It’s been a busy week for NASA. On Monday, the Deep Space Climate Observatory unveiled a new “Blue Marble” snapshot of Earth, taken from 1 million miles away. Then two days later, the Kepler mission revealed a celestial cousin of our home world, the closest physical replica of Earth that has ever been detected.
Today, we have updates from the New Horizons mission and the discovery of flowing glaciers on the edges of Sputnik Planum, the flat icy plains on the western hemisphere of Pluto’s heart, known as Tombaugh Regio.
“All around the periphery and in the interior are geological wonders,” said New Horizons co-investigator Bill McKinnon of Washington University in St. Louis.
McKinnon presented a close-up image of Sputnik Planum’s northwestern edge, where the frigid flatland meets rugged cliffs and crater walls. The landscape at this margin looks “degraded”, he said, as if eroded over time.
The likely cause of this erosion is flowing glaciers, akin to the large ice masses that creep across Greenland here on Earth.
“If you look carefully, you can actually see a pattern that indicates the flow of viscous ice towards the cliff boundary,” McKinnon said. The key difference is our glaciers consist of ice water, whereas those on Pluto are composed of frozen nitrogen.
“Water ice at Pluto’s temperatures won’t move anywhere. It’s immobile and brittle, “McKinnon said. “The kind of ices that we think make up the planet [Pluto] — nitrogen ice, carbon monoxide ice, methane ice — these ices are soft and malleable even at Pluto conditions, and they will flow in the same ways that glaciers do on Earth.”
These nitrogen glaciers also exist on the southern portion of Sputnik Planum, where the New Horizons team has spotted a second mountain range, which they’ve informally named Hillary Montes. These icy peaks take the name Edmund Hillary, one of the first two people to successfully summit Mount Everest in 1953. (Pluto’s first mountain range was named after Tenzing Norgay, Hillary’s Nepalese guide.)
The glaciers have slid between the two mountain ranges and created frozen “ponds” in some craters, McKinnon said.
The surface of Pluto is cold — 38 Kelvin or -391 degrees Fahrenheit — so how could ice be so dynamic when the temperature is so frigid?
When asked to speculate, New Horizons project leader Alan Stern said that even though the outer surface of Sputnik Planum looks frozen, its inner layers could be made of flowing ice or might even be liquid.
“If you get down tens of meters, the pressure from the overburden of ice can actually change the properties of the [frozen] nitrogen, so that it’s getting warmer. So it’s much more able to flow. There may even be conditions where you can get liquid nitrogen flowing below a deep layer of ice,” Stern said.
McKinnon agreed: “Even at 38 kelvin solid nitrogen would creep.”
Flyover of Hillary and Norgay Montes. Features as small as one-half mile (1 kilometer) across are visible. Video by NASA/JHUAPL/SwRI.
All of those revelations came from images taken before the New Horizons’ closest approach on July 14. The second set of new surprises came from the moments after the probe’s flyby, as it stared back at Pluto.
The New Horizons atmosphere team recorded a luminous haze surrounding the icy planet, as it eclipsed the probe’s view of the sun.
“We’ve known for 25 years that Pluto has an atmosphere, but this is our first picture of the atmosphere,” said Michael Summers, New Horizons co-investigator of George Mason University in Fairfax, Virginia.
The haze extends at least 100 miles above the surface, which is five times farther than the team had originally predicted. The team suspects that the sun’s UV rays cause chemical reactions with methane gas in Pluto’s atmosphere. The result is two chemicals — ethylene and acetylene — that produce the haze layers in the sky. Then, at some point, another chemical reaction produces tholins, a dark hydrocarbon compounds that settle onto Pluto’s surface as a reddish hue.
These dark spots regions around Pluto’s equator “are likely the tholins that were raining out or falling out of the hazes, said New Horizons deputy project scientist Cathy Olkin of Southwest Research Institute.
But as one moves toward the poles, you notice brighter, light-colored concentrations of ice.
She blames the odd distribution of ice on Pluto’s extreme tilt — its poles lean at 120 degrees — and the planet’s eccentric orbit. It takes 248 years for the planet to round the sun, but sometimes it’s much closer to the sun than other times. As Pluto circles the sun, the team suspects that seasonal changes transport these ices around the planet.
“Some parts are kind of baked, like near the equator. Other parts receive the condensations of the ices,” Olkin said. “There is one glaring difference in this pattern and that’s Tombaugh Regio.”
Pluto’s heart is a light-colored stain on what should otherwise be a dark belt of tholins encircling the planet. What gives?
“What’s really special about Tombaugh Regio is we’re seeing methane, nitrogen and carbon monoxides ice there. On the other northern part of Pluto, we’re see methane and nitrogen, but not carbon monoxide,” Olkin said. “So maybe what we’re seeing with Tombaugh Regio is a source region for some of these specific ices, that complicates the story of this seasonal transport.”
So many questions remain, but luckily, the team has only parsed through a thimbleful of data so far.
“95 to 96 percent of the data is still on the spacecraft,” Stern said, who also presented results showing that Pluto’s moon Charon has much less atmosphere than the dwarf planet.
For the next couple of months, the team will only receive occasional images, as the probe beams down critical data from other instruments, like the student dust counter.
“Then in September the spigot will open again and the sky will be raining presents,” said Stern.