Life on Titan ?

Over the past few years, the scientific community has realized that numerous forms of life evolve in extreme conditions on Earth.For instance, we found out bacteria being able to support temperatures up to 113°c and living in boiling water of thermal sources. We also found fishes living in the abyss of oceans and supporting pressures 500 times higher than the atmospheric pressure.Another example: the well known Yellowstone National Park ( USA ) is filled with billions of bacteria living in water whose temperature is around 70°c.

The intensive exploration of the most inhospitable places on Earth has shown that what we call now the extremophiles are capable of living without sunlight or oxygen.Where there is not much oxygen and no sunlight, one can encounter blind spiders or other insects with no pigment and being able to draw their energy from the mixture of hydrogen and sulfur released by volcanic chimneys.All those observations push us into thinking that any exotic life is a possibility in other hostile worlds like Europa,Titan or even Jupiter.

Titan seems to be a very hostile sphere, notably in terms of temperature.Our temperature model derived from the Stefan's law shows that the surface temperature is expected to be around -114°c ( 159K or -173°F) on the basis of an atmospheric pressure of 1.5 bars.Hence, the atmosphere is likely to generate a greenhouse effect that increases the environmental temperature by about 74°c.It's unsufficient to allow the presence of liquid water on the surface.Now, liquid water appears to be the essential solvent from which life has emerged.But, on Titan, contrary to the other bodies in the Solar System, a liquid of hydrocarbon ( ethane, methane...) or ammonia might be present at the surface, perhaps allowing the development of organic molecules, amino acids,proteins or DNA.It is often said that Titan puts together the ingredients of the primitive soup, the chemistry that led to life on Earth. A famous laboratory experiment was carried out in the 1950's by Stanley Miller to try to figure out the secrets of the origin of life.He submitted a mixture of methane ( CH4), hydrogen ( H2), ammonia (NH3) and water vapour ( H2O) to electric sparks for many days in a closed system. The sparks were supposed to represent lightning in the early stages of our planet and the previous molecules were believed to represent the major components of the early Earth's atmosphere.After several days of electric sparks, he obtained organic compounds ( HCN, HCHO...) and amino acids which are used to make proteins.Did this process occured on Titan? It's hard to say but one thing is striking : the resemblance between Titan's atmosphere and Earth's atmosphere.The atmosphere of those two bodies is mostly composed of nitrogen ( 78% for Earth and around 90% for Titan ) contrary to Venus and Mars which are mostly composed of carbon dioxide.Nevertheless, Titan is very poor in oxygen meaning that there is no life breathing oxygen or using oxygen in the feeding process.


A form of life on Titan?

If there is life on Titan, what might it look like? An answer implies that we analyse the basis and the environment of life on Earth and that we try to imagine the elements of the Titan crust.On Earth, water acts not only as a solvent for the living creatures to combine, it is also the major compound of life herself.For instance, plants and jelly fishes have an abundance of water higher than 90% of their weight. Human beings are also mostly composed of water in a proportion of around 65%.To sum up, it is hard to conceive that life can be envisaged without the water molecule.Likewise, it is also hard to admit that we can encounter a form of life which is not based on the carbon element since it is present in large amounts in every species on Earth.Carbon is the second major constituent of life: it represents around 30% of the human weight and calcium, magnesium and other oligo elements are in a very small proportion.Another particularity of life is that it is always based on DNA ( Deoxyribo Nucleic Acid ) except that one knows one exception.The fact that water is the most common molecule on Earth leads us to say that life has emerged by using what was the most common molecule. However, carbon is relatively rare in our environment while it is abundant in every living creature. Carbon has chemical and physical properties that enable it to combine very well with oxygen, hydrogen or water which is less the case for Silicon. That is the reason why an element so rare in the crust of our planet has contributed to build the most complex molecular structures in nature.

On Titan, the most fundamental differences with Earth are the gravity and the distance to the Sun.Most scientists believe that there would be no life on Earth if the sphere were 5% farther from the Sun or 1% closer to the Sun.Too far from the Sun and the planet would be too cold for life to evolve and too close and our planet would resemble Venus. But Titan is so much different that the theories that apply to Earth may not work for Titan.Titan presents a major advantage of being protected from the noxious solar radiations by a very thick atmosphere largely denser than our own atmosphere.On this point, it is a less hostile environment for any living creature to evolve than Mars, for example.Nevertheless, as we've said, the temperature on Titan might be below -100°c erasing the probability of discovering liquid water in some place though thermal sources are likely to raise this possibility.With a solvent like ethane or methane, one can imagine a completely different chemistry of life from what we know.The species would not absorb oxygen like human beings or carbon dioxide like plants as carbon dioxide is frozen and oxygen is very rare.According to Marc Lafferre and Guilhem Cournot, one might encounter living creatures absorbing hydrocarbon gas ( methane, ethane...) which are relatively common in the atmosphere ( around 5% of the atmosphere's composition) and releasing hydrogen which will recombine again with nitrogen or hydrocarbon molecules in the atmosphere to form other hydrocarbon molecules or ammonia under the action of ultraviolet light that splits the molecules.Hence we could see lakes or oceans of hydrocarbon ( ethane, methane...) with glaciers of ammonia rich ice resulting from a snow of ammonia falling from time to time.Living species would consist mostly of hydrocarbon ( methane...).So, the major elements of life would be carbon and hydrogen.The oxygen would be present in a smaller proportion in the structure of life because of the low environmental temperatures that favour combinations with carbon and hydrogen rather than hydrogen with oxygen or carbon with oxygen which are frozen on the surface of Titan ( frozen carbon dioxide and frozen water ).The french chemist André Brack in 1993 pointed out that a form of life based on silicon could be envisaged : the species would be mostly composed of silicon, an element very abundant in the soil ( around 25% of the composition of the crust on Earth ).In other words, the creatures would be made of sand.They would breathe oxygen or hydrogen and would release silicon dioxide ( Si O2 ) or Si H2. Silicon like carbon has the ability to create bonds simultaneously with four other elements and to give birth to multiple molecules. However, the bonds are particularly rigid compared to carbon.As a result, the species would be very limited and their development and adaptation capabilities would be slown down. The main advantage of this form of life would be to resist higher environmental temperatures.On Titan, the low temperatures diminish the probability of encountering this hypothetical form of life.On the other hand, Venus could have harboured that kind of life because of the very hot environmental temperatures.Furthermore, on Titan, a form of life involving the chemistry of nitrogen and ammonia appears less credible since carbon doesn't play a key role in this configuration.

If the distance plays a crucial role for the appearance of life, the gravity of the saturnian moon plays a key role for the way life will develop and evolve.The low gravity of Titan allows creatures to be thinner with thin bones if they have bones.To sustain their own weight, they don't need to have powerful legs or paws.The fish moves in a liquid less dense than water.So, he has no interest of being heavy and short according to the Archimedes' Law. As said Ralph Lorenz, Titan is a heaven for hypothetical birds: they benefit from a low gravity and a thick air.They can be heavier than on Earth and their wings can be shorter which is also better in a cold environment.A denser air enables birds to carry out fewer movements of the wings to advance at the same speed.

It seems, today, that the environmental temperature on Titan is by far too cold to allow the development of an Earth like life, based on water and carbon.But the evolution of the Solar System and the Sun could become more and more favourable for the development of a life on Titan very soon.According to Chris Mc Kay,who studied notably the possibilities of terraforming on Mars,Titan might become warm enough, over the next five billion years, to host life. The Sun is expected to cool down very progressively and in a few billion years, it will become a red giant, a less energetic star, extending far beyond the orbit of our planet.As a result, it will be largely closer to Titan and the amount of energy received by the satellite, though less energetic, will be sufficient to raise the overall temperature so that water appears in its liquid form.Less ultraviolet light will attain the atmosphere of Titan so that the shield of red haze that resulted from the action of those particles and that prevented the light from reaching the soil will disappear and an Earth like meteorology will emerge.The current prebiotic molecules will then be able to combine and evolve towards the development of amino acids, proteins and life.


A model for methane based life

In this model developed by Marc Lafferre and Guilhem Cournot ( 2004 ), the mean temperature at the surface of the satellite is around -114°c ( -173°F or 159K). This temperature is well above the theoritical temperature thanks to a greenhouse effect that raises the temperature by about 73°c (163°F or 346K).This mean temperature is too high to allow the presence of liquid methane despite high pressures that delay the boiling point.On the other hand, seas and lakes of ethane rich hydrocarbon can be observed.Ethane ( C2H6 ) is in its liquid form between -88°c and -183°c.So, the evaporation process is weak. The atmosphere consists mostly of nitrogen and contains a large amount of one of the most powerful greenhouse gases one knows,that is to say methane ( CH4) which represents up to 5% of the atmosphere. The ultraviolet radiations from the Sun engender complex photochemical reactions in the upper atmosphere by breaking the methane molecules ( CH4) into various compounds such as acetylene ( C2H2) or Ethane (C2H6) which combine with other molecules to form tholin,a kind of red sludge.The ethane molecules formed in the upper atmosphere will then fall as rain in small quantities.

Under those circumstances, an ecosystem based on methane has emerged.The solvent that enabled life to develop on Earth, that is water, is replaced here by liquid hydrocarbon. The living creatures have not developed on the basis of water or carbon dioxide since both molecules are frozen at the surface of Titan. In other words, the species are not made up of water ( H2O ) contrary to life on Earth. The key compounds of life on Titan are carbon, hydrogen,methane and ethane.Those elements or molecules are more flexible and more volatile than oxygen and water in the harsh local climate conditions.The creatures absorb the hydrocarbon liquid as we absorb water and most of them, including plants, breathe the methane gas to produce the organic material and expire H2.The birds, for instance, drink "ethane rich hydrocarbon liquid", breathe H2 released by plants and expire CH4.In a sense, that would be the biological cycle of methane.

As the solar energy reaching the Titan soil is very weak at this distance from the Sun, the forms of life prospering on Titan are less pigmented than on Earth.Obviously, we don't encounter animals as colourful as the animals we can see in a tropical area on Earth. Finally, animals tend to have white or slightly red colours.They have developed a biological structure that enable them to store and manage the little energy they receive.Furthermore,they benefit from a low gravity: that's the reason why the skeleton of Titan mammals is generally thinner than that of Earth's mammals.Titan is a paradise for birds: they benefit not only from a low gravity, seven times smaller than on Earth, but also from a dense air which makes possible a smaller frequency in the movement of wings.Thereby, the bird spends less energy to fly though the air is heavier and more difficult to displace.

Hence, Titan appears to be a major source of speculation and research for exobiologists in their quest for the secrets of the origin of life.No doubt that the exploration of Titan will allow us to better understand the chemistry of every planet in the Solar System and perhaps the chemistry that led to life on Earth.

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