Pluto is defying scientists’ expectations in many ways

Ask Alan Stern to name the most surprising thing learned from NASA’s New Horizons mission to Pluto, and he’ll tell you to rephrase the question.

“A better question would be, ‘ What isn’t puzzling or mysterious?’ Because that’s a much shorter list,” said Stern, the planetary scientist in charge of the mission. “Almost everything we see on Pluto and in its atmosphere is puzzling.”

After traveling through the solar system for nine years, the New Horizons spacecraft made its closest approach to the Pluto system last July. Equipped with seven extremely sensitive scientific instruments, it took high- resolution images of the dwarf planet’s surface features, observed its five satellites, monitored its atmosphere and measured its interaction with the solar wind — all as it whizzed past its targets at speeds of up to 30,800 mph.

A suite of five papers published this week in the journal Science illuminate some of the findings from this data, as well as some of the mysteries.

So far, the spacecraft has transmitted back to Earth just 40 percent of the data it collected. Scientists say most of it has defied their expectations.

“There really wasn’t much that turned out the way we thought it would,” said planetary scientist Randy Gladstone of the Southwest Research Institute in Boulder, Colo., a co-investigator on the mission.

One of the biggest surprises was the diversity of landscapes on the dwarf planet’s surface.

In a paper about the geology of Pluto and its largest moon, Charon, scientists described an ancient, heavily cratered terrain on the dwarf planet’s surface that could be 4 billion years old. But they also spotted wide, smooth plains that look as if they are still being resurfaced today.

Elsewhere, the study authors saw towering mountains of ice measuring 2 to 3 miles high, and evidence of active glaciers made of nitrogen that are carving wide troughs across parts of Pluto’s exterior.

And then there are the features that no one can quite make sense of — regions with parallel ridges and grooves that the scientists are calling “washboard terrain,” and narrow ridges that are oriented north to south that have been dubbed “bladed terrain.”

“We’ve got several hypotheses floating around on the team, but there’s not a lot of convergence or consensus on how these might have formed,” said Jeff Moore of the NASA Ames Research Center in Mountain View, Calif., who led the geological study.

Moore explained that even though the temperature on Pluto’s surface averages just 40 degrees above absolute zero (nearly minus-400 degrees Fahrenheit), the dwarf planet is hardly frozen in time.

The slow decay of radioactive materials embedded in the silicate rock that makes up most of Pluto’s interior produces a meager amount of heat that eventually makes its way to the surface. That energy, coupled with a bit of warmth from the distant sun, can be enough to mobilize the nitrogen glaciers and fuel other geological changes on Pluto.

Charon, too, turned out to be significantly more geologically active than the researchers had anticipated.

“We expected it would be a boring cratered ball,” Moore said. “Instead we can see there was a lot going on geologically in its early history.”

Moore said Charon’s southern hemisphere showed more signs of recent resurfacing than the northern hemisphere. The researchers think this could be the result of cryovolcanism, a process similar to volcanoes on Earth except they emit water and other volatile compounds instead of molten rock.