Humanity has loved the very remote dwarf planet Pluto from a distance–a very great distance–ever since its discovery in 1930 by the American astronomer Clyde Tombaugh. From where it resides in the frigid outer limits of our Solar System, this little, icy world has been an object of mystery, fascination, and affection–perhaps simply because it is so very far away. Mysteries have a way of snatching at the human imagination with a captivating relentlessness, and Pluto has remained a bewitching, bewildering mystery for almost a century. On July 14, 2015, after spending almost ten years on a dangerous and difficult journey through our Solar System, NASA’s heroic New Horizons spacecraft succeeded in making its historic visit to Pluto and its moons, and the beloved little world began to reveal to the fascinated, curious eyes of astronomers its many long-held, captivating secrets. In January and February 2016, New Horizons scientists revealed that Pluto had again disclosed some of its secrets–it is a world where glaciers of nitrogen ice carry mysterious floating hills under a hazy atmosphere, that appears to be blue, when it is observed with infrared vision.
New Horizons has completed its flyby of Pluto and its five moons, and is now continuing on its way deeper and deeper into a region of our Solar System known as the Kuiper Belt. The Kuiper Belt is dark and cold, and it is here that a mesmerizing multitude of frozen icy objects–the nuclei of comets–circle around our Sun. Indeed, the Kuiper Belt is so far away that our Star can do little to light up and warm this frozen region so far from its fierce fires.
The Kuiper Belt is located beyond the orbit of the deep blue-banded outermost known major planet, the gaseous ice-giant Neptune, which is the eighth planet from our Sun. Pluto is a relatively large inhabitant of this region, where it dances around our Sun along with a vast number of alien, sparkling, frozen worldlets.
Pluto was designated a major planet soon after its discovery. However, in later years, the growing realization among astronomers that this mysterious, beloved little icy “oddball” is only one of several relatively large frozen spheres inhabiting the Kuiper Belt prompted the International Astronomical Union (IAU), in 2006, to formally define what a “planet” is. This resulted in poor Pluto being eliminated from the pantheon of major planets inhabiting our Solar System. Now, reclassified as a mere dwarf planet, Pluto, nonetheless continues to be a small world of mystery.
The Sad Saga Of A Small, Icy Worldlet
The Pluto saga began less than a century ago when the astronomer Clyde Tombaugh (1906-1997) first spotted it as a faint little pinhead of light. Tombaugh was on the hunt for the elusive Planet X, when he found Pluto instead. Planet X is a hypothetical, hidden giant planet that may lurk in the cold blackness beyond the orbit of Neptune. This example of scientific serendipity occurred when Tombaugh, a young farmer’s son from Kansas, was searching the outer limits of our Solar System using a telescope in Flagstaff, Arizona.
Like other Kuiper Belt Objects (KBOs), Pluto is composed of a mixture of ice and rock. It is also very tiny, being a mere 1/6 the mass of Earth’s Moon and approximately 1/3 its volume. Pluto also sports a highly inclined and eccentric orbit as it circles our Star from a distance of about 20 to 49 Astronomical Units (AU). One AU is the average distance between Earth and Sun, which is 93,000,000 miles. Pluto periodically wanders closer to our Star than Neptune. However, both bodies are lucky because an orbital resonance with Neptune prevents the two from fatally crashing into one another.
The Kuiper Belt is a very remote domain of our Solar System, situated beyond the realm of the quartet of giant gaseous planets that inhabit the outer Solar System: Jupiter, Saturn, Uranus, and Neptune. The Belt reaches out from the orbit of Neptune to approximately 50 AU. Neptune’s average distance from our Sun is about 30.1 AU.
Pluto possesses a frozen quintet of icy little moons: Charon, Nix, Hydra, Kerberos, and Styx. Charon is by far the biggest of this frigid batch of distant moons, and it has a diameter that is about 50% the size of Pluto’s. Some astronomers think that Pluto and Charon compose a binary system because the barycenter of their orbits is not situated within either of the two small worlds. Charon was discovered in 1978 by the American astronomer James Christy. This relatively large moon-world is frequently considered to be an enormous chunk that was blasted off Pluto itself as the result of a catastrophic collision with another object that was galloping through the Kuiper Belt too close to Pluto.
For most of the 20th century, astronomers wrongly believed that poor little Pluto is a solitary body, where it circles our Star in the dark, deep freeze of the outermost region of our Solar System–very far from the wonderful light and comforting warmth of our Sun. However, in 1992, the first KBO (other than Pluto and Charon) was discovered. This discovery revealed that Pluto is very far from being alone where it dwells in the Kuiper Belt. Since 1992, many other KBOs have been discovered that bear a haunting resemblance to Pluto, and these kindred worldlets also display eccentric orbits. The most important of these little bodies is the scattered disc object named Eris that was discovered in 2005. Eris is about 27% more massive than Pluto. This brought home the realization that Pluto is only one of many other KBOs, and this new understanding resulted in its reclassification and demotion from major planet to dwarf planet. However, not all astronomers accept poor Pluto’s demotion, and they argue that Pluto should not have lost its original designation as the ninth major planet from our Star–and that the other kindred dwarf planets, that have more recently been discovered, should also be given major planet status along with Pluto.
Launched on January 19, 2006, New Horizons zipped past the enormous gas-giant Jupiter in February 2007, in order to get a gravity kick, while also conducting a scientific study of this behemoth of a planet–by far the largest in our Solar System. As part of New Horizons’ extended mission, it will fly deeper and deeper into the mysterious Kuiper Belt, in order to make some revealing up close and personal observations of one or two of the mysterious icy denizens of this distant, dark, and cold domain. The Kuiper Belt is actually a remnant of our Solar System’s ancient formation about 4.56 billion years ago. Its icy inhabitants are left-over planetesmals- -which are the comet-like building blocks of the four planetary giants of the outer Solar System.
Our Solar System’s denizens have been classfied into three categories. The first category includes the quartet of rocky, inner terrestrial planets (Mercury, Venus, Earth, and Mars), while the second category includes the four magnificent gaseous giants of the outer limits. Pluto and Charon now have been freshly classified as inhabitants of a third category of planetary objects termed ice dwarfs. Ice dwarfs possess solid surfaces, but in contrast to the surfaces of the four terrestrial rocky planets, the strange surfaces of these frozen objects are largely made up of icy material.
New Horizons promises to reveal the ancient, haunting, and intriguing saga of the secretive origins of the inhabitants of this outermost domain. The plucky spacecraft is set to explore–for the very first time–how ice dwarfs like Pluto and Charon have evolved over the eons.
Hazy Blue Skies When Observed With Infrared Eyes
On January 29, 2016, a revealing image obtained from NASA’s New Horizons spacecraft was released that offered the first tantalizing peek at Pluto’s atmosphere in infrared wavelengths. It is the first image of the atmosphere made with data obtained from the New Horizons Ralph/Linear Etalon Imaging Spectral Array (LEISA) instrument.
In the intriguing image, sunlight can be seen emanating from above and behind Pluto. The image was taken on July 14, 2015, while New Horizons was about 112,000 miles away. The image covers LEISA‘s full spectral range (1.25 to 2.5 microns), which is divided into thirds, with the shortest third being placed into the blue channel, while the middle third is placed into the green channel, and the longest into the red channel.
The blue ring around the enchanting ice dwarf is the result of sunlight being scattered from haze particles that compose a heavy population in Pluto’s ethereal atmosphere. Astronomers think that the haze is really a photochemical smog that is being caused by the action of sunlight on methane and other molecules. This action produces a complex concoction of hydrocarbons such as ethylene and acetylene. These hydrocarbons accumulate to create tiny particles, that are a mere fraction of a micrometer in size, which then scatter sunlight to form the blue haze. The new infrared image, when used in conjunction with some previous images made at shorter, visible wavelengths, provides astronomers with a new understanding concerning the size distribution of these particles.
White patches that can be seen in the image around Pluto’s limb are believed to be caused by sunlight skipping off more reflective or smoother regions of Pluto’s frigid surface–with the largest white patch situated in the western section of the informally dubbed Cthulhu Regio. Future LEISA observations returned to scientists on Earth should show the remainder of the blue haze, that is missing from the lower section of this image.
Mysterious Floating Hills Carried By Glaciers Of Nitrogen Ice
In February 2016, New Horizons astronomers announced their findings that nitrogen ice glaciers on Pluto appear to be carrying along with them a large number of isolated hills that may be fragments of water ice originating in Pluto’s uplands. These hills individually measure one to several miles across, according to data and images derived from NASA’s New Horizon spacecraft.
The strange hills, which are located in an enormous plain of ice informally dubbed Sputnik Planum, exist within a vast heart-shaped feature on Pluto’s surface. The hills of ice are probably miniature versions of the larger, jumbled mountains observed on Sputnik Planum’s western border. These strange hills provide yet another example of Pluto’s fascinating and active geological history.
Water ice is less dense than nitrogen-dominated ice. For this reason, the astronomers have proposed that these mysterious hills of water ice are floating in a bizarre, alien sea of frozen nitrogen. The hills travel over time in much the same way as icebergs do in the cold Arctic seas of our own planet. The scientists believe that the hills are probably portions of the rugged uplands of Pluto that have split off, and are now being carried by the nitrogen glaciers into Sputnik Planum. Indeed, “chains” of the floating hills form along the flow paths of the glaciers. When the hills enter the cellular terrain of central Sputnik Planum, they become susceptible to the convective movements of the nitrogen ice, and are forced to float to the edges of the cells. This is the area where the hills cluster in groups that extend up to 12 miles across.
The feature that is informally called Challenger Colles–in honor of the crew of seven astronauts who lost their lives in the space shuttle Challenger disaster on January 28, 1986–is located at the northern edge of the image. Challenger Colles appears to be a particularly large collection of these strange hills of water ice, measuring approximately 37 by 22 miles. This feature is situated close to the boundary with the uplands, which is away from the cellular terrain, and can possibly represent a location where some of the hills have been stranded because the nitrogen ice is particularly shallow in this area.
In addition, the image also shows the inset in context next to a larger view that reveals most of Pluto’s encounter hemisphere. The image was obtained from New Horizons’ Multispectral Visible Imaging Camera (MVIC) instrument. Because North is up, illumination is observed in the upper-left of the image. The image resolution is about 1050 feet per pixel, and the image measures a little over 300 miles long and approximately 210 miles wide. It was obtained at a range of about 9,950 miles from Pluto–about 12 minutes before New Horizons’ closest approach to Pluto on July 14, 2015.