Speaking of the ground, if you landed near the equator, as the Huygens probe did, your first step would lead you across sand. Sand? Just take a sample with you into a warm room. You will soon realize not everything that appears to be sand is in fact silica, like the sand on Earth. The fine grains of the Titan desert consist of water ice with additional organic compounds that rained down from the tholin haze. However, the sand was created in a way similar to terrestrial sand—by erosion. Like the entire surface of this moon, the mountains on Titan consist of ice, which at these temperatures is as hard as granite. They erode just like granite due to rain and creeks digging a bed into them. In the large deserts the grains of sands clump together into dunes shaped by the wind. They can reach heights of up to 300 meters. They are not as soft as sand dunes on Earth, and over time they may solidify.
These are not the only obstacles you might encounter. Depending upon the season, bigger problems may be posed by rivers, lakes, and oceans. The climate conditions cause these to be concentrated in the polar regions. The three largest lakes, Kraken Mare, Ligeia Mare, and Punga Mare, are called ‘mares,’ pronounced 'mar-ray and mar-rays,' since their sizes of over 100,000 square kilometers make them comparable to inland seas on Earth. The largest and so far only lake in the southern hemisphere is called Ontario Lacus, and is approximately the same size as Lake Ontario. (‘Mare’ and ‘lacus’ are Latin for sea and lake, respectively.)
These seas and lakes are probably only a few hundred meters deep. Ligeia Mare, for instance, as radar scans by Cassini have shown, has a depth of only 170 meters. Of course they do not consist of water, which would freeze, but of liquid hydrocarbons like methane and ethane. The entire volume of these compounds surpasses that of Earth’s by a hundredfold! If you took a Porsche along on your voyage you could fill up its tank directly from these lakes, though you would have to bring along extra oxygen for the internal combustion engine, since there is hardly any oxygen on Titan. Due to the low oxygen level, there is no fire hazard, even though the lakes consist of pure fuel.
What about water sports? Diving is out of the question, unless you have a submarine. You can also leave your surfing equipment at home. The waves on Ontario Lacus only reach a maximal height of two centimeters. This is not only due to the lack of a breeze, which can get stronger in spring and fall, but also due to liquid methane being 'stickier' than liquid water. Physicists would say water has a lower viscosity. If we overcome the issue of heat insulation, we could recommend snorkeling to people vacationing on Titan. After all, these lakes are transparent, allowing you to see all the way to the bottom.
You might have problems swimming, though. First of all, you would need a perfectly insulated swimsuit. A spacesuit would not suffice, as Francesca experienced. If you recall, she could not walk through the shallow methane lake. Even if your suit insulated you from the coldness of space, the direct contact with the fluid methane would suck out all the heat and you would freeze almost instantly.
Secondly, the human body consists mostly of water, which has almost twice the density of methane. Therefore you would sink like a stone unless you brought along a gas-filled innertube. A boat would be an alternative, but you wanted to go snorkeling, you crazy tourist.
On Earth, lakes consist of water. When temperatures fall below zero, due to an anomaly that does not occur in any other substance, water expands as it freezes, so ice is lighter than water. This keeps ice at the surface. Otherwise, as water froze, it would sink to the bottom. Then more water would freeze and sink, until the lake was frozen solid. This would kill animals and plants that survive because of the insulation of the ice layer on the surface. On Titan, lakes might theoretically freeze from the bottom up because, unlike water, methane shrinks as it solidifies.
So why do we still see methane ice floating on Titan’s lakes, just as ice does on Earth, and see no totally frozen lakes? The methane ice is using something similar to the inner tube. While it freezes, it incorporates tiny nitrogen bubbles from the atmosphere. Remember, nitrogen has a share of more than 95 percent of Titan’s ‘air.’ This gas is lighter than methane and keeps the ice afloat.
No Destination for Mountain Climbers
If you want to go mountain climbing, you might choose a different destination. Titan offers nothing comparable to the Himalayas on Earth, with Mt. Everest's peak reaching 8.85 kilometers, or to Mons Olympus on Mars, with its height of 20 kilometers. The surface of Titan is generally rather flat, with few hill or mountains, but there are some. The Xanadu region near the equator, for instance, which is as large as Australia, is crisscrossed by mountain ranges reaching a height of up to 2 kilometers. These were probably not created by shifting continental plates, as happened on Earth, but by the shrinking of the moon, like wrinkles on aging skin. During the 4.5 billion years of its existence, Titan might have shrunk by 7 kilometers. The rigid ice crust could not contract accordingly and thus it ruptured. At the cracks, the edges pushed over each other and created mountains. Rain and creeks have probably cut into these ice mountains in ways similar to how streams have dug into limestone on Earth. This also means you will probably find the occasional cave on Titan.
Impact craters are rare on Titan, for the same reason they are rare on Earth. First, large meteorites will break up in the dense atmosphere, and second, the climate will quickly erode the existing craters. Thus most of the surface is between 100 million and a billion years old, which is young in geological terms. The largest crater, Menrva, has a diameter of 440 kilometers. Most craters lack structures typical of those on other moons, such as an elevated area in the center. Researchers suspect this would have been caused by the heat of an impact melting part of the icy ground, which then resolidified.
Radar data sent by Cassini also indicate structures reminiscent of volcanoes. Here, water containing ammonia might reach the surface, which could be a source for replenishing the nitrogen in the atmosphere. There is no clear-cut proof yet, though scientists definitely hypothesize Sotra Patera—patera being Latin for bowl—which was discovered by Cassini, to be such a cryovolcano. This object is part of a mountain range with a height of 1,000 to 1,500 meters. On top there are craters, and the flanks appear to be covered with frozen ‘lava,’ which in this case consists of water ice. If you are curious, Titan’s montes are named after mountains in Tolkien’s Middle Earth, and its colles—small hills —after characters from his books.
If you plan longer hikes on Titan, you might want to bring snowshoes. The ‘sand’ is a problem. This loose material possesses only a third of the density found in earth sand, which means you have less buoyancy, just like in the methane lakes, and you will automatically sink in deeper. You might have only a seventh of your weight, which makes every step easier, but you know how exhausting it can be when walking in loose or wet sand on Earth. On Titan you would sink in three times as deep.
The situation could become particularly complicated after it rains, when the sand is mixed with liquid methane. The quicksand described in this novel might actually be the rule rather than the exception.
The Changing Seasons
The best travel season for you depends upon your personal preferences. If you prefer a calm vacation in the eternal twilight, without a lot of disturbances, maybe with a nice view of a dune, you should travel to the equatorial regions in summer or winter. If you want some action, better choose spring or fall—or the right region during winter. Cassini, for instance, arrived near Saturn during the southern summer. At that time, the area near the South Pole was rather dry, and the North Pole was wet (as it was winter there). The North Pole experiences spring while the South Pole is cooling off in its autumn. Researchers are not yet sure what effects the seasons really have, and whether the lakes near the North Pole dry out completely during its summer. Currently, it is believed that the lakes remain, at least in part. Therefore, you might want to choose a lodging in the North if you are interested in water sports.
What should a visitor from Earth bring along to such a place? Almost everything. Fres
h linens definitely would not be your main problem. You can produce water from water ice, and oxygen from the water, but you would have to pack everything else into your spaceship. Most of all, you will need energy, both for the heating system and for generating water and oxygen. You cannot rely on solar cells, because even in the case of a clear sky the sun would not provide nearly enough energy. Therefore you need a nuclear power plant or at least an RTG, a radioisotope generator, which generates energy from the heat released by the decay of plutonium. Of course, your little house should be shielded well and offer a safe airlock. While neither nitrogen nor methane, the two main components of the atmosphere on Titan, are poisonous, it also contains HCN, also called hydrogen cyanide or prussic acid. That would not agree with you, for sure. At least there would be no vacuum inside your house, so you don’t have to adapt laboriously to the pressure difference when getting ready to go outside.
The Underground
Titan has an average density of 1.88 grams per cubic centimeter. This is significantly higher than the density of water, so this moon must have a rocky core. The silicate core is surrounded by several layers of water ice in various conditions. Most probably there is a liquid ocean above the core, and then an ice crust above that with a thickness of 100 kilometers.
The existence of such an ocean has not yet been directly detected. This means nobody has ever gone swimming in it, but this would also apply to the groundwater layer below your house here on Earth. The radar scans by Cassini clearly indicate the existence of an ocean. The probe measured the exact positions of 50 locations on the surface during several fly-bys, and changes were indicated from one recording to the next. This can only be explained by an ice crust floating on top of an ocean.
Where does the heat come from to keep the ocean liquid? Well, the water does not absolutely have to reach 0 degrees for several reasons. First, if ten percent ammonia is dissolved in the water, the melting point drops below 0 degrees. Remember, ammonia has been suspected of being the source of the atmospheric nitrogen. Second, radioactive materials in the core of Titan might be generating heat. Third, the tidal friction of various layers against each other—caused by Saturn—creates heat. And finally, there might also be chemical processes at the transition between rock and water that could generate energy. The ocean is probably kept liquid by a combination of any or all these effects. Titan is not extraordinary in this respect—we conclude that there are also oceans below an ice crust on Enceladus, and/or the Jupiter moon of Europa.
However, the ice ocean on Titan might be slowly freezing. This was indicated by Cassini’s most recent measurements of the moon’s gravitational field. According to these, the water has a high density, which is only possible if additional chemicals are dissolved in it. This suggests a salty ocean rivaling the Dead Sea on Earth. Accordingly, the ice crust below the mountains of Titan would reach deeper into the interior than below other terrains. This would be a reason for assuming a continuous freezing process.
The Birth of Titan
Titan came into being at about the same time as its planet Saturn, i.e. 4.5 billion years ago, after the sun had ignited inside the planetary nebula. Out here, at a distance of 9.5 astronomical units from the sun—an astronomical unit being the mean distance from the center of the sun to the center of Earth—the protoplanetary nebula cooled off more quickly than in the inner solar system, nearer the hot primal sun, where water more likely existed in liquid form or as water vapor. Furthermore, the lighter elements predominated out here—hence the creation of gas planets rather than rocky planets.
After the temperature had fallen sufficiently, first the firmer and then the more volatile compounds condensed down to water vapor, which then froze into ice crystals. When these particles met, they merged into larger clumps that in turn combined into even bigger conglomerates. This finally created planetesimals, which were still undifferentiated. This means that they had neither core nor crust, and rock and ice were still randomly mixed. This is the ‘usual’ co-accretion theory, which works for most planets and their moons, though not for Earth and its companion, Luna. Titan is also an exception, since it accounts for such a large part of the mass of all Saturn moons.
Most probably there was an 'accident' during the formation of the moon; several proto-moons collided and finally formed Titan. The other moons were created from the leftover material. This would also explain why the orbital plane of Titan is different from those of its siblings.
An alternative model assumes Saturn capturing Titan, which had formed in a different orbit. Could it even have been a planet of its own? The nitrogen isotope percentages in its atmosphere seem to indicate so. They suggest an origin in the Oort cloud, which today orbits far outside our solar system and represents something like the landfill of solar evolution. According to this model, Titan would even be a little bit (100 million years) older than Saturn.
Life on Titan?
Saturn, and thus its moon Titan, orbit far outside what scientists call the 'habitable zone,' a narrow area surrounding the sun. Does this mean that this moon is unsuitable for life?
No.
The habitable zone is defined as a region in which planetary surfaces support liquid water. Nothing more and nothing less. Our neighboring star Proxima Centauri was recently in the news, since it possesses several planets that fall under this definition. This does not mean that life actually exists there. In the case of Proxima b, one of the planets of Proxima Centauri, this is complicated by it always pointing the same side to its sun. That side would be hot, and the other side cold. In addition, it is probably regularly blasted with radiation by its sun.
Titan, on the other hand, resides in a cold but also very calm zone. Unlike Jupiter, Saturn does not bombard its moons with radiation. While water cannot exist on its surface, it would be anthropocentric to claim life always requires water. Titan does not have water in its liquid state, but has lots of organic chemical compounds, just like primal Earth did.
The astrobiologists are still basically skeptical, as they should be. A prebiotic chemistry is to be expected, meaning organic molecules reacting with others, which is also typical for life, but nothing more. Yet there are a few hints of more than this happening. While analyses found sources of hydrogen and acetylene, these substances do not appear in the atmosphere. If there were methane-based forms of life, both would be perfect providers of energy. Do the inhabitants of Titan ‘consume’ these substances? (The Titan life forms in the novel are based on this assumption.) Or are hitherto unknown chemical processes responsible for this fact?
A common argument against life on Titan is the fact that its low temperatures would slow down all reactions. However, there is no fixed reaction speed for life. After all, Titan has been around for 4.5 billion years. Life that developed there might grow, move, and think a bit more slowly—but it could exist.
And finally, it would be conceivable that such life could be based on a totally different foundation than anything we can imagine right now. In 2015, for instance, researchers investigated whether and how cells in methane lakes could form membranes, undoubtedly an important prerequisite for life. They found acrylonitrile to be suitable for this, a substance that has been detected in the atmosphere of Titan.
Even prussic acid, or hydrogen cyanide, which is lethal for us, offers interesting possibilities. Its molecules can form polyimines, a form of polymer with a double bond between the carbon and the nitrogen atom. These molecular chains can be activated by solar energy, among other things, similar to chlorophyll. Interestingly enough, the Titan atmosphere is rather transparent for the suitable photons (light particles). A life form based on prussic acid would therefore not even notice the layer of haze that to us makes the surface of Titan appear in eternal twilight.
If there is a chance of life somewhere in the solar system, scientists agree that it would be on Titan. Mars and Venus are much more likely devoid of life. A habitability index developed by astrobiologists therefore puts Titan first in line right behin
d Earth, while Mars is only the runner-up.
The Exploration of Titan
The Dutch astronomer Christiaan Huygens discovered Titan on March 25, 1655, using a 50-power telescope that he had developed together with his brother Constantijn. He simply called it Luna Saturni, literally, ‘Moon of Saturn.’ Saturn was thus the second planet after Jupiter (not counting Earth) where moons were discovered. Huygens even managed to calculate the orbital period of this moon. His result was 16 days and four hours, close to the actual value of a fraction under 16 days. Once other astronomers found additional Saturnian moons, Huygens simply called it ‘my moon,’ and the name ‘Huygens’ moon’ became common.
In 1847 the English astronomer John Herschel then suggested mythological names for all the moons of Saturn. These names became accepted toward the end of the 19th century.
The dense atmosphere of Titan set narrow limits for observations from Earth. Gerard Kuiper managed to detect methane in its atmosphere in 1944.
The first visits by space probes were a bit disappointing. In 1979, Pioneer 11 only managed to prove very cold temperatures on Titan. In 1980 Voyager 1 was the first to measure the density, composition, and temperature of the atmosphere. The surface still could not be seen.
This changed in 2004 when the Cassini probe reached the system. Cassini used radar to survey Titan. Most importantly, it managed to drop the Huygens lander module down to the surface on January 14, 2005. The first chapter of this novel is a faithful recreation of this landing—except for the fictitious ending. Even later, Cassini provided many interesting pictures and data about this moon. Almost all we know about Titan was derived by scientists from these data.
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