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Geologically active Pluto may have ice volcanoes, subsurface ammonia

New Horizons Pluto Panoramic

Artist’s concept of the New Horizons spacecraft during its encounter with Pluto. Image Credit: NASA / JHUAPL / SwRI

Pluto’s surface may harbor ice volcanoes, and its geological activity could be driven by a subsurface layer of ammonia and water ice, members of the New Horizons team reported at the 47th annual meeting of the American Astronomical Society’s Division for Planetary Sciences, held this week in National Harbor, Maryland.

The mission team is presenting more than 50 discoveries from the July 14 flyby at the conference, emphasizing the findings have significantly altered their views of this small world.

Cryovolcanoes South of Sputnik Planum

(Click to enlarge.) Image Credit: NASA / JHUAPL / SwRI

By combining images taken of Pluto’s surface by the spacecraft’s instruments, the mission team created a 3-D topographic map showing detailed features of Wright Mons and Picard Mons, two mountains on Pluto’s surface.

Color is used in the map to depict elevation changes, with brown areas marking higher elevations, blue marking lower ones, and green marking intermediate ones.

Both mountains are tens of miles across. Wright Mons has a height of two miles and Picard Mons has one of 3.5 miles. Large holes at their summits strongly suggest they are ice volcanoes, also known as cryovolcanoes.

On Earth, holes at the tops of large mountains indicate the presence of volcanoes, said New Horizons postdoctoral researcher Oliver White of NASA’s Ames Research Center in Moffett Field, California.

Volcanoes on our planet eject molten rock, but on Pluto, they would likely spew a semi-liquid mixture of water ice, nitrogen, ammonia, and methane.

With broad bases and gentle slopes, the mountains might be shield volcanoes, White noted.

“If they are volcanic, then the summit depression would likely have formed via collapse as material is erupted from underneath. The strange hummocky texture of the mountain flanks may represent volcanic flows of some sort that have traveled down from the summit region and onto the plains beyond, but why they are hummocky, and what they are made of, we don’t yet know,” White said.

The identification of these mountains as volcanic remains tentative pending additional data. If they are volcanoes, they could hold vital clues to Pluto’s geological evolution. Beyond Earth, the only known shield volcanoes are on Mars.

“Nothing like this has been seen in the deep outer Solar System,” said New Horizons Geology, Geophysics, and Imaging (GGI) team leader Jeffrey Moore, also of NASA’s Ames Research Center.

An image of a possible cryovolcano on Pluto

The informally named feature Wright Mons, located south of Sputnik Planum on Pluto, is an unusual feature that’s about 100 miles (160 kilometers) wide and 13,000 feet (4 kilometers) high. It displays a summit depression (visible in the center of the image) that’s approximately 35 miles (56 kilometers) across, with a distinctive hummocky texture on its sides. (Click to enlarge.) Image & Caption Credit: NASA / JHUAPL / SwRI

Pluto’s active geology, including possible cryovolcanism, could be driven by a mixture of ammonia and water ice in its mantle, scientists speculate.

Located between a planet’s outer crust and inner core, the mantle could be undergoing convection, through which hot material rises up while cooler material sinks down.

On Earth, convection is a slow process that takes millions of years and drives tectonic plate movement.

If mantle convection is taking place inside Pluto, it is likely being driven by the presence of ammonia, which acts as an anti-freeze. Convection would not be possible with water ice alone, which has a temperature more than 300 °F below zero (–185 °C).

Scientists had previously thought that water ice alone was present beneath the dwarf planet’s surface.

The lack of craters in regions such as Sputnik Planum and the presence of mountains indicates a geologically active surface below which lies a moving mantle.

Researchers Jay Melosh, Alex Trowbridge, and Andy Freed compared a convection model using a mantle of pure water ice with one composed of five percent ammonia to determine which would generate mantle convection and subsequent geological activity.

They found that the mantle containing the small level of ammonia lowered the temperature under which water ice can become viscous (semifluid), resulting in convection and the geological activity New Horizons found on Pluto’s surface.

The model could be used to determine geological activity on other planetary bodies, they noted.

In terms of age, Pluto’s surface is diverse, with some crater-rich regions that date back four billion years and other, craterless areas such as Sputnik Planum, just ten million years old.

There are also areas with moderate numbers of craters, indicating a surface that is “middle aged”.

To date, more than one thousand craters have been mapped on Pluto’s surface, reported Kelsi Singer of Boulder, Colorado’s Southwest Research Institute (SwRI).

The sheer variety of  sizes and appearances will likely give scientists important insights into the formation of the outer Solar System, she said.

Such variation in the ages of surface terrain is a sign that Pluto may have been geologically active through most of its history, meaning Sputnik Planum is hardly an anomalous area.

Scientists were also surprised by a lack of small craters on both Pluto and its large moon Charon. Small craters are produced via impacts of very small Kuiper Belt Objects (KBOs). The fact that craters on Pluto and Charon are large suggests KBOs formed at larger sizes than previously thought, about several tens of miles across.

Crater Counts on Pluto

Locations of more than 1,000 craters mapped on Pluto by NASA’s New Horizons mission indicate a wide range of surface ages, which likely means Pluto has been geologically active throughout its history. Image & Caption Credit: NASA / JHUAPL / SwRI

The notion that KBOs may have been born large means New Horizons’ next target, 2014 MU69 , with a size of 30 miles (40–50 km) across might be composed of the primordial material that made up the building blocks of the Solar System.

New data on Pluto’s atmosphere reveals it to be colder and more compact than pre-flyby models predicted. Far less of this atmosphere is escaping into space than previously thought.

According to Leslie Young of SwRI, Pluto’s nitrogen atmosphere, produced on the surface when nitrogen ice sublimates into gas, was believed to be escaping at a rate that eroded half a mile of the planet’s surface since the formation of the Solar System, before the flyby.

Instead, the escape rate turns out to be thousands of times lower, having eroded only about half a foot of surface over four billion years.

The mechanism by which Pluto’s atmospheric gases escape is similar to the one that drives atmospheric escape from Earth and Mars and not like that which drives the escape of gases from comets, as scientists believed before the New Horizons encounter.

Pluto’s four small moons surprised scientists with their rapid, chaotic rotations. Most Solar System moons have synchronous rotations with their parent planets.

Video Courtesy of NASA

In contrast, Pluto’s small moons spin very rapidly and behave “like spinning tops”, reported New Horizons co-investigator Mark Showalter of the SETI Institute.

Hydra, the furthest of the small moons, rotates 89 times each orbit around Pluto. Nix rotates approximately ten percent faster than it did just three years ago.

The moons’ chaotic behavior is believed to be caused by torque or force from Charon, preventing the small moons from synchronously revolving around Pluto.

Like Comet 67P/Churyumov-Gerasimenko, both Hydra and Kerberos appear to have formed from a merger of two initially separate objects, suggesting the system may have had more moons in the period following the impact that created Charon.

“The New Horizons mission has taken what we thought we knew about Pluto and turned it upside down,” said NASA director of planetary science Jim Green. “It’s why we explore – to satisfy our innate curiosity and answer deeper questions about how we got here and what lies beyond the next horizon.”

Pluto's minor moons – Styx, Nix, Kerberos, and Hydra.

Data from NASA’s New Horizons mission indicates that at least two — and possibly all four — of Pluto’s small moons may be the result of mergers between still smaller moons. If this discovery is borne out with further analysis, it could provide important new clues to the formation of the Pluto system. Image & Caption Credit: NASA / JHUAPL / SwRI


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.

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