Mysterious, magical, mystifying Titan, the largest moon of the ringed-planet Saturn, possesses a strange surface enshrouded in a dense orange hydrocarbon mist–that long kept it well hidden from the curious eyes of astronomers, who could not penetrate its blanket of alien smog. The mysterious surface of this magical moon was at last unveiled in 2004, when the Cassini Mission’s Huygens Probe was deliberately separated from the Cassini Orbiter to begin its historic dive down to the long-enshrouded and secret surface of Titan. However, this bewitching, bothersome, and bewildering moon-world will not reveal its many secrets without a struggle. In March 2017, planetary scientists announced that a recent NASA-funded study has shown how hydrocarbon lakes and seas that linger on this hydrocarbon-tormented moon-world might sometimes erupt with dramatic nitrogen bubbles. The idea of nitrogen bubbles forming fizzy froth on Titan’s hydrocarbon lakes and seas helps solve yet another one of the more bewitching, enchanting unsolved mysteries Cassini has unveiled during its time exploring Titan: the so-called “magic islands” that–presto chango–first appeared in earlier observations, only to disappear–and then reappear again!
Of the hundreds of moons that do their mesmerizing dance around the major planets of our Solar System, only Titan possesses a dense atmosphere and large liquid reservoirs on its surface–making it in some ways eerily similar to a terrestrial planet, like our Earth. The terrestrial planets of our Solar System’s well-lit and warm inner kingdom are Mercury, Venus, Earth, and Mars.
Our Earth and Titan have nitrogen-laden atmospheres–more than 95 percent nitrogen in Titan’s case. However, unlike our own planet, Titan has very little oxygen. The rest of Titan’s atmosphere is primarily methane with trace quantities of other gases, including ethane. Furthermore, at the frigid temperatures that characterize Saturn’s great distance from the brilliant, melting heat of our Star, the methane and ethane can flow on Titan’s surface as liquid.
Because of this, planetary scientists have speculated for a very long time about the possibility of hydrocarbon lakes and seas existing on this misty orange moon-world. The data that was obtained, from the NASA/European Space Agency (ESA) Cassini-Huygens mission, lived up to expectations. Ever since it arrived at the Saturn system back in 2004, after making a long and treacherous journey through interplanetary space, the heroic spacecraft revealed to the prying eyes of scientists that almost two percent of Titan’s surface is covered in liquid.
The Cassini-Huygens mission is a collaborative NASA/ESA/Italian Space Agency robotic spacecraft that is currently surveying the Saturn system The spacecraft was originally constructed to carry two components. One is the ESA-designed Huygens Probe named in honor of the Dutch astronomer and mathematician Christiaan Huygens (1629-1695), who discovered the moon-world Titan. Huygens also observed the famous rings of the gas-giant planet Saturn. The second component, the NASA designed Cassini Orbiter, was named for the Italian-French astronomer Giovanni Dominico Cassini (1625-1712), who discovered Saturn’s many other moons. The Cassini Orbiter reached the frigid realm of the ringed-planet on July 1, 2004, and on December 25, 2004, the Huygens Probe was deliberately separated from the Cassini Orbiter–that it had been riding piggy back–and began its slow descent down to the long-veiled surface of this mysterious, magical moon. The Huygens Probe sent back to scientists on Earth a veritable treasure chest filled with valuable information about this very secretive moon-world. At last, Titan’s hidden, veiled face was revealed. The mission will continue until 2017, when Cassini plunges down into the clouds of the planet that it has been watching for so long.
The gas-giant planet, Saturn–the second-largest planet in our Star’s family–along with its icy retinue of lovely and majestic rings, sparkling moons, and dazzling moonlets, orbits our Sun about ten times farther out than our own planet. Astronomers received their first batch of revealing information about Titan’s veiled face from the successful dive down to its surface by the Huygens Probe, that sent back to Earth amazing images as it floated down to Titan’s hydrocarbon-tormented, slashed surface. Once on the ground of this bewildering moon–whose surface has been compared to creme brulee in its consistency–the Huygens Probe continued to show scientists Titan’s strange face.
These images, when combined with other studies using instruments from the Cassini Orbiter, unmasked some of Titan’s bewitching geological features including lakes and river channels flowing with the hydrocarbons methane, ethane, and propane. Titan’s alien surface revealed eerily familiar features such as mountains and sand dunes, as well as a surface scarred with numerous craters. Titan’s waving, rippling dunes are formed from ferocious and powerful winds that blast loose particles up from Titan’s surface, only to toss them downwind. The sands of Titan are unlike the familiar sands of our own planet. Titan’s “sand” is likely composed of extremely tiny particles of solid hydrocarbons–or, perhaps, ice trapped within hydrocarbons–with a density of approximately one-third that of the sands of Earth. This indicates that, working in combination with the low density of Titan’s “sand” particles, they carry only the tiny weight of about four percent that of terrestrial sand. Titan’s particles of “sand” are about the same puny weight as freeze-dried grains of coffee.
Titan’s alien, frigid climate–including heavy hydrocarbon downpours and powerful, rushing winds–carve surface features that are eerily like those of own Earth, and it also undergoes seasonal weather variations, similar to those of our own planet. Indeed, with its pools of liquids, both on its surface and beneath its surface, along with its primarily nitrogen atmosphere, Titan sports a methane cycle that is similar to our planet’s water cycle. However, this faraway moon-world has much more frigid temperatures than those of Earth–about -179.2 degrees Celius.
This smoggy largest moon of Saturn, which is the second-largest planet in our Solar System–after the gigantic behemoth, Jupiter–is almost as big as the planet Mars! Situated in the outer kingdom of our Solar System, swathed in orange mist, and extremely cold, Titan’s chemical atmosphere is frozen. This intriguing atmosphere is made up of a mix of compounds that many scientists propose is comparable to that of Earth’s primordial atmosphere–that existed long before life had emerged (prebiotic). Titan’s bizarre orange atmosphere, that contains large amounts of “smoggy” hydrocarbons, creates a blanketing shroud of obscuring smog that is so extremely dense that it showers “gasoline-like” rain down to the truly tortured surface of this frigid moon-world.
NASA’s Voyager 1 spacecraft was the first to reach Titan back in 1980. Despite this heroic journey through interplanetary space, by one of NASA’s earliest missions, Voyager 1 failed in its efforts to obtain close-up images of Titan’s veiled surface because it was unable to cut through the obscuring dense shroud of orange smog. Voyager 1’s historic images revealed only a handful of minor color and brightness variations in Titan’s atmosphere. In 1994, the Hubble Space Telescope (HST) did succeed in obtaining some revealing images of Titan’s veiled face–showing the existence of a dazzling continent dubbed Xanadu–named after the “Xanadu” mentioned in Samuel Taylor Coleridge’s romantic poem Kubla Khan. Titan’s real Xanadu sparkles as if it were paved with a myriad of diamonds.
Hydrocarbon Lakes And Seas
There are three large seas on Titan, that are all situated near the moon’s north pole. These very alien hydrocarbon seas are surrounded by numerous smaller lakes pooling in the northern hemisphere. Only one solitary lake has been detected in Titan’s southern hemisphere.
The precise composition of these reservoirs of liquid remained a mystery until 2014, when the Cassini radar instrument was first used to discover Ligeia Mare, which is the second largest sea on Titan–roughly the same size as Lake Huron and Lake Michigan combined. However, Ligeia Mare is not filled with water–like Earth’s Great Lakes–but is instead methane-rich. A study published in the Journal of Geophysical Research: Planets, which made use of the radar instrument in a different mode, independently confirmed this result. The study was based on data collected with Cassini’s radar instrument during flybys of Titan that occurred between 2007 and 2015,
“Before Cassini, we expected to find that Ligeia Mare would be mostly made up of ethane, which is produced in abundance in the atmosphere when sunlight breaks methane molecules apart. Instead, this sea is predominantly made of pure methane,” explained Dr. Alice Le Gall in an April 26, 2016 NASA Press Release. Dr. Le Gall is a Cassini radar team associate at the French research laboratory LATMOS, Paris, and lead author of that study.
In Titan’s atmosphere, nitrogen and methane react to form a wide variety of organic materials. Many planetary scientists propose that the heaviest materials tumble down to the surface. Dr. Le Gall and colleagues believe that when these compounds reach the sea, either by directly falling from the air, by way of rain or through Titan’s rivers, some are dissolved in the liquid methane. The insoluble compounds, such as benzene and nitriles, tumble down to the sea floor.
Presto Chango: Solving Titan’s “Magic Island” Mystery
In the extremely frigid temperatures found at Titan, cold liquid methane rains down from the sky and collects in the that moon’s strange and alien rivers. Even slight alterations in temperature, air pressure or composition can cause the nitrogen to quickly separate out of solution in a way that has been compared to the fizz that results when opening a bottle of carbonated soda.
NASA’s Cassini spacecraft has discovered that the composition of Titan’s seas and lakes varies from one place to another, with some reservoirs being considerably more well-endowed than others in ethane rather than methane. “Our experiments showed that when methane-rich liquids mix with ethane-rich ones–for example from a heavy rain, or when runoff from a methane river mixes into an ethane-rich lake–the nitrogen is less able to stay in solution,” explained Dr. Michael Malaska in a March 15, 2017 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Malaska, of the JPL, led the study.The JPL is in Pasadena, California.
This results in bubbles–many, many, many bubbles.
The liberation of nitrogen, termed exsolution, can also happen when methane seas grow slightly warmer in temperature during the seasonal changes on Titan. A fizzy frothy liquid, however, could also result in certain problems for any future robotic probe sent to swim through or float upon Titan’s hydrocarbon seas. Excess heat flowing out from a probe could potentially result in the formation of bubbles around its structures. For example, propellers used for propulsion could result in this type of fizz–making it very hard to steer the probe or even keep it stable.
Planetary scientists have proposed several potential explanations for the appearing, disappearing, reappearing “magic islands” of Titan. One of the proposals provides details about the mechanism that could be creating the enchanting, fizzy patches of bubbles–if, indeed, the bubbles really are the true cause of this magical mystery.
“Thanks to this work on nitrogen’s solubility, we’re now confident that bubbles could indeed form in the seas, and in fact may be more abundant than we’d expected,” commented Dr. Jason Hofgartner of JPL in the March 15, 2017 JPL Press Release. Dr. Hofgartner serves as a co-investigator on Cassini’s radar team, and is a co-author of the study.
While identifying how nitrogen travels between Titan’s liquid reservoirs and its atmosphere, the scientists teased the nitrogen out of a simulated ethane-rich solution, while the ethane froze to the bottom of their small, simulated Titan “lake.” Water is less dense in its solid phase than in its liquid phase. Ethane, however, is not water. As a result, ethane ice would form at the bottom of Titan’s frigid pools–and, as the ethane crystalized, it would make no room for the dissolved nitrogen gas. This is the mechanism that would make it come bubbling out.
Imagine hydrocarbon lakes, bubbling with nitrogen, on a mysterious, mystifying distant, orange, and very smoggy cold moon. This very dramatic scene becomes even more intriguing because the motion of nitrogen on Titan doesn’t travel in only one direction. This means that it has to get into the methane and ethane before it can escape.
“In effect, it’s as though the lakes of Titan breathe nitrogen. As they cool, they can absorb more of the gas, ‘inhaling.’ And as they warm, the liquid’s capacity is reduced, so they ‘exhale,'” commented Dr. Malaska in the March 15, 2017 NASA Press Release.
Something similar happens, on our own planet, with carbon dioxide absorption by Earth’s oceans.
Cassini‘s final close flyby of Titan–its 127th targeted visit–occurred on April 22, 2017. During the last flyby, Cassini sent its radar beam sweeping over Titan’s hydrocarbon seas–for the very last time.
Cassini is about to make its final farewell to the Saturn system. The last flyby over the mysterious, mystifying Titan’s surface also bent the spacecraft’s course in order for it to begin its last series of 22 dives through the gap between Saturn and its innermost rings–a final feat known as Cassini’s Grand Finale. The dramatic end will come when the 20-year-old mission concludes with Cassini’s fatal dive into the atmosphere of Saturn on September 15, 2017.
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