Distant Icy Worlds: The Dancers And Their Dance
By Judith E Braffman-Miller
Performing their mysterious, mesmerizing dance in the alien gloom of a perpetually frigid twilight, trans-Neptunian objects (TNOs) keep themselves well-hidden from the prying eyes of curious astronomers who seek to find them in our Solar System’s most secretive and remote corners. The first TNO to be discovered was the ice dwarf planet Pluto in 1930–but it took until 1992 for astronomers to discover a second TNO, named 1992 QB, orbiting our Star–and as of February 2017, more than 2,300 TNOs have made their debut in the Minor Planet Center’s List of Transneptunian Objects. In November 2017, a team of astronomers released their new findings in respect to this very mysterious population of icy, dancing worldlets. The scientists propose that heat generated by the gravitational tug of moons that were born as the result of massive collisions could extend the lifetimes of liquid water oceans sloshing beneath the crust of these icy small worlds inhabiting our Solar System’s outer limits–thus greatly expanding the number of locations where extraterrestrial life might be found, since the presence of liquid water is necessary to support the emergence and evolution of life as we know it. Astronomers now estimate that dozens of these worlds exist far from the light and warmth of our Sun.
“These objects need to be considered as potential reservoirs of water and life. If our study is correct, we now may have more places in our Solar System that possess some of the critical elements for extraterrestrial life,” commented study lead author Dr. Prabal Saxena in a November 30, 2017 NASA Press Release. Dr. Saxena is of NASA’s Goddard Space Flight Center (GSFC) in Greenbelt, Maryland. The paper describing this new research is published in the November 24, 2017 issue of the planetary science journal Icarus.
These frigid worlds, that dance around in the everlasting gloom of our Solar System’s deep-freeze, are found beyond the orbit of the banded, blue ice-giant planet Neptune–the outermost major planet from our Sun. In this frigid, faraway domain of ice, Pluto and its quintet of moons lurk in the semi-darkness, accompanied by a madding crowd of others of their frozen kind. This captivating population of TNOs are considered to be much too cold for liquid water to pool on their surfaces, where temperatures usually plummet to an unimaginable -350 degrees Fahrenheit. However, evidence has been accumulating showing that some of these frozen “oddballs” may contain liquid water whirling around under their crusts of ice. The bulk densities of these objects are similar to that of other bodies suspected by astronomers of having subsurface oceans, and an analysis of light reflected from some TNOs provides tantalizing hints that crystalline water ice and ammonia hydrates are present. At the frigid temperatures of these icy worldlets, water ice takes on an amorphous, disordered form instead of the regularly ordered crystals normally seen in warmer regions–such as that of snowflakes falling on the surface of our own planet. In addition, radiation from space causes crystalline water ice to experience a sea-change into the amorphous form that breaks down ammonia hydrates–and cannot be expected to survive for long on TNO surfaces. This indicates that both compounds may have originated from an interior layer of life-loving liquid water that erupted to the icy surface, in a process termed cryovolcanism (icy volcanism).
Most of the long-lived heat within TNOs results from the decay of radioactive elements that were incorporated into these objects as they were forming. This heat can be sufficient to melt a layer composed of the icy slush, thus creating a subsurface ocean of liquid water–and, possibly, allowing it to linger beneath the object’s icy shell for billions of years. But as radioactive elements decay into more stable ones, they no longer can release heat and, as a result, the interiors of these frigid objects gradually grow colder and colder–ultimately freezing any subsurface ocean of liquid water back into ice. The new research found that the gravitational dance with a moon can churn out sufficient additional heat within a TNO to significantly extend the lifetime of the subsurface ocean–before it again freezes to ice.
The orbit of any moon will evolve in its gravitational waltz around its parent object in order to reach the most stable state possible. This stable state would be circular, and aligned with the equator of its parent-body–additionally keeping the moon’s spin at a rate that forces it to keep the same side perpetually facing its parent. Massive collisions between these celestial objects can create new moons when material is sprayed into orbit around the larger object. That is because this material ultimately coalesces into one or more moons under the influence of gravity. Because smash-ups occur in a variety of differing directions and speeds, they are not likely to create moons with perfectly stable orbits–at least at first. As a smash-up-created newborn moon adusts to its more stable orbit around its parent world, mutual gravitational attraction causes the interiors of the parent world and its newborn moon-offspring to stretch and relax–over, and over, and over again. The repetitious steps of this gravitational dance generates friction that liberates heat in a process termed tidal heating.
In The Mysterious Domain Of TNOs
A TNO is defined as any minor planet in our Solar System that circles our Sun at a greater average distance (semi-major axis) than Neptune–which amounts to 30 astronomical units (AU). One AU is equivalent to the distance between Earth and Sun, which is about 93,000,000 miles. A dozen minor planets with a semi-major axis greater than 150 AU and a perihelion greater than 30 AU have been detected. These objects are termed extreme trans-Neptunian objects (ETNOs).
Of the 2,300 TNOs, appearing in the Minor Planet Center’s List of Transneptunian Objects, 2,000 have a perihelion–when they are closest to our Star–that is farther out than Neptune. As of November 2016, 242 of these TNOs have had their orbits well-enough determined to attain a permanent minor planet designation.
The most massive known TNO is Eris, followed by Pluto, 2007 OR10, Makemake, and Haumea. The Kuiper Belt, Scattered Disk, and Oort Cloud are the trio of conventional divisions of this volume of space. However, there are a few exceptions to this general rule. For example, the unusual–and very red–object, named Sedna, does not fit neatly into any of these three categories.
TNOs are classified into two main groups according to their distance from our Sun:
Kuiper Belt Objects
The Kuiper Belt is the distant home of icy objects with an average distance to our Sun of 30 to about 55 AU. Kuiper Belt Objects (KBOs) sport close-to-circular orbits with a small inclination from the ecliptic. There are two distinct groups of KBOs:
*Resonant Objects are KBOs that are locked in an orbital resonance with Neptune.
*Classical Kuiper Belt Objects (cubewanos) have no such resonance. These icy objects move in almost circular orbits, that have not fallen under Neptune’s powerful gravitational influence.
Scattered Disc Objects (SDOs)
The scattered disc is the distant domain of frozen objects that are situated even farther from our Sun than their KBO cousins. SDOs usually travel around our Star in highly irregular (elliptical) orbits, and also have a large inclination from the ecliptic. There are three distinct classes of SDOs:
*Scattered-near (typical SDOs) are scattered near objects with orbits that are non-resonant, non-planetary-crossing, and also have a Tisserand parameter (relative to Neptune) of less than 3. The Tisserand parameter (or variant) is a value that is calculated from several orbital elements of a relatively small object and a larger perturbing body.
*Scattered-extended (detached objects) are scattered-extended objects that have a Tisserand parameter (relative to Neptune) that is greater than 3, and they also have a time-averaged eccentricity greater than 0.2.
*Sednoids are an additional extreme sub-grouping with perihelia so remote that their orbits cannot be explained by perturbations from the quartet of giant gaseous planets inhabiting our Star’s outer region, nor can they be explained by interaction with the Galactic tides of our Milky Way.
Launched on January 19, 2006, NASA’s New Horizons spacecraft completed a five-month-long observational flyby of the Pluto system, revealing a treasure chest filled with important new discoveries about the mysterious attributes of this small, intriguing icy world. Currently, New Horizons is well on its way to our Solar System’s frozen outer limits in order to observe other small worlds dwelling in the Kuiper Belt. As part of New Horizons’ extended mission, into this unexplored domain, it will help shed new light on this distant and dimly-lit region of our Solar System. The Kuiper Belt is the birthplace of lingering relics of our Solar System’s primordial birth, and this intriguing population of icy, dancing objects have preserved in their frozen hearts some very important long-lost secrets of our primeval Solar System. New Horizons promises to reveal the amazing story of how our Sun and its family of objects were born about 4.56 billion years ago.
On New Year’s Day 2019, the New Horizons spacecraft is scheduled to fly past its second target–after having completed its successful rendezvous with the Pluto system. The next small, frozen KBO–that is New Horizons’ second target–currently goes by the bland name of 2019 MU69 (short for 486958 2014 MU69)–however, it will be given a new and more exciting name soon. Actually, many astronomers suspect that little MU69 may really be a duo of distant, frozen worlds–a binary KBO flying around our Star near the outermost edge of our planetary system.
Distant Icy Worlds: The Dancers And Their Dance
Dr. Saxena’s team used the equations for tidal heating to calculate its contribution to the “heat budget” for a great variety of both discovered and hypothetical TNO-moon systems. These systems include the Eris-Dysnomia system. Eris is the second-largest of all currently known TNOs–with Pluto being the largest.
“We found that tidal heating can be a tipping point that may have preserved oceans of liquid water beneath the surface of large TNOs like Pluto and Eris to the present day,” commented study co-author, Dr. Wade Henning, in the November 30, 2017 NASA Press Release. Dr. Henning is of NASA GSFC and the University of Maryland, College Park.
“Crucially, our study also suggests that tidal heating could make deeply buried oceans more accessible to future observations by moving them closer to the surface. If you have a liquid water layer, the additional heat from tidal heating would cause the next adjacent layer of ice to melt,” explained Dr. Joe Renaud in the same NASA Press Release. Dr. Renaud is of George Macon University in Fairfax, Virginia. He is also a co-author of the study.
Even though the presence of liquid water is necessary for the emergence and evolution of life as we know it, by itself it is not enough. Life as we know it also requires a supply of chemical building blocks, as well as a source of energy. Hidden deep beneath the oceans on our planet, there are certain geologically active areas that house entire ecosystems that thrive in total darkness. These strange forms of life can exist because hydrothermal vents–referred to as “black smokers”–contribute the necessary ingredients in the form of energy-rich chemicals dissolved in superheated water. Tidal heating, as well as heat derived from the decay of radioactive elements, could both create such life-sustaining hydrothermal vents, according to the team of scientists.
The team of astronomers also plan to develop and use even more precise models of tidal heating and TNO interiors. This would help them to determine how long tidal heating can extend the lifetime of a subsurface sloshing ocean of nurturing liquid water, as well as how the orbit of a moon evolves as tidal heating dissipates energy. The astronomers also hope to discover at what point a liquid water ocean forms–whether it forms quickly or if it depends on a significant buildup of heat first.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she especially loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field.
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