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Science articles discuss wonders, complexity of Pluto system

Pluto's haze.

Pluto’s Haze: About 20 haze layers are seen; the layers have been found to typically extend horizontally over hundreds of kilometers, but they are not strictly parallel to the surface. For example, white arrows indicate a haze layer about three miles (five kilometers) above the surface on the left, which has descended to the surface at the right. (Click to enlarge) Image & Caption Credit: NASA/JHU-APL/SwRI

Five articles published in the March 18 issue of the journal Science present a comprehensive view of the Pluto system based on all data sent back by NASA’s New Horizons spacecraft over the last eight months. A detailed image (above) of Pluto’s atmospheric haze taken by Ralph / Multispectral Visual Imaging Camera (MVIC), in which about 20 layers can be seen stretching over more than hundreds of kilometers, was also published by the mission team.

The articles are titled “Surface Compositions across Pluto and Charon“, “The Atmosphere of Pluto as Observed by New Horizons“, “Pluto’s Interaction with its Space Environment: Solar Wind, Energetic Particles, and Dust“, “The Geology of Pluto and Charon through the Eyes of New Horizons“, and “The Small Satellites of Pluto as Observed by New Horizons“.

Approximately half of the 50 gigabits of data collected during the July 14, 2015, flyby have now been transmitted from the spacecraft’s digital recorders. The return of all data is expected sometime in October.

Pluto’s complexity and beauty surprised many scientists, who expected to find a heavily cratered but dead world so far from the Sun. Instead, they found a previously unimaginable level of complexity, with interactions between surface and atmosphere still not well understood.

In spite of its cold temperatures, only tens of degrees above absolute zero, both Pluto’s surface and atmosphere are highly active and amazingly diverse.

“Observing Pluto and Charon up close has caused us to completely reassess thinking on what sort of geological activity can be sustained on isolated planetary bodies in this distant region of the Solar System, worlds that formerly had been thought to be relics little changed since the Kuiper Belt’s formation,” noted Jeff Moore of NASA Ames Research Center and lead author of the geology paper.

Mountains, Craters, and Plains on Pluto

New Horizons’ views of the informally named Sputnik Planum on Pluto (top) and the informally named Vulcan Planum on Charon (bottom). Image Credit: NASA/JHU-APL/SwRI

Pluto’s diverse landscapes, which range from the smooth, crater-free young surface of Sputnik Planum to ranges of mountains made up of water ice to cryovolcanoes and adjacent pitted regions, are presumed to be the result of interactions between several different volatiles, including methane, nitrogen, and carbon monoxide, with inert water ice.

These interactions occur in the form of endless cycles of evaporation and condensation, explained Lowell Observatory’s Will Grundy, another lead author of the geology study.

On Earth, the only substance that condenses and evaporates is water. On Pluto, at least three compounds – methane, nitrogen, and carbon monoxide – do so, resulting in Pluto’s cycles being far more complex and, therefore, not well understood, he added.

Somehow, possibly through the radioactive decay of rock in its core, Pluto maintains enough of an internal heat source to power active geology, enabling this cycle, plus resurfacing areas such as Sputnik Planum and fueling cryovolcanism. That heat could sustain a subsurface ocean of ammonia-rich water beneath a bedrock of water ice. Long ridges along Pluto’s surface suggest that ocean could be in the process of freezing.

Except for Sputnik Planum, most areas on the surface of Pluto’s encounter side are ancient. A variety of terrains including grooves, channels, and cliffs result from different mixtures of methane, nitrogen, carbon monoxide, and water ice. Stranger than terrains seen on most planets and moons, these terrains include a wrinkled surface with sharp ridges and pitted plains.

Numerous craters in the dark region known as Cthulhu Regio were likely created more than four billion years ago during a tumultuous period in the Solar System’s history known as the Late Heavy Bombardment, Moore said.

Surface diversity on Pluto

Surface diversity on Pluto showing the light-colored ice of Sputnik Planum, and the dark, heavily cratered, ancient terrain of Cthulhu Regio. Image Credit: NASA/JHU-APL/SwRI

Sputnik Planum itself features soft glaciers of mostly nitrogen ice that flow on top of a harder, water-ice bedrock. Valleys in this region, similar to those on Mars and Saturn’s moon Titan, are difficult for scientists to explain because nitrogen on Pluto cannot exist in the liquid state required to form them. Some suspect the channels were created when water ice was eroded by heavier nitrogen ice.

Blue skies on Pluto

Pluto’s blue sky. Image Credit: NASA/JHU-APL/SwRI

Also baffling are Pluto’s mountains of water ice, which rise several kilometers. Before seeing them, scientists did not believe soft volatile ices could form mountains.

The many layers of Pluto’s hazes surprised scientists as well. New Horizons showed Pluto’s atmosphere to be both cooler and more compact than previously thought. Estimates of Pluto’s atmospheric escape rate turned out to be too high, and the primary gas being lost to space is not nitrogen but methane.

“The thought was that Pluto’s atmosphere was escaping like a comet, but it is actually escaping at a rate much more like Earth’s atmosphere,” said Fran Bagenal of the University of Colorado, lead author of the study on particles and plasma.

Because the atmosphere close to Pluto’s surface is 99 percent nitrogen, scientists are hard-pressed to explain why methane is the gas escaping.

While upper atmosphere of Pluto is colder than expected, the temperature of its lower atmosphere varies with location, explained SETI Institute senior scientist David Hinson. Pluto’s atmosphere has a bluish color and ranges from its surface up to 200 kilometers. Surface pressure was found to be 11 microbars.

NASA's New Horizons captured this high-resolution enhanced color view of Charon just before closest approach on July 14, 2015.

Charon. (Click to enlarge) Image Credit: NASA/JHU-APL/SwRI

The surface of Pluto’s largest moon, Charon, is ancient, covered with craters, and composed largely of grayish water ice. A dark red region known as Mordor Macula, located near its north pole, is thought to be made up of hydrocarbons, especially methane.

The methane could come from Charon’s interior, or it could be part of Pluto’s escaping atmosphere, which interacts with ultraviolet light from the Sun, then freezes onto Charon’s surface.

Charon is less geologically active than Pluto, and its surface indicates a violent past. The Pluto system is hypothesized to have been formed by a violent impact approximately four billion years ago.

A long gash that stretches 1,800 km across Charon’s surface suggests that, like Pluto, the large moon once had a subsurface ocean, which froze long ago. That process caused the surface to bulge and fracture, producing the gash.

The small moons Styx, Nix, Kerberos, and Hydra spin rapidly and have extreme axial tilts that scientists cannot easily explain. Their surfaces are icy but with brightness and color different from those seen on Pluto and Charon.

Family Portrait of Pluto’s Moons

Family portrait of Pluto’s moons. (Click to enlarge) Image Credit: NASA/JHU-APL/SwRI

Nix and Hydra have diameters of about 25 miles (40 km), whereas Styx and Kerberos are about six miles (10 km) wide. None of them are large enough to be spherical, and all have cratered, ancient surfaces.

The latest photos sent back by New Horizons includes two (in the second image from above) that shows the polygonal terrain of Sputnik Planum juxtaposed with the dark, cratered region on Charon known as Vulcan Planum, site of a strange mountain surrounded by a moat.

Water-ice rich Vulcan Planum, which contains deep troughs, was likely formed by cryovolcanism that occurred when Charon’s underground ocean froze.

Another image (third from above) released by the mission team is a close-up of Sputnik Planum that combines red, blue, and infrared images captured by Ralph/MVIC.

Dark Cthulhu Regio is visible at the bottom west of the image. An enhanced color photo of the area beneath Sputnik Planum was created with a combination of Long Range Reconnaissance Imager (LORRI) panchromatic images and Ralph/MVIC color pictures.

New Horizons Principal Investigator Alan Stern will present a public lecture, “The Exploration of Pluto”, on Tuesday, March 22, at the 2016 Lunar and Planetary Science Conference from 7:30–8:30 p.m. CDT.

Press briefings discussing New Horizons’ findings at Pluto and NASA’s Dawn mission’s findings at Ceres, along with Stern’s lecture, will be live streamed from the conference on Monday, March 21, and Tuesday, March 22, at this link.

Video Courtesy of Video


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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