March 17--Ask Alan Stern to name the most surprising result 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 the mission's principal investigator. "Almost everything we see on Pluto and in its atmosphere is puzzling."
The New Horizons spacecraft made its closest approach to the Pluto system on July 14, 2015, taking high-resolution images of the dwarf planet's surface features, observing its satellites, monitoring its atmosphere and measuring its interaction with the solar wind -- all as it sped past at 30,800 mph.
A suite of five papers publised Thursday in the journal Science illuminate some of the findings from this data, as well as some of the mysteries.
So far, the spacecraft has beamed just 40% of the data it collected back to Earth, but scientists say it's enough to know that the Pluto system has defied most of their expectations.
"There really wasn't much that turned out the way we thought it would," said Randy Gladstone of the Southwest Research Institute in Boulder, Colo., a co-investigator on the mission.
One of the biggest surprises for scientists was the astonishing diversity of landscapes on the dwarf planet's surface.
In a paper on the geology of Pluto and its largest moon, Charon, the authors describe ancient, heavily cratered terrain on the dwarf planet's surface that could date back 4 billion years. At the same time, they also spotted wide, smooth plains that appear so young that researchers say they are still being resurfaced today.
The authors also report towering 2- to 3-mile-high mountains of water ice and evidence of active glaciers made of nitrogen that carve wide troughs across the dwarf planet's surface.
And then there are the features that no one can quite make sense of yet -- regions with parallel ridges and grooves that the scientists are calling "washboard terrain," and blade-like ridges that are oriented north-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 Moffett Field, Calif., who lead the geological study.
Moore explained that even though the temperature on Pluto's surface averages just 40 degrees above absolute zero, or about -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 dwarf planet's surface. That energy, coupled with the warmth from the distant sun, can be enough to mobilize the nitrogen glaciers and other geological changes on the planet.
In addition, Moore said Charon turned out to be significantly more geologically active than the researchers were expecting.
"We expected it would be a boring cratered ball," he said. "Instead we can see there was a lot going on in geologically in its early history."
Moore said the hemisphere south of Charon's equator showed more signs of recent resurfacing than the northern hemisphere. The researchers think this could be due to cryovolcanism, which is something like a water-ice volcano.
And there were more surprises as well: Hal Weaver, a co-investigator on the mission from Johns Hopkins Applied Physics Laboratory said his team was shocked at how reflective the surfaces of Pluto's four small satellites -- Styx, Nix, Kerberos and Hydra -- turned out to be.
For example, if the surface of the Earth's moon were as reflective as the surface of Hydra, our moon would be about 10 times brighter than it is now, he said.
Weaver said his team is still trying to work out how these surfaces have remained so bright after 4 billion years of exposure to cosmic rays, solar wind ions, ultraviolet light and collisions with darker Kuiper belt debris -- all of which should have caused them to grow darker.
He said the group was also surprised that the rotation rates of the moons are not even remotely synchronous. As you can see in the video below, the small moons behave like spinning tops, rotating about their axes much faster than they revolve around the system's center of mass.
Hydra, the outermost moon is particularly crazy, completing a rotation in only 10.3 hours.
Another research team lead by Gladstone reports that Pluto's atmosphere is not escaping nearly as fast as was expected, especially considering how small Pluto is and how much weaker its gravity is compared with Earth's or Mars'. However, data collected by New Horizons also suggests that Pluto's atmosphere is much colder at the highest altitudes than was thought, which could keep Pluto's gases in place.
"It's always fun to have your models validated, but it is way more fun to have them trashed," he said. "Finding out your are completely wrong is a great part of science."
As to what might be cooling off the atmosphere, scientists say they are still not sure.
New Horizons also detected some strange layers in the haze on Pluto that researchers cannot yet explain.
There were other surprises as well. A look at the surface composition of the dwarf planet revealed that the changes in brightness across different regions are among the most extreme in the solar system.
"In just about every discipline it was like, wow, holy cow, what's up with that?" said Gladstone.
For the most part, the researchers expressed delight that their expectations were so wrong.
"A lot of us were inspired to go into science to puzzle out mysterious things," Moore said. "In that case, Pluto is a gold mine."
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