Pluto’s Sputnik Planum resurfaced by convection; backlit images reveal haze, surface details
The process of convection regularly resurfaces the ices that comprise the polygonal terrain of Pluto’s Sputnik Planum – an area of 900,000 square kilometers on the left side of its “heart” feature – approximately every 500,000 years.
Two studies published in the June 2 issue of the journal Nature discuss how mission scientists combined topographic and compositional data collected by NASA’s New Horizons spacecraft with computer models to determine the depth of Sputnik Planum‘s surface ice and the rate at which that ice is flowing.
Ranging in size from 10 to 30 miles (16 to 48 km) across, the region’s icy, churning polygonal cells are estimated to be less than one million years old – extremely young on geological timescales.
Mission scientists describe the convection process as similar to the movement that occurs within a lava lamp.
Surface ice on Sputnik Planum is comprised largely of nitrogen, with small amounts of methane and carbon dioxide.
The ice is capable of flowing at Pluto’s surface temperature of minus 235 degrees Celsius.
It moves at a rate of several centimeters per year, which may seem slow but is actually fast in geological time.
“For the first time, we can really determine what these strange welts on the icy surface of Pluto really are,” said New Horizons science team co-investigator William McKinnon of Washington University in St. Louis, Missouri.
“We found evidence that even on a distant cold planet billions of miles from Earth, there is sufficient energy for vigorous geological activity, as long as you have ‘the right stuff ‘, meaning something as soft and pliable as solid nitrogen,” he explained.
Scientists theorize the convection is driven by heat in Pluto’s interior generated by the radioactive decay of elements that were incorporated into the dwarf planet when it initially formed around four billion years ago.
When internal heat warms a reservoir of ice several miles deep, that ice floats to the surface in large blobs, where it subsequently cools and sinks in an ongoing cycle.
“It’s a vigorously – from a geological point of view – churning layer of solid nitrogen, and it’s [as if] the heart of Pluto is truly beating,” McKinnon said.
One of the computer models confirmed convection can occur even if the ice is just several miles deep. It also showed the convection cells are very broad.
Over millions of years, blobs of solid nitrogen can merge with one another.
Ridges visible between Sputnik Planum‘s polygonal cells mark areas where nitrogen ice that cooled sank back down, in the process creating features shaped like an “X” or a “Y” marking locations where several of the cells once adjoined one another.
Scientists studying images of Sputnik Planum have seen evidence of nitrogen ice glaciers having moved into the plain, away from the mountains of water ice that border the region.
In addition to this area having no impact craters, there is also evidence that water ice flows on top of nitrogen ice, in the process pushing large boulders into troughs that separate the individual polygonal cells.
“This activity probably helps support Pluto’s atmosphere by continually refreshing the surface of ‘the heart’,” McKinnon said. “It wouldn’t surprise us to see this process on other dwarf planets in the Kuiper Belt. Hopefully, we’ll get a chance to find out someday with future exploration missions there.”
New Horizons Principal Investigator Alan Stern of the Southwest Research Institute (SwRI) in Boulder, Colorado, described Sputnik Planum as “one of the most amazing geological discoveries in 50-plus years of planetary exploration. The finding by McKinnon and others on our science team that this vast area – bigger than Texas and Oklahoma combined – is created by current day ice convection is among the most spectacular of the New Horizons mission.”
On the same day that the two Nature articles were published, the New Horizons mission released a silhouetted image of Pluto backlit by the Sun, taken just 19 minutes after the July 14, 2015, closest approach. The image reveals Pluto’s atmospheric hazes in stunning detail, including a bright area that could be a low-lying cloud.
The image also provides a view of rugged terrain on Pluto’s night side as well as images of Sputnik Planum and the Norgay Montes mountain region on the encounter hemisphere.
Captured by the Ralph / Multispectral Visual Imaging Camera (MVIC), the photo was taken from a high phase angle, meaning when the Sun was on the opposite side of Pluto, a view mission scientists describe as Pluto’s “Twilight Zone“, from a distance of 13,400 miles (21,550 km).
With a resolution of 1,400 feet (430 meters) per pixel, this image shows a level of detail in Pluto’s layered atmospheric hazes and on the dwarf planet’s night side that could not be captured upon approach. Two insets showing close-ups of regions on the top and bottom of the silhouetted image were released alongside it.
The top inset shows Pluto’s complex, layered hazes as illuminated by sunlight from behind.
A bright, wispy area visible in the center of the inset might be a cloud in Pluto’s lower atmosphere. Extending tens of miles in diameter, the 140-mile (230-kilometer) feature is illuminated by sunlight from a low angle, giving it an unusually bright appearance.
Computer models indicate methane clouds can form in Pluto’s atmosphere. If this feature is a cloud, it is the only one to be detected by New Horizons as of now.
Also visible in the top inset are the southern sections of Sputnik Planum‘s nitrogen ice plains and the Norgay Montes mountain range.
The bottom inset offers a rare, detailed view of the topography of Pluto’s night side. This terrain is visible only because it is lit from behind by sunlight filtered through Pluto’s atmospheric hazes. It is a rugged area, with sharp peaks and broad valleys.
The silhouetted high-resolution close-up, which has far better quality than low-resolution images taken on approach, shows an area with a diameter of 460 miles (750 km), offering a rare detailed view of the non-encounter hemisphere in twilight.
Astrophysicist Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona, and a member of the New Horizons science team, discusses the work involved in transforming raw images into detailed photos in a June 3 blog entry titled “Processing Pluto’s Pictures“.
He provides insight into the way multiple images of features taken both in advance of and during the encounter are combined to eliminate blurring and bring out the finest structure and detail in photos of Pluto and its moons.
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.