Methane + no volcanism = life on Titan?

[Ann]

On April 10 this year, bystander posted a NASA JPL-Caltech | Cassini Solstice Mission article summarizing a paper by Jeff Moore et al, claiming that Saturn's biggest moon Titan is shaped by weather, not volcanism, and that Titan may be a geologically dead world.

If so, does that mean that precisely because Titan is geologically dead (if that is the case), the atmosphere, the hydrocarbon lakes or other parts of the surface of Titan probably contain living organisms?

I'm asking because there has been quite a bit of speculation about the methane in Titan's atmosphere. Wikipedia writes this about Titan's methane:

Energy from the Sun should have converted all traces of methane in Titan's atmosphere into more complex hydrocarbons within 50 million years—a short time compared to the age of the Solar System. This suggests that methane must be somehow replenished by a reservoir on or within Titan itself. That Titan's atmosphere contains over a thousand times more methane than carbon monoxide would appear to rule out significant contributions from cometary impacts, since comets are composed of more carbon monoxide than methane. That Titan might have accreted an atmosphere from the early Saturnian nebula at the time of formation also seems unlikely; in such a case, it ought to have atmospheric abundances similar to the solar nebula, including hydrogen and neon.[40] Many astronomers have suggested that the ultimate origin for the methane in Titan's atmosphere is from within Titan itself, released via eruptions from cryovolcanoes.[41][42][43] A possible biological origin for the methane has not been discounted (see below)

So if there is really no cryovolcanism to explain the methane in Titan's atmosphere, is it likely then that the methane was made by living organsims?

Mars, too, has some methane in its atmosphere, although in very small amounts. Wikipedia writes:

Methane has been detected in the Martian atmosphere with a mole fraction of about 30 ppb;[12][98] it occurs in extended plumes, and the profiles imply that the methane was released from discrete regions. In northern midsummer, the principal plume contained 19,000 metric tons of methane, with an estimated source strength of 0.6 kilogram per second.[99][100] The profiles suggest that there may be two local source regions, the first centered near 30° N, 260° W and the second near 0°, 310° W.[99] It is estimated that Mars must produce 270 ton/year of methane.[99][101]

The implied methane destruction lifetime may be as long as about 4 Earth years and as short as about 0.6 Earth years.[99][102] This rapid turnover would indicate an active source of the gas on the planet. Volcanic activity, cometary impacts, and the presence of methanogenic microbial life forms are among possible sources. Methane could also be produced by a non-biological process called serpentinization involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars

I'm asking this because I have read a bit of speculation that methane is a good tracer of life on other planets. And the methane in the atmospheres of Mars and Titan, which are worlds that seem life-friendly for other reasons, has been cited as possible biomarkers there.

My question is how good methane is as a tracer of life. If, for example, NASA manages to build the kind of telescope that can detect and analyze atmospheres on planets in other solar systems, how should we react to the news that the atmosphere of such an exoplanet has been found to contain methane?

Ann

[dougettinger]

I am chemically confused. If methane is turned over every 4 Earth years, what does it become? Do carbon and hydrogen combine with other elements? Why does Titan still have methane? I thought methane was one the primordal molecules of the solar system.

4/19/2011

[BMAONE23]

Methane is CH4
__H
H-C-H
__H
And Oxygen is O2
O=O
These interact with each other by taking 1 methane molecule and 2 Oxygen molecules (O2)
You then have
1 Carbon
4 Hydrogen &
4 Oxygen

These recombine with the introduction of combustion in an oxygen rich and methane poor environment to create
CO2 and 2-H2O
CHHHH & OO OO=
OCO & HOH HOH
This is the chemical interaction between the two reactants Methane and Oxygen
Any form of combustion will cause this reaction

Methane (CH4) is a molecule in which 4 hydrogen atoms are bonded to one carbon atom. If you have a gas stove, lighting the stove causes the methane to react with oxygen in the atmosphere to release heat and the atoms recombine to form carbon dioxide and water vapor.

The Methane on Titan is likely stable due to the extreme lack of oxygen

[dougettinger]

Excellent answer. Thanks.

4/19/2011

[Colin Robinson]

Whether methane is a biomarker depends on what sort of environment you find it in. Are conditions such that any methane tends to break down, or get transformed into something else? If that's the case, yet methane is still present, then it must be replenished from somewhere. Living organisms and volcanism are two possible sources, but not necessarily the only ones.

As BMAONE23 mentioned, methane on Earth is destroyed through oxidation. That would not happen on Titan. But Ann's quote from WP referred to another process that destroys methane -- photolysis -- molecules being broken up by energy from the sun, which is known to occur in Titan's upper atmosphere. Not only do methane molecules get broken up (leading to formation of more complex organics), also some of the hydrogen that was in them escapes into space.

Living things on Titan's surface could put some of the lost methane back together by combining organic molecules with hydrogen defusing down from the upper atmosphere. It would make biological sense for them to do that, because they would derive energy from the reaction. All the lost methane could be restored in this way, except for that inconvenient fact that hydrogen atoms are escaping from the atmosphere. It would be a bit like trying to do a series of small jigsaw puzzles, while pieces were being thrown out of the window!

So logically, whether or not Titan has living things, its atmospheric methane must have been replenished quite recently from some subsurface reservoir. If it didn't happen by means of volcanism, it may have happened some other way. For instance by a meteor strike, massive enough to melt lots of ice and release methane from clathrates. One fairly radical suggestion is that Titan's entire atmosphere was formed fairly recently, by a sudden event of this nature.

All things considered, I think Titan is more likely to have life if it does have volcanoes than if it doesn't. That way, new pieces for the jigsaw puzzles are getting thrown into the playroom all the time.