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Is Titan's atmosphere biogenic in origin?
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 there, it certainly couldn’t play an active role in any biological processes. Liquid methane and ethane would have to take the place of water, which I think is quite feasible since hydrocarbons are excellent solvents for a wide range of organic chemicals. The low temperatures would allow a lot of unstable compounds that can’t exist at room temperature to be used, and so could allow a far richer chemistry than is available to organisms living at room temperature. Without catalysis, chemical reactions are extremely slow at these temperatures, which would reduce unwanted side reactions and allow Titanian organisms far better control of their internal chemistry than organisms living at room temperature (it might mean that they have a slow metabolism compared to those on Earth, though). I’d imagine that Titanian life is more likely to be consist of microscopic single-celled organisms than green bug-eyed monsters, after all the only living organisms on Earth for most of its history were bacteria. It wasn’t until there was high levels of free oxygen in the atmosphere that large, highly metabolically active multicellular life forms appeared here. Titan doesn’t have oxygen in its atmosphere, instead its atmosphere closely resembles the anoxic atmosphere of Earth a couple of billion years ago (with all of the CO2 and H2O frozen out of course). I'm looking forward to the results from the Cassini/Huygens mission, they should be very interesting. Does anyone know if any missions are planned to Neptune, which I understand also has lots of nitrogen and methane in its atmosphere (and looked very Earthlike in the Voyager photos)? Hugh |
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Is Titan's atmosphere biogenic in origin?
In article ,
Hugh wrote: 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 there, it certainly couldn’t play an active role in any biological processes. I nitpick only to be polite: water can't exist as a liquid on the surface of Titan, but what about underground, in a subtitanian liquid water zone analogous to Ganymede's hypothetical ocean? Am I remembering correctly that a water-ammonia mix can have a melting point as cold as 175ish K? -- "The keywords for tonight are Caution and Flammable." Elvis, _Bubba Ho Tep_ |
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Is Titan's atmosphere biogenic in origin?
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Is Titan's atmosphere biogenic in origin?
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Is Titan's atmosphere biogenic in origin?
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Is Titan's atmosphere biogenic in origin?
In article ,
Hugh wrote: On 14 Jul 2004 16:51:33 -0400, (James Nicoll) wrote: In article , Hugh wrote: 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 there, it certainly couldn’t play an active role in any biological processes. I nitpick only to be polite: water can't exist as a liquid on the surface of Titan, but what about underground, in a subtitanian liquid water zone analogous to Ganymede's hypothetical ocean? Am I remembering correctly that a water-ammonia mix can have a melting point as cold as 175ish K? You're absolutely right, I'd forgotten that ammonia can greatly reduce the freezing point of water. There could well be a substantial water/ammonia ocean beneath Titan's surface, so there could be subterranean water-based life on Titan. I still like the idea of hydrocarbon-based life in Titan's methane oceans though :-) A little googling shows that not only do some models of Titan have eutectic water-ammonia mixes, with water ice above and a mixture of water-ice and clathrates below but far more depressingly, I knew that at one point and it wasn't all that long ago. I wasn't aware that Titan ever had open seas (first 10^8 years), though. Makes one think... In any case, a world-ocean of ammonia and water could be one of several biomes, and variation with selective pressure might keep a continual process of organisms trying to colonize the colder surface regions. Admittedly there are limits to what v&ns can accomplish. -- Take the piston rings out of my stomach, And the cylinders out of my brain Extract from my liver the crankshaft, And assemble the engine again! [from 'The Dying Aviator'] |
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Is Titan's atmosphere biogenic in origin?
Mordecai wrote:
(Hugh) wrote in message ... gases such as hydrogen. With their weak gravitational field, small bodies such as Titan (which is about the same size as Earth's moon) Titan is not about the same size the same size as earths moon, nor even close. Titan is, in fact, larger than Mercery, and the second largest moon in the solar system. Not trying to be rude, just pointing it out Titan is nearly as large as Mars, with about 75% of its diameter, a bit less than half its volume, and about a quarter of its mass (Titan is made of lighter stuff). |
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