Enshrouded in a dense golden hydrocarbon mist, Saturn’s largest moon Titan is a mysterious mesmerizing world in its own right. For centuries, Titan’s veiled, frigid surface was completely camouflaged by this hazy golden-orange cloud-cover that hid its icy surface from the prying eyes of curious observers on Earth. However, this misty moisty moon-world was finally forced to show its mysterious face, long-hidden behind its obscuring veil of fog, when the Cassini Spacecraft’s Huygens Probe landed on its surface in 2004, sending revealing pictures back to astronomers on Earth. In September 2018, astronomers announced that new data obtained from Cassini show what appear to be gigantic, roaring dust storms, raging through the equatorial regions of Titan. The discovery, announced in the September 24, 2018 issue of the journal Nature Geoscience, makes this oddball moon-world the third known object in our Solar System–in addition to Earth and Mars–where ferocious dust storms have been observed. The observations are now shedding new light on the fascinating and dynamic environment of Titan, which is the second largest moon in our Solar System, after Ganymede of Jupiter.
Cassini’s successful mission of exploration to the Saturn system is over, but planetary scientists are left with a cornucopia filled with important new information that Cassini/Huygens sent back to Earth before its mission ended. A collaborative NASA/European Space Agency/Italian Space Agency mission, the robotic spacecraft was made up of two components. The first was the European Space Agency’s (ESA’s) Huygens Probe, that had been named in honor of the Dutch mathematician and astronomer Christiaan Huygens (1629-1695), who discovered Titan. The Huygens Probe also closely observed Saturn’s lovely system of gossamer rings. The second component, the NASA-designed Cassini Orbiter, was named after the Italian-French astronomer Giovanni Dominico Cassini (1625-1712), who discovered four of Saturn’s other intriguing, numerous, and icy moons.
After a long and dangerous journey through the space between planets, the Cassini/Huygens Spacecraft reached Saturn on July 1, 2004. On December 25, 2004, the Huygens Probe was purposely liberated from the Cassini Orbiter. Huygens then began its historic descent through the dense blanket of golden-orange fog to at last lift the veil hiding Titan’s long-hidden face.
“Titan is a very active moon. We already know that about its geology and exotic hydrocarbon cycle. Now we can add another analogy with Earth and Mars: the active dust cycle, in which organic dust can be raised from large dune fields around Titan’s equator,” Dr. Sebastien Rodriguez explained in a September 24, 2018 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Rodriguez is an astronomer at the Universite Paris Diderot, France, and the paper’s lead author. The JPL is in Pasadena, California.
Titan is certainly an exotic, fascinating, frigid oddball, that bears an eerie resemblance to the primordial Earth before life emerged and evolved on our planet (prebiotic). In fact, Titan is the only moon in our entire Solar System with a substantial atmosphere. It is also the only known world–other than Earth–where stable bodies of surface liquid exist.
But there is an important difference. On our own planet, lakes and seas are flowing with water, while Titan’s lakes and seas are filled primarily with methane and ethane, that slosh around within these liquid reservoirs. In this never-before-seen cycle, the hydrocarbon molecules evaporate and condense into clouds that send an exotic “rain of terror” back down to this strange moon-world’s carbon-slashed surface.
Like our own Earth, Titan’s atmosphere is mainly made up of nitrogen–but with the added ingredient of a small amount of methane. It is the only other world in our Solar System that is actually known to have a cycle of liquids that shower back down to the surface again, in Titan’s case as large, lazy drops of hydrocarbon rain. The clouds of Titan pour torrential rains of gasoline down to the surface of this tormented moon. Even though the ingredients are different, this cycle is similar to our own planet’s water cycle. Many planetary scientists propose that Titan contains a subsurface ocean of sloshing liquid water.
The Face Behind The Veil
Titan is a little larger than Mercury–the smallest major planet inhabiting our Solar System. Indeed, Titan would have been classified as a major planet in its own right if it orbited our Sun instead of Saturn. The Huygens Probe images lifted the veil from the face of this distant moon-world, revealing a youthful surface that is both smooth and relatively free of impact craters. Huygens also found that this icy, hydrocarbon-saturated moon’s climate includes those heavy rains of gasoline, as well as raging, roaring winds. Some of Titan’s surface features were found to be hauntingly akin to certain surface features on Earth.
Titan orbits Saturn once every 15 days and 22 hours. Like Earth’s large Moon, in addition to many other moons in our Solar System, Titan’s rotational period is precisely the same as its orbital period. This means that Titan only shows one face to its parent-planet, while the other face is always turned away.
Most of the moons of our Sun’s family circle the quartet of large gaseous planets located in our Solar System’s outer limits: Jupiter, Saturn, Uranus and Neptune. The four solid inner planets–Mercury, Venus, Earth and Mars–are almost entirely moonless. Earth is the only inner planet that hosts a large Moon, while Mars sports only a pathetic duo of misshapen little Moons (Phobos and Deimos), that are either captured asteroids that escaped from the Main Asteroid Belt between Mars and Jupiter, or are instead the outcome of a primordial collision between Mars and a large protoplanet.
Titan has three large seas. However, the seas of Titan are not filled with water, but are filled instead with swirling liquid hydrocarbons. All three of Titan’s exotic seas are close to its north pole, and they are surrounded by many smaller hydrocarbon-filled lakes in the northern hemisphere.
The precise chemical composition of these very alien lakes and seas remained unknown until 2014, when Cassini’s radar instrument detected Ligeia Mare, now known to be Titan’s second-largest hydrocarbon-filled lake. Ligeia Mare is brimming with an abundance of sloshing methane, and this enormous liquid reservoir is approximately the same size as two of Earth’s Great Lakes combined–Lake Michigan and Lake Huron. Many planetary scientists think that the seabed of Ligeia Mare may be blanketed with a thick layer of sludge that is composed of organic-rich compounds.
Indeed, the mixture of nitrogen and methane that whirl around in Titan’s swirling thick golden-orange atmosphere create a variety of organic compounds. It has been suggested that the heaviest materials float down to the surface of this hydrocarbon-slashed moon. When these organic compounds tumble down into Titan’s lakes and seas–either by raining down from the clouds in alien showers of hydrocarbons, or by traveling along with Titan’s strange rivers–some are dissolved in the liquid methane. The compounds that manage to survive this ordeal, and do not dissolve (such as nitrites and benzene), float down to the alien sea floors of this oddball moon-world.
Titan’s atmosphere is approximately 95% nitrogen. However, in a way that dramatically differs from Earth’s own mostly-nitrogen atmosphere, Titan’s atmosphere has very little oxygen. Indeed, the remainder of Titan’s atmosphere is almost entirely composed of methane–along with small qunatities of other gases, such as ethane. At the extremely cold temperatures that are found at Saturn’s great distance from the heat of our Star, Titan’s methane and ethane can accumulate on its icy surface to form pools of liquid.
For this reason, astronomers have for years considered the possibility that hydrocarbon lakes and seas might exist on the surface of this misty moisty moon. The data derived from Cassini/Huygens validated this prediction. During its long and productive mission, now over, Cassini revealed that almost 2% of Titan’s entire bizarre surface is coated wth gasoline-like liquids.
Titan has a radius that is about 50% wider than Earth’s Moon. It is approximately 759,000 miles from its parent-planet Saturn, which itself is about 886 million miles from our Sun–or 9.5 astronomical units (AU). One AU is equal to the average distance between Earth and Sun, which is 93,000,000 miles. The light that streams out from our Star takes about 80 minutes to reach Saturn. Because of this vast distance, sunlight is 100 times more faint at Saturn and Titan than on Earth.
Astronomers are still debating Titan’s origin. However, its intriguing atmosphere does provide a hint. Several instruments aboard the Huygens spacecraft measured the isotopes nitrogen-14 and nitrogen-15 in Titan’s atmosphere. The instruments revealed that Titan’s nitrogen isotope ratio most closely resembles that seen in comets that exist in the remote Oort Cloud–which is a sphere composed of hundreds of billions of icy comet nuclei that circle our Star at the amazing distance of between 5,000 and 100,000 AU. This shell of icy objects extends half way to the nearest star beyond our own Sun.
The nitrogen that exists in Titan’s atmosphere indicates that it likely formed early in our Solar System’s 4.56 billion-year-old history. This means that Titan probably was born within the same cold disk of gas and dust that gave birth to our Sun (protostellar nebula), instead of forming in the warmer disk that eventually created Saturn.
Despite this oddball moon’s many exotic attributes, it actually sports one of the most Earth-like surfaces in our Solar System. Titan may also experience volcanic activity, but its volcanoes would erupt with different ingredients than the molten-rock lava that shoots out from the volcanoes of Earth. In dramatic contrast to what occurs on our own planet, Titan’s volcanoes erupt icy water “lava” (cryovolcanism). Titan’s entire alien surface has been sculpted by gushing methane and ethane, which carves river channels, and fills its enormous great lakes with liquid natural gas.
Vast regions of dark dunes also extend across Titan’s exotic landscape, especially around its equatorial regions. Unlike Earth’s sand, the “sand” that creates Titan’s dunes is composed of dark grains of hydrocarbon that resemble coffee grounds. The tall linear dunes of this misty moisty moon-world appear to be quite similar to those seen in the desert of Namibia in Africa. Because Titan’s surface is pockmarked by relatively few impact craters, its surface is considered to be quite young. Older surfaces display heavier cratering than more youthful surfaces, whose craters have been “erased” by resurfacing. This resurfacing is caused by processes that cover the scars left by old impacts as time goes by. Our own planet is similar to Titan in this respect. The craters of Earth are erased by the ongoing processes of flowing liquid (water on Earth), powerful winds, and the recycling of Earth’s crust as a result of plate-tectonics. These processes also occur on Titan, but in modified forms. In particular, the shifting of the ground resulting from pressures coming from beneath (plate tectonics), also appear to be at work on this veiled moon-world. However, planetary scientists have not seen signs of plates on Titan that are analogous to those of our own planet.
A Distant, Dusty Moon
Titan experiences changing seasons–just like Earth. In particular, Titan’s seasons change around the equinox, when our Sun passes Titan’s equator. At this time, huge clouds can form in tropical areas, resulting in violent methane storms. Cassini observed these ferocious methane storms during several of its flybys over Titan.
Dr. Rodriguez and his colleagues also spotted a trio of odd equatorial brightenings in infrared images obtained by Cassini during Titan’s 2009 northern equinox. At the time, the scientists speculated that the brightenings might also be the same kind of methane clouds observed in tropical areas. However, this proved not to be the case. A later investigation conducted by the astronomers revealed that these brightenings were caused by something entirely different.
“From what we know about cloud formation on Titan, we can say that such methane clouds in this area and in this time of year are not physically possible. The convective methane clouds that can develop in this area and during this period of time would contain huge droplets and must be at a very high altitude–much higher than the 6 miles that modeling tells us the new features are located,” Dr. Rodriguez explained in the September 24, 2018 JPL Press Release.
The scientists also ruled out the possibility that the mysterious features actually exist on Titan’s surface in the form of frozen methane rain or icy lava erupted from cryovolcanoes. Such surface features would show a different chemical signature and would be visible for much longer periods of time than the bright features observed in this study. The bright features were visible from time spans of only 11 hours to five weeks.
Additional modeling showed that the strange features must be atmospheric–but nevertheless close to Titan’s surface–probably creating a very thin layer of tiny solid organic particles. Because they are located directly above the dune fields surrounding Titan’s equator, the only possible explanation left is that these mysterious spots are really clouds of dust churned up from the dunes.
Organic dust forms when organic molecules, resulting from the interaction of sunlight with methane, grow large enough to tumble down to the surface of Titan. Dr. Roderiguez continued to explain that, even though this is the first-ever observation of a dust storm on Titan, the discovery is not especially surprising.
“We believe that the Huygens Probe, which landed on Titan in January 2005, raised a small amount of organic dust upon arrival due to its powerful aerodynamic wake. But what we spotted here with Cassini is at a much larger scale. The near surface wind speeds required to raise such an amount of dust as we see in these dust storms would have to be very strong–about five times as strong as the average wind speeds estimated by the Huygens measurements near the surface and with climate models,” Dr. Rodriguez added.
The existence of such powerful roaring winds kicking up violent and powerful dust storms suggests that the underlying sand can be set in motion, too, and that the giant dunes covering Titan’s equatorial regions are still active and continually changing.
The screaming winds could be carrying the dust raised from the dunes across great distances, contributing to the global cycle of organic dust on Titan. These would result in effects similar to those that occur on both Earth and Mars.