With its gossamer rings sparkling with a mesmerizing frozen sea of icy moonlets and twirling icy fragments and particles, the gas-giant Saturn is arguably the most beautiful planet in our Solar System. This second-largest planet in our Sun’s family, orbiting our Star about ten times farther than Earth, Saturn is so light that it could float on water–providing that a basin could be found that is large enough to contain it. A denizen of the cold outer region of our Solar System, this sixth planet from our Sun is also the proud parent-planet of a very special misty moisty moon-world, the hydrocarbon tormented, smoggy orange shrouded moon dubbed Titan. Titan is the largest moon in the Saturn system, as well as the second-largest moon inhabiting our entire Solar System, after Ganymede of Jupiter. In August 2017, only weeks away from its dramatic, mission-ending fatal dive down into the clouds of the ringed-planet, that it has been circling since 2004, NASA’s Cassini spacecraft is waiting to make one last distant encounter with Titan before it meets its fate in the clouds of Saturn.
The Cassini spacecraft is being kept busy during its last days, circling Saturn every week as part of its final mission–appropriately named its Grand Finale. On a few of its previous orbits, Titan has been close enough to influence Cassini’s orbit, causing the spacecraft to approach Saturn a tiny bit closer or a tiny bit farther away. A handful of those distant passes even nudged Cassini into the outer limits of Saturn’s famous, magnificent system of rings.
Cassini will visit Titan one last time before it comes to the end of the road on September 11, 2017. The spacecraft will continue dispatching precious scientific information back to astronomers until it finally loses contact with Earth.
Numerous flybys of Titan were planned from the mission’s beginning as a way to explore the bewitching and mysterious moon-world, swathed as it is in a heavy, impenetrable blanket of orange hydrocarbon smog. Torn and tormented by alien rivers and seas of ethane, methane, and propane, and pummeled by lazy, large drops of hydrocarbon rain, Titan is both eerie and mystifying as it orbits its beautiful ringed parent-planet in the distant, cold kingdom of the outer giant planets of our Sun’s family–Jupiter, Saturn, Uranus, and Neptune. Because of Titan’s veil of heavy smog, the geological features of its surface were well-hidden from the prying eyes of curious astronomers until the Cassini/Huygens orbiter and lander at last arrived there–and started to lift the veil from the hidden face of this magical moon-world.
The Cassini-Huygens mission is a collaborative NASA/European Space Agency/Italian Space Agency robotic spacecraft that was initially constructed to be composed of two components: One is the European Space Agency’s (ESA’s) Huygens Probe, named for the Dutch mathematician and astronomer Christiaan Huygens (1629-1695), who discovered Titan, and who also studied Saturn’s system of rings. 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 many, many moons. After a long, treacherous journey through interplanetary space, that carried it from Earth to Saturn, Cassini-Huygens at last reached Saturn on July 1, 2004. On December 25, 2004, the Huygens Probe was deliberately liberated from the Cassini Orbiter, and began its descent down to the mysterious surface of the heavily shrouded, distant moon-world–sending back to Earth an abundance of information about Titan. At last, Titan’s hidden face was unveiled–revealing its well-kept secrets.
The new Huygens Probe images of Titan show a smooth, young surface, pockmarked by relatively few impact craters. This frigid moon-world’s climate includes ferocious, rushing winds, as well as alien downpours of heavy hydrocarbon rain. Both the wind and rain carve out surface features that bear a haunting resemblance to some of the surface features of our own planet, such as sand dunes, rivers, lakes, seas, and deltas. Indeed, the planetary scientists studying the images now propose that Titan may be similar to the way Earth was before life had emerged and evolved out of non-living substances.
Titan orbits its gas-giant parent-planet once every 15 days and 22 hours. In a way that is similar to Earth’s own large Moon, and a number of other moons circling the quartet of giant planets in our Solar System’s outer limits, its rotation period is exactly the same as its orbital period. This basically means that Titan is tidally locked in synchronous rotation with Saturn–always showing only one face to its planet.
Titan has three large seas filled with liquid hydrocarbons that are all situated close to its north pole, and they are all surrounded by many smaller hydrocarbon lakes in the northern hemisphere. Only one lone lake has been seen in Titan’s southern hemisphere.
The precise composition of these lakes and seas was unknown until 2014, when the radar instrument aboard Cassini was the first to show that Ligeia Mare–the second largest of Titan’s seas–is heavily laden with methane. Ligeia Mare is approximately the same size as two of the Great Lakes on Earth combined–Lake Huron and Lake Michigan. The seabed of Ligeia Mare is thought to be covered by a sludge layer of organic-rich compounds.
The methane and nitrogen that exist in Titan’s atmosphere react together to form a variety of organic materials. Many scientists think that the heaviest materials float down to the surface of the moon-world. When these compounds enter the sea, whether by directly tumbling down from the air as hydrocarbon rain, or by way of Titan’s rivers, some are dissolved in the liquid methane. The compounds that fail to dissolve, such as nitrites and benzene, sink down into the floor of this alien sea.
Both Earth and Titan sport atmospheres that are dominated by nitrogen–more than 95% nitrogen in Titan’s case. However, unlike Earth, Titan’s atmosphere contains very little oxygen. Indeed, the remainder of Titan’s atmosphere primarily contains methane, along with traces of other gases–such as ethane. At the truly cold temperatures that characterize Saturn’s great distant from our fiery, searing-hot, glaring Star, Titan’s methane and ethane can exist on the surface in their liquid phase.
For this reason, for years, astronomers contemplated the possibility that hydrocarbon lakes and seas might exist on the surface of this misty moisty moon. The data derived from the Cassini/Huygens mission lived up to their expectations. Since its arrival at the Saturn-system, the Cassini spacecraft has unveiled over 620,000 square miles of Titan’s bewitching, well-hidden surface–and it has shown that almost two percent of Titan’s entire surface is covered with liquid.
Lifting Titan’s Mysterious Orange Veil
Titan is just a little bit larger than the planet Mercury–the smallest major planet inhabiting our Sun’s family. Planetary scientists were eager to explore this planet-size moon-world ever since NASA’s Voyager 1 spacecraft whizzed by it back in 1980. Alas, Voyager 1 was not able to pierce through the heavy golden fog that blankets Titan’s secretive surface.
Because Titan possesses a strong gravitational pull–as a result of its large size– mission scientists were able to take advantage of the situation by using Titan’s gravity to bend Cassini’s course as it circled Saturn. Indeed, a single close flyby of Titan provided more of a change in velocity than the entire 90-minute engine burn necessary to slow it down so it could be snared by Saturn’s powerful gravity upon its arrival.
Cassini/Huygens’ four engineers, who were given the task of plotting the spacecraft’s course years in advance, were able to use Titan as their “linchpin”. Numerous passes by the misty moon provided the equivalent of enormous quantities of rocket propellant. By using Titan’s gravity, the fortunate engineers were able to stretch Cassini’s orbit out farther from Saturn–for example, to dispatch the spacecraft toward Saturn’s mid-size, icy moon, Iapetus. Using this technique, the engineers made good use of Titan flybys to alter the orientation of Cassini’s orbit during the course of the mission–for example, when they lifted the spacecraft from the plane of the rings in order to observe them from high above, along with high southern and northern latitudes on Saturn and its many moons.
A Fond Farewell To A Misty Moon
The Cassini spacecraft has made 127 close flybys of Titan over the course of its 13-year mission at the Saturn system–along with many more distant observations of this misty moon-world. When Cassini deliberately dropped the ESA’s Huygens Probe, it floated down through Titan’s dense atmosphere to finally land on its strange surface in January 2005.
The Cassini mission has proven to be highly successful. Among Cassini’s many important discoveries, it revealed that there were indeed bodies of open liquid hydrocarbons pooling on Titan’s surface–just as many planetary scientists had suspected for years. However, it was surprising that Titan’s lakes and seas are primarily located at its poles, with almost all of the liquid existing at northern latitudes–at least, this true, for the current era. Cassini also revealed that most of Titan is devoid of lakes, and that there are vast regions of linear dunes existing closer to the equator that are similar to those observed on Earth–in places such as Namibia. The plucky spacecraft peered at giant hydrocarbon clouds looming over Titan’s poles–as well as feathery bright clouds that floated across the landscape, dropping a heavy rain of gasoline that darkened this hydrocarbon tormented moon’s strange surface. There were also some tantalizing hints that an ocean of life-loving liquid water might slosh around beneath Titan’s icy surface crust.
At first, the images that Cassini returned to scientists on Earth were spotty. However, each new encounter contributed to the earlier ones. Over the course of the entire mission, Cassini’s radar managed to image approximately 67% of Titan’s surface, using the spacecraft’s saucer-shaped, large antenna to bounce signals off the orange-shrouded moon’s strange and intriguing surface. Images derived from Cassini’s infrared spectrometer, radar, and imaging cameras, gradually added more and more previously unknown details, building up an increasingly more complete, high-resolution picture of Titan.
“Now that we’ve completed Cassini’s investigation of Titan, we have enough detail to really see what Titan is like as a world, globally,” commented Dr. Steve Wall in an August 11, 2017 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Wall is deputy lead of Cassini’s radar team at the JPL, which is located in Pasadena, California.
Planetary scientists now have acquired enough information to understand the distribution of Titan’s surface features (such as seas, dunes, and mountains) as well as the behavior of its atmosphere over time. The scientists also are now able to begin pieceing together how surface liquids might migrate from pole to pole.
However, uncertainties remain. One question, that still remains unanswered, is how the methane in Titan’s atmosphere is being replenished, since it is constantly being broken down over time by sunlight. Planetary scientists have also observed evidence of volcanism, with methane-laden water serving as that strange moon’s “lava”–but a definitive detection has not been achieved.
Cassini’s observations could still provide more enticing clues. Planetary scientists have been watching for summer rain clouds to form at the north pole, as their models have predicted. Cassini detected rain clouds at the south pole, during Titan’s southern summer, back in 2004. However, clouds at high northern latitudes have been few in number.
“The atmosphere seems to have more inertia than most models have assumed. Basically, it takes longer than we thought for the weather to change with seasons,” commented Dr. Elizabeth Tuttle in the August 11, 2017 JPL Press Release. Dr. Tuttle is a Cassini imaging team associate at Johns Hopkins Applied Physics Laboratory, in Laurel, Maryland.
The slow formation of northern summer clouds might make a better match with models that predict a global reservoir of methane, Dr. Tuttle continued to explain. “There isn’t a global reservoir at the surface, so if one exists in the subsurface that would be a major revelation about Titan,”she added. This shows how valuable Cassini’s long-term monitoring of Titan’s atmosphere has been, because the monitoring is providing data that can be used to test models and theories.
Cassini’s Grand Finale
The Cassini spacecraft made its final close flyby of Titan on April 22, 2017. This last close encounter with the misty moon-world provided the spacecraft with a much needed nudge over Saturn’s rings. The push also helped Cassini begin its final series of orbits, enabling it to soar between the rings and Saturn.
During that flyby, Cassini’s radar played a starring role–its observation requirements determining how the spacecraft would be oriented as it flew low over the surface one last time at an altitude of 606 miles. One of the mission’s top priorities was to have one final peek at Titan’s so-called “magic islands”. These “magical” surface features first appeared and then vanished in separate observations taken years apart. On the last flyby there were no magic islands to be seen. Cassini’s radar team is still trying to solve the mystery of Titan’s “magic islands”–trying to understand what the features might have been. The leading candidate explanations are either bubbles or waves.
Of greatest interest to the radar team was a set of observations in which the instrument was used to explore the mysterious, hidden depths of several of the small hydrocarbon lakes that mark Titan’s north polar region. In the future, the scientists will be working to tease out important information gathered from these alien lakes in order to determine their composition, in terms of methane versus ethane.
As Cassini made its last close flyby with the misty moisty orange moon, before its Grand Finale, the Cassini radar team imaged a long swath of Titan’s once-veiled surface, revealing terrain observed on the very first Titan flyby in 2004.
“It’s pretty remarkable that we ended up close to where we started. The difference is how richly our understanding has grown, and how the questions we’re asking about Titan have evolved,” Dr. Wall noted.