Latest on Pluto: flowing nitrogen ice, lofty haze, 'a dream come true'
Fresh data from New Horizons show smooth icy landscapes feathering into ancient, cratered terrains, providing signs of geological activity that could still be under way.
New images from NASA's New Horizons mission to the Pluto-Charon system reveal that a vast icy plain, first seen in detail last week, sports glacier-like ice streams that spill into craters and encircle water-ice mountains – including a new range comparable in height to the Appalachian Mountains on Earth.
At the same time, close-ups of the dark equatorial region reveal a crater-pocked landscape within a portion of the region the team has dubbed Cthulhu Regio.
The contrasting landscape underscores Pluto's unexpected complexity, revealed when the New Horizons spacecraft swooped to within 7,800 miles of the dwarf planet's surface on July 14.
Even though portions of Pluto's surface show signs of geological activity that still could be underway today, "some parts of Pluto are fundamentally ancient," said William McKinnon, a planetary geophysicist at Washington University in St. Louis and a member of the New Horizons science team, during a briefing Friday.
"Pluto has a very complicated story to tell," added the mission's lead scientist, Alan Stern, with the Southwest Research Institute in Boulder, Colo. "There is a lot of work we need to do to understand this very complicated place."
So far, New Horizons has beamed back to data-hungry scientists only about 4 to 5 percent of the observations the craft's suite of seven instruments gathered during the historic flyby.
As if to underscore the level of patience the mission demands, from now to mid-September most of the data the craft is scheduled to return will focus on engineering information, although a smattering of science data will come with it. Once these data are in hand, the science spigot will reopen with a vengeance and flow for about a year.
Much of the new information focused on Pluto's big heart, which the team has informally named Tombaugh Regio. It's east and west lobes are strikingly different, Dr. Stern said. Ice deposits on the eastern lobe, as well as those that appear to be flowing out of the region's southern tip, look much thinner than those to the west, suggesting that the the western lobe is the source for the icy veneer covering geological features to the east and south.
A Texas-sized patch of this western lobe, Sputnik Planum, hosts the broad, polygon-shaped ice patches that look like garden pavers, revealed on June 17. But on the planum's northern and southern boundaries, these features appear to be flowing.
To the north, the ice buts up against what the team interprets as scarps leading to higher terrain. Researchers have detected what look to be ice streams flowing up to, then along, the scarp faces. In one location, nitrogen ice appears to be flowing into an ancient crater an impactor punched into the edge of the highlands. Its entry point appears to be a gap that erosion cut into the crater rim.
At Pluto's surface temperature of about minus 390 degrees Fahrenheit, water ice would be extremely hard and brittle. But ices of nitrogen, methane, and carbon monoxide "are soft and malleable, even at Pluto conditions," Dr. McKinnon said. "They will flow."
Researchers have known for years that Pluto's surface contained nitrogen ice, he noted.
"But to see evidence for recent geological activity is simply a dream come true," he added.
Sputnik Planum's smooth appearance, devoid of any impact craters, "tells us that this is a really a young unit," probably no older than a few tens of millions of years, McKinnon said. Based on modeling estimates of the heat coming from Pluto's interior – at temperatures still frigid by terrestrial standards – "there's no reason why this stuff cannot be going on today."
To the south, the ice appears to be flowing around mountains, including the new range the team has informally named Hillary Montes and feathering itself across lower elevations that make up Cthulhu Regio.
New Horizons also delivered surprises about Pluto's atmosphere, and confirmed that if Charon has one, it is exceedingly tenuous.
Regarding Pluto's atmosphere, New Horizons' data "are basically changing the way we think about Pluto's atmosphere," said Michael Summers, a researcher at George Mason University in Fairfax, Va., and another member of the New Horizons science team. "We're basically having to start from scratch."
Looking back at Pluto as it headed away from the planet and deeper into the Kuiper Belt, New Horizons revealed that the planet's haze – light scattered off of particles in the atmosphere – extended far higher than models had predicted. The haze also revealed a distinct structure to the atmosphere and even provided hints of weather features on Pluto, Dr. Summers noted.
The evidence for weather appears in layers of haze, one about 30 miles above the surface and the other some 50 miles high. The team also saw features that looked to some like waves.
Ultimately, the haze extends up to 80 miles above the surface, where atmospheric temperatures are relatively warm. Up there, the heat triggers reactions between methane and other molecules that form tiny, complex-hyrdocarbon particles known as tholins, which grow heavy enough to fall back to Pluto's surface. Previously, researchers thought that the tholins would form lower in the atmosphere, where temperatures are colder. These tholins are ruddy particles, thought to be responsible for Pluto's reddish tint.
Tholins also are likely to be responsible for the apparently ice-free equatorial region's distinctively dark regions – tholins baked by what, for Pluto, would be its tropical sun.
In addition, the atmosphere appears to be have flipped a switch, losing about half its mass in the past two years.
"That's pretty astonishing to an atmospheric scientist," Summer said.
Pluto has entered a stage in its orbit where it is heading away from the sun. Indeed, when the mission was still being evaluated by the planetary-science community, "there was a real interest in trying to get to Pluto while it still had a substantial atmosphere," Stern said. "We wanted to get there while there was still an atmosphere to study."
But between 1989 and two years ago, the atmosphere seemed stable, despite Pluto's outbound course. This led some to suggest that maybe the atmosphere doesn't collapse at Pluto heads toward its most distant point from the sun.
New Horizons appears to have detected what could be the first stages of that collapse, just as New Horizons arrived, Stern said.
"It would be an amazing coincidence, but there are some on our team who would say: I told you so," he said. "We'll see if this is, in fact, what's happening or if it's a more-complicated story."