An Enigmatic Environment ( Pre-Cassini / Huygens Approach )


A Liquid Surface ?

What's beneath the hazy and opaque layer of Titan's atmosphere? this question lets perplexed even the specialists of planetology.All the spacecrafts sent towards Saturn have not been able to provide any clear indication upon the surface of the saturnian moon.The orange sphere appears as opaque as Venus.But it is not from the Venusian chemistry that we will be able to guess what the surface looks like: the solar energy absorbed by Titan is by far lower ( around 15 watt/m²/s²) than that absorbed by Venus ( 2650 watt/m²/s²).The greenhouse effect generated by the atmosphere of Titan is not as pronounced as that generated by the atmosphere of Venus, notably because the atmospheric pressure is largely lower ( 1.5 bars compared to 95 bars ).So, it is almost impossible that the surface of the satellite is covered with seas of water. However, the cold conditions at this distance from the Sun allow the possibility of liquid methane or liquid ammonia on the surface.Methane is more volatile than ammonia, CO2 is more volatile than ammonia and ammonia is more volatile than water.If the environmental temperature is below 0°c as everybody knows, water is in its solid form ( ice, frost deposit, snow).At -180°c, methane gas becomes liquid. If the temperature conditions on Titan are those, one can envisage lakes or oceans of methane and a meteorology involving methane instead of water.But, as we've said, no visible clouds have been detected yet in the hazy atmosphere.It is believed that methane gas ( CH4) in the atmosphere ,whose presence has been demonstrated, splits under the action of ultraviolet light to form various molecules and notably ethane (C2H6).As a result, any lake or sea on Titan might contain a large amount of Ethane as Jonathan Lunine had shown in his model.The mixture of methane and ethane would slow down the evaporation process so that clouds are rarer.As said Ralph Lorenz, co author with Jacqueline Mitton of "Lifting Titan's Veil", a book devoted to Titan, Ethane would act on methane like sugar acts on water, preventing evaporation.The photochemistry occuring in the atmosphere would also produce Acetylene ( C2H2) but in smaller proportions than ethane. Let's imagine now that the environmental temperature on the surface is warmer than expected and that it is well over -100°c: in this case, one can envisage seas or lakes of ammonia (NH3). Ralph Lorenz emphasized the possibility that water and ammonia might have melted to form ammonia rich ice: he showed that a mixture with water and 30% ammonia would freeze at -97°c.A this temperature, one have no chance of encountering carbon dioxide in the form of gas: carbon dioxide freezes at -78°c. Let's remember that Mars Polar Lander could have photographed a landscape covered with frozen carbon dioxide at the level of the polar cap on Mars: unfortunately, the mission failed.So, what is the Titan's surface made of ? : Seas with high concentrations of methane,ethane, acetylene or ammonia, or simply a soil covered with frozen carbon dioxide or water ice ?: it looks like a scientific speculation but when we see the wrong predictions of economists and financial analysts, what can we expect from the predictions of planetologists? The atmosphere's composition may provide clues of what could be below the gas layer : as we've shown, the two main compounds of the atmosphere are nitrogen and methane.Conversion of Ammonia to nitrogen by the action of ultraviolet light seems to be the most obvious way for Titan to acquire a nitrogen atmosphere ( N,N²): this element push us into thinking that the surface of Titan is ammonia rich. We can also imagine that methane is present in a large amount at the surface if the temperature conditions are sufficiently low.


Volcanoes on Titan?

Is Titan geologically active? In other words, are there active volcanoes? and if there are active volcanoes,what kind of lava is expelled? all these questions have still no answers but we can expect surprises on that subject.Indeed, the exploration of the solar system has shown that many planets or satellites harbour volcanoes which are currently active ( or have been active).The most impressive volcanism in the solar system is undoubtedly on Io, a moon of Jupiter, 3130 km across and orbiting 400 000 km around the largest planet one knows.It seems paradoxical that a moon smaller than our moon is able to expel melted material up to 300 km into space while the satellite of our planet is completely inactive.The reason for this phenomenon lies in the storm of gravitational pulls and tensions generated by Jupiter, Europa, Ganymede and Callisto.Like the Earth and the moon, Io undergoes permanent tides but in larger proportions.Unlike Io, Mars and Venus (by far wider) have been volcanically active a long time ago and seem today inactive.The exploration of Mars has unveiled that the tallest volcano in the solar system, the Olympus Mons ( 27 km high ) is today extinct ( unless it is sleeping ).The numerous volcanoes on Venus also seem to be extinct.On the basis of those observations, we are entitled to imagine that Titan may be the place of a giant volcanism like on Io.But, can we compare Titan and Io since Titan is almost twice as large as Io and roughly twice as far from the Sun ? Perhaps that the main factor for the probability of volcanism on Titan is the tidal forces undergone by Titan in interaction with Saturn, Rhea, Dione, Hyperion and the other saturnian moons.According to Jacqueline Mitton and Ralph Lorenz, Titan is submitted to tides 400 times the tides that one encounters on Earth. But Titan, like our moon, always turn the same face towards Saturn. As a result, the variations of level of an hypothetic ocean on Titan would be around 9% depending on the changes of distance from Saturn ( a consequence of the elliptical orbit ).We have also to take into account the gravitational influence of the other moons that orbit around Saturn.All those phenomena are likely to shake the material in the interior of the satellite so that an active volcanism can be sustained. If there is a Io like volcanism on Titan,the temperature near the soil might be higher than expected (-180°c) and allow the presence of liquids like ammonia or water in certain places.The abundance of nitrogen in the atmosphere ( up to 95% of the gas layer) could come, to a large extent, from ammonia rich ice or ammonia rich oceans heated by lava flows allowing ammonia to boil.Ammonia would then be disintegrated or broken up by the action of ultraviolet light at a given altitude, releasing nitrogen particles. The presence of methane in the atmosphere could also be the outcome of interactions between carbon dioxide ( CO2 ) and ammonia (NH3): Both molecules are expected to be frozen at the surface: contrary to Venus and Mars, the CO2 gas is almost absent in the atmosphere: it seems obvious that the atmosphere is certainly too cold to enable the CO2 to be in its gas form: let's recall that below -78°c, the carbone dioxide is frozen: so, any probe could photograph a landscape covered with frozen carbon dioxide (white like snow).On this carbon dioxide soil could evolve glaciers of ammonia and in places where the temperature is extreme ( below -180°c),lakes of hydrocarbon ( CH4, C2H6, C2H2...) could be encountered.The action of volcanic eruptions and lava flows could play a key role for the development of the atmosphere ( in any case, in the lower atmosphere).If there is indeed a volcanism on Titan, what kind of volcanism is it ? one idea advanced by scientists is that there might be what is called a cryovolcanism: the volcanoes eruptions might be very different from what we encounter on Earth: the material ( or lava ) thrown out would not be molten silicate rock but icy material ( cryolava resulting from cryomagma), a blend of water and ammonia for example. What would the volcanoes look like? Rock volcanoes take a variety of forms, depending on the properties of the lava, that is its gas content and its viscosity. If the volcanoe is gassy, with high pressure and viscous lava, it will be explosive. On the other hand, a low viscosity magma without much gas pressure will be quiet ( common in Hawaii). On Venus, the lava is very viscous but does not cool down very rapidly. Submitted furthermore to a very high atmospheric pressure, the volcanoes take the form of pancake shaped domes.Jeff Kargel led experiments to guess the shape of hypothetic volcanoes on Titan: these experiments suggest that ammonia spiked cryolavas would be quite viscous.With low gravity ( a gravity seven times weaker than on Earth ), it seems that volcanoes on Titan could also take the form of pancake shaped domes.If there are lakes and seas on Titan, one can imagine that numerous volcanoes may erupt under those lakes and seas: and what happens if a cryomagma column encounters a pocket of methane? an explosive eruption follows?


Geysers and Other Natural Phenomena on Titan ?

Looking for liquid sources, we might find it in geysers which lie in places where the geothermal heat flow is much higher than the planetary average. On Earth, one encounters those phenomena in Iceland, in New Zealand or in the Yellowstone National Park ( USA ).The molecule expelled is water: a column of water is heated very quickly reaching the boiling point and the vapour pressure resulting from it causes the eruption. Can we encounter geysers at the surface of other planets or satellites? The moon of Neptune, Triton, has shown that geysers of nitrogen were present.Hence, geysers are not a particularity of our blue planet and it is easy to think that Titan is likely to harbour geysers.Those geysers would certainly be different from the geysers we commonly encounter.Indeed, the nature of the geysers will be closely linked to the surface temperature: if the environmental temperature is around -180°c ( that is -292° F or 93.15 K ), methane can be in its liquid form and ammonia,water and carbon dioxide are frozen.In this case, geysers would rather consist of methane or ammonia than water or carbon dioxide.In those extreme conditions,a liquid like water thrown out by the geyser would freeze too rapidly. On the other hand, a liquid like methane or ethane projected by the geyser would rather remain liquid.The greater the volatility of the liquid thrown out by the geyser, the greater the chance of the liquid to fall as a liquid.That's the reason why we favour geysers of ammonia, ethane or methane on Titan. The geysers might be more powerful on Titan than on Earth because of the low gravity (a little weaker than the gravity of our moon): a similar underground pressure upwards on Titan and on Earth doesn't undergo the same gravitational pull: as a result, the flow can go up higher even if the atmospheric pressure is one and a half times higher than on Earth.

What about the erosion process on Titan? Winds, rivers, rainfall, rockclash and meteors are the main sources for the formation of eroded landscapes and sand dunes. Most specialists believe that rainfall is much less erosive on Titan than on Earth because the drops fall much more slowly in the weak gravity. Moreover, a droplet of methane is less dense than a droplet of water and the frequency of rainfall is expected to be far lower on Titan than on Earth. The erosive power of winds is also expected to be weaker.As we've already said, two main factors can be shown up to explain slower winds: there is very little sunlight to drive the winds and the rotation speed of Titan is relatively slow. Rivers of methane might be less erosive than rivers of water on Earth since the flow is limited by the weak gravity and the slow winds.


A Human Experience on Titan

We would feel seven times lighter on Titan than on Earth and walking might be a little like walking on our moon or like walking at the bottom of a swimming pool. As pointed out Ralph Lorenz and Jacqueline Mitton,we would also have to support a pressure of 1.5 bars and an air four times denser than air on our own planet.Contrary to what one could imagine, a sprint on Titan would be hard because every leap takes a longer time than on Earth to be carried out.It needs less energy but the frequency of contacts with the ground is weaker because of the weak attraction that delays the reception of every leap.New synchronisation movements would have to be developed in order to increase the running speed.To a certain extent, running on Titan might be a little like running on a giant trampolin except that every landing of every leap would be slower than on Earth. Skiing on Titan might be not as fun as on Earth: less force is available to overcome the static friction as the gravity is lower, the weight of the individual is weaker and the air is thicker. Nevertheless, a snow of hydrocarbon like ethane might allow a better slide than snow of water because of the physical properties.As noticed Ralph Lorenz, Titan put together favorable conditions to learn to fly: we are lighter and the air is denser.As a result, the air is easier to push against. With large artificial wings, it might be possible to take off and fly few seconds with a weak risk of injury.Sailing on Titan might be less hard than on Earth but slower because of a weaker density of the liquid ( ethane rich, methane rich, ammonia rich ) and slower winds. Liquids such as methane or ethane have a density about 600 kg by cube metre or around 2/3 that of water. So, is it possible to swim in a liquid of this density without sinking?In fact, it will depend on the way we move: the problem is to be able to swim fast enough to generate some lift to compensate your extra weight. It has been imagined that the combination of a slightly less dense fluid and a much lower gravity enables you to push yourself about half way out of the water like a dolphin.Furthermore, Titan would be a great place to play basketball: a professional basket player might be able to take off his feets by more than five metres.The dunks would be an easier task than on Earth as he would have a higher jumping power and more time to adjust his dunks. How would it look like to play tennis?At the beginning, it would be certainly more difficult to adapt than basketball.The ball would be very light ( a little more than the weight of three ping pong balls).Your racquet would also be very light ( a little more than the weight of a tennis ball ).In spite of a denser air, the ball could be propelled straight at a higher speed than on Earth: the fastest serve of Andy Roddick ( 153 mph ) would be a middling serve on this moon.This game would become less a game of strength and more than ever a game of accuracy: slices and top spin would become more than on Earth key elements to avoid unforced errors.We would often be obliged to jump to hit the ball after the huge rebounds. Any planet with a gravity lower than on Earth, such as Titan or Mars, makes the balls more volatile and less controlable.A good means of checking that idea is to go to the mountain where the gravity is a little lower and the air thinner and play tennis: you would see the difference of feeling with a ball apparently lighter and that flies more than at the sea level.Obviously, if we set up a colony on Titan, we would undoubtedly adapt the sports to the local physical conditions.

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