NATIONAL HARBOR, MARYLAND—Researchers on NASA’s New Horizons mission have discovered evidence on Pluto for what appears to be two cryovolcanoes—volcanoes built out of frozen ice that once oozed molten ice from the inside of the dwarf planet. The discovery points to an internal heat source that, at some point in Pluto’s past, drove the melting of interior reservoirs of volatile ices, such as nitrogen and methane, that then erupted at the surface. It also suggests that the cryovolcanoes were a way for Pluto to periodically rejuvenate surface supplies of these volatile ices, which sublimate into the thin atmosphere and are eventually lost to space.
The features would be the first large ice volcanoes in the solar system, says Jeff Moore, a New Horizons scientist at NASA Ames Research Center in Mountain View, California, though the team is not yet willing to say the discovery is definite. “These sure look very suspicious and we’re looking at them closely.” The results were announced here today in a series of talks and an accompanying press conference at a meeting of the American Astronomical Society.
The two features, provisionally named Wright Mons and Piccard Mons, are mounds of ice with rims that tower as high as 5 or 6 kilometers above the surface. They are located just outside the southern tip of Sputnik Planum, the smooth, bright “heart” of Pluto. The rims, more than 150 kilometers across, encircle pits that are nearly as deep as the mountains are tall. The shape was unmistakably volcanolike, says Oliver White, a New Horizons scientist also at Ames Research Center. “When you see a big mountain with a big hole on the top, it generally points to one thing.” Moore says the surface textures suggest that past eruptions, which may have occurred a billion years ago, would have oozed out water ice that was plastic rather than liquid and runny. “It’s more like toothpaste extruding out of a tube,” he says. He says the volcanoes themselves are probably made out of water ice, since nitrogen and methane ices are too soft to support such a heavy, tall feature.
During the eruptions, the volcanoes could eject methane and nitrogen as gases, along with the molten water ice, Moore says. He adds that it’s interesting that the volcanoes are seen at the edge of Sputnik Planum, a deep basin that was likely created by a large impact. The stress of such an impact could have opened up conduits in the icy crust, he says. A similar situation can be found on Mars, where volcanoes are found flanking the Hellas Basin. “You have to look to the other red planet to find something similar,” says Alan Stern, the New Horizons principal investigator at Southwest Research Institute (SWRI) in Boulder, Colorado. “It’s truly amazing—it’s like something from a terrestrial planet.”
On seeing the images at the conference, Dave Stevenson, a planetary scientist at the California Institute of Technology in Pasadena, unaffiliated with New Horizons, says he’s “neutral” as to whether the features are in fact volcanoes. “There wasn’t enough there for me to pass judgment,” he says. But he says it’s “not ridiculous” to think that Pluto has enough residual heat, from radioactive elements in its rock, to drive cryovolcanism.
Bob Pappalardo, a planetary scientist at the Jet Propulsion Laboratory in Pasadena, California, also wants to see more evidence. If the results hold up, he says, they could increase “the relative role of cryovolcanism in resurfacing icy bodies.” Planetary scientists have previously invoked tectonics—the cracking of a body by forces in the crust—as a way of opening up planetary interiors and allowing resurfacing. A good example of this is the fissures on Enceladus, a moon of Saturn, which are currently spewing water out into space. Volcanolike features have been detected on Triton, a moon of Neptune, and Titan, a moon of Saturn. But White says neither of the two moons support features as big as the Wright and Picard mons. “There’s nothing of this scale,” he says.
Another surprise revealed at the conference was the discovery that Pluto’s atmosphere is much colder and more compact than thought. Previously, UV light from the sun was thought to heat up atmospheric molecules, exciting them enough that they jump off into space. But the new haze molecules that have been detected on Pluto act as coolants for the atmosphere, absorbing some of the energy of that UV light. That means that Pluto has held onto a lot more of its nitrogen than was thought, says Randy Gladstone, a New Horizons scientist at SWRI. “Escape is currently not significant,” he says. Stern points out, however, that long-term climate cycles in Pluto’s tilt and orbit can cause its climate to vary wildly, which means that the atmospheric loss rate may have been much higher in the past.
Because of the extreme distances and power limitations of the radio antenna, data from the New Horizons encounter, which in July flew within 12,500 kilometers of Pluto’s surface, is still being beamed back to Earth. Just 20% of the data has been retrieved, Stern says.
The mission is not yet over. Last week, mission managers at Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, completed maneuvers to steer the spacecraft toward a smaller follow-on target, 2014 MU69, a Kuiper Belt object less than 50 kilometers across. Current plans have that flyby occurring on 1 January 2019, at a distance of 20,000 kilometers.