Back in the early 2000s, planetary scientist Alan Stern was challenged to predict what the New Horizons probe would find when it got to Pluto. Stern refused to take the bait. “Something wonderful,” is all he would say.
Sure enough, that’s what the spacecraft found when it sped by the dwarf planet last July at more than 30,000 m.p.h.—a tortured, highly varied landscape that pointed to a living, geologically active world rather than an inert blob hovering at the frozen edge of the solar system.
Even now, three months after New Horizons’ close encounter, scientists are just beginning to get a handle on what’s going on with Pluto and it’s large, equally intriguing moon Charon. But what they know already, laid out in a new paper in Science, is impressive—and deeply perplexing.
A mix of stunningly sharp images and data from the probe’s suite of spectrometers and other instruments reveal that Pluto’s “bedrock,” which includes rugged mountains that reach up to 11,000 feet high, are made of water ice covered nearly everywhere with other ices—frozen nitrogen, methane, and carbon dioxide.
On the steep sides of the mountains, those other ices amount to no more than a dusting, says William McKinnon, a planetary scientist and New Horizons team member from Washington University. “Either they were never coated, or the lighter ices slide off.” But a bright, flat, heart-shaped region labeled informally as Tombaugh Regio, about the size of Texas, is almost entirely covered with a much thicker layer. A significant chunk of Tombaugh Regio, known as Sputnik Planum, is especially smooth.
“We can’t find a single crater,” says Stern, of the Southwest Research Institute in Boulder, Colorado. Given the chunks of rock and ice flying around in Pluto’s neighborhood, that can only mean one thing. “The area has been recently resurfaced,” Stern says—as recently as 100 million years ago, and perhaps even more recently than that.
That would only be possible if Pluto had a source of heat other than the Sun, whose energy at a distance of about three billion miles is vanishingly feeble. Elsewhere in the solar system, icy moons such as Jupiter’s Io and Saturn’s Enceladus—which spout volcanoes and geysers—are heated by friction caused by tidal forces as they orbit massive nearby planets. But Pluto, says Stern, “is out there all by its lonesome.” Charon and Pluto’s other moons don’t have sufficient mass to account for the planet’s heat.
One plausible source is the decay of radioactive minerals, including uranium, in Pluto’s rocky core, which makes up more than half of the dwarf planet’s mass. “That’s certainly enough to explain some of what we see,” McKinnon says. It would allow the non-water ices in Tombaugh Regio to flow like glaciers or ice sheets on Earth, creating the bizarre, patchy features seen on Sputnik Planum. These are presumably caused by hotspots beneath the ice that make it churn and flow, albeit slowly.
Radioactive heat, however, doesn’t explain the mountains at the edge of Sputnik Planum. At Pluto’s distance from the Sun, water ice is as hard as rock, and it takes a lot of energy to thrust it high above the surrounding terrain. “It’s not clear what their origin is,” says McKinnon, “but clearly it happened. Something created these. Right now, it’s still perplexing.”
A shroud of gas forms around Pluto when its elongated, egg-shaped orbit takes it closest to the Sun. In theory, that atmosphere—mostly nitrogen, with some carbon dioxide mixed in—should refreeze and coat the planet’s surface as it once again ventures away from the Sun.
That happened in 1989. And yet, telescopic observations since then revealed that the atmospheric density remained unchanged—which suggested that it hadn't returned to its solid, icy state.
But, New Horizons’ measurements tell a different story: Pluto’s atmospheric pressure at the surface is half of what the astronomers had calculated. “Either they got it wrong,” Stern says, “or the atmosphere is now collapsing. If so, it’s very sudden.”
The scientists were also surprised to see distinct layers of haze in the atmosphere. “What’s creating them? We don’t know,” says Stern. “We also see evidence of wind streaks on the surface, and what look like dune fields, which suggests that the atmosphere was substantially thicker in the past, more like the atmosphere of Mars.”
The truth might be out there. It just needs more time to arrive. Thus far, only about 15 percent of the images and data taken by the probe during its close encounter have been beamed back to Earth.
“We’re still doing emergency room triage,” says Stern. “The really detailed analysis is yet to come.”