In the outer reaches of the solar system, where temperatures can plummet to below −200°C, volcanoes spew slushy ice rather than molten rock. Such cryovolcanism has been detected on large asteroids and the moons of several gas giants, and now researchers studying Pluto have spotted what they have proposed is the largest swath of cryovolcanically processed terrain known on the dwarf planet.
On 14 July 2015, NASA’s New Horizons spacecraft made its closest approach to Pluto. From a distance of 12,500 kilometers, the piano-sized craft captured images and spectra of Pluto’s surface. Some of the first data to be beamed back via the spacecraft’s radio telecommunications system—at roughly 2 kilobits per second, which is leisurely compared with the transmission speeds of even 1990s era dial-up modems—revealed a surprising sight: a landscape that was incongruously lacking in impact craters and covered in mounds several kilometers high separated by broad depressions.
It’s hard to describe this terrain, even using geological terminology, said Kelsi Singer, a planetary scientist at the Southwest Research Institute in Boulder, Colo., and a member of the New Horizons team. “Hummocky” is the correct gist, but the mounds are more blended than that word implies, she said. “They’re not really individual hills. They’re much more interconnected.” Irrespective of the specific terms one uses to describe the landscape, the terrain is clearly unlike anything seen elsewhere in the solar system, said Singer. “We immediately thought that it looked intriguing.”
Singer and her colleagues focused on a roughly 600- × 300-kilometer swath of Pluto’s surface. That region, near the Sputnik Planitia ice sheet that forms the left side of Pluto’s conspicuous heart-shaped terrain, is anchored by two soaring features known as Wright Mons and Piccard Mons. These geological formations, rising 4 and 7 kilometers, respectively, above the surrounding terrain, were previously postulated to be volcanoes.
Hills and Few Craters
Using data collected by three different instruments aboard New Horizons, the researchers analyzed the geomorphology and chemical composition of the unusual terrain surrounding Wright Mons and Piccard Mons. Singer and her colleagues found that the region’s hill-like structures tended to have flat or gently rounded tops and that boulders and ridges were prevalent across the landscape. Craters were far and few between. And the landscape was largely composed of water ice with perhaps other species like methane or ammonia mixed in, the team found.
Singer and her collaborators also concluded that the geological structure on Wright Mons previously hypothesized to be its caldera was, in fact, not a caldera at all. There aren’t any features associated with flowing material, which would be expected of a caldera, said Singer. “The center of Wright Mons has the same lumpy look as the top and outside of Wright Mons.”
Recent Slurries of Ice
The best explanation for the odd landscape is cryovolcanism, Singer and her colleagues suggested. A slurry of ice, erupting repeatedly over time from many different vents, could have plausibly sculpted the terrain. “We can’t find another hypothesis that would explain the features better than some form of icy volcanism,” said Singer.
And on the basis of the relative dearth of impact craters pockmarking the region, the team concluded that the eruptions probably occurred within the past 1 billion years. That’s relatively recently, said Sarah Fagents, a volcanologist at the University of Hawai‘i at Mānoa in Honolulu not involved in the research. “It’s the last quarter of the solar system’s history.”
Because cryovolcanism implies that there’s heat to melt ice, that says something about Pluto’s interior, the researchers suggested. Singer and her colleagues proposed that Pluto’s interior structure might be particularly efficient at storing heat leftover from the dwarf planet’s formation. One possible physical explanation is that Pluto’s interior contains crystalline structures known as clathrates. These structures, which consist of a lattice of water molecules surrounding another substance, such as methane, are known to be extremely insulating. Perhaps a layer of clathrates in Pluto’s interior helps retain heat, the researchers hypothesized. “It’s kind of like your insulating mug,” said Singer. “If you put something warm in it, you can keep the heat in for longer.”
These results were published in March in Nature Communications.
It’s even possible that Pluto has retained enough heat to power current episodes of cryovolcanism, roughly 4.5 billion years after the dwarf planet formed, the researchers suggested. “We can’t say that it’s stopped,” said Singer.
—Katherine Kornei (@KatherineKornei), Science Writer