The Other Side of the Rainbow

"Toto, I’ve a feeling that we’re not in Kansas anymore." Dorothy in The Wonderful World of Oz  by L. Frank Baum.

Did the navigation instruments make a wrong turn? Did we crash land on Earth? When the Hugyens landing craft separated from the main Cassini spacecraft and landed on the surface of Titan (the first spacecraft to land on another world in the outer solar system), on January 14, 2004, scientists looked at not what appeared to be an alien world at all, but one that looked eerily familiar. The craft had landed on what appeared to be a floodplain strewn with pebbles that could have been found in many places on Earth, but with one crucial difference, it was Titan. In fact, Titan has been found to have many similarities to Earth. Some scientists have speculated that we were looking at a possible earlier Earth--the one exception is that methane replaces water as the liquid medium due to the extremely cold temperatures on the moon. Hal Clement, in his book Half-Life explored this possibility with a tale of a mission that was sent to Titan. The pioneers were sent there to explore the possible origins of life on Titan, information that scientists felt would help them deal with a series of plagues that were threatening the human race with possible extinction back on Earth. If Cassini and Hugyens had been a spaceship that transported humans, Stephen Baxter’s Titan is the book to read.
Planetary scientists had expected a world much like the Jovian ice moons, but instead we got something very unique. Prior to the Cassini-Hugyens mission, science fiction authors, too, speculated on what type of world Titan would be. Some including Robert A. Heinlein saw on Titan a world of monsters, in his juvenile novel The Puppet Masters. In the story, a species that called Titan home enslaves the human race by taking over their bodies. Harry Harrison in Star Smashers of the Galaxy Rangers envisioned Titan as being home to villainous bug-eyed monsters that kidnap a heroine. It sounds lame and cliché, but as with many of Harrison’s novels it is meant to be comedic in nature and it certainly fits the bill.
Other authors have speculated about intelligent life on Titan including Joan Vinge in Eyes of Amber about a very unusual culture unlike anything on Earth,The Stars Are Gods by Gord Eklund and Gregory Benford and Code of the Lifemaker  by James Hogan speculate about a highly developed machine culture on Titan.
Other authors see Titan as being an economically and politically essential world of the future solar system in such novels as Robert A. Heinlein’s The Rolling Stones, Larry Niven’s World of Ptavvas, Kim Stanley Robinson’s Mars trilogy in which Titan plays a role of being a primary source of nitrogen for the terraforming of Mars and Ben Bova’s Titan, part of his Grand Tour series of novels.
Titan was first observed on March 25, 1655 by the Dutch astronomer Christaan Hugyens (namesake of the landing craft on Titan) who had been inspired by fellow astronomer Galileo who first observed the Jovian moons in 1610 through one of his improved telescopes. Hugyens named the world Saturni luni which simply meant “Saturn’s moon.”  As more satellites were being discovered orbiting Saturn, including four by Giovanni Cassini (the namesake of the craft that carried Hugyens to the Saturnian system), the naming became problematic as consensus could not be reached. Titan was eventually the agreed upon name in 1847 with the publication of Results of Astronomical Observations Made at the Cape of Good Hope by John Herschel in which the suggestion for the name was first made.
In spite of a new name, the moon showed nothing unusual. It orbits Saturn once every fifteen days and twenty-two hours. Like our moon, its orbit is identical to its rotational period so that the same face of Titan shows to its mother planet. Perhaps, if anything was unusual about Titan, it was its size. At 5,150 kilometers across, it was bigger than the planet Mercury and about one and a half times the size of our moon. In fact, for a while, it was believed the largest moon in the solar system.

The lofty title was broken when data from Voyager 1, in 1980, showed that a blanketing atmosphere was giving Titan its bulk. Titan was now relegated to second largest moon in the solar system behind the Jovian moon Ganymede.
What Titan lost in its crown of being the largest moon it made up now with being the only moon in the solar system with a substantial atmosphere. In fact, Titan’s atmosphere isactually denser than that of Earth. Coupled with the dense atmosphere and its lighter gravity, the dream of human flight that has eluded humankind since time began here on Earth would be possible on Titan. Voyager may have provided the first evidence of an atmosphere on Titan, but it was Spanish astronomer Josep Sola who in 1903 first speculated that Titan had an atmosphere after observing limb darkening on the moon. Gerard Kuiper, in 1944, using a spectroscopic technique to first calculate an atmospheric pressure on the moon; his reading, groundbreaking in its day, fell short of today’s known pressure.
Like Earth, the atmosphere of Titan is predominately nitrogen. However, that is where the similarity ends. The other abundant gases in Titan’s atmosphere include methane along with many different complex hydrocarbons as well as some hydrogen. One early science fiction story by Alan Nourse, Trouble on Titan, speculated about a jet that ran on methane/ammonia from Titan’s atmosphere. Arthur C. Clarke’s Imperial Earth describes a solar system in which Titan’s atmosphere provides the hydrogen needed for interplanetary travel.
Looked at through a good telescope Titan may appear to be a distant sun with its orange glow. A reaction in the atmosphere provides its unique radiance. As ultraviolet light from the Sun strikes the moon’s atmosphere it creates more complex hydrocarbons, including tholins, out of the methane.
An interesting question now arises. Calculations estimate that the methane in Titan’s atmosphere would have been depleted with exposure to ultraviolet light within fifty million years--a mere moment in time in the life of the solar system. Methane, however, is still present in Titan’s atmosphere. How is the methane being replaced?
One suggestion is that the methane is being replenished by life on Titan. On Earth, a large contributor of methane to our atmosphere comes from ruminant flatulence. Could it be that there are cows on Titan producing methane? Unlikely, but it is not out of the question that there could be microbial cells. Scientists, however, being a conservative group, are looking for alternatives to the suggestion that lifeforms are the source of the methane in Titan’s atmosphere.
Another intriguing idea is that the methane came from comets. The problem with this theory is that it would be expected that there should also be a higher concentration of carbon monoxide. The carbon monoxide readings of Titan’s atmosphere shows that methane is probably not of cometary origin.
Then there is the theory that the methane is produced by cryovolcanoes; cryovolcanoes are ice volcanoes that erupt volatile gases rather than molten rock. Saturnalia, a novel by Grant Callin, describes a volcano-ridden Titan with a fictional volcano larger than the largest known volcano in the solar system, Olympus Mons. Recent observations by Cassini would seem to rule the volcanic origin out. Cassini’s sensors did, however, show lava flows, but they were frozen. At the same time, Titan’s surface appears to be remarkably free of impact craters. Something was happening that has erased the evidence and for all intents and purposes, it still looks like volcanoes are a possibility. Something happened, but what?
Perhaps a modified version of the volcanic theory?  Perhaps Titan is not continuously geologically active, but only sporadically (unlike Earth), working on a system of starts and stops with geologically active periods, the first having happened 4.5 billion years ago, when Titan first formed, then around 2.5 billion years ago when the convection of the core of Titan was initiated and most recently, the episode with the onset of ice crust convection of Titan; this latest episode was sufficient to obliterate most of the surface features of the moon. This model not only explains many of the features on the moon such as the presence of frozen volcanic flows but also the lack of impact craters and the even the detailed isotopic composition of the atmosphere.
Now there is an explanation of a feasible non-biological explanation for the methane in Titan’s atmosphere. Science fiction readers and writers may now decide to drop Titan from their list of interesting worlds, but it does not have to be so. In fact, Titan holds a lot more surprises to intrigue many science fiction readers and writers.  
Titan has a weather system very much like that of the earth, but with one crucial difference. On Earth, the weather is based on the hydrological (water-based) cycle. Any water on Titan would be frozen solid with surface temperatures hovering around -179 degrees Celsius--colder than would be expected with a thick blanketing atmosphere. Ironically, it is this same atmosphere that actually has an anti-greenhouse effect in that it prevents a lot of the energy of the Sun from ever reaching the moon’s surface to heat it up. If it did, Titan would be a very different world that we see today. On Titan, methane replaces the role of water. On Titan there is methane rain.
Not only is there methane rain, but it falls in deluges unlike anything that we would experience on Earth even with our water-based system. Why the difference? The sunlight that strikes Titan’s atmosphere is meager (Titan lies at approximately 1.4 billion kilometers from the Sun as opposed to 93 million miles for Earth). With so little energy striking the atmosphere of Titan, the evaporation rate is one centimeter of methane per year contrasted to that of the Earth which is at about one meter of water per year. Clouds on the Earth only hold a few centimeters of moisture before being released as rain. The clouds of Titan, on the other hand, hold up to ten meters of moisture. This coupled with the low evaporation rate means that the weather of Titan is characterized by torrential downpours and flash floods.
The atmosphere of Titan is not finished surprising scientists. Earth’s winds are governed by what is known as Hadley circulation. Warm air rises at the equator and settles down as it cools towards the poles. The winds are deflected by a shear force created by the rotation of the planet causing the air to descend around thirty degrees latitude north and south. As a result, the air in this region would be expected to be very dry. It is in keeping with the fact that Earth’s deserts are generally located at these latitudes. This Hadley circulation also creates a global band of low pressure which on Earth tends to be confined to the equator by our oceans. On Titan, with no confining oceans, this low pressure zone moves from equator to pole, making the entire world, in effect, a tropical environment, this in spite of bone-chilling temperatures.
Even more chilling than the atmosphere of Titan is the fact that we may be looking at the future of our own planet. As our planet begins to warm up, it will have a greater capacity to hold moisture. More moisture held in the atmosphere in turn will result in less frequent rainfall but when it does come it will be in the form of torrential downpours
When the Hugyens lander descended to Titan’s surface near the equator in January of 2005 no lakes were found. There was no evidence of liquid bodies at all, in sharp contrast of what was later found in the polar regions. No bodies of liquid but a lot of evidence of liquid in the recent past complete with what appeared to be dark drainage channels meandering into darker expanses. Hugyens scientists were somewhat apprehensive about landing Hugyens as it was thought that the dark region was a tar like substance, but when Hugyens landed in it, instead of a gooey mess, the site was very solid. In fact, the landing area was a sea of water ice pebbles that were water ice carved out by the action of liquids. Hugyens may have missed being caught in one of Titan’s flooding methane rains, but why the dryness at the equator?
Titan’s rotation is slower than the earth’s-the prevailing winds flow in the same direction as the rotation from east to west- and as a result the entire equatorial region dries out to become a sea of sand.
The strangeness of the winds of Titan is such that scientists may have to rethink some of our theories of air circulation here on Earth. On Earth we have dunes in our deserts that align with the two dominant wind directions. Observations by the Cassini spacecraft while in orbit about the moon show a dune orientation different than what is expected. In fact, even in simulation experiments have trouble duplicating the dune directions of Titan.

In spite of the lack of lakes at the landing site of Hugyens, there are lakes on the surface of Titan. The lakes of Titan are, in fact, the only bodies of liquid that have been found on a world other than our own. They were first speculated to exist with the passing of Titan by both of the Voyager spacecraft, 1 and 2, but it was indirect evidence. The atmospheric temperature and composition supported the theory that the surface of Titan had potential hydrocarbon lakes. It was only in 1995, when that other wonder of human technological achievement, the Hubble Telescope, showed direct evidence of large bodies of liquid methane on Titan either in scattered pockets or ocean wide expanses. With the arrival of the Cassini spacecraft in 2004, that there was hope that actual visual representation of these liquid methane seas would be found as the Sun’s light reflected off of the liquid surface, but it was not to be. Where were these liquid seas as indicated by the Hubble Telescope?
In a July 2006 flyby, radar imagery from Cassini showed a number of large smooth patches on the surface in the northern latitudes of the moon as well. The Cassini-Hugyens scientists were ready to make their announcement. In January 2007, Cassini-Hugyens scientists announced to the world that Titan did indeed have hydrocarbon lakes. The lakes are made up of liquid ethane along with methane, nitrogen and other hydrocarbons.

In February 2007, further confirmation of the announcements were radar observations of the Cassini spacecraft, during a flyover of the north polar region, showed a large number of liquid methane or ethane bodies, one that is larger than Lake Superior, the largest inland freshwater body of water on Earth.
During a flyby in July 2007 in the south polar region, a dark feature named Ontario Lacus was a suspected lake with a possible shoreline that was verified by radar imagery. Science fiction author J. Matthew Neal wrote a novel about this very feature in Ontario Lacus which describes a possible lifeform that is found in the lakes of Titan, with amazing regenerative powers.
Looking at the lakes of Titan add a further element of mystery to this moon. On Earth, the lakes are active with wave action, even more so during a storm. On Titan, one would expect that the waves of Titan lakes would be even bigger than those on Earth. But there is nothing but absolute stillness, a photograph freezing a moment in time.  
The surprises of Titan do not end there. It is theorized that Titan is made up of a rocky core surrounded by several layers of different forms of ice crystals. Some scientists speculate that there may even be a liquid layer made up of water and ammonia between the ice layers-the presence of ammonia allows water to stay in its liquid state to extremely low temperatures. Adding further to this evidence of a liquid interior are electrical measurements of Hugyens that showed that at a depth of 43 kilometers below the surface, there was an electrically conductive layer with water being the prime candidate to account for the readings. Further, radar readings from Cassini show that the crust of Titan rotates at a faster rate than the core; a possible conclusion is that there is a liquid layer between allowing for the differing rates of spin. Once we have water in an environment, the possibility of life as we know it increases dramatically. Titan may indeed have life in the underground waterways.
Like that one missing piece of a puzzle that you agonize over to complete the picture, even the sophisticated Cassini craft had one limitation in that it was unable to penetrate the blanketing atmosphere of Titan to look for a secondary magnetic field that the magnetic field of Saturn would induce should an underground ocean be indeed present. Scientists are already drawing up plans to look for this elusive magnetic signature in the coming decades
Titan is an enigma in that it shows an uncanny similarity to the life-bearing planet that we live on while at the same time being so very alien to us. It is certainly a world that is worth the effort to explore not only from the standpoint of understanding of an alien world but also of understanding our world as well. It is also providing scientists some knowledge of extremes of weather that we here on Earth may one day experience here on our own planet should conditions continue as witnessed by the weather of Titan.
It also provides us with a potential to look way back into Earth’s past and see a possible transition from chemical replicators to true life forms. Perhaps if a generation starship were sent beyond the confines of the solar system for millions of years of voyaging, who knows what they may come back to should they return to the solar system. Perhaps humans on Earth will find that we have intelligent neighours right on our galactic doorstep, true Titan. We can only wonder at the possibilities.  
Further Reading:
1.      Adamkovics, M. et al. 2007. "Widespread Morning Drizzle on Titan." Science. 318:962.
2.      Artemieva, N. and Lunine, J. 2003. "Cratering on Titan: impact melt, ejecta, and the fate of surface organics." Icarus.164:471-480.
3.      Barnes, J. et al. 2006. "Global-scale surface spectral variations on Titan seen from Cassini/VIMS." Icarus. 186 (1):242.
4.      Bevilacqua, R. et al. 1980. "Resonances and close approaches I." The Titan-Hyperion case. Earth, Moon and Planets. 22(2): 141-152.
5.      Brown, R. Lebreton, J. P. and Hunter, J. (eds). 2009. Titan from Cassini-Hugyens. Springer.
6.      Cassini, G. 1673. "A Discovery of two New Planets about Saturn, made in the Royal Parisian Observatory" by Signor Cassni, Fellow of both the Royal Societys, of England and France. Philosophical Transactions.  8L1673):5178-5185.
7.      Coates, A. et al. 2007. Discovery of heavy negative ions in Titan’s ionosphere. Geophys. Res. Lett. 34:L22103.
8.      Coustenis, A. 2005. "Formation and evolution of Titan’s atmosphere." Space Science Reviews. 116:171-184.
9.      Coustenis, A and Taylor, F. 2008.  "Titan: Exploring an Earthlike World."  World Scientific.
10.  Darell, F. 2010. "Molecular hydrogen in Titan’s atmosphere: Implications for tropospheric and thermospheric mole fractions." Icarus. 208:878-886.
11.  Dermott, S. and Sagan, C. 1995. "Tidal effects of disconnected hydrocarbon seas on Titan." Nature. 374:238-240.
12.  Emily, L. et al. 2006. "A large cloud outburst at Titan’s south pole." Icarus. 182:224-229.
13.  Fortes, A. 2000. "Exobiological implications of a possible ammonia-water ocean inside Titan." Icarus. 146(2):444-452.
14.  Grasset. O. et al. 2000. "On the internal structure and dynamic of Titan." Planetary and Space Science.  48(7-8):617-636.
15.  Harland, D. 2002. Mission to Saturn: Cassini and Hugyens Probes. Springer.
16.  Ivanov, B. et al. 1997. "Atmospheric entry of large meteoroids." Planetary and Space Science. 45:993-1007.
17.  Jacobson, R. et al. 2006. "The gravity field of the saturnian system from satellite observations." The Astronomical Journal. 132(6):2520-2526.
18.  Kuiper, G. 1944. “"Titan: a Satellite with an Atmosphere." Astrophysical Journal. 100:378.
19.  Lancaster, N. 2006. "Linear Dunes on Titan." Science. 312:724-727.
20.  Longstaff, A. 2009. “Is Titan (cryo)volcanically active?” Astronomy Now. 19.
21.  Lorenz, R. 2003. "The Glitter of Distant Seas." Science. 302:403-404.
22.  Lorenz, R. 2010. "Winds of Change on Titan." Science. 329:519-520.
23.  Lorenz, Ralph and Mitton, Jacqueline. 2002. Lifting Titan’s Veil: Exploring the Giant Moon of Saturn. Cambridge University Press.
24.  Lovett, R. March 2008. "Saturn Moon Titan May Have Underground Ocean." National Geographic.
25.  Lunine, J. and Lorenz, R. 2009. "Rivers, Lakes, Dunes and Rain: Crustal Processes in Titan’s Methane Cycle." Annual Review of Earth and Planetary Sciences. 37: 299-320.
26.  Mahaffy, P. 2005. "Intenisve Titan Exploration Begins." Science. 308:969-970.
27.  McKay, C. and Smith, H. 2005. "Possibilities for methanogenic life in liquid methane on the surface of Titan." Icarus. 178(1): 274-276.
28.  McKay, C. et al. 1991. “The greenhouse and antigreenhouse effects on Titan.” Science. 253:1118-1121.
29.  Mitri, G. et al. 2007. "Hydrocarbon Lakes on Titan." Icarus. 186(2):385-394.
30.  Niemann, H. et al. 2005. "The abundances of constituents of Titan’s atmosphere from the GCMS instrument on the Hugyens probe." Nature. 438:779-784.
31.  Ramou, R. et al. 2006. "The Latitudinal Distribution of Clouds on Titan." Science. 311:201-205.
32.  Raulin, F. 2005. "Exo-astrobiological aspects of Europa and Titan." Space Science Review. 116(1-2):471-487.
33.  Raulin, F. and Owen T. 2002. "Organic chemistry and exobiology on Titan." Space Science Review. 104(1-2):377-394.
34.  Richardson, J. et al. 2004. "Titan’s Surface and Rotation: New Results from Voyager 1 Images." Icarus. 170(1):113-1124.
35.  Shiga, D. 2006. "Huge ethane cloud discovered on Titan." New Scientist. 313:1620.
36.  Sotin, C. et al. 2005. "Release of volatiles from a possible cryovolcano from near-infrared imaging of Titan." Nature. 435:786-789.
37.  Stofan, E. et al. 2007. "The lakes of Titan." Nature. 445(1):61-64.
38.  Sushil, K. et al. 2006. "Titan’s methane cycle." Planetary and Space Science. 54(12):1177.
39.  Tobie, G. et al. 2005. "Titan’s internal structure inferred from a coupled thermal-orbital model." Icarus. 175 (2):496-502.
40.  Tobie, G. et al. 2006. "Episodic outgassing as the origin of atmospheric methane on Titan." Nature. 440:61-64.
41.  Waite, J. et al. 2007. "The Process of Tholin Formation in Titan’s Upper Atmosphere." Science. 316:870.
42.  Wye, L et al. 2009. "Smoothness of Titan’s Ontario Lacus: Constraints from Cassini RADAR specular reflection data." Geophysical Research Letters. 36:L16201.