In Our Solar System’s Dark Deep Freeze, Poor Pluto Is Much Too Cold
By Judith E Braffman-Miller
In the dark, distant, and mysterious outer fringe of our Solar System, the dwarf planet Pluto dwells with a multitude of others of its frozen kind–circling our Star in a domain of everlasting twilight. Astronomers are only now first beginning to explore this unknown territory that exists far from our Sun–a swath of space where its welcoming warmth and shimmering golden rays of light can barely reach. NASA’s New Horizons spacecraft was launched on January 19, 2006, to undertake a decade-long, three-billion-mile treacherous journey through the amazing sea of interplanetary space to finally reveal many of the well-kept secrets of our Solar System’s previously unexplored, and very distant, outer limits, where Pluto dwells in a region known as the Kuiper Belt. The gas composition of a planet’s atmosphere generally determines how much of our Star’s heat gets imprisoned in the atmosphere. In November 2017, a team of astronomers announced new findings obtained from the New Horizons spacecraft in 2015, showing that poor Pluto is much colder than it should be, even in this frozen place–and that its atmospheric haze is to blame.
New Horizons began a year-long download of revealing new images, as well as other data, over the Labor Day weekend in 2015. The images, that were then downlinked, unveiled much of Pluto’s mysterious surface–observed at resolutions as good as 440 yards per pixel. The images displayed new features such as nitrogen ice flows that may have oozed out of mountainous regions before they finally flowed down onto Pluto’s icy plains. Other features revealed in these images included networks of valleys that may have been carved by material meandering over the dwarf planet’s surface. The images also showed large areas that appeared to be eerily similar to the jumbled chaos terrains of Jupiter’s moon Europa. Both small worlds display chaotically mixed up mountainous terrain.
Our Solar System’s Twilight Zone
The new study, published in the November 16, 2017 issue of the journal Nature proposes a new and unusual cooling mechanism, controlled by haze particles, to explain Pluto’s frigid atmosphere.
“It’s been a mystery since we first got the temperature data from New Horizons. Pluto is the first planetary body we know of where the atmospheric energy budget is dominated by solid-phase haze particles instead of by gases,” explained first author Dr. Xi Zhang, assistant professor of Earth and Planetary Sciences at the University of California Santa Cruz (UCSC) in a November 15, 2017 UCSC Press Release. Dr. Zhang is of UCSC.
The Pluto saga started back in the 1930s, when the young American astronomer, Clyde Tombaugh (1906-1997) was given the task of hunting for the elusive, and possibly non-existent, Planet X. For decades, many astronomers have proposed the hidden existence of a large major planet lurking mysteriously within the cold darkness beyond Neptune–the most distant known major planet from our Star. Tombaugh did succeed in making a remarkable discovery–but he did not find what he was looking for, he found something else. In a beautiful example of scientific serendipity, Tombaugh spotted a very distant, dim pinpoint of light. The tiny tidbit of light was not coming from Planet X–it was coming from Pluto, a complex and intriguing little frozen world far, far away. Good things sometimes come in small packages.
New Horizons’ July 2015 flyby of Pluto and its bewitching quintet of sparkling, frozen moons, provided the first close-up peek into the Kuiper Belt, a faraway region situated in our Solar System’s twilight zone. This successful mission explored a new frontier in space, and the information that it obtained will help astronomers reach a better understanding of the origins of our Sun and its family of planets, moons, and other objects. The Kuiper Belt is the distant birthplace of a myriad of dancing icy worlds, worldlets, and icy fragments that range in size from boulders to dwarf planets–like Pluto. Kuiper Belt Objects (KBOs) preserve, in our Solar System’s deep freeze, important clues about the ancient birth of our Sun and its family.
Pluto is a relatively large denizen of the Kuiper Belt, and it was originally classified as the ninth major planet from our own Sun. However, with an increased understanding among astronomers that this bewitching, bewildering little world is only one of a large number of other similar icy and rocky objects dwelling in the Kuiper Belt, the International Astronomical Union (IAU) was forced to formally define the somewhat controversial term “planet”–and Pluto lost its lofty classification as the ninth major planet from our Sun. Currently, re-classified as a dwarf planet, this small world remains an interesting object of considerable mystery, affection, and sometimes intense controversy among members of the astronomical community.
For most of the 20th century, astronomers generally considered Pluto to be an isolated, icy world inhabiting the dimly lit outer region of our Solar System–far from our Star. However, this idea came to an end in 1992 when the very first KBO (other than Pluto and its largest moon Charon) was detected, and astronomers came to the realization that Pluto is not as far from the madding crowd as originally thought. Indeed, a second KBO, dubbed Eris, was detected in 2005, and it rivaled Pluto in size. The Kuiper Belt apparently is a remote realm that is heavily populated with a multitude of miniature icy worldlets, which were born early in our Solar System’s history. This multitude of KBOs are also termed transneptunian objects.
Since 1992, other frozen small worlds, similar to Pluto, have been spotted in the Kuiper Belt. These kindred objects display similar eccentric orbits that are like Pluto’s. Pluto’s highly inclined and eccentric orbit takes it from 20 to 49 astronomical units (AU) from our Sun. One AU is equivalent to the average distance between Earth and Sun, which is about 93,000,000 miles.
Pluto is only about two-thirds the diameter of Earth’s Moon, and it probably contains a rocky core that is encased within a mantle of water ice. More exotic forms of ice, such as methane and nitrogen frost, coat Pluto’s frozen surface. Because of its size and lower density, Pluto’s mass is approximately one-sixth that of Earth’s Moon, but it is more massive than the dwarf planet Ceres–the largest denizen of the Main Asteroid Belt located between Mars and Jupiter–by a factor of 14.
Pluto travels along a 248-year-long elliptical orbit that can carry it as far as 49.3 AU from our Star. From 1979 to 1999, Pluto was actually closer to our Sun than Neptune, and in 1989, Pluto traveled to within 29.8 AU of our Sun. This provided astronomers with a rare opportunity to study this frozen, far away small world.
Because Pluto’s orbit is so extremely elliptical, when it travels close to our Sun, its surface ices sublimate. This means that Pluto’s surface ices undergo a sea-change directly from solid to gas, and then rise temporarily to create a thin atmosphere. Little Pluto’s weak gravitational grasp (only about six percent of Earth’s) results in its tenuous atmosphere becoming much more extended in altitude than Earth’s atmosphere. Pluto grows much colder during the part of each orbit that carries it away from our Sun. During this time, when Pluto again wanders back into our Solar System’s distant twilight zone, most of its atmosphere is believed to freeze and then fall as snow to the surface of this weird and wonderful world.
Pluto’s quintet of icy moons are named Charon, Nix, Hydra, Kerberos, and Styx. Of these five tiny, icy moons, Charon is by far the largest. Indeed, Charon is almost 50% the size of Pluto itself. Charon was discovered by the American astronomer James Christy back in 1978, and it is so big that some astronomers refer to the Pluto.-Charon system as a double dwarf planet. The distance between the duo is 12,200 miles.
The Hubble Space Telescope (HST) photographed Pluto and Charon in 1994 when Pluto was about 30 AU from our planet. These images revealed that Charon is of a grayer hue than Pluto (which is redder). This suggests that they have different surface compositions and structure. Charon’s orbit around Pluto takes 6.4 Earth-days, and a single Pluto rotation (one Pluto day) takes 6.4 Earth-days. For this reason, Charon neither rises nor sets. Instead, Charon looms above the same area on Pluto’s surface, and the same side of Charon always faces its parent-dwarf-planet (tidal locking). When compared with the majority of the planets and moons in our Solar System, the Pluto-Charon system is tipped on its side, like the greenish-blue, ice-giant planet, Uranus. Pluto’s rotation is retrograde. This basically means that it rotates backwards, from east to west. Uranus, and the cloud-enshrouded inner planet, Venus, also display retrograde rotations.
Because Pluto and Charon are small worlds that are very far from Earth, they are difficult to observe from our planet. Back in the late 1980s, Pluto and Charon floated in front of one another numerous times for several years. This allowed astronomers to make valuable observations of these rare events, and also helped them to make rudimentary maps of each little world, showing areas of relative brightness and darkness.
Late in 2014 and early in 2015, image animations revealed the mutual orbital ballet between Pluto and Charon around their center of mass. Beginning in the spring of 2015, New Horizons began its detailed observations of Pluto including hunts for more moons and for rings. Various studies continued through its close approach on July 14, 2015, at a distance of 8507 miles, and after.
A tiny world that shows mountains made of water ice-based bedrock, Pluto also has a dark surface coloring that appears to be the result of carbon residues called tholins. These are created by solar ultraviolet rays or charged particles that tumble down upon mixtures of methane and nitrogen. Frozen gases on Pluto’s surface include methane, nitrogen, and carbon monoxide. These were spotted by ground-based telescopes, and are currently believed to be thin layers resting on top of the “bedrock” of water ice.
Pluto has the distinction of being the only world to be named by a little girl. In 1930, 11-year-old Venetia Burney of Oxford, England, mentioned to her grandfather that the new discovery be named for the Roman god of the underworld. He forwarded the name to the Lowell Observatory–and it was selected.
In Our Solar System’s Dark Deep Freeze, Pluto Is Much Too Cold
Pluto is much colder than it should be–even at its great distance from our Star. The new study, published in Nature, involves the absorption of heat by Pluto’s haze particles. The particles then emit infrared radiation, cooling the atmosphere by radiating energy into space. The upshot is an atmospheric temperature of approximately -333 degrees Fahrenheit, rather than the predicted -280 degrees Fahrenheit.
According to Dr. Zhang, the excess infrared radiation from the haze particles, zipping around in Pluto’s atmosphere, should be detectable by the upcoming James Webb Space Telescope (JWST), scheduled for launch in 2019. JWST should be able to provide confirmation of Dr. Zhang and his team’s hypothesis.
Indeed, extensive layers of atmospheric haze can be observed in images of Pluto taken by New Horizons. The haze is produced by chemical reactions in the upper atmosphere, where ultraviolet radiation from the Sun ionizes nitrogen and methane, which then react to create very small hydrocarbon particles that are only tens of nanometers in diameter. As these extremely tiny particles float down through Pluto’s atmosphere, they bump into one another and stick together, thus creating aggregates that grow larger and larger as they tumble down, eventually collecting on Pluto’s surface.
“We believe these hydrocarbon particles are related to the reddish and brownish stuff seen in images of Pluto’s surface,” Dr. Zhang commented in the November 15, 2017 UCSC Press Release.
The astronomers are interested in studying the effects of haze particles on the atmospheric energy balance of other planetary bodies, such as the ice-giant planet Neptune’s large moon Triton, as well as the smoggy orange moon, Titan, of Saturn. These findings may also shed new light on studies of exoplanets, enshrouded in hazy atmospheres, that dwell in the distant families of stars beyond our own Sun.
Dr. Zhang’s co-authors are Dr. Darrell Strobel, a planetary scientist at Johns Hopkins University and co-investigator on the New Horizons mission, and Dr. Hiroshi Imanaka, a scientist at NASA Ames Research Center in Mountain View, California. Dr. Imanaka studies the chemistry of haze particles in planetary atmospheres.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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