Towering mountains of water ice rise up to 3500 meters tall on Pluto, above smooth plains covered in veneers of nitrogen and methane ice, NASA’s New Horizons team announced today. The discovery, along with the finding that parts of the dwarf planet’s surface are crater-free and therefore relatively young, points to a place that has been geologically reworked in the recent past. “It could even be active today,” said John Spencer, a New Horizons team member at Southwest Research Institute (SWRI) in Boulder, Colorado, at a press conference today at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.
The team also showed off new images of unexpectedly smooth surfaces on Pluto’s moon Charon—which, without an atmosphere, was expected to have an even more battered surface than Pluto. Radioactive elements in both bodies’ interiors could provide some of the heat needed for geological mountain building or ice flows that repave the surface. But Pluto, and especially Charon, are far too small for this heat to persist. The giant impact thought to have formed the two worlds could also provide a source of energy, but that probably happened billions of years ago.
“It’s going to send a lot of scientists back to the drawing boards,” said Alan Stern, the mission’s principal investigator at SWRI, at the press conference. Scientists outside the team suggest that the puzzlingly youthful surfaces could be explained if the dwarf planet and its moon were formed in a far more recent impact event, or if their reservoirs of water ice were mixed with other compounds that can melt and flow and lower temperatures.
Although the number of TV crews parked outside APL has diminished considerably since the historic flyby on 14 July, the power of Pluto to dazzle continues to grow. The New Horizons team still has not retrieved data from the moment of close approach, which came on Tuesday as the probe swooped within 12,500 kilometers of the surface, 33 times closer than the moon is to Earth. Those images will come much later, over the course of 16 months, after the spacecraft completes its observations and can devote itself to beaming back data. At distances of about 4.7 billion kilometers, it takes 4.5 hours for New Horizons to communicate with Earth, and the data returns in trickles of a few kilobits per seconds.
But the early images are still providing scientists with plenty to chew on. One surprise was the discovery of the rugged water ice mountains in a dark, equatorial region next to a bright, heart-shaped region. (The team said it would informally name the “heart” Tombaugh Regio, after Pluto’s discoverer.) The frigid temperatures on Pluto mean that water ice is hard and doesn’t move or melt easily: It is Pluto’s bedrock. Seeing it protrude in mountains at the surface suggests that layers of other, more volatile ices—methane, nitrogen, and carbon monoxide—might only be a thin veneer of materials. Yet if these layers are too thin, they would be lost completely relatively quickly as they sublimate into the atmosphere and erode into space, Stern says. That means that there must be a way of replenishing these more volatile ices from within Pluto’s interior—perhaps through volcanoes of ice, called cryovolcanoes. “We haven’t found geysers and we haven’t found cryovolcanoes, but this is very strong evidence that will send us looking,” he says.
Smooth surfaces on Pluto's moon Charon imply geological reworking in the recent past.
Geoffrey Collins, a planetary scientist at Wheaton College in Norton, Massachusetts, unaffiliated with the team, is amazed by the images. “Clearly we’re seeing internal activity on the surface of Pluto and Charon,” he says. “Something is pulling apart their ice crusts.” Collins is excited because there is no way to explain the activity with conventional models of heat loss. “If the Charon-Pluto impact happened more recently, all the problems would be solved,” he says.
Jonathan Lunine, a planetary scientist at Cornell University who is not affiliated with the mission, agrees that the most curious discovery is the youthful surfaces of both bodies. “How do you keep these things warm for so long?” he asks. But he would rather find a mechanism besides a more recent impact event, which he calls “special pleading.” A giant impact is more likely to have occurred near the start of the solar system 4.5 billion years ago, when the Kuiper belt—the distant shell of icy bodies in which Pluto resides—harbored more potential impactors than it does today. But Lunine says it could be that the dynamics of the Kuiper belt are different from those in the rest of the solar system. Another mechanism to get water ice to move and flow more readily, he suggests, is to mix it with other compounds, such as ammonia. Ammonia-water mixes have been proposed for other icy bodies in the outer solar system, but they have never been identified directly, he says. “Maybe that’s happening here.”
Nancy Chabot, another planetary scientist at APL who is not affiliated with the mission, says the most important discovery today will end up being the ice mountains. “It’s going to be something people talk about for a while,” she says. The mountains—and their implication of mountain-building activity—runs counter to the expectation that Kuiper belt objects are cold, pristine relics. “We talk about these things as time capsules from the early solar system,” she says. That notion must evolve, she says. “Even though they are primitive bodies, they are also active bodies.”
NASA is planning to reveal more images at press conferences on Friday, 17 July, and a week later, on 24 July. After that, downloads of image data from the spacecraft will pause until September, while the mission concentrates on retrieving near real-time data from particle and plasma measuring instruments. Even once the full dataset is retrieved, sometime toward the end by 2016, the mission will not be over. In August, the team will choose between two small Kuiper belt objects for an extended mission. If granted funding, New Horizons will steer toward an encounter with one of those small bodies in 2019.
With additional reporting by Richard Kerr
*See Science’s full coverage of Pluto, including regular updates on the New Horizons flyby.