Far, far away, in the distant suburbs of our Solar System, where our Sun’s intense heat and light can do little to warm and illuminate this eerie region of frigid and perpetual twilight, a sparkling host of icy objects–some large, some small–dance in orbit around our Star. Here, in our Solar System’s deep freeze, our Sun can shine with only a weak, dim fire, and hang in the alien sky looking like an especially large, silvery-bright star swimming in a twinkling sea of a million, billion, trillion stars. On July 14, 2015, after making a treacherous ten-year journey through interplanetary space, NASA’s New Horizons spacecraft succeeded in becoming the very first space mission to reach this mysterious and unexplored remote corner of our Solar System–the home of the ice dwarf planet Pluto, and its five bewitching moons. New Horizons has sent many revealing images back to Earth of the Pluto system, solving one mystery after another about this strange place. In February and March 2016, New Horizons astronomers announced some additional findings: Pluto’s largest moon, Charon, shows signs of once having had a subsurface ocean, and Pluto itself sports a chain of exotic mountains capped by methane snow.
The New Horizons spacecraft has now traveled well past Pluto, on its way to making still more historic observations of what dwells in the far suburbs of our Solar System. New Horizons made its closest flyby of Pluto on the morning of July 11, 2015, almost 8,000 miles above its exotic, alien frozen surface. Indeed, the July 2015 flyby over the ice dwarf planet and its quintet of moons is providing a close-up and personal view of our Solar System’s Kuiper Belt, a remote region in the outer limits of our Sun’s family. This very successful spacecraft, zipping into a new frontier in space, will enable scientists to obtain a greater understanding of how our Solar System was born and how it evolved through time. The very distant Kuiper Belt is the home of a glittering multitude of tumbling icy worldlets and other frozen objects that range in size from boulders to dwarf planets, that are about the same size as Pluto. Kuiper Belt Objects (KBOs) preserve in the dark deep-freeze of our Solar System’s outer region, some very important information about the primordial Solar System.
Mysterious and mesmerizing, the Kuiper Belt is located far beyond the beautiful, banded, deep blue, gaseous ice-giant Neptune–the outermost of the eight major planets from our Sun. Astronomers are only now first beginning to explore this far and frigid frontier, where literally trillions of icy, sparkling objects twirl around our Star. Pluto is a relatively large denizen of the Kuiper Belt, and it was classified as the ninth major planet from our Sun soon after its discovery. However, in later years, many astronomers came to the conclusion that Pluto should be reclassified. This is because more recent observations revealed that this captivating, frozen little “oddball” is just one out of many similar objects. In 2006, the International Astronomical Union (IAU) was forced to formally define the controversial term planet. As a result, poor, pitiful, unlucky Pluto, was booted out of the pantheon of major planets and demoted to dwarf planet status. Nevertheless, this strange, small world continues to draw both attention and affection from planetary scientists.
According to the IAU’s definition, a planet must be in orbit around its parent-star, must possess a sufficiently hefty mass for its self-gravity to overcome rigid body forces so that it assumes hydrostatic equilibrium, and becomes spherical in shape–and it also must have cleared its neighborhood of other objects lingering around in its orbit.
Pluto is circled by a quintet of moons: Charon, Nix, Hydra, Kerberos, and Styx. Charon is by far the largest of the quintet, and it boasts a diameter approximately half that of Pluto. The little icy world, that is Pluto, was named for the Roman god of the underworld.
The Pluto saga began in the 1930s, when Clyde Tombaugh (1906-1997), who was then a young American astronomer, was given the task of searching for a mysterious, elusive, and possibly non-existent Planet X. Planet X was, at the time, considered to be a hypothetical, well-hidden, giant world, inhabiting the frigid darkness beyond Neptune. Tombaugh did indeed discover a faint and remote point of light–but it was not the sought-after Planet X. Instead, that little speck of distant, dim light was tiny Pluto.
For most of the 20th century, astronomers considered Pluto to be an isolated, icy, small world, dwelling in the distant twilight region of our Solar System, far from the madding crowd of other objects that are in orbit around our Star. However, a change in perspective came in 1992, when a large number of other small icy denizens of the Kuiper Belt were discovered. At this point, it became apparent that Pluto had not cleared its neighborhood of other objects lingering around its orbit. Therefore, Pluto lost its major planet status in 2006, when the IAU found it necessary to come up with an acceptable definition of a planet. Pluto was not as far away from the madding crowd of Solar System denizens as once thought.
Since 1992, a significant number of additional small, frozen worlds that are akin to Pluto–showing similar eccentric orbits–have been detected. Pluto displays a highly inclined and eccentric orbit that carries it from 20 to 49 astronomical units (AU) from our Sun. One AU is equal to the mean separation between Earth and Sun, which is about 93,000,000 miles.
New Horizons was launched on January 19, 2006, on its unprecedented, historic decade-long dangerous journey to our Solar System’s as yet unexplored distant twilight zone. The successful spacecraft began a year-long download of new and revealing images, as well as other data, over the Labor Day weekend in 2015. Images that were then derived more than doubled the area of Pluto’s mysterious surface observed at resolutions as good as 440 yards per pixel. The images showed strange surface features, on that distant small world, such as nitrogen ice flows, believed to have oozed out of mountainous regions down onto the planet’s network of valleys. These may have been carved by slushy material gushing all over Pluto’s surface. The images also unveiled dunes, as well as bizarre regions resembling the chaos terrains on Europa, a moon of Jupiter. Europa’s chaos terrains were once thought to be unique in our Solar System, but Pluto is now known to possess very similar jumbled and chaotic mountainous regions.
The images also displayed the most heavily cratered–and therefore oldest–terrain yet detected by New Horizons on Pluto. This ancient, cratered terrain is intriguingly located next to the almost crater-free–and therefore youngest–icy plains. Heavily cratered surfaces suggest an old surface, while relatively crater-free surfaces suggest a young surface that has been rendered smooth and almost entirely unblemished by craters, as a result of recent resurfacing that erases older craters.
Charon’s Primordial Ocean?
Pluto’s largest moon, Charon, was discovered in 1978 by the American astronomer James Christy. The new observations show that this large companion of Pluto may have once had a subsurface ocean that has long since frozen over and expanded, pushing outward. This outward expansion is thought to be the cause of widespread stretching and fracturing of the icy moon’s surface. This is because water expands as it freezes.
The side of Charon, viewed by the New Horizons spacecraft in July 2015, displays what has been described as a system of “pull apart” tectonic faults. This observation is expressed on Charon’s surface in the form of valleys, scarps and ridges–with the valleys sometimes over 4 miles deep. Charon’s tectonic landscape reveals that the moon somehow managed to expand in its past. Charon’s surface fractured as it stretched. For example, a feature named Serenity Chasma is part of an extensive equatorial belt of chasms on Charon. This system of faults and fractures is at least 1,100 miles long and in places some chasms are 4.5 miles deep. As a comparison, Earth’s Grand Canyon is 277 miles long and just over a mile deep.
The outer layer of Charon is primarily made up of water ice. This layer was kept toasty when Charon was a young moon, as a result of the decay of radioactive elements–as well as Charon’s own internal heat of formation. Many planetary scientists think that Charon could have been warm enough to force the water ice to melt deep down, thus forming a subsurface ocean. As Charon cooled off over time, this subsurace ocean would have frozen and expanded. Because water expands when it freezes, this could have lifted the outermost layers of the large moon and created the massive chasms observed today.
Walking In A Methane Wonderland
In March 2016, the New Horizons team announced the discovery of a chain of exotic snowcapped mountains extending across an enormous dark region on Pluto’s surface informally dubbed Cthulhu Regio (pronounced kuh-THU-lu). Cthulhu is a vast region that stretches almost 50% around Pluto’s equator, starting from the west of the great nitrogen ice plains named Sputnik Planum. Cthulhu is slightly larger than the state of Alaska, and measures about 1,850 miles long.
Cthulhu’s appearance is known for its dark surface, which planetary scientists suggest may be the result of being well-coated by a layer of dark tholins. Tholins are complex molecules that form when methane is exposed to sunlight. Cthulhu’s geology displays a great variety of differing landscapes–from mountainous to smooth, and to heavily cratered and fractured.
There is a mountain range located in southeast Cthulhu that is 260 miles long. The mountain range is located among craters, with narrow valleys dividing its peaks. The upper slopes are covered with a bright material that dramatically contrasts with the dark red color of the surrounding plains.
Many planetary scientists suggest that the mysterious bright material may be mostly methane that has condensed as ice onto the peaks originating from Pluto’s thin atmosphere. “That this material coats only the upper slopes of the peaks suggests methane ice may act like water in Earth’s atmosphere, condensing as frost at high altitude,” explained Dr. John Stansberry in a March 3, 2016 NASA Press Release. Dr. Stansberry is a New Horizons science team member from the Space Telescope Science Institute in Baltimore, Maryland. Compositional data obtained from the Ralph/Multispectral Visible Imaging Camera (MVIC) aboard New Horizons indicates that the location of the bright ice on the mountain peaks correlates almost precisely with the distribution of methane ice.