A Mysterious Poisonous Ice Cloud Haunts A Tortured Moon-World
By Judith E Braffman-Miller
Titan is a veiled, frigid moon-world, enshrouded beneath a thick orange hydrocarbon fog, as it circles its ringed-gas-giant parent-planet Saturn in the outer Solar System–far from the golden welcoming warmth of our Sun. Titan is a frozen world, like its icy sister-moons that orbit the gaseous, giant denizens of our Solar System’s outer limits: Jupiter, Saturn, Uranus, and Neptune. Almost as big as Mars, Titan is both the largest moon of Saturn, as well as the second-largest moon in our Sun’s family–after Ganymede of Jupiter–and it certainly would have been classified as a planet if it orbited the Sun instead of Saturn. In October 2017, a team of astronomers with NASA’s Cassini mission announced that they found evidence of a bizarre, toxic hybrid cloud looming high above Titan’s south pole. This recent discovery is a new demonstration of the complex chemistry taking place in Titan’s alien, thick, orange atmosphere–in this case, cloud formation in the hydrocarbon-tormented moon’s stratosphere–and part of a group of processes that ultimately helps deliver a strange stew composed of diverse organic molecules to Titan’s odd surface.
Although this floating, wispy, and poisonous cloud is invisible to the human eye, it was successfully discovered at infrared wavelengths by the Composite Infrared Spectrometer (CIRS), aboard the Cassini spacecraft. The cloud haunts the sky of its oddball moon at an altitude of approximately 100 to 130 miles, high above the methane rain clouds that pelt Titan with large, heavy, and lazy drops of a drenching liquid hydrocarbon downpour. The toxic cloud lurks in Titan’s troposphere–the lowest region of its atmosphere–and it covers a large expanse near its south pole, from about 75 to 85 degrees south latitude.
“This cloud represents a new chemical formula of ice in Titan’s atmosphere. What’s interesting is that this cloud’s ice is made of two molecules that condensed together out of a rich mixture of gases at the south pole,” explained Dr. Carrie Anderson in an October 18, 2017 NASA Press Release. Dr. Anderson is of NASA’s Goddard Space Flight Center (GSFC) in Greenbelt, Maryland.
Earlier data obtained from CIRS helped identify the presence of hydrogen cyanide ice in clouds hovering over Titan’s south pole, in addition to other poisonous chemicals in the dense, orange, smoggy blanket that characterizes the distant moon’s exotic atmosphere.
Titan’s stratosphere shows a global circulation pattern that shoots out a current of warm gases from the hemisphere where it’s summer to the winter pole. This circulation reverses direction when seasons change, and this results in an accumulation of clouds at whichever pole is having its winter. Soon after its arrival at Saturn, Cassini found evidence of this phenomenon at Titan’s north pole. Years later, close to the end of the successful spacecraft’s 13 years of examining the Saturn system, a similar buildup of exotic clouds was observed at Titan’s south pole.
Oddball Orange Moon
Titan was discovered on March 25, 1655 by the Dutch astronomer Christiaan Huygens, who was inspired by Galileo’s discovery, in 1610, of Jupiter’s four large Galilean moons: Io, Europa, Ganymede, and Callisto. However, for a very long time this distant moon-world, orbiting Saturn, has been cloaked in mystery. This is because Titan’s secretive surface was covered by a thick, dense, and impenetrable orange fog composed of hydrocarbons–and no telescope on Earth was able to pierce through this smoggy blanket to observe its surface. However, Titan has recently revealed many of its well-kept secrets, thanks to the space-borne Cassini Mission that dispatched a probe–the Huygens Probe–down into the thick orange clouds to observe the hydrocarbon-rich surface of Titan in January 2005. The Cassini Mission ended in September 2017, when the very successful orbiter was intentionally crashed down into the clouds of the ringed-planet that it had been observing for more than a decade.
Like other worlds inhabiting the outer Solar System, Titan is situated far from our Sun’s heat. Indeed, Titan is a frigid moon-world, and the structure of its chemical atmosphere is frozen. This “oddball” moon is famous for its bizarre lakes and seas filled with liquid hydrocarbon, as well as for its weird shroud of orange smog. Often called a “world in its own right”, many planetary scientists propose that this bewitching and bizarre moon may display an eerie similarity to the way our own planet was, very long ago, well before life emerged here. By studying Titan as it now is, scientists may be able to get a precious peek into a remote past when our planet was young. For this reason, Titan’s atmosphere is especially interesting because it is thought to be composed of an icy mixture of chemicals similar to those believed to have been present in our planet’s primordial atmosphere. Titan’s atmosphere is composed primarily of nitrogen–just like Earth’s. However, Titan’s atmosphere also holds much greater quantities of “smoggy” chemicals like methane and ethane–and this smog is so dense that it actually showers a terrible rain of “gasoline-like” liquids down to the tortured surface of this exotic moon.
Titan itself is mostly composed of rocky material and water ice. Its surface is geologically youthful and generally smooth, with few impact craters–although mountains and several ice volcanoes (cryovolanoes) have been detected. Smooth surfaces, that display few impact craters, usually indicate a young surface. Older surfaces are generally heavily scarred with craters, that have been excavated by numerous impacts, that occurred over a long period of time.
Titan’s climate–including its rain of terror and its fierce winds–creates surface features that eerily resemble those on Earth. These features include rivers, lakes, and seas filled with methane and ethane, as well as dunes and deltas that are related to seasonal weather patterns similar to those on our own planet. With its liquids (both subsurface and surface) and its primarily nitrogen atmosphere, Titan’s methane cycle closely resembles Earth’s water cycle–but at a much lower temperature of about -179.2 degrees Celsius.
Titan–like the other moons of both Saturn and Jupiter–is believed to have been created through co-accretion, which is a process similar to that believed to have formed the major planets of our Solar System. As the youthful gas-giants–Jupiter and Saturn–formed, they were encircled by disks of material that eventually coalesced into a myriad of mostly icy moons. While Jupiter is surrounded by its four large Galilean moons that sport very regular, planet-like orbits, Titan is clearly the dominant moon of the Saturn system, and it overwhelms the other, mid-sized icy moons of its parent-planet. Titan possesses both a high orbital eccentricity not readily explained by the process of co-accretion working alone. One proposed model for the formation of Titan is that Saturn’s system started with a group of moons–similar to Jupiter’s quartet of Galilean moons–but that they were jostled by a disruptive series of gigantic impacts, which would continue to go on to create Titan. Saturn’s many mid-sized, icy moons, such as Iapetus and Rhea, were formed from the debris left in the wake of those giant impacts. Such a violent beginning could also explain Titan’s orbital eccentricity.
In 2004, an analysis of Titan’s atmospheric nitrogen hinted that it had possibly been derived from material existing in the distant Oort cloud and not from sources present during co-accretion of materials surrounding Saturn. The very remote Oort cloud is a distant sphere that surrounds our entire Solar System– it is the frigid home of a dancing multitude of icy, frozen comet nuclei.
Back in 1980, NASA’s Voyager I spacecraft made an unsuccessful (but, nevertheless, heroic) effort to obtain some close-up pictures of Titan’s heavily veiled surface. Alas, it was not able to cut through Titan’s thick cover of orange hydrocarbon clouds. However, the Cassini-Huygens mission, a joint NASA/European Space Agency/Italian Space Agency robotic spacecraft, that has now ended its mission, succeeded in obtaining the precious information that astronomers had long searched for. The NASA-developed Cassini Orbiter was named after the Italian-French astronomer Giovanni Dominico Cassini (1625-1712) who is credited with the discovery of four of Saturn’s many moons. The European Space Agency’s (ESA’s) Huygens Probe, named for Christiaan Huygens, successfully performed the feat of cutting through Titan’s blanket of smog to explore its well-hidden surface.
After a long and difficult journey through interplanetary space, Cassini-Huygens finally arrived at the Saturn system on July 1, 2004. On December 25, 2004, the Cassini Orbiter was deliberately separated from the Huygens Probe, that it had been carrying piggy-back, and sent the probe whizzing off to Titan. Arriving at Titan on January 14, 2005, Huygens floated down through the thick orange clouds to finally lift Titan’s veil– seeing, for the first time, its long-hidden face. It then obtained images of the surface of this oddball moon, and sent them back to Earth.
Titan is 50% larger than Earth’s Moon, and it is 80% more massive. It is also larger than Mercury–the smallest of the eight known major planets in orbit around our Sun. However, Titan is only about 40% as massive as Mercury. Orbiting Saturn once every 15 days and 22 hours, Titan, like Earth’s Moon, and many of the other icy moons that circle the gaseous giant planets of our Solar System’s outer realm, has a day (rotational period) that is exactly the same as its orbital period. This is because Titan is tidally locked in synchronous rotation with its parent-planet, and it permanently shows only one face to Saturn. Because of this, there is a point on Titan’s surface (sub-Saturnian point), from which Saturn would always appear to be looming directly overhead.
Saturn’s irregularly shaped, small moon, Hyperion, is locked in a 3:4 orbital resonance with Titan. A “slow and smooth” evolution of the resonance–in which Hyperion wandered away from a chaotic orbit–is thought by many astronomers to be an improbable explanation for this resonance. Instead, the favored theory is that Hyperion likely formed in a stable orbital island, whereas the much more massive Titan absorbed ejected objects that had wandered too close to its gravitational embrace.
Titan is 3,201 miles in diameter, 1.06 times that of the planet Mercury, 1.48 that of Earth’s Moon, and 0.40 that of our own planet. Before Voyager I arrived, back in 1980, Titan was believed to be slightly larger than Ganymede of Jupiter (3, 270 miles). Thus, Titan was mistakenly thought to be the largest moon in our Solar System. However, this proved to be an overestimation resulting from Titan’s opaque, very dense atmosphere–which reaches many miles above its surface and increases its apparent diameter. Titan’s diameter and mass (and thus its density) are similar to those of the Galilean moons Ganymede and Callisto of Jupiter. Based on its bulk density, Titan’s composition is calculated to be approximately 50% water ice and 50% rocky material. Even though Titan is similar in composition to two of Saturn’s mid-sized, icy moons, Enceladus and Dione, it is considerably more dense as a result of gravitational compression.
Titan is thought to be differentiated into several layers, with a 2,100 mile rocky center blanketed beneath several layers of different forms of crystalline ice. Some planetary scientists suggest that Titan’s interior may still be hot enough for a liquid layer, composed of a “magma” of water ice and ammonia, to be sandwiched between the icy crust and deeper layers composed of high pressure forms of ice. The presence of ammonia enables water to remain liquid even at temperatures as frigid as -97 degrees Celsius. The Cassini probe detected evidence for a layered structure in the form of natural extremely low radio waves in Titan’s atmosphere. Planetary scientists think that Titan’s surface is a poor reflector of extremely low-low-frequency radio waves. For this reason, it has been proposed that these low-frequency radio waves may be reflecting off the liquid-ice boundary of a hidden ocean sloshing beneath the surface.
Titan has the distinction of being the only moon in our Solar System with a significant atmosphere–and it is the only Solar System object known to possess a dense, mostly nitrogen atmosphere, that is similar to Earth’s. The composition of Titan’s atmosphere is 98.4% nitrogen with the remaining 1.6% composed primarily of methane (1.4%) and hydrogen (0.1-0.2%). There are also trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene and propane. In addition, there are also other gases present in Titan’s atmosphere, such as cyanoacetylene, hydrogen cynanide, carbon dioxide, carbon monoxide, cyanogen, argon and helium.
The hydrocarbons are believed to form in Titan’s upper atmosphere, as a result of the breakup of methane by our Sun’s ultraviolet light–resulting in the thick orange smog that heavily veils Titan’s surface.
Energy emanating from our Star should have changed all traces of methane in Titan’s atmosphere into more complex hydrocarbons within a time span of 50 million years–a mere blink of the eye compared to the age of our 4.56 billion year old Solar System. This finding suggests that methane must be replenished by a reservoir either on or within Titan itself. The origin of the methane in Titan’s atmosphere is generally thought to be its interior, shot out to the surface as a result of eruptions from ice volcanoes (cryovolcanoes).
A Mysterious And Poisonous Gas Cloud
One simple way to think about the cloud structure of Titan is that different gases will condense into ice clouds at varying altitudes. This has been compared to layers in a parfait dessert. However, precisely which cloud condenses, and where it condenses, depends on how much vapor exists, as well as on the temperatures–which become colder and colder at lower altitudes in Titan’s stratosphere. However, what actually occurs is somewhat more complex. This is because each type of cloud forms over a range of altitudes, and this makes it possible for some ices to condense simultaneously (co-condense).
Dr. Anderson of GSFC and his colleagues used Cassini’s CIRS in order to make sense of the complex tangle of infrared fingerprints derived from numerous molecules present in Titan’s atmosphere. This instrument separates infrared light into its component colors, much like raindrops creating a rainbow, and it measures the strengths of the signal at the different wavelengths.
“CIRS acts as a remote-sensing thermometer and as a chemical probe, picking out the heat radiation emitted by individual gases in an atmosphere. And the instrument does it all remotely, while passing by a planet or moon,” explained Dr. F. Michael Flasar in the October 17, 2017 NASA Press Release. Dr. Flasar is the CIRS Principal investigator at GSFC.
The newly discovered poisonous cloud, which the astronomers call a high-altitude south polar cloud, displays a strong and distinctive chemical signature that showed up in three sets of Titan observations conducted from July to November 2015. Titan’s seasons last for seven Earth years, and because of this it was late autumn at the south pole the entire time.
Because the spectral signatures of the various ices did not match those of any individual chemical, the team of scientists went on to conduct laboratory experiments in order to condense mixtures of gases simultaneously. In order to this, they used an ice chamber that simulates the conditions existing in Titan’s dense atmosphere, testing pairs of chemicals that had infrared fingerprints in the right part of the spectrum.
Initially, they allowed one gas to condense before the other. However, the best result was obtained when the scientists introduced hydrogen cyanide and benzene into the chamber, allowing them to condense simultaneously. Benzene, by itself, does not display a distinctive far-infrared fingerprint. But when the benzene was allowed to co-condense with hydrogen cyanide, the far-infrared fingerprint of the co-condensed ice displayed a close match to the observations conducted using CIRS.
However, more studies are needed to help scientists understand the structure of the co-condensed ice particles. Many researchers think that these frozen tidbits will be both disorderly and lumpy, rather than sporting a well-defined crystal structure.
Dr. Anderson and his colleagues had already discovered an example of such co-condensed ice particles in the CIRS data that had been obtained back in 2005. Those observations were conducted near Titan’s north pole, approximately two years following the winter solstice in the orange moon’s northern hemisphere. This cloud had formed at a considerably lower altitude, below 93 miles, and sported a different chemical composition: hydrogen cyanide and cyanoacetylene, one of the more complex organic molecules detected in Titan’s exotic atmosphere. Titan’s southern cloud had been detected about two years earlier during that moon’s southern winter solstice. One proposed explanation for this is that the mixtures of gases were slightly different in the two cases. Alternatively, the temperatures had heated up a bit by the time the north polar cloud had been spotted by astronomers. A combination of both explanations is also possible.
“One of the advantages of Cassini was that we were able to flyby Titan again and again over the course of the thirteen year mission to see changes over time. This is a big part of the value of a long-term mission,” Dr. Anderson explained in the October 17, 2017 NASA Press Release.
The Cassini mission’s “Grand Finale” came on September 15, 2017.
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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