For such a small world, Pluto has an incredible diversity of features, including flowing glaciers, curiously pitted terrains, hazy skies, and multi-colored landscapes. Now scientists from the New Horizons mission have revealed that the distant dwarf planet is even weirder than they thought, with potential ice volcanoes, floating mountains, and misbehaving moons.
Scientists presented this new set of observations from the New Horizons spacecraft, which flew past Pluto in July, on Monday at the Division for Planetary Sciences annual meeting, and the data are showing that Pluto is not what anyone expected.
The team gets “an A for exploration” and “an F for predictive ability,” said Alan Stern, New Horizons’ principal investigator. “The Pluto system is baffling us.”
Two pits near Pluto’s south pole could be icy volcanic calderas. The pits are located at the summits of two enormous mountains, Wright Mons and Piccard Mons. Each mountain is a couple of miles (several kilometers) tall and at least 60 miles (100 kilometers) wide, similar in size and shape to Hawaiian shield volcanoes. But instead of fiery lava, Pluto’s volcanoes would spew ices, perhaps nitrogen, carbon monoxide, or a watery slurry dredged from a buried ocean.
Jeff Moore of NASA’s Ames Research Center said during a conference presentation that the team is not yet ready to conclude these features are indeed volcanoes, “but they look very suspicious.”
If they’re real, they would be the first volcanoes spotted in the outer solar system. And though the team plans to confirm the discoveries with additional data, some members are already pretty convinced.
“When you see a big mountain with a hole in the top, it generally points to one thing,” said Oliver White, also from NASA Ames. “I’m having difficulty unseeing these volcanoes.”
Anarchic, Floating Mountains
Pluto’s mountains may be more like icebergs in the ocean than mountains on Earth. Made of water ice, these big blocks of material are probably floating on a “sea” of nitrogen ice, Moore revealed. In some regions, these mountains are as large as the Rockies but are still buoyant enough to rise high above denser nitrogen and carbon monoxide ices. “Even the largest mountains of Pluto could simply be floating,” Moore said during his presentation.
Near the western edge of the ice field known as Sputnik Planum, giant sheets of water ice can be fractured and rearranged, producing what Moore refers to as “anarchic terrain.” Jumbled chains of angular blocks, some as much as 25 miles (40 kilometers) across and 3 miles (5 kilometers) high, form mountains that stretch chaotically near the otherwise smooth, young plain. New analyses suggest that Sputnik Planum could be just 10 million years old. It was basically “born yesterday,” Stern said. “It’s a huge finding, that small planets can be active, on a massive scale, billions of years after their formation.”
Enormous Fractures, Buried Ocean
Some parts of Pluto’s surface, such as Sputnik Planum, are incredibly smooth. Others are curiously pitted or look like an alien version of snakeskin. Still other regions are riven with enormous fractures, such as Virgil Fossa, to the west of Sputnik Planum. Such cracks look as though they formed when Pluto expanded and busted up its crust, and that may be just what they are. “A slowly cooling and freezing ocean will lead to expansions,” explained Bill McKinnon of Washington University in St. Louis. If Pluto’s crust conceals a buried water ocean—as scientists think is likely—then as that ocean slowly freezes and grows, it could be stressing Pluto’s crust and producing these enormous fissures.
Small, Cool Atmosphere
Before the flyby, scientists thought Pluto had a puffy atmosphere that was maybe seven or eight times as voluminous as Pluto. That atmosphere, primarily nitrogen, was also thought to be escaping so fast that some 0.6 mile (1 kilometer) of ice on the Pluto surface would have sublimated and vanished over its 4.6-billion-year lifespan.
Now New Horizons scientists say that idea is almost completely wrong. Pluto’s atmosphere isn’t nearly as voluminous as they had anticipated, and it isn’t escaping nearly as quickly as predicted. “With the new rate, it’s something like half a foot [of vanished ice],” says Leslie Young of the Southwest Research Institute. Most of Pluto’s nitrogen is instead staying close to the dwarf planet. Though puzzling, the observation could be explained by the presence of hydrogen cyanide high in Pluto’s atmosphere. Nobody expected to find hydrogen cyanide in such quantities, but it would have a significant cooling effect on the atmosphere, keeping it snuggled in close to Pluto.
Pluto’s four small moons have finally been revealed, and Nix, Styx, Kerberos, and Hydra are, like most things about this system, weirder than scientists had guessed. Kerberos and Hydra look as though they’re made of two smaller objects that slowly collided and stuck together, similar to the duck-shaped comet that the Rosetta spacecraft is now orbiting. “At some point in the past, there were more than just the four [small] moons of Pluto—there were at least six,” Mark Showalter of the SETI Institute, said at a press conference.
Adding to the weirdness are the rapid rotation rates of the small moons. Hydra wins the race, spinning around itself once every 10 hours, but all the moons are pirouetting more quickly than expected. “We simply have not seen a satellite system that does this,” Showalter said. Plus, Nix has an odd, reddish crater on one face that scientists can’t fully explain yet. And Kerberos, which scientists guessed would be the dark sheep of the bunch, is actually just as bright as its three small siblings.