Saturday, July 18, 2015

New Horizons Reveals Pluto's Mysterious Frozen Plains and Extended Atmosphere

Jim Green, director of Planetary Science at NASA Headquarters in Washington, left, Alan Stern, New Horizons principal investigator at Southwest Research Institute (SwRI) in Boulder, Colorado; Randy Gladstone, New Horizons co-investigator at SwRI in San Antonio; Fran Bagenal, New Horizons co-investigator, University of Colorado, Boulder; and Jeffrey Moore, New Horizons co-investigator at NASA’s Ames Research Center in Moffett Field, California are seen during a New Horizons media briefing where new images and science findings collected by the spacecraft during the historic flyby of Pluto were discussed Friday, July 17, 2015 at NASA Headquarters in Washington. Photo Credit: (NASA/Bill Ingalls)

In the latest data from NASA’s New Horizons spacecraft, a new close-up image of Pluto reveals a vast, craterless plain that appears to be no more than 100 million years old, and is possibly still being shaped by geologic processes. This frozen region is north of Pluto’s icy mountains, in the center-left of the heart feature, informally named “Tombaugh Regio” (Tombaugh Region) after Clyde Tombaugh, who discovered Pluto in 1930. “This terrain is not easy to explain,” said Jeff Moore, leader of the New Horizons Geology, Geophysics and Imaging Team (GGI) at NASA’s Ames Research Center in Moffett Field, California. “The discovery of vast, craterless, very young plains on Pluto exceeds all pre-flyby expectations.”

This fascinating icy plains region — resembling frozen mud cracks on Earth — has been informally named “Sputnik Planum” (Sputnik Plain) after the Earth’s first artificial satellite. It has a broken surface of irregularly-shaped segments, roughly 12 miles (20 kilometers) across, bordered by what appear to be shallow troughs. Some of these troughs have darker material within them, while others are traced by clumps of hills that appear to rise above the surrounding terrain. Elsewhere, the surface appears to be etched by fields of small pits that may have formed by a process called sublimation, in which ice turns directly from solid to gas, just as dry ice does on Earth.

This annotated view of a portion of Pluto’s Sputnik Planum (Sputnik Plain), named for Earth’s first artificial satellite, shows an array of enigmatic features. The surface appears to be divided into irregularly shaped segments that are ringed by narrow troughs, some of which contain darker materials. Features that appear to be groups of mounds and fields of small pits are also visible. This image was acquired by the Long Range Reconnaissance Imager (LORRI) on July 14 from a distance of 48,000 miles (77,000 kilometers). Features as small as a half-mile (1 kilometer) across are visible. The blocky appearance of some features is due to compression of the image.   Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
This annotated view of a portion of Pluto’s Sputnik Planum (Sputnik Plain), named for Earth’s first artificial satellite, shows an array of enigmatic features. The surface appears to be divided into irregularly shaped segments that are ringed by narrow troughs, some of which contain darker materials. Features that appear to be groups of mounds and fields of small pits are also visible. This image was acquired by the Long Range Reconnaissance Imager (LORRI) on July 14 from a distance of 48,000 miles (77,000 kilometers). Features as small as a half-mile (1 kilometer) across are visible. The blocky appearance of some features is due to compression of the image. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Scientists have two working theories as to how these segments were formed. The irregular shapes may be the result of the contraction of surface materials, similar to what happens when mud dries. Alternatively, they may be a product of convection, similar to wax rising in a lava lamp. On Pluto, convection would occur within a surface layer of frozen carbon monoxide, methane and nitrogen, driven by the scant warmth of Pluto’s interior.

Pluto’s icy plains also display dark streaks that are a few miles long. These streaks appear to be aligned in the same direction and may have been produced by winds blowing across the frozen surface.

Artist’s concept of the interaction of the solar wind (the supersonic outflow of electrically charged particles from the Sun) with Pluto’s predominantly nitrogen atmosphere. Some of the molecules that form the atmosphere have enough energy to overcome Pluto’s weak gravity and escape into space, where they are ionized by solar ultraviolet radiation. As the solar wind encounters the obstacle formed by the ions, it is slowed and diverted (depicted in the red region), possibly forming a shock wave upstream of Pluto. The ions are “picked up” by the solar wind and carried in its flow past the dwarf planet to form an ion or plasma tail (blue region). The Solar Wind around Pluto (SWAP) instrument on the New Horizons spacecraft made the first measurements of this region of low-energy atmospheric ions shortly after closest approach on July 14. Such measurements will enable the SWAP team to determine the rate at which Pluto loses its atmosphere and, in turn, will yield insight into the evolution of the Pluto’s atmosphere and surface. Also illustrated are the orbits of Pluto’s five moons and the trajectory of the spacecraft.   Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Artist’s concept of the interaction of the solar wind (the supersonic outflow of electrically charged particles from the Sun) with Pluto’s predominantly nitrogen atmosphere. Some of the molecules that form the atmosphere have enough energy to overcome Pluto’s weak gravity and escape into space, where they are ionized by solar ultraviolet radiation. As the solar wind encounters the obstacle formed by the ions, it is slowed and diverted (depicted in the red region), possibly forming a shock wave upstream of Pluto. The ions are “picked up” by the solar wind and carried in its flow past the dwarf planet to form an ion or plasma tail (blue region). The Solar Wind around Pluto (SWAP) instrument on the New Horizons spacecraft made the first measurements of this region of low-energy atmospheric ions shortly after closest approach on July 14. Such measurements will enable the SWAP team to determine the rate at which Pluto loses its atmosphere and, in turn, will yield insight into the evolution of the Pluto’s atmosphere and surface. Also illustrated are the orbits of Pluto’s five moons and the trajectory of the spacecraft. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

The Tuesday “heart of the heart” image was taken when New Horizons was 48,000 miles (77,000 kilometers) from Pluto, and shows features as small as one-half mile (1 kilometer) across. Mission scientists will learn more about these mysterious terrains from higher-resolution and stereo images that New Horizons will pull from its digital recorders and send back to Earth during the next year. 

The New Horizons Atmospheres team observed Pluto’s atmosphere as far as 1,000 miles (1,600 kilometers) above the surface, demonstrating that Pluto’s nitrogen-rich atmosphere is quite extended. This is the first observation of Pluto’s atmosphere at altitudes higher than 170 miles above the surface (270 kilometers).

The New Horizons Particles and Plasma team has discovered a region of cold, dense ionized gas tens of thousands of miles beyond Pluto — the planet’s atmosphere being stripped away by the solar wind and lost to space.

“This is just a first tantalizing look at Pluto’s plasma environment,” said New Horizons co-investigator Fran Bagenal, University of Colorado, Boulder.

This figure shows the locations of the sunset and sunrise solar occultations observed by the Alice instrument on the New Horizons spacecraft. The sunset occultation occurred just south of the “heart” region of Pluto, from a range of 30,120 miles (48,200 km), while the sunrise occurred just north of the "whale tail", from a range of 35,650 miles (57,000 km).   Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
This figure shows the locations of the sunset and sunrise solar occultations observed by the Alice instrument on the New Horizons spacecraft. The sunset occultation occurred just south of the “heart” region of Pluto, from a range of 30,120 miles (48,200 km), while the sunrise occurred just north of the "whale tail", from a range of 35,650 miles (57,000 km). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

"With the flyby in the rearview mirror, a decade-long journey to Pluto is over — but, the science payoff is only beginning,” said Jim Green, director of Planetary Science at NASA Headquarters in Washington. "Data from New Horizons will continue to fuel discovery for years to come.” 

Alan Stern, New Horizons principal investigator from the Southwest Research Institute (SwRI), Boulder, Colorado, added, “We’ve only scratched the surface of our Pluto exploration, but it already seems clear to me that in the initial reconnaissance of the solar system, the best was saved for last."

Beginning an hour and half after closest approach, the Solar Wind Around Pluto (SWAP) instrument observed a cavity in the solar wind — the outflow of electrically charged particles from the Sun — between 48,000 miles (77,000 km) and 68,000 miles (109,000 km) downstream of Pluto. SWAP data revealed this cavity to be populated with nitrogen ions forming a “plasma tail” of undetermined structure and length extending behind the planet.

This figure shows how the Alice instrument count rate changed over time during the sunset and sunrise observations. The count rate is largest when the line of sight to the sun is outside of the atmosphere at the start and end times. Molecular nitrogen (N2) starts absorbing sunlight in the upper reaches of Pluto’s atmosphere, decreasing as the spacecraft approaches the planet’s shadow. As the occultation progresses, atmospheric methane and hydrocarbons can also absorb the sunlight and further decrease the count rate. When the spacecraft is totally in Pluto’s shadow the count rate goes to zero. As the spacecraft emerges from Pluto’s shadow into sunrise, the process is reversed. By plotting the observed count rate in the reverse time direction, it is seen that the atmospheres on opposite sides of Pluto are nearly identical.   Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
This figure shows how the Alice instrument count rate changed over time during the sunset and sunrise observations. The count rate is largest when the line of sight to the sun is outside of the atmosphere at the start and end times. Molecular nitrogen (N2) starts absorbing sunlight in the upper reaches of Pluto’s atmosphere, decreasing as the spacecraft approaches the planet’s shadow. As the occultation progresses, atmospheric methane and hydrocarbons can also absorb the sunlight and further decrease the count rate. When the spacecraft is totally in Pluto’s shadow the count rate goes to zero. As the spacecraft emerges from Pluto’s shadow into sunrise, the process is reversed. By plotting the observed count rate in the reverse time direction, it is seen that the atmospheres on opposite sides of Pluto are nearly identical. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Similar plasma tails are observed at planets like Venus and Mars. In the case of Pluto’s predominantly nitrogen atmosphere, escaping molecules are ionized by solar ultraviolet light, “picked up” by the solar wind, and carried past Pluto to form the plasma tail discovered by New Horizons. Prior to closest approach, nitrogen ions were detected far upstream of Pluto by the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument, providing a foretaste of Pluto’s escaping atmosphere.

“This is only the beginning for Pluto atmospheric science” says New Horizons scientist Andrew Steffl of the Southwest Research Institute in Boulder, Colorado. “Next month, the full Alice occultation dataset will be sent to Earth for analysis. Even so, the data we have now show that Pluto’s atmosphere rises higher above its surface, in relative terms, than does the Earth’s.” 

New Horizons is part of NASA’s New Frontiers Program, managed by the agency’s Marshall Space Flight Center in Huntsville, Alabama. The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, designed, built and operates the New Horizons spacecraft and manages the mission for NASA’s Science Mission Directorate. SwRI leads the mission, science team, payload operations and encounter science planning.

3 comments:

  1. Those "irregular segments" look almost exactly like a scum of ice that has formed with bubbles under it in shallow frozen puddles I've seen on Earth - so perhaps that is exactly what it is, albeit I haven't a clue to the ices' composition (methane? CO2?). But if I had to hazard a guess I would say something under the 'ice' is outgassing, forming the cracks & "bubbles". It might even explain some pits (ice 'sinkholes'). Just a theory, so I guess it's about as valid as "their" (meaning NASA).

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