Pluto surprises scientists with atmospheric haze and flowing ice

Artist’s impression of how the surface of Pluto might look, according to one of the two models that a team of astronomers has developed to account for the observed properties of Pluto’s atmosphere, as studied with CRIRES. The image shows patches of pure methane ice on the surface and a tenuous haze in the horizon. At the distance of Pluto, the Sun appears about 1,000 times fainter than on Earth. Image Credit: ESO / L. Calçada

Pluto’s complex surface geology and unusual atmosphere continue to baffle scientists almost two weeks after the spacecraft’s closest approach to Pluto as NASA’s New Horizons spacecraft returns data from the historic July 14, 2015, flyby.

Images taken by the Long Range Reconnaissance Imager (LORRI), released at a press conference on Friday, July 24, show evidence of flowing ices in the area informally dubbed Sputnik Planum, on the west side of Pluto’s famous heart feature.

A sheet of ice in this bright region appears to have flowed or could still be flowing in a manner similar to the movement of glaciers on Earth. Mission co-investigator John Spencer noted the only other places in the Solar System experiencing this kind of activity are the planets Earth and Mars, both of which are active worlds.

Annotated image of the northwestern region of Pluto’s Sputnik Planum, swirl-shaped patterns of light and dark suggest that a surface layer of exotic ices has flowed around obstacles and into depressions, much like glaciers on Earth. Image Credit: NASA/JHU-APL/SwRI

New Horizons’ Ralph instrument identified an abundance of nitrogen, methane, and carbon monoxide ices in Sputnik Planum.

Bill McKinnon, deputy leader of the mission’s Geology, Geophysics, and Imaging (GGI) team, pointed out that icy deposits from Sputnik Planum appear to have flowed into a dark, crater-filled area adjacent to the southern part of the heart.

“At Pluto’s temperatures of minus-390 degrees Fahrenheit (–234.4 ºC), these ices can flow like a glacier,” McKinnon said.

Patterns of swirling light and dark at the north of Sputnik Planum suggests surface ice also has flowed there, making its way into troughs.

Annotated image of the southern region of Pluto’s Sputnik Planum. The large crater highlighted in the image is about 30 miles (50 kilometers) wide, approximately the size of the greater Washington, D.C., area. Image Credit: NASA/JH U-APL/SwRI

A new mountain range discovered on the western side of Sputnik Planum has been informally dubbed Hillary Montes after explorer Sir Edmund Hillary, one of the first two people to successfully reach the top of Mount Everest in 1953.

Using several close-approach images of Sputnik Planum and Hillary Montes, NASA created a simulated video flyover of these regions of Pluto’s surface.

The images in the video below were acquired by the LORRI instrument on the New Horizons as the spacecraft flew by Pluto’s surface from a distance of 48,000 miles (77,000 kilometers) on July 14. Features as small as one-half mile (1 kilometer) across are visible.

Video courtesy of NASA/JHU-APL/SwRI

Also released was a sharper image (below) of Pluto’s encounter side, generated by combining four LORRI images with color data provided by Ralph. The individual images were all taken on July 13 from a distance of 280,000 miles (450,000 km) with twice the resolution of the last pre-encounter image taken on July 13 and released the next day. They show features as small as 1.4 miles (2.2 km). (More images are available here.)

Four images from New Horizons’ LORRI were combined with color data from the Ralph instrument to create this global view of Pluto. (The lower right edge of Pluto in this view currently lacks high-resolution color coverage.) The images, taken when the spacecraft was 280,000 miles (450,000 kilometers) away, show features as small as 1.4 miles (2.2 kilometers), twice the resolution of the single-image view taken on July 13. Image Credit: NASA/JHU-APL/SwRI

The New Horizons team recently released a striking image (below left) of Pluto taken by the spacecraft from a position behind the dwarf planet, which, backlit by the Sun, appears surrounded by a thin atmospheric halo. Mission members described the photo as “a breathtaking farewell to New Horizons.” For one member of that team, the images highlight just how dynamic the distant sentinel at the edge of our Solar System is.

“I think this data means that Pluto is a world of unexpected complexity and riches,” Alan Stern, New Horizons principal investigator, told SpaceFlight Insider in an exclusive interview.

Pluto backlit by the Sun. Image Credit: NASA/JHU-APL/SwRI

In the image on the left, taken by NASA’s New Horizons spacecraft around midnight EDT on July 15, Pluto’s atmosphere, backlit by the Sun, rings its silhouette like a luminous halo. This global portrait of the atmosphere was captured when the spacecraft was about 1.25 million miles (2 million kilometers) from Pluto and shows structures as small as 12 miles across. The image, delivered to Earth on July 23, is displayed with north at the top of the frame.

“Ninety-five percent of the data is still up in the Kuiper Belt with the spacecraft,” Alan Stern said. “It should take us somewhere between four and five hundred days to get the remainder of the data captured during the flyby. Think of it like an orbiter, like MAVEN at Mars or Cassini at Saturn, where it’s just data coming down all the time. We’re not taking observations all the time. But, we took this vast amount and now we’re starting to get it back.”

In the image below right, two distinct layers of atmospheric haze are visible, taken by New Horizons only seven hours after its closest approach to Pluto. While the haze rises as high as 80 miles (130 km) above the planet’s surface, the distinct layers figure prominently at 50 miles (80 km) and 30 miles (50 km).

Scientists surmise that the hazes are produced when ultraviolet sunlight breaks down particles of methane gas in Pluto’s atmosphere. Methane is a simple hydrocarbon; breaking it down produces more complex hydrocarbons such as ethylene and acetylene, both of which have been identified in the atmosphere.

These complex hydrocarbons condense into ice particles as they fall to the colder, lower regions of Pluto’s atmosphere, producing atmospheric haze. When complex hydrocarbon ices interact with the Sun’s ultraviolet rays, they are converted into dark hydrocarbons known as tholins. The presence of tholins on Pluto’s surface is presumed to be responsible for its reddish color.

Finding hazes so high in Pluto’s atmosphere was a surprise to mission scientists, who had thought atmospheric temperatures 20 miles (30 km) or more above the surface would be too warm to produce this effect.

Image of Pluto’s hazes; false-color inset reveals a variety of structures, including two distinct layers. (Click to enlarge.) Image Credit: NASA/JHU-APL/SwRI

One hour after closest approach to Pluto, New Horizons’ REX radio experiment had made observations that Pluto’s atmosphere has an unexpectedly low surface pressure – only 1/100-thousandth that of the pressure on the surface of Earth, which is about half of the amount that had been previously calculated from Earth-based observatories.

A possible explanation for the decrease in surface pressure on Pluto may be due to half of its atmosphere having recently frozen out onto its surface as it orbits farther from the Sun.

NASA has also released a second animation (below) depicting the hazes in Pluto’s atmosphere.

“We knew that a mission to Pluto would bring some surprises, and now – 10 days after closest approach – we can say that our expectation has been more than surpassed,” noted John Grunsfeld, associate administrator for NASA’s Science Mission Directorate. “With flowing ices, exotic surface chemistry, mountain ranges, and vast haze, Pluto is showing a diversity of planetary geology that is truly thrilling.”

While the July 14, 2015, flyby might have been a high point of the mission – New Horizons is not finished yet. If the spacecraft can receive the funding to continue its research, it will be sent to review other destinations out in the Kuiper Belt.

“We have two targets, and we have to choose between them because they are in different directions and we have to fire the engines this fall and then if NASA approves the extended mission’s budget, we will fly by whichever one that is selected. In either case, it will be early 2019 flybys,” Stern said.

Alan Stern will be making a recommendation to NASA in August 2015 on which of those two KBOs the New Horizons spacecraft should next visit. The preferred flyby target is 2014 MU₆₉ (PT1), a 40–70 km object. Another slightly bigger object, 2014 PN₇₀ (PT3), could also be targeted, but more fuel is required for the probe to maneuver for a flyby. The choice will be decided later in August 2015. (UPDATE.)

Video courtesy of NASA/JHU-APL/SwRI

The animation in the above video shows several steps in the sunlight-driven chemistry: (1) Ultraviolet sunlight breaks apart methane in Pluto’s upper atmosphere; (2) this leads to the buildup of complex hydrocarbons, such as ethylene and acetylene; (3) clumps of these hydrocarbons condense as ice particles to form the hazes; (4) the hazes are chemically converted to tholins, which fall to the surface and darken Pluto.

Tagged: Alan Stern Kuiper Belt Lead Stories LORRI NASA New Horizons Pluto Solar System

Laurel Kornfeld

Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.