Pluto continues to deliver surprises

OXON HILL, Md. — At this point, the only thing unsurprising about Pluto is that it continues to offer up surprises.

A wide variety of landscapes, ongoing surface transformations and a family of wildly spinning moons are among the riddles reported by the New Horizons mission team November 9 at a meeting of the American Astronomical Society’s Division for Planetary Sciences.

“Pluto is like a graduate course in planetary science,” mission leader Alan Stern said at a news briefing. “It’s going to take the larger planetary science community many years to digest all this.”

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The New Horizons spacecraft, which buzzed the dwarf planet on July 14, has so far sent back only about 20 percent of the data it acquired from the Pluto system. And every new nugget continues a story that’s pretty familiar by now: Pluto is a weird place.

Terrains both new and old sit side-by-side on Pluto’s surface. Some heavily cratered regions are roughly 4 billion years old, about as old as Pluto itself. Others, like the now famous heart, appear to have been laid down within the last 10 million years, judging by the total lack of craters.

Dramatic landscapes are coming into focus as images streaming in over the last couple of months have let researchers create topographical maps. One 320-kilometer-long crack informally dubbed Virgil Fossa features walls roughly 4 kilometers high, about twice the depth of the Grand Canyon.

Two mountains look strangely similar to shield volcanoes back on Earth. On Pluto, though, the volcanoes would spew ice, not rock. “There’s nothing like this seen in the outer solar system,” says Oliver White, a planetary scientist at the NASA Ames Research Center in Moffett Field, Calif. The mountains aren’t definitely volcanoes, but researchers aren’t sure what else to call them. “Whatever they are, they’re definitely weird,” says White.  

Pluto’s atmosphere is much colder and more compact than researchers thought, which implies that it’s escaping into space at a much lower rate than predicted. Calculations before the encounter suggested that the planetwide ice level has dropped by nearly 1 kilometer over Pluto’s lifetime as the ice sublimated into the atmosphere and then drifted into space. But at the current escape rate, the dwarf planet would have knocked only about 15 centimeters off its icy crust. “It’s dangerous to go from a snapshot to 4.5 billion years,” cautions Leslie Young, a planetary scientist at the Southwest Research Institute in Boulder, Colo. “But we’re scientists; that’s what we do.” 

Whirling far above Pluto, four tiny satellites — Nix, Hydra, Kerberos and Styx — are also behaving unexpectedly (SN Online: 11/2/15). Pluto’s gravity should have slammed on the brakes and slowed down their spins. But the rapidly twirling moons seem to be unfazed.  Hydra, the outermost moon, whips around its axis about 89 times during each loop around Pluto and Charon. Nix, meanwhile, appears to be flipped nearly upside down while the other three tiny moons might be spinning on their sides. “This is unprecedented,” says planetary scientist Mark Showalter of the SETI Institute in Mountain View, Calif., who discovered Kerberos and Styx several years after New Horizons launched. “We’ve never seen anything like this before, and we still don’t know what to make of it.”

Lots more data are yet to come, including some very high-resolution images that have yet to see the light of day. If the first 20 percent of the data are any indication, Pluto is probably not done surprising researchers. “Pluto and its system of satellites really outsmarted us,” says Stern. “I think it’s fair to say that New Horizons gets an A for exploration … but we get an F for predictive ability.”