From: mightyhugh@yahoo.co.uk   
      
   Titan is unique among all the planetary moons in having a dense   
   atmosphere. This atmosphere has a number of unusual characteristics   
   that it shares with Earth's atmosphere, and which I think are   
   consistent with it being biogenic.   
      
   Titan is the largest moon of the gas giant, Saturn, and is similar in   
   size to Earth’s moon. When the solar system formed (4.6 billion years   
   ago), the planets condensed out of a disk of material swirling around   
   the sun. Near the sun, solar heat vaporised the more volatile   
   components of this accretion disk, leaving behind the nonvolatile   
   (rocky and metallic) components. Hence, the "terrestrial" planets   
   nearest the sun - Mercury, Venus, Earth, Mars - are composed mainly of   
   rock and metals such as iron (the metals are concentrated mainly in   
   the core of these planets). Their inventory of volatile substances   
   (such as water, ammonia, carbon dioxide) is modest or nonexistent.   
      
   At the distance the gas giants including Saturn (and presumably Titan)   
   formed from the sun, volatile substances were present in abundance.   
   The largest component of the accretion disk at that distance was   
   hydrogen, which would have had a tendency to hydrogenate any substance   
   capable of undergoing hydrogenation. Free oxygen would tend to be   
   converted into water, nitrogen into ammonia and carbon into   
   hydrocarbons. Temperatures at these distances from the sun are much   
   lower than they are where the terrestrial planets formed, and here   
   water and ammonia exist as nonvolatile ices rather than liquids or   
   gases.   
      
   Large gas giants such as Saturn have a gravitational field so powerful   
   that they can hoover up and retain even highly volatile, lightweight   
   gases such as hydrogen. With their weak gravitational field, small   
   bodies such as Titan (which is about the same size as Earth's moon)   
   would have trouble attracting and retaining volatile gases. The moons   
   of the gas giants are thus formed mainly of rock, water/ammonia ices,   
   carbonaceous residues and perhaps solid carbon dioxide. Titan is the   
   only one with a substantial atmosphere.   
      
   Titan's atmosphere consists mainly of nitrogen, with a hydrocarbon   
   component (primarily methane). Like Earth's atmosphere, it has a   
   stratosphere, a zone where the atmospheric temperature rises rather   
   than falls with altitude. Earth and Titan are the only bodies in the   
   solar system with an atmosphere having this structure. On Earth, the   
   presence of the stratosphere is the reason we have oceans of liquid   
   water today - if the stratosphere wasn't there, water vapour would be   
   able to reach the upper atmosphere, where it would be rapidly   
   photodissociated and escape into space. On Titan, the stratosphere   
   probably performs a similar role. In fact, methane has a sufficiently   
   low molecular weight that it could probably escape from Titan's feeble   
   gravity even without being photodissociated.   
      
   Our atmosphere is easily recognised as biogenic because it is in a   
   state of chemical disequilibrium. Its two main constituents are   
   nitrogen and oxygen. The most stable state for these two substances is   
   not as separate gases, but as the nitrate ion. In the cores of   
   lightning bolts, temperatures are high enough for oxygen and nitrogen   
   to combine to produce nitric oxide. Nitric oxide then rapidly   
   undergoes further oxidation to form nitrate. This reaction would   
   slowly but surely remove Earth’s atmosphere, if it weren’t for the   
   continual activity of denitrifying bacteria (which convert nitrate   
   back into gaseous nitrogen). Denitrifying bacteria are responsible for   
   the nitrogen in our atmosphere. The other major constituent, oxygen,   
   is produced by photosynthesis. Apart from a minor inert gas component,   
   our atmosphere today is entirely biogenic in origin.   
      
   The atmosphere of Titan would also appear to be in disequilibrium. Its   
   two main constituents are nitrogen and methane. Although not as   
   powerful an oxidising agent as oxygen, nitrogen does nonetheless   
   combine exothermically with hydrogen to form ammonia (in fact I think   
   the reaction 3CH4 + 2N2 -> 4NH3 +3C may be exothermic - can anyone   
   confirm this?). Titan is known to have a "hydrological" cycle based   
   around methane and ethane, so presumably it has storms and lightning   
   bolts as well. Some experiments on the effects of electric discharge   
   through gas mixtures were carried out in the 1950s, AFAIK a lightning   
   bolt passing through a gaseous mixture of nitrogen and methane   
   produces a tarry, nonvolatile residue containing a complex mix of   
   hydrocarbons and amines (and probably some ammonia and hydrogen as   
   well). At Titan’s near-liquid air temperatures, this mixture will be   
   completely solid apart from any hydrogen produced (which will escape   
   to space because Titan’s gravity is too weak to hold it). Titan’s   
   atmosphere would long since have been converted into tar and ammonia   
   ice without some continual process of renewal. In fact, I doubt   
   whether Titan’s gravitational field would be strong enough to capture   
   significant amounts of gaseous nitrogen from the accretion disk   
   present when the planets formed, instead I think it is far more likely   
   to have been produced by the denitrification of ammonia ice. Likewise,   
   methane has such a low molecular weight that I can’t see very much of   
   it being captured from an accretion disk, even if Titan could hold   
   onto it afterward (and I’m not sure that it can without a   
   stratosphere). To me it looks far more likely that the nitrogen and   
   methane are both byproducts of an ongoing process, which have   
   accumulated over time to produce the atmosphere and oceans we see on   
   Titan today. I think that process is very probably life itself - what   
   else could keep regenerating nitrogen and methane unfailingly for   
   billions of years? The data on atmospheric composition from the   
   Huygens probe will be very interesting, since with luck it could find   
   further evidence of chemical disequilibrium, strengthening the case   
   for life. It will be even more interesting if it can analyse Titan’s   
   seas - does anyone know if it can do this?   
      
   If life is present on Titan, it must be completely different from life   
   as we know it. Titan is so cold that water ice is a quartzlike mineral   
      
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