Titan - Is the source of so much Methane being overlooked?

Question

Is the source of so much Methane on Titan being overlooked?

Tons of very intelligent and knowledgeable people already looked at it, so the answer in undoubtedly "no", so I guess my question is more along the line: "why so? I don't understand why there's so much confidence that the source of Methane on Titan is not biological."

Why am I actually asking this?

I remember a huge excitement about Methane having been found on Mars' atmosphere before the excitement waned because the amounts were too low and it didn't appear to repeat (ex: with season). Methane is, I learnt, a pretty volatile molecule that will degrade quickly and, thus, needs to be replenished continuously - any methane found needs to have been produced recently (in geological terms). Additionally, it is a known biological "waste" product of biological processes on earth, hence the initial excitement about the discovery on Mars.

In early periods of life on Earth (Archaean Eon?) the Methane producing "bacteria" (the Archaea methanogens) possibly ruled the Earth, probably producing lots of Methane that would be released into the atmosphere (akin to Titan's atmosphere of nowadays, I would guess). Oxygen-producing life dramatically changed the atmosphere compositions, making it toxic to the methanogens which became relegated to swamps, underground, guts of animals etc (wherever it's oxygen-free) and made all methane react with the oxygen.

Doesn't Titan's atmosphere give strong reasons to believe the hypotheses that there's methane-producing life pumping out all that methane? In a similar way to what might have been Earth's past before the great oxygenation event, allowing life millennia of methane production to end up with a thick atmosphere, and similar to how Earth accrued a strong envelope of Oxygen after millennia of oxygen production?

One of the hypotheses for so much Methane is some geochemical process but .. there's so many bodies in the solar system, from warm to cold, from tiny to large, and yet isn't Titan the only one with (remotely) so much Methane?

Answer 1

It is not being overlooked. There are studies which are being carried out about the feasibility of life on Titan based on methane.

It is true that Titan has all the necessary conditions for life:

• It is not in thermodynamic equilibrium
• It has more than enough of carbon based molecules
• It has a fluid environment (ethane/methane) for chemical reactions to take place
• Its temperature is low enough to permit bonding of molecules (though not like on the Earth)
• It has other requirements for life, such as metals, a changing geology with (hydrocarbon) oceans and rocky surfaces etc.

However, having necessary conditions does not mean that life has arisen or it is there.

The main problem for explaining methanogenic life on Titan is that there is no mechanism that explains its production by process that life as we know is capable of. The most common method of methane production by methonogenic bacteria is:

$$\require{mhchem}\ce{CO2 + 4H2 -> CH4 + 2H2O}$$

However, there is little or no $\ce{CO2}$ in the atmosphere of Titan. There is a well understood methane cycle on Titan; however, the origin and replenishment are still mysteries.

In a way, there is too much methane on Titan compared to other bodies in solar system. The problem with life in Titan is not how it produces methane, rather, how it uses methane. Now we have to develop a way by which lifeforms use methane much like we use water. Though this may be possible, there are some issues with this:

• Titan is very cold. It's surface temperature is around -180°C. The main problem is that we know of no catalyst at this temperature, without which life would not be feasible.

• Water is a very good solvent, helping in transporting substances in a cell. How methane would behave in such circumstances is not very well understood, though a model of nitrogen based cell membrane that could exist in methane has been modeled.

• Though methane producing organisms were once dominant on Earth when it had atmosphere similar to Titan, there is one difference – there is no water vapor on Titan.

• Water has one crucial property – It is denser at ~ 4°C than at its freezing point. This means that while the surface of water may be frozen, the lower part of a liquid body will still be able to support life. Methane has no such property however, and at the low temperatures (on Titan), freezing of whole hydrocarbon lakes can occur, which is not conductive to life.

At present, geochemical processes offer a better explanation compared to biological processes, and as such is accepted by the scientific community. As a matter of principle, scientific community is conservative in accepting such things (having burnt their fingers more than once). Extraordinary claims require extraordinary proofs. In case of extraterrestrial hydrocarbon based life, no such proof has been offered. However, the hunt for life on Titan goes on.

There is a proposed Titan Saturn System Mission; however budget constraints have postponed this mission. In a way, the mission selections are biased towards finding water based habitable planets/moons (which would be directly useful for us) rather than find exotic life.