The Science of Pluto: The Gates of the Planetary Underworld (Part 1)

A computer-generated impression of the view from the surface of Pluto. The Sun looks like a very bright star on the sky (upper right), while Charon, Pluto’s biggest satellite, looms large near the horizon (at left). The scientific study of the Pluto-Charon system, has been one of expanding horizons and increased awareness of our planetary family and our place in it. Image Credit: ESO/L. Calçada

A computer-generated impression of the view from the surface of Pluto. The Sun looks like a very bright star on the sky (upper right), while Charon, Pluto’s biggest satellite, looms large near the horizon (at left). The scientific study of the Pluto-Charon system, has been one of expanding horizons and increased awareness of our planetary family and our place in it. Image Credit: ESO/L. Calçada

“Come wander with me, she said,
Into regions yet untrod;
And read what is still unread
In the manuscripts of God.”

— Henry Longfellow, “The Fiftieth Birthday of Agassiz” (1857)

 

 

Like a candle in the dark, the history of the scientific study of the Universe is one of ever-expanding horizons and an increased awareness of the world around us and our place in it. The discovery of Pluto in the early 20th century, and the subsequent study of this distant world of unknowns in the outer reaches of the Solar System, is a perfect example of this constant, slow transition from ignorance toward knowledge and enlightenment about the great cosmic dark of which we are an integral part.

For millenia, the heavens were thought to be a tidy and well-understood place. Against the backdrop of countless fixed stars which were the abode of deities and mythic heroes, seven bright wanderers were restlessly roaming the skies, forever affecting and dictating the lives of men. But during the course of a few hundred years, the heavens were changed forever. Italian astronomer Galileo Galilei decisively showed that far from being the enigmatic representatives of the supernatural, these bright wanderers in the sky were physical worlds instead much like our own, while both German astronomer Johannes Kepler and British mathematician Isaac Newton laid the foundations for our current scientific understanding of the Cosmos.

Meanwhile, the heavens were slowly enriched with additional faraway planetary worlds. The discoveries of Uranus and Neptune during the 18th and 19th centuries nearly doubled the expanse of the Solar System and opened the way for the discovery of a new realm of planetary bodies in the early 20th century. Not long after Albert Einstein had revolutionised our understanding of the physical Universe with his theories of special and general relativity, American astronomer Clyde Tombaugh was opening the gates of the planetary underworld with his discovery of Pluto in 1930.

Pluto and Charon, as seen by the Hubble Space Telescope. Long characterised as 'the astronomers' planet, Pluto is set to be revealed in all its glory just one week from now, courtesy of the New Horizons mission. Image Credit: NASA/JPL-Caltech

Pluto and Charon, as seen by the Hubble Space Telescope. Long characterised as the astronomers’ planet, Pluto is set to be revealed in all its glory just one week from now, courtesy of the New Horizons mission. Image Credit: NASA/JPL-Caltech

Yet, contrary to all the other known planets of the Solar System, Pluto quickly turned out to be a misfit. It was soon determined that the orbit of this distant world was highly eccentric and inclined by a whopping 17 degrees relative to the plane of the ecliptic on which all of the other planets orbit the Sun. Furthermore, the mass estimates for the newly discovered world took a nosedive in the decades that followed. Far from being an Earth-mass planet as was originally thought in the immediate aftermath of its discovery, the 2,300-km-wide Pluto was finally found to be at least 500 times less massive than Earth, by the time its bigger moon Charon was discovered in 1978. And even though the distant world has been studied extensively by ground- and space-based telescopes, due to its large average distance from the Sun of approximately 5.8 billion km, it has more or less remained as enigmatic as when it was first spotted by Clyde Tombaugh 85 years ago.

Even some of Pluto’s basic properties like its mass and bulk density aren’t known with certainty. For instance, the discovery of Charon in 1978 helped astronomers to infer the combined mass of the Pluto-Charon system, but not the exact masses for both planetary objects separately, something which requires detailed in-situ measurements that can only be made by robotic spacecraft, similar to the way that NASA’s Voyager 2 spacecraft helped to refine astronomers’ previous mass estimates of Neptune during its close flyby of the ice giant planet in 1989. Estimates of Pluto’s internal composition are similarly ambiguous. The small, faraway world is thought to have an average density of 2.2 grams per cubic centimeter, which is twice that of water, meaning that its interior must be composed between 25 and 50 percent of various ices that are mixed with rocky material. Since Pluto is located in the far reaches of the outer Solar System where water ice is abundant, it is believed that the latter dominates the planet’s interior and surface as well. Nevertheless, a whole list of important questions regarding Pluto’s interior remain unanswered. For instance, the composition of its rocky material is currently unknown. As for the mixture of ices that may lie below the surface, scientists have speculated that besides water ice, nitrogen, carbon monoxide, and methane ices may be also present. Determining the exact ratio of these different kinds of ices to Pluto’s underground rocky material is important in acquiring a better measurement of the planet’s bulk density, which in turn could help to shed more light to the chemical interactions that are taking place between Pluto’s interior and surface.

One other important question regarding Pluto’s interior is whether the latter is differentiated or not. The answer to this question would provide great insights into Pluto’s thermal and geologic evolution since it was formed 4.5 billion years ago. All of the Solar System’s major planets, as well as dwarf planets like Ceres and major asteroids like Vesta, have the different constituents of their internal material separated in distinct layers, generally in the form of a core, a mantle, and an upper crust. Through this process, all the heavier material, like iron, falls toward the planet’s center, while all the lighter silicate and icy material creates the mantle and the upper crust. Planetary differentiation is driven by many different mechanisms that are mainly caused by the decay of radioactive elements in the planets’ interiors, tidal heating from the gravitational interaction with nearby massive objects, and the impact heating that results from the collision of large objects on planetary surfaces. In the case of Pluto, radioactive heating is a process which is believed to be taking place today as well and could have played a huge role in the planet’s thermal evolution early in its history. Nevertheless, scientists need more solid data in order to have a better picture of the processes that have shaped Pluto in the past. “Thermal evolution of Pluto & Charon is a key question for scientists to answer,” writes Dr Kimberly Smith, a research scientist at NASA’s Ames Research Center in California. “But thermal models are dependent on interior structure. At present we do not know whether Pluto or Charon are homogeneous (i.e. same material throughout) or differentiated (split into core and crust, or maybe core, subsurface ocean and crust).”

The best-ever Hubble images of Pluto reveal a striking contrast between bright and dark areas on the surface of the enigmatic distant planet, hinting at an active, ever-changing world. Image Credit: NASA/ESA/SWRI

The best-ever Hubble images of Pluto reveal a striking contrast between bright and dark areas on the surface of the enigmatic distant planet, hinting at an active, ever-changing world. Image Credit: NASA/ESA/SWRI

Equally important questions remain unanswered regarding Pluto’s surface and atmosphere as well. Lying so far from the Sun, one would assume that Pluto would be more or less a dead and frozen chunk of rock and ice. Yet decades of observations have helped to uncover a long series of tantalising evidence which show otherwise. While taking advantage of a stellar occultation by Pluto in 1988, astronomers were able to discover that the planet harbors a thin atmosphere of mainly nitrogen with traces of methane and carbon monoxide, making Pluto only the fourth planetary body in the Solar System besides Earth, Titan, and Triton to harbor a nitrogen-dominated atmosphere. Pluto’s rarefied atmosphere is so thin that it has an average pressure at least 100,000 times lower than that of Earth, causing its surface ices to sublimate directly from solid to gas form as the seasons change on the faraway world. As the ices constantly sublimate on the surface of Pluto during the planet’s 248-year-long orbital revolution around the Sun, they are re-distributed on the entire surface, covering it with fresh material, which many scientists think could be carried away around the planet through blowing winds. The presence of winds in the Plutonian atmosphere, if confirmed, would be an additional line of evidence that Pluto is an active and ever-changing world.

The latter has become readily apparent in recent years, through a series of observations with NASA’s Hubble Space Telescope. While studying the distant planet with the help of the orbiting observatory during the course of the last 15 years, astronomers were surprised to image a series of brightness and color variation on the face of Pluto that has been caused by the change of seasons on the faraway world. More specifically, Hubble had taken images which show the northern hemisphere becoming redder and brighter with time, while the southern hemisphere has gotten progressively darker. This is believed to be a natural cycle of the atmospheric methane being broken up from the ultraviolet radiation of the distant Sun as the northern hemisphere becomes hotter with the coming of summer. In addition, this process might also be the result of Pluto having reached perihelion in the late-1980s which has somewhat raised Pluto’s surface temperature a bit, causing many surface ices to sublimate into gas (as if an average summer surface temperature on Pluto of -220 degrees Celsius can be considered “hot”).

A baffling aspect of these observations by Hubble were a series of dark surface features on Pluto that were mixed with the brighter ones. Furthermore, astronomers were surprised to see that the mass of Pluto’s atmosphere had progressively increased, even though Pluto was past its closest point to the Sun in its orbit, which meant that its atmosphere should start to progressively freeze out on its surface instead. “It’s baffling,” says Dr. Mike Brown, an astronomy professor at the California Institute of Technology. “For now, we can only guess. We know there’s methane on Pluto. Here’s what we think happens: Sunlight hits the methane and breaks it apart into its chemical components — hydrocarbons. Over millions of years this process makes a dark reddish-brown oil or tar like substance that sticks to the ground. These darker areas spread larger as they absorb more sunlight and cause additional frost to sublimate. Now, Pluto is headed away from the Sun again.It will gradually get colder and colder and its atmosphere will re-freeze to its surface. In fact, that should have already started happening, but apparently it has not. It’s a mystery.”

Pluto as seen from the New Horizons spacecraft on July 3. Image Credit: NASA/JHUAPL/SWRI/Bjorn Jonsson

Pluto as seen from the New Horizons spacecraft on July 3. Image Credit: NASA/JHUAPL/SWRI/Bjorn Jonsson

There has also been much speculation about Pluto’s theorised similarity with Neptune’s largest moon, Triton. The latter was the one to really steal the show during Voyager 2’s close flyby of Neptune in 1989. Even though it has one of the coldest surfaces ever measured in the entire Solar System (just 38 Kelvin above absolute zero), Triton was found to be a geologically active world, despite its great distance of 4.5 billion km from the Sun. When Voyager 2 flew over the moon’s northern hemisphere a day after the Neptune encounter, it transmitted back to the unsuspecting scientists on Earth images of geyser-like eruptions of gaseous nitrogen coming from its surface, which formed plumes up to 8 km high into Triton’s tenuous atmosphere. Could Pluto harbor something similar? This debate has been an ongoing one among the scientific community. “Both Pluto and Triton have a dark equatorial region and bright poles, commented Dr. Amanda Zangari, during a recent “Ask Me Anything” session on reddit, a postdoctoral researcher at the Southwest Research Institute in Boulder, Colo., for the New Horizons mission, which is just one week away from its long-awaited scheduled close flyby of the Pluto system on July 14. “Among the [New Horizons] team, we’ve been impressed at how much they match. Pluto’s dark patch is quite different though. Triton’s got geysers, plumes and cantaloupe terrain and not a lot of craters. We won’t know if Pluto is like that until we get closer, but it’s within the realm of possibility.”

“A proxy for the Pluto system that many of have had in our minds, is Neptune’s large moon Triton,” said Will Grundy, co-investigator for the New Horizons mission at the Lowell Observatory, in Flagstaff, Ariz., during a recent NASA Google+ Hangout. “And the reason for that is that it’s the same size as Pluto, [has] a lot of the surface ingredients and it’s probably a captured Kuiper Belt Object like Pluto. However, the environment that Triton inhabits now in a retrograde orbit around Neptune, is very very different from the dynamical environment that Pluto is in, so I wouldn’t go out on a limb and say it’s gonna look like Triton. It’s not, but it may have many common features with Triton.”

These and many more are some of the fundamental questions regarding the properties of the Pluto-Charon system, which NASA’s New Horizons mission aims address. The latter is the culmination of 85 years of research and study of what has been literally a frontier for both planetary science as well as our own deeper understanding about our planetary neighborhood at large. The scientific study of Pluto has been characterised by a continuing increase in our awareness on both fields, one which is only now starting to reach its peak. “Before the discovery of the first KBOs in the 1990s, Pluto seemed like an oddball, because we lacked the technology to be able to probe deeper into that region and see that it was just the first of a new population of small planets and smaller bodies,” commented Dr. Alan Stern, principal investigator for New Horizons mission at the Southwest Research Institute, during a past interview for AmericaSpace. “With the discovery of the Kuiper Belt, however, a new realization has arisen that what we’re seeing are indeed a new class of planets, dwarf planets. And that’s how science works. You make discoveries of things that you didn’t expect to that force you to reconsider and change your views on a certain field of study. With the help of modern-day instruments, we are seeing that large terrestrial and gas giant planets are quite the minority among the planets of our Solar System.”

We’re just a week away from seeing what new realisations and knowledge the Pluto system will have in store for us.

 

The second part of the article, which will focus on Pluto’s largest moon Charon, will appear tomorrow.

 

Stay with AmericaSpace for regular updates and LIVE COVERAGE of New Horizons’ approach and flyby of the Pluto system.

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