piano, on Tuesday made its closest approach past Pluto, flying within 12,500 kilometers of its surface and making a first-ever reconnaissance of an object in the Kuiper belt, the region of icy worlds beyond Neptune. But images from Pluto are being returned to Earth in a trickle over the course of 16 months, because of the vast distances and the modest power of New Horizon’s radio antenna. NASA Planetary Science Division Director Jim Green says the spacecraft has returned only 1% to 2% of the data so far.
In pictures NASA released on Wednesday, the big surprise was mountains of water ice rising 3500 meters up from strikingly smooth, crater-free surfaces. The lack of craters—also seen on Charon, Pluto’s largest moon—is evidence for youthfulness, and geological activity that could pave over the surfaces in fresh icy materials. This was unexpected, because many thought that the internal heat sources within Pluto and Charon, leftover from their formation in a giant impact billions of years ago, would have dissipated long ago.
Larry Soderblom, a retired scientist from the U.S. Geological Survey in Flagstaff, Arizona, who helped explore Neptune’s moon Triton on NASA’s Voyager mission, is impressed by both the similarities and differences between that world and Pluto. Pluto is the largest Kuiper belt object; Triton is thought to be a captured one. Both harbor smooth surfaces that suggest repaving driven by internal heating. But where that activity on Triton can be driven by the tidal pull of Neptune, scientists are scratching their heads over what could be driving it on Pluto. There are other differences between the worlds, Soderblom says: Triton lacks Pluto’s tall mountains and its rugged, ropy pits. “Everywhere we go, we’re surprised,” he says. “We should know better by now.”
NASA is planning its next press conference on 24 July. After that, image retrievals from New Horizons’ cameras will 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 steer the spacecraft to an encounter with it in 2019. The $720 million mission is being operated by Johns Hopkins University’s Applied Physics Laboratory in Laurel, Maryland.
With additional reporting by Richard Kerr.
*See Science’s full coverage of Pluto, including regular updates on the New Horizons flyby.
(Video credit: NASA/JHUAPL/SWRI)