Mystifying, bewitching, and swathed in a heavy, dense shroud of orange hydrocarbon mist, Titan circles its immense gas-giant parent-planet, Saturn, and is a remarkable world in its own right. Slashed by strange rivers and seas of ethane, methane, and propane, and pelted by large and lazy drops of hydrocarbon rain, Titan is an eerie, tormented, and mysterious moon-world orbiting its magnificent and beautiful ringed parent-planet, in the distant outer realm of the giants–Jupiter, Saturn, Uranus, and Neptune. The four enormous and gaseous wonderland worlds are unlike the quartet of much smaller rocky denizens of the inner Solar System–Mercury, Venus, Earth, and Mars. Because of its dense orange blanket of smog, the geological features of Titan’s surface were hidden from the prying eyes of curious astronomers until 2004 when the Cassini/Huygens orbiter and lander finally arrived there–and started to unveil its long-hidden face. In April 2016, a team of planetary scientists announced yet another important revelation about this moon-world–a large sea on Titan is composed primarily of pure liquid methane, with the seabed itself possibly well-coated in a sludge of carbon-and nitrogen-rich material, as well as showing strange shores surrounded by wetlands.
Of the hundreds of bewitching moons in our Sun’s family, Titan is remarkable for being the only one boasting a dense atmosphere and large liquid reservoirs on its surface, rendering it in many ways more like the four rocky, terrestrial planets of the warm and well-lit inner Solar System. Indeed, both Earth and Titan possess atmospheres dominated by nitrogen–more than 95 percent nitrogen in Titan’s case. However, unlike our Earth, Titan’s atmosphere has very little oxygen; the remainder of its atmosphere is primarily composed of methane and trace quantities of other gases–such as ethane. At the truly frigid temperatures found at the Saturn system’s great distance from our Sun, Titan’s methane and ethane can exist on the surface in their liquid form.
For this reason, for many years astronomers considered the possibility that hydrocarbon lakes and seas might exist on this fantastic moon-world. Data that finally arrived courtesy of the joint NASA and European Space Agency’s (ESA’s) Cassini-Huygens mission lived up to their expectations. Since arriving at the Saturn system in 2004, the Cassini spacecraft has revealed more than 620,000 square miles of Titan’s long-hidden, bewildering surface–and it has shown that almost two percent of Titan’s entire surface is covered in liquid.
The Realm Where The Ringed Planet Reigns
Saturn, along with its frozen retinue of icy rings, dazzling moons, and sparkling moonlets, orbits our Sun about ten times farther out than the Earth. Astronomers received their first collection of detailed data about Titan when the Cassini/Huygens orbiter and lander arrived there in 2004. The Huygens lander successfully obtained revealing images when it drifted down to Titan’s tormented, hydrocarbon-slashed surface, as well as when it was still floating slowly and softly down through the moon’s thick, foggy, orange atmosphere–which has 1.4 times greater pressure than that of our own planet. These pictures, when combined with other studies using instruments aboard the Cassini orbiter, reveal to curious planetary scientists that Titan’s geological features include lakes and river channels filled with methane, ethane, and propane. Titan’s strange surface also shows mountains and sand dunes–and it is pockmarked by craters. The rippling dunes form when fierce winds sweep up loose particles from the surface and then tosses them downwind. However, the sands of Titan are not like the sands on our Earth. Titan’s “sand” is both bizarre and alien, probably composed of very small particles of solid hydrocarbons–or, possibly, ice imprisoned within hydrocarbons–with a density of about one-third that of the sand on our own planet. Furthermore, Titan’s gravity is low. In fact, it is only approximately one-seventh that of Earth. This means that, working in combination with the low density of Titan’s sand particles, they carry only the small weight of a mere four percent that of terrestrial sand. Titan’s “sand” is about the same light-weight as freeze-dried grains of coffee!
Titan’s alien climate–including its heavy hydrocarbon rain and fierce winds–forms surface features that are similar to those on Earth, and it experiences seasonal weather changes–just like our own planet. In fact, with its liquids pooling both on its surface and beneath its surface, along with its mostly nitrogen atmosphere, Titan has a methane cycle that is comparable to Earth’s water cycle–although at the much more frosty temperature of about -179.2 degrees Celsius.
Titan is the largest moon of the gas-giant planet Saturn, as well as the second-largest moon in our entire Solar System–after Jupiter’s enormous Ganymede. Indeed, this smoggy orange moon is almost as big as the planet Mars! Because Titan is situated in the outer domain of our Solar System, circling Saturn–which is the sixth major planet from our Star, the Sun–it is extremely cold, and its chemical atmosphere is frozen. This very interesting atmosphere is composed of a mix of compounds that many astronomers think are comparable to those that existed in our own planet’s primordial atmosphere. Titan’s strange, dense, orange atmosphere contains large quantities of “smoggy” hydrocarbons. This very heavy shroud of obscuring smog is so extremely dense that it showers “gasoline-like” rain down on the tortured surface of this distant moon-world.
NASA’s Voyager 1 spacecraft was the first to visit Titan back in 1980. Although Voyager 1 made a truly heroic journey, it proved unsuccessful in its efforts to obtain close-up pictures of Titan’s veiled surface. This adventurous, early space mission was not able to obtain the desired images because it could not cut through the dense orange smog–and the resulting images showed only some minor brightness and color variations in Titan’s atmosphere. In 1994, the Hubble Space Telescope (HST) did succeed in obtaining some precious and revealing images of Titan’s well-hidden surface–showing the existence of a bright and sparkling continent dubbed Xanadu–after the “Xanadu” of Samuel Taylor Coleridge’s romantic poem Kubla Khan. Titan’s Xanadu glitters as if lit by the cold fires of a multitude of sparkling rhinestones.
The Cassini-Huygens mission is a collaborative NASA/European Space Agency/Italian Space Agency robotic spacecraft that is observing the Saturn system. The spacecraft was initially constructed to sport two components: One is the European Space Agency-designed Huygens Probe named in honor of the Dutch mathematician and astronomer Christiaan Huygens (1629-1695), who discovered Titan. Huygens also studied the rings of Saturn. The second component, the NASA-designed Cassini Orbiter, was named for the Italian-French astronomer Giovanni Dominico Cassini (1625-1712) who discovered four of Saturn’s other moons. After a long, difficult journey through interplanetary space, that took it from Earth to Saturn, Cassini-Huygens finally reached the realm of the ringed planet on July 1, 2004. On December 25, 2004, the Huygens Probe was deliberately severed from the Cassini Orbiter, and began its descent down to the long-veiled and hidden surface of Titan–sending back, to waiting astronomers on Earth, an abundance of valuable information about the mysterious moon-world. Titan, at last, had its hidden face unveiled–revealing its well-kept secrets. The mission will continue until 2017.
Titan possesses a smooth, young surface, scarred by comparatively few impact craters. The climate of this frigid moon–including its fierce winds and showers of hydrocarbon rain–carves out surface features that bear an eerie resemblance to those on Earth, such as lakes, sand dunes, rivers, seas, and deltas. Indeed, planetary scientists propose that Titan bears a haunting resemblance to Earth, and is believed to be similar to the way our planet was before life had a chance to evolve out of non-living substances (prebiotic).
Titan circles its giant parent-planet once every 15 days and 22 hours. Like Earth’s own large Moon, and a number of other moons dancing around the quartet of giant gaseous planets of our Solar System’s outer realm, its rotation period is identical to its orbital period. This means that Titan is tidally locked in synchronous rotation with Saturn–always showing only one face to its planet.
The Methane Sea Of A Distant Moon-World
Titan has three large hydrocarbon seas that are all located near its north pole, and they are surrounded by numerous smaller lakes in the northern hemisphere. Only one large lake has been detected in the southern hemisphere.
The exact composition of these bodies of liquid remained unknown until 2014, when the radar instrument aboard Cassini was first used to show that Ligeia Mare, the second largest sea on Titan, is richly endowed with methane. Ligeia Mare is similar in size to two of the Great Lakes on Earth combined–Lake Huron and Lake Michigan! The new study, published in the February 25, 2016 issue of the Journal of Geophysical Research: Planets, which used the radar instrument in a different mode, independently confirmed the 2014 result. “It’s a marvelous feat of exploration that we’re doing extraterrestrial oceanography on an alien moon,” commented Dr. Steve Wall in an April 26, 2016 Jet Propulsion Laboratory (JPL) Press Release. Dr. Wall, who is of the JPL in Pasadena, California, is Cassini radar team deputy lead.
Dr. Alice Le Gall commented in the same JPL Press Release that “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.” Dr. Le Gall, a Cassini radar team associate, is of the French research laboratory LATMOS, in Paris, and lead author of the new study.
The new study is based on data gathered by Cassini’s radar instrument during flybys of Titan between 2007 and 2015.
There are several potential explanations that could account for the sea’s composition of pure methane, Dr. Le Gall added. “Either Ligeia Mare is replenished by fresh methane rainfall, or something is removing ethane from it. It is possible that the ethane ends up in the undersea crust, or that it somehow flows into the adjacent sea, Kraken Mare, but that will require further investigation,” she noted.
In their research, the planetary scientists combined several radar observations of heat given off by Ligeia Mare. They also studied data collected from a 2013 experiment that bounced radio signals off Ligeia Mare. The results of that experiment were presented in a 2014 paper led by Cassini radar team associate Dr. Marco Mastroguiseppe of Cornell University in Ithaca, New York, who also was part of the new study.
During the 2013 experiment, the radar instrument spotted echoes emanating from the seafloor and determined the depth of Ligeia Mare along Cassini’s track over that strange methane sea. This represented the first-ever detection of the bottom of an extraterrestrial sea. The astronomers were amazed to find that depths in this distant, alien sea were as great as 525 feet at the deepest point along the track the radar pointed.
Dr. Le Gall and her team used the newly acquired depth-sounding information in order to separate the contributions made to the sea’s detected temperature by the liquid sea and the seabed, which provided new insights into their compositions.
“We found that the seabed of Ligea Mare is likely covered by a sludge layer of organic-rich compounds,” she noted.
Methane and nitrogen present in Titan’s atmosphere react together to create a variety of organic materials. Many planetary scientists think the heaviest materials somersault down to the surface of hydrocarbon-slashed Titan. Dr. Le Gall and her team propose that when those compounds splash into the sea, either by directly falling from the air as hydrocarbon rain, or through Titan’s rivers, some are dissolved in the liquid methane. The compounds that do not dissolve, such as nitrites and benzene, sink down into the floor of this exotic sea.
The research proposes that the shoreline surrounding Ligeia Mare is possibly porous and may be saturated with liquid hydrocarbons. The data span a period running from local winter to spring, and the astronomers expected that–in a way similar to seasides on Earth–the surrounding solid terrains on Titan would warm much more rapidly than the sea.
However, this did not prove to be the case. Instead, Cassini’s measurements did not suggest any great difference in temperature between the surrounding shore and the methane sea over this span of time. This finding indicates that the terrains surrounding the lakes and seas on Titan are wet with liquid hydrocarbons, which would cause them to warm up and cool down in much the same way as the methane sea itself.
Dr. Wall commented in the April 26, 2016 JPL Press Release that “Titan just won’t stop surprising us.”