New Horizons Mission to Pluto and Charon

 

By Brooklyn Tessier, Corinne Mathews, Jessie Kraus and Jaewoo Jung

 

The New Horizons satellite was launched in January 2006 for a decade long mission to observe the dwarf planet Pluto (Fig. 1), its moon Charon (Fig. 1) and the Kuiper Belt Objects. It made its flyby of Pluto in 2015 and will continue its secondary mission of observing and studying other Kuiper Belt Objects for the next decade. The New Horizons was able to provide new discoveries and confirm predictions made about Pluto and its dwarf twin satellite Charon. With the use of the right instrumentation, the New Horizons mission has revealed that Pluto, along with its moon Charon, has a surprisingly complex geology with a unique atmosphere and climate.

The New Horizons satellite mission is providing information that is relevant and important to share with the public. It was believed that Pluto was a cold, dead dwarf planet, but this theory is far from true. Pluto has an active and diverse geology as does Charon. Pluto’s blue atmosphere is fascinating scientists with multiple haze layers different from what has been seen before. Although Charon may have little to no atmosphere, its gravity may be pulling in gases escaping from Pluto. The data collected will challenge the general public’s perception of Pluto, Charon, and other Kuiper Belt Objects.  Sharing the new findings and discoveries of the New Horizons with the general public will help people better understand the far reaches of our solar system.

New Horizons Instrumentation

New Horizons is a spacecraft designed specifically to collect the scientific data NASA felt would answer the most important questions about Pluto, Charon and its system.  Specific mission objectives included, mapping the surface composition and temperatures of Pluto and Charon, investigating the geology and atmosphere of Pluto and Charon and searching for rings and other satellites. Once a list of important questions was generated, specific scientific instruments were created to measure and collect this data. Keeping in mind the payload needed to be light and energy efficient,  NASA selected 7 instruments (Fig. 2):

Long-Range Reconnaissance Imager (LORRI)

– A long range camera with high resolution.

Solar Wind At Pluto (SWAP)

– A spectrometer for measuring solar wind and the escape rate of the atmosphere.

Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI)

– Like SWAP, PEPSSI measures ions leaving the atmosphere, but gives more mathematical data.

Alice

– An ultraviolet image spectrometer that collects data about the composition of the atmosphere surrounding Pluto and Charon.

Ralph

– A telescope and colour camera.

Student Dust Counter (SDC)

– A student made instrument which continually collects space dust.

Radio Science Experiment (REX)

– Measures the composition and temperature of the atmosphere.

Each of these instruments contains several sensors to collect the necessary data, but could also serve as a backup if one of the other instruments were to fail¹.  

Pluto’s Geology

Before the New Horizons mission, we knew very little about the geology of Pluto, scientists only had theories and guesses based on blurry images.  Even our largest telescopes were unable to focus on surface details of Pluto. All of that changed with the flyby of the New Horizons and the spectacular images LORRI and Ralph were able to capture.  

Sputnik Planitia

Hosting some of the most interesting geological discoveries on Pluto is Tombaugh Regio, nicknamed Pluto’s Heart, because of its shape (Fig. 3). The Tombaugh Regio has two geologically distinct lobes. Sputnik Planitia, the eastern lobe of Pluto’s Heart, is a smooth plain.  Pluto was believed to be a frozen planet riddled with craters. When the images came back, scientists found huge areas with absolutely no cratering.² The lack of cratering was a shocking discovery.  Cratering helps estimate the age of a planet much the same way a tree shows its age with rings.  This means that geologically speaking Pluto is very young, with a broad sweeping plain, Pluto’s surface appears to be less than 100 million years old. Pluto must have geological processes that were recently resurfacing Pluto.³  In a solar system more than 4 billion years old, scientists are now looking for and finding evidence of cryovolcanoes, glaciers, and ice flows which altered Pluto’s surface, and may be active even today.

Nitrogen Ice Plains    

The nitrogen ice plains of Sputnik Planitia are not completely flat. Images show there is a pattern of raised cells, each between 16 – 40 km wide throughout Sputnik Planitia (Fig. 4).  The most common explanation for this phenomenon is that part of Pluto is acting like a giant lava lamp. Mission scientists believe the pattern of the cells stems from the slow thermal convection of the nitrogen-dominated ices that fill Sputnik Planitia. A deep reservoir is filled with solid nitrogen. At the deepest points, likely several kilometers deep, the internal heat from Pluto warms the nitrogen making it lighter, sending great big blobs floating to the surface. At the surface these large cells cool and sink, and the cycle begins again.⁴  A heat source at the center of Pluto conflicts with scientific beliefs before the New Horizons mission. Pluto was thought to be a frozen solid mass due to its small size and distance from the sun. So, what is generating the internal heat? Radioactive material inside Pluto’s core could be decaying and provide a warm interior.² It is possible Pluto was involved in a huge collision billions of years ago, which may have generated an internal heat. Or, Pluto’s warm interior could be caused by something else entirely, however, the nitrogen ice plains suggest Pluto has a warm core.⁵ This curious discovery is shifting how scientists think about Pluto and other bodies in the outer solar system.

Ice Water Mountains

Another intriguing discovery, Pluto has ice water mountains thousands of meters high.  It was assumed, and later verified, that these mountains are made of ice water, because at Pluto’s surface temperature water ice is as hard as rock.  A mountain of ice at the temperature and gravity of Earth, would collapse under its own weight, but Pluto has a lower gravity, and is much colder, water ice is the only element on Pluto that would be as solid as rock on Earth.⁶

Earth has plate tectonics which push our mountains to great heights. Pluto doesn’t have tectonics, nor does it have gravitational interactions with larger bodies which have been known to create mountains on moons orbiting larger planets. Something is generating the mountainous landscape on Pluto.⁷ How are these mountain ranges created? Perhaps it has something to do with the internal heat of Pluto, or maybe it is an indication of a liquid subsurface ocean.

Subsurface Ocean

The possibility of a liquid layer inside Pluto has scientists very excited.  Other geological features hint that Pluto had or has a subsurface ocean beneath its crust.  As well as a possible explanation for mountain ranges, a liquid layer would explain cracking and fissures along Pluto’s surface.  Pluto has an icy spider, a radiating pattern of fractures hundreds of kilometers long in every direction. The longest fracture, informally named Sleipnir Fossa, spans almost 600 kilometers.⁸  There must be pressure underneath the surface, causing Pluto’s solid outer crust to expand to form these cracks. Scientists have discovered that the rocky surface material acts as a blanket, trapping the small amount of internal heat inside Pluto.This could be enough heat to melt a significant amount of ice, and maybe even keep it in a liquid state.⁹  A subsurface ocean would also explain why Pluto has a wonky orbit. It doesn’t rotate smoothly, it wobbles as though Sputnik Planitia is heavier than the rest of Pluto.

The idea of liquid water on the far edges of the solar system is so astonishing that scientists will be investigating this possibility for a long time.

Bladed Terrain

A bladed ice ridge terrain has been discovered on Tartarus Dorsa (Fig. 5). Hundreds of kilometers of parallel ridges can be seen topped with bladed formations. These features are thought to be penitentes, long thin blades of hardened ice found at high altitudes. These geological formations have only been seen on Earth prior to the New Horizons mission, but unlike Earth, the blades on Pluto tower hundreds of feet into the air.  These ice blades suggest a complex climate and geology on Pluto.¹⁰ On Earth when the right atmospheric conditions exist, sublimation (evaporation from solid to gas) and erosion create these unusual features.

Penitentes on Earth (Fig. 6) are made of water, while on Pluto they consist of methane. Although Pluto is much colder than Earth, and the air is thinner, the snow and ice on the surface are made from methane and nitrogen instead of water.¹¹ The creation of methane ice penitentes on Pluto undergoes the same process as water ice penitentes do on Earth..  Scientists have concluded the surface and air of Pluto are far more dynamic than previously thought.¹⁰

Pluto’s Atmosphere

One of New Horizons goals was to examine the composition of Pluto’s atmosphere.¹² New horizons used REX, ALICE, LORRI and visible imaging cameras.¹² ALICE discovered that Pluto’s atmosphere was primarily composed of nitrogen with lesser amounts of methane, carbon dioxide, acetylene, ethylene, and ethane. ¹² There are exciting discoveries about Pluto’s atmosphere found by New Horizons. Pluto has a large atmospheric radius, although its air is very thin. Pluto’s atmosphere goes through pressure changes which creates weather and wind, and Pluto has seasonal changes in its atmosphere causing it to get thinner or thicker with temperature changes. These points about Pluto’s atmosphere are made in comparison to Earth’s atmosphere in this video

Using the data obtained from New Horizons scientists concluded that Pluto’s atmosphere is also unique in that the haze particles are acting as the solar radiation absorbers not the gas particles.¹³ This is unlike other planets’ atmosphere where solar radiation is absorbed by gas particles.¹³

Haze layers

Images from New Horizons shows haze layers over Pluto with multiple layers.¹² ¹³ These layers can be seen below (Fig. 7).  Light is unable to penetrate past 350 km up from ground level.¹³ LORRI was able to determine the more prominent layers to be at altitudes of 10, 30, 90 and 190 km. Scientists believe the bluish coloring of the haze suggests that small ion particles are a part of the composition.¹² These small particles are the reason why earth’s sky appears blue, small particles scattering the sunlight. The haze is expected to be a combination produced by photochemistry and ion chemistry using hydrocarbons and nitrogen.¹³  Pluto’s haze layers were expected to behave and have a composition similar to Saturn’s moon Titan.¹³ This is due to previous observations and predictions of Pluto’s atmosphere and how it compared to Titan’s atmosphere. New Horizons found evidence that suggests that the haze composition is different from Titan’s.¹³ There is very little change in Pluto’s atmospheric pressure which leads to very weak horizontal winds.¹²

 

Atmospheric Temperature

Pluto is located far from the sun and it is known that temperatures are well below freezing. New Horizons was able to find that the temperature in Pluto’s atmosphere had strong temperature inversions, or a reversal of normal temperature behavior, entering and exiting altitudes below 20 km.¹² Temperature inversions are layers of atmosphere where the temperature decreases less than normal. Temperature inversions are possible reasons for Pluto multiple haze layers. These observations are consistent with measurements from earth based stellar occultation.¹² REX also noted that temperature inversions are stronger entering than exiting.¹²   This means that temperature will rise or drop faster going to lower altitudes compared to rising to higher altitudes. REX was also able to detect differences in temperatures horizontally which was not previously seen.¹² Pluto’s upper atmosphere was noted to be cooler than the lower atmosphere.¹² At lower altitudes temperatures can range depending on surface conditions and whether or not frozen nitrogen is present.¹² The upper atmosphere temperature was lower than what was predicted with theoretical models. This suggests that an unknown cooling agent is present. This cooling medium may be located within the haze layers that reside in the upper altitudes.¹³ ¹²

Atmospheric escape

Atmospheric escape occurs in all planets when the moon has enough gravitational pull to draw in an atmosphere. There are many reasons for atmospheric escape but, Pluto’s responsible mechanism was concluded to be the result of thermal Jeans.¹² Thermal Jeans is the upwards escape of air molecules due to thermal evaporation. The models for Pluto’s predicted escape rate of nitrogen and methane via thermal Jeans were based off of Titan’s model due to its similarity.¹⁴ The escape rates for the two major gases, nitrogen and methane were calculated using Pluto’s surface pressure and radius.¹² New Horizons found that the rate at which nitrogen escapes is 10,000 X slower than what was expected.¹² Reasons for this may be due to the cooler temperatures in the upper atmosphere and the unidentified cooling medium.¹² The methane escape rate however, does falls within expected rates.¹²

Charon

Welcome to Charon, one of Pluto’s moons, and one of the greatest astronomy mysteries that is being researched currently. We had only slight thoughts of what the shape, size, and markings of Charon would be. Then, just like Pluto, New Horizons made discoveries that astronomers and other scientists would not believe: like crater formations, the tectonics shapes, mountains with moats and the red spot on the Northern Hemisphere.

Charon’s atmosphere

The New Horizons mission did not detect an atmosphere on Charon, Pluto’s largest moon. There is currently no evidence to prove that it has any atmosphere at all, and if one is found it would likely be very thin as Charon has a small mass and a smaller gravitational attraction, any gases would likely evaporate into space.¹⁵ It is theorized that any atmosphere would be composed of gases escaping from Pluto.  In fact, scientists believe that Charon may be stealing gases evaporating from the ice on Pluto’s surface. Pluto and Charon orbit close to each other, so gases from Pluto’s atmosphere are likely being pulled in by Charon’s gravity.¹⁶ What has led scientists to investigate this theory?  It seems Pluto’s moon has been caught red-handed. Images have shown that Charon has a large red spot on its north pole (Fig. 8), nicknamed Mordor Macula, from The Lord of the Rings.  The red colour comes from tholins, “chemically complex, tar-like hydrocarbons that are produced when nitrogen and methane ices are exposed to radiation from the sun”.¹⁶ When the question was posed, where did the nitrogen and methane come from, scientists looked to Pluto’s surface, which is also believed to contain tholins.  The complex chemistry involved in creating tholins suggests Pluto and Charon each have a more dynamic chemical activity than scientists suspected.

How We Saw Charon

LORRI  and Ralph were incredibly crucial for the information of the Geology of Charon, because they took high-quality pictures, with different scans, and  filters, and gave us a map of what Charon would look like.¹⁷ These instruments provided scientists with the information to deduce new understandings of Charon.

Before New Horizons, Charon was only seen using spectrometry (which is defined as using wavelengths and lights to understand what elements are found, as each element has their own light signature)¹⁸, and by telescope. Through the spectrometer, we were able to figure out the elements that make Charon are primarily rock and ice water.¹⁹ However, we had undefined  images of what Charon looked like before New Horizons mission. The closest we could see was through the Hubble Telescope (Fig. 9), seeing only light and an idea that Charon was half the size of Pluto. New Horizons gave us a detailed look at Charon; when scientists saw this moon, it was not what they expected.²⁰

Craters of Charon

Charon’s craters became an exciting research point for scientists because of the difference between the craters Oz Terra (Northern Hemisphere) and Vulcan Planum (Southern Hemisphere).²¹ Looking at Fig. 10, we can see that the craters of Oz Terra are more faded than the craters of Vulcan Planum, scientists began to research to why that happened. Scientists believe that Charon’s southern half appears much younger than the north because there are fewer craters between Oz Terra and Vulcan Planum, and faint ridge lines in the northern hemisphere are evidence of significantly more resurfacing .²² Resurfacing is when a moon or planet’s upper crust has a tectonic change that will change the outer surface look.¹⁹  Craters tell us the age of that planetary object or moon. Craters can tell us the age because asteroids, meteors, or comets were colliding into planets or moons. More craters on a surface of a planet or moon suggests it is older due to the fact that many more rocks and dense objects were colliding into it.²³ Charon has many impact craters, but there are far fewer in the south compared to the north; however, we cannot see if there are more impact craters at the northern red pole, Mordor Macula.²⁴ If we know the age of Charon and its formation it will give us a greater insight into how our solar system was formed and how other solar systems can be formed.

Tectonics of Charon

Tectonics are plates of crust that shape rocky planetary or moon surfaces; Charon’s tectonics compared to Earth are noticeably different. Earth’s tectonic plates collide or diverge from each other, which causes the expansion of the earth or forms mountains, and other high ridges. Scientists believe that Charon’s tectonic plates are only diverging from each other.²⁵ Charon has many fault lines and cracks, scientists believe that ice-water came up from those faults and resurfaced Charon, giving Charon some smooth texture.¹⁹ One theory from NASA scientist, Paul Schenik, is that cold volcanism may have occurred, “The team is discussing the possibility that an internal water ocean could have frozen long ago, and the resulting volume change could have led to Charon cracking open, allowing water-based lavas to reach the surface at that time.”²⁶

Conclusion

The information scientists had about Pluto and Charon before the flyby was minuscule compared to the amount of data that New Horizons has provided. The theory of a cold, dead, dwarf planet was wrong. With the use of the right instrumentation, the New Horizons mission has revealed that Pluto, along with its moon Charon, has a surprisingly complex geology with a unique atmosphere and climate. We had no idea of Pluto’s amazing geological features, or of the possibility of a subsurface ocean. Scientists didn’t understand the composition of Pluto’s blue, hazy  atmosphere, or that gases escape to its moon, Charon. Scientists could only make a guess about an atmosphere on Charon, and had absolutely no idea of the red spot, Mordor Macula. All of these findings are changing the way society thinks about our solar system, and this is only the beginning. As we continue to process the information from New Horizons scientists will be mining the data to reveal all the secrets of Pluto and the other Kuiper Belt Objects.

References

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