News from Titan

[Colin Robinson]

.. Just goes to show that 'reasonable assumptions' don't make a lot of difference when it comes to actually measuring the reality of the situation, eh ?

What subsequent discussion has shown, is that conclusions which seem reasonable to some planetary scientists may be questioned by others.

Here is what Chris McKay says about the question of isotopic ratios:

...it is impossible to predict any isotopic effect that this life might have on C. On Earth, methanogens produce CH4 from CO2+H2, or from organic material derived from CO2. The net reaction is CO2 + 4H2 => CH4 + 2H2O and thus methanogens on Earth are a net source of CH4 in a world of CO2. The enzymes that mediate these reactions create methane with a large isotopic enrichment of 12C over 13C of ~5%. On Titan, it has been predicted that methanogens would produce CH4 by C2H2 + 3H2 => 2CH4 (eg. McKay and Smith 2005). This is obviously not a net source of CH4: it merely recycles CH4, thereby undoing the photolysis of CH4 and there is no a priori reason to expect the resulting CH4 to exhibit an isotopic shift from these reactions.

Source: Cassini: Making Sense of the News

The paper you quote mentioned the "Fischer–Tropsch reaction of the H2 with CO2".
How relevant this is, would depend what levels of CO2 and H2 there are in the bulk composition of Titan. If there is lots of CO2 subsurface, does that imply that any subsurface bodies of liquid water would be expected to contain lots of dissolved CO2? That would mean a more earthlike chemical situation subsurface than at the surface and in the atmosphere…

Now what might they be ?

For instance, life-forms which, like us, are based on complex carbon chemistry, but which flourish at a much lower temperature than Earth life, and in an atmosphere where the predominant carbon compound is not CO2.

[Selfsim]

What subsequent discussion has shown, is that conclusions which seem reasonable to some planetary scientists may be questioned by others.

No consensus = no mainstream, eh? (smiley face).

Here is what Chris McKay says about the question of isotopic ratios:

Source: Cassini: Making Sense of the News

The paper you quote mentioned the "Fischer–Tropsch reaction of the H2 with CO2".
How relevant this is, would depend what levels of CO2 and H2 there are in the bulk composition of Titan. If there is lots of CO2 subsurface, does that imply that any subsurface bodies of liquid water would be expected to contain lots of dissolved CO2? That would mean a more earthlike chemical situation subsurface than at the surface and in the atmosphere…

It appears to be just a cursory note (added perhaps for completeness ?) ... The reference link (37) leads to a paper discussing FTT synthesis around Earth's hydrothermal vents, which takes place at 390 degrees C and 400 bars. I would think Cassini would have picked up something to support such conditions, if it existed.

Interestingly, from the Huygens' GCMS spectrographic readings, the abundance of atmospheric CO₂ at the surface, is of (roughly) the same order of magnitude as the Ar⁴⁰ and C₂H₆ (Ethane), and yet in the text, they say that CO₂ has only been 'tentatively' identified. I don't get the 'tentative' in this statement (??). Admittedly, the absolute values of all three are only tiny (tens of parts per million) .. but I don't see that justifying the term 'tentatively'.
I guess the idea of CO₂ existing as a part of the bulk would be for testing the idea that Titan may have started out life along the lines of a fizzy cometary body. If subsurface liquid water can exist, so can dissolved CO₂.
Also interesting, is that CO exists in the overall atmosphere, (averaged), at roughly the same levels as the Ethane and yet this doesn't seem to have turned up in the GCMS readings (?)

Overall in this instance, I'd have to agree that McKay's warblings make more sense .. but why would 'methanogens' ever exist in the first place ? What are 'methanogens' ? And what relevance would Earth-based metabolic processes have on Titan, if the speculated version of life is completely alien as far as Earth-life is concerned ? Why worry about water based hydrates on a moon which is dominated by Nitrogen and Methane ? How can this possibly have anything to do with the evolution of speculated 'methanogens' ? Surely as far as speculation goes, either there's methane based alien life or, (somehow), there's Earth-like water based life ? If one scientist proposes ammonia-water based clathrates, (due to tidal bulging and density measurements), and another (McKay) is on about an alien methane based 'world', where are the common boundaries for agreement/consensus (or mainstream) in all this ? What effect does ammonia have on the evolution of water-based life ? (Kills it stone dead, if you ask me !)

This is why this 'life' speculation nonsense leads nowhere .. the only way to establish constraints on human's wild imaginings is to produce real data. Up until that happens, its all just speculation .. good for only sci-fi !

For instance, life-forms which, like us, are based on complex carbon chemistry, but which flourish at a much lower temperature than Earth life, and in an atmosphere where the predominant carbon compound is not CO2.

Well why would they ever be 'like us' and exhibit anything even vaguely like our metabolism ?? Other than being constrained by a paucity of data and imagination, why assume that such life is at all viable in the first place ?

Regards

[Noclevername]

Well why would they ever be 'like us' and exhibit anything even vaguely like our metabolism ?? Other than being constrained by a paucity of data and imagination, why assume that such life is at all viable in the first place ?

"Paucity of data" is the key phrase. We know carbon-based life exists. We have zero evidence that any other kind is even possible.

[Selfsim]

Well why would they ever be 'like us' and exhibit anything even vaguely like our metabolism ?? Other than being constrained by a paucity of data and imagination, why assume that such life is at all viable in the first place ?

Actually I should clarify: we are constrained by a paucity of data and and over-abundance of imagination.
I highly doubt that we have a paucity of imagination .. (smiley-face)

[Selfsim]

"Paucity of data" is the key phrase. We know carbon-based life exists. We have zero evidence that any other kind is even possible.

Right … so until we discover non-carbon based life, its just a moot point … so Chris McKay has raised a moot point .. terrific contribution!

Cheers

[Noclevername]

Right … so until we discover non-carbon based life, its just a moot point … so Chris McKay has raised a moot point .. terrific contribution!

Re-reading it, the article quoted from specifically mentioned complex carbon compounds as a basis for hypothetical Titanian life. It is that chemistry that McCay was referring to. My contribution was not so great. D'oh!

[Selfsim]

I find it difficult to reconcile the measured readings with the intent of the article.

According to the GCMS Huygens readings (Fig 1) , H₂ had a count readings of:

~ 10^4.1 at 120 to 130 kms altitude;
~ 10⁴ at 75 to 77 kms altitude;
~ 10⁵ at ground level.

So we have a depletion of about one order of magnitude over 130 kms column depth, and that's of a gas which forms 2,000 parts per million (ppm) of the avg atmosphere. Where is the 'loss' of H₂ at the surface ?

There were no significant counts of acetylene (C₂H₂) at any measured altitude. At best, acetylene's average atmospheric proportion is a mere ~4 ppm^{Footnote 1}, and can occur due to photodissociation of methane, (at high altitudes), but is still well within the detection range of GCMS. If it was there, it would have been detected .. and so too, would any depletions with altitude. Where is the acetylene ? Where is its hypothesised depletion (??). Surely it just simply isn't at the initially assumed altitudes and in significant amounts ?

It seems McKay is saying the 'depletion' occurs when the expectation/prediction wasn't reflected in the readings .. well, so what ? Surely that just means the initial model, resulting in the expectation, was inaccurate … how does that justify invoking a liquid methane based life-form ?

Ethane (C₂H₆) was only significant at ~10³ counts at ground level, falling off to ~10² at 120 to 130 kms. Ethane forms a mere 20 ppm of the net atmosphere, so this depletion doesn't seem particularly 'significant' to me.

Where is the significant 'depletion' of ethane and acetylene, which is the focus of the McKay article ?:

The depletion of ethane and acetylene become significant in the astrobiological sense because of this latest report of a hydrogen flux into the surface. This is the key that suggests that these depletions are not just due to a lack of production but are due to some kind of chemical reaction at the surface.

I'd have to read further to fully understand what this is about but as usual, McKay seems to have adopted an initial, hypothetically consistent model as reality, and then starts invoking new biogenic processes to justify the model.

Why not just review the assumptions of the initial model in the light of the new readings, as opposed to barrelling along, assuming his initial hypothesis about the existence of a methanogenic life-form (speculated in a 2005 paper), is a better explanation than an initially inaccurate model, (which was based on remote Earth-based measurements) ?

This just doesn't add up to me.

Regards

Footnote 1: The measured value of acetylene came from ground based spectroscopic radiance measurements (ISO/SWS/Grating - 1997). The production of the acetylene is assumed to be ~65 kms. Why did the GCMS instrument not find at least some at the 75 to 77 kms range, if the range and amount assumptions are valid ?

[Colin Robinson]

No consensus = no mainstream, eh? (smiley face).

Well, if scientists agreed about everything, that would be more like a monolith than a mainstream!

.. but why would 'methanogens' ever exist in the first place ? What are 'methanogens' ?

Methanogens are organisms whose metabolism produces methane (CH4). They are generally found in conditions where there is little or no free oxygen (O2). They get energy by combining hydrogen with molecules that contain carbon.

Why worry about water based hydrates on a moon which is dominated by Nitrogen and Methane ?

Titan's atmosphere is dominated by nitrogen and methane. Planetary scientists are interested not only in the atmosphere, but also in what's underneath the atmosphere -- the surface, the subsurface, the bulk composition.

It is actually difficult to make sense of Titan's atmosphere without also considering the subsurface, because the atmosphere is not a closed system -- hydrogen atoms are constantly escaping into space...

And what relevance would Earth-based metabolic processes have on Titan, if the speculated version of life is completely alien as far as Earth-life is concerned ?

The "speculated version of life" is not "completely alien", because theoretical discussion of possible life on Titan is based on familiar principles of chemistry and thermodynamics.

The sort of question that has been considered by David Grinspoon, Chris McKay and others is this -- are there sources of energy on Titan which could support organisms such as microbes?

The thing is, a microbe without an energy source would be the equivalent of a perpetual motion machine. That really would be something "completely alien", from the point of view of mainstream science.

This is why this 'life' speculation nonsense leads nowhere .. the only way to establish constraints on human's wild imaginings is to produce real data. Up until that happens, its all just speculation .. good for only sci-fi !

Selfsim, a couple of days ago you said...

My point is that if we are truly interested in exploration, we need to focus on keeping an eye out for geological mechanisms we haven't observed before... I'm sure there will be many other physical phenomena we haven't seen yet, but nonetheless still occur, and may or may not be common in a given search space.

My point is that if we are truly interested in exploration, we should also keep an eye out for chemical and biological mechanisms we haven't observed before.

To do this, it is necessary to develop conceptual models of what might be happening, and then test the conceptual models for their internal consistency, consistency with basic scientific principles, and consistency with fresh data as it comes in...

I find it difficult to reconcile the measured readings with the intent of the article...Where is the significant 'depletion' of ethane and acetylene, which is the focus of the McKay article ?

What do you think the "intent of the article" is? Is McKay saying he himself has found evidence for depletion of ethane, acetylene and hydrogen? Or does he refer to recent papers by other scientists on these points -- scientists such as Roger Clark and D.F.Strobel -- and go on to discuss a number of different hypotheses which are compatible with those findings?

[Colin Robinson]

I'd have to read further to fully understand what this is about

True.

but as usual, McKay seems to have adopted an initial, hypothetically consistent model as reality, and then starts invoking new biogenic processes to justify the model.

I suggest it might be better to wait until you do fully understand the argument, before you make this sort of criticism of one of the scientists involved.

Why not just review the assumptions of the initial model in the light of the new readings,

Actually, that is "just" what McKay and other scientists have been doing...

Since they found that levels of ethane on the surface of Titan were not as their model predicted, they have been reviewing the initial model and trying to work out specifically where it was wrong.

There are a number of logical possibilities:

1. Conceivably, rate of ethane production in Titan's upper atmosphere is a lot less than expected.
2. Another possibility is that the rate of ethane production is as expected, but the process has not been happening for as long as the model assumed, because the current atmosphere of Titan emerged comparatively recently.
3. A third possibility is that ethane does not accumulate at the surface in the expected amounts because it gets broken down into methane again due to a catalysed reaction with hydrogen.

The third possibility is consistent with D.F.Strobel's findings in his 2010 paper about hydrogen diffusion in Titan's atmosphere. Strobel's paper is based on data from the Cassini orbiter about hydrogen levels very high in the atmosphere, as well as on data obtained by Huygens as it descended.

[Colin Robinson]

What effect does ammonia have on the evolution of water-based life ? (Kills it stone dead, if you ask me !)

Ammonia is poisonous to some organisms, but certainly not to all. Right here on Earth, there are species that can tolerate high levels of ammonia. See
Siegel SM Life and the outer planets 1. Performance of terrestrial organisms in ammonia-rich systems (published in the journal Life Sciences and Space Research)

[Selfsim]

Methanogens are organisms whose metabolism produces methane (CH4). They are generally found in conditions where there is little or no free oxygen (O2). They get energy by combining hydrogen with molecules that contain carbon.

Methanogens are liquid water based archaea. Water is a polar solvent. Life chemistry is critically dependent on solubility. No liquid water exists on the surface of Titan. Titan is dominated by non-polar solvents and tholin solids. McKay conflates the term 'methanogen' with a 'possible' new hypothesised Titanian life form, which is based on liquid methane, which is also a poor solvent. Why would one assume that a second abiogenesis based on liquid methane, would miraculously result in a process which produces methane by C₂H₂ + 3H₂ => 2CH₄ ? What is the reason for thinking that it would ? How could that reaction work in a non-solvent environment ?
The methaogenic archaea on Earth adapted and evolved from our common water-based ancestor to do what it does in an O₂ depleted environment. What would be the basis for assuming the Titanian life form would end up performing the same process, which the Earth methanogens 'worked out' by adaption utilising liquid polar water based catalytic solvents over long periods ?
H₂ should be depleted if the above reaction is occurring. From the Huygens probe data, it isn't.
It is actually in greater abundance at the surface .. the opposite effect from what a 'metabolising ground-based Titantian methanogen' would have.

Other than McKay and Smith's hypothetical and imaginative speculations, I cannot remotely envisage why such a lifeform could even remotely function in the same generalised way as Earth-life ... why would it ?

Titan's atmosphere is dominated by nitrogen and methane. Planetary scientists are interested not only in the atmosphere, but also in what's underneath the atmosphere -- the surface, the subsurface, the bulk composition.

It is actually difficult to make sense of Titan's atmosphere without also considering the subsurface, because the atmosphere is not a closed system -- hydrogen atoms are constantly escaping into space.

.. Well let's see now .. hydrogen is a light element. It shoud be escaping in huge quantities from the atmosphere to space. And yet, the Huygens readings measured more at the surface (where McKay's Titanian life form supposedly consumes it). But the H₂ is supposedly caused by photolysis in the upper atmosphere .. which subsequently ecapes. So, either the upper altitude H₂ is being pushed back down to the ground, or its leaking out of the ground (perhaps somehow from the bulk subsurface liquid water-ammonia clathrates). But the surface is frozen solid. How can H₂ be produced from frozen solid ground ? Cassini hasn't ever spotted any thermal hot-spots ...
It seems to me the ground H₂ is more likely to be being transported by some solid tholin precipitation effect from higher altitudes.
Either way, I don't get why McKay keeps saying that H₂ is being depleted at the surface ? This is not evident from the Huygens' data.

The "speculated version of life" is not "completely alien", because theoretical discussion of possible life on Titan is based on familiar principles of chemistry and thermodynamics.

McKay's own definition of 'alien' is life which emerged completely independently from Earth's.
Everything in the observable universe is related via the known fundamental laws. Does this then mean that nothing in the observable universe is 'alien' ?

The sort of question that has been considered by David Grinspoon, Chris McKay and others is this -- are there sources of energy on Titan which could support organisms such as microbes?

To me, this is putting the cart before the horse. Just because there may be a source of free energy, does not imply the existence of life everywhere there is available free energy.

The thing is, a microbe without an energy source would be the equivalent of a perpetual motion machine. That really would be something "completely alien", from the point of view of mainstream science.

No, a microbe without an energy source would be a dead microbe !

...
My point is that if we are truly interested in exploration, we should also keep an eye out for chemical and biological mechanisms we haven't observed before.

To do this, it is necessary to develop conceptual models of what might be happening, and then test the conceptual models for their internal consistency, consistency with basic scientific principles, and consistency with fresh data as it comes in...

The 'conceptual models' are meaningless without an empirical evidence basis. There is no data with which to build a viable non-Earth based biological model. (Show me one which works ...)

I find it difficult to reconcile the measured readings with the intent of the article...Where is the significant 'depletion' of ethane and acetylene, which is the focus of the McKay article ?

What do you think the "intent of the article" is? Is McKay saying he himself has found evidence for depletion of ethane, acetylene and hydrogen? Or does he refer to recent papers by other scientists on these points -- scientists such as Roger Clark and D.F.Strobel -- and go on to discuss a number of different hypotheses which are compatible with those findings?

I was asking for clarification of the term 'depletion'. This is the term introduced by McKay. He has not clarified in the article exactly what it means. From the Huygens measurements, there is almost zilch acetylene (required as the primary energy source for his Titanians), more H₂ (the secondary energy source for his Titanians), and more ethane at the surface than at altitude.
Quite possibly, it may just be that this particular article has been dumbed down for publication and may have left out key context points (??).

Can you provide a link to the 2010 Strobel/Clark et al model paper ?

[Colin Robinson]

Can you provide a link to the 2010 Strobel/Clark et al model paper ?

Here is a link to Strobel's paper about hydrogen diffusion...

Darrell Strobel Molecular hydrogen in Titan's atmosphere: implications of the measured tropospheric and thermospheric mole fractions

No, a microbe without an energy source would be a dead microbe !

Well, here is a point we agree on. Microbes cannot live without an energy source.

Then there is the further question of what counts as an energy source for microbes. As I understand it, the laws of thermodynamics mean that living things (just like functioning machines) require an energy gradient, a situation where energy can flow. "Gibbs free energy" is one of the concepts used to describe what is thermodynamically necessary...

It's also worth noting that very different systems can be kept in motion via similar chemical reactions. For instance, bacteria are not much like motor cars, but both cars and bacteria can get energy by oxidizing petroleum.

Why would one assume that a second abiogenesis based on liquid methane, would miraculously result in a process which produces methane by C₂H₂ + 3H₂ => 2CH₄ ?

It depends what you mean by "assume". If you mean something like "believe to be true" or "believe to be almost certain", then I don't know of any scientist who is "assuming" (in that sense) that abiogenesis has happened on Titan.

However, the word "assume" has another, quite different sense, of using a proposition as a starting point for a line of reasoning. You can "assume" something, in this sense, without believing it to be true or even likely.

It is a matter of asking questions like: "IF this were true, what consequences would logically follow?"

One reason for doing this, is that the logical consequences can sometimes falsify the starting point.

For instance, someone could reason as follows: "If there are microbes living on moon X, they would die without a source of Gibbs free energy. However, moon X lacks any source of Gibbs free energy. Therefore, any microbes on moon X are likely to be dead..."

That may be one reason why McKay and others have considered whether or not there are thermodynamically viable energy sources on Titan.

The answer is yes: the fact that Titan's atmosphere contains hydrogen molecules (H₂), as well as compounds such as acetylene and ethane, means that an organism — or, for that matter, a machine — could obtain energy by catalysing a chemical reaction.

McKay conflates the term 'methanogen' with a 'possible' new hypothesised Titanian life form,

The suffix -gen comes from Greek "genes", meaning "producer of". A methanogen is, literally, a producer of methane, just as a pathogen is a producer of disease. If Titan has organisms producing methane, then it has methanogens.

which is based on liquid methane, which is also a poor solvent.

It's true that methane is a weaker solvent than water. Which means an organism would need to use proportionally more solvent to do the same amount of chemistry...

The methaogenic archaea on Earth adapted and evolved from our common water-based ancestor to do what it does in an O₂ depleted environment.

Actually, they may not have needed to "adapt" to low-O₂ conditions.

A few billion years ago, when life on Earth was getting started, our planet as a whole was a low-oxygen environment. Later, after O₂ started to build up in Earth's atmosphere, certain organisms adapted to tolerate and use the O₂. See Wikipedia: Great Oxygenation Event.

[Selfsim]

Here is a link to Strobel's paper about hydrogen diffusion...
Darrell Strobel Molecular hydrogen in Titan's atmosphere: implications of the measured tropospheric and thermospheric mole fractions

Thanks for that .. much appreciated.
Rather complex. It still seems to stand against McKay's statements about 'depletion of H2'. It isn't depleted. Its still equally 'abundant' at ground level as at higher altitudes (?!?!).

I think the story they're trying to weave is:
i) H₂ formed from the irreversible photolysis of CH₄, escapes in large volumes at high altitudes.;
ii) the mole fractions of H₂ at high altitudes and ground level are the same. (McKay predicted a big difference .. which was in error by 2 orders of magnitude).
iii) (ii) above, is now being interpreted (mostly by McKay) as implying that in order to maintain the same H₂ mole fractions in both locations, there must be a corresponding loss (depletion) of H₂ at ground level by some kind of process (he's suggested biological);
iv) McKay also argues that the (comparative) absence of acetylene could be for the same reason (ie: biology eating it for energy).
v) Strobel has modelled a flux to sustain the uniform gradient in H₂ mole fractions in both locations .. (this results in an H₂ 'hose' effect pouring H₂ from high altitudes down to ground level .. to maintain the balance for the high altitude losses. (Perhaps tholin precipitation transports it to the ground (??)).
iv) The question is how is the volume of CH₄ sustained, given that it is constantly destroyed at altitude ? The obvious answers are either something at ground level produces it chemically by combining reversibly, the H₂ with C (requiring energy .. where does this come from? .. ie: catalysis, chemical reaction, etc), or it is leaking out of the ground, already formed ? The original theorised ubiquitous 2 CH₄ => C₂ + 4H₂ reaction is not viable, when taking the measurements into account.

I hope I got all that right .. its a pretty sketchy and complex model though … filled with assumptions. McKay is still not abandoning his hypothetical 'methanogenic life form' though, in spite of there being no acetylene, no change in C₁₂/C₁₃ isotope ratios .. and no measured depletion of H₂. He says he's not prepared to abandon his ideas just yet, (not surprisingly).

That may be one reason why McKay and others have considered whether or not there are thermodynamically viable energy sources on Titan.

The answer is yes: the fact that Titan's atmosphere contains hydrogen molecules (H₂), as well as compounds such as acetylene and ethane, means that an organism — or, for that matter, a machine — could obtain energy by catalysing a chemical reaction.

Titan's atmosphere contains insufficient acetylene (almost none). The presence of H₂ means nothing without a viable catalyst, and the 2 CH₄ => C₂ + 4H₂ reaction everywhere, is not viable. (Reference Strobel).

The suffix -gen comes from Greek "genes", meaning "producer of". A methanogen is, literally, a producer of methane, just as a pathogen is a producer of disease. If Titan has organisms producing methane, then it has methanogens.

Semantics, I think...

It's true that methane is a weaker solvent than water. Which means an organism would need to use proportionally more solvent to do the same amount of chemistry…

Huh ?
Solubility is a function of attraction and association between solute and solvent ionic molecules. Dissolution is dependent on the degree of the relative polar or non polar natures of the solute and solvent molecules respectively, and is thus a property of the compounds involved .. the proportions of the quantities would not be particularly relevant. Methane lacks the polarity of water, and thus would not be a good transporter of polar molecules within a cell. AFAIK, life is critically dependent on transport of polar molecules within cells. I can't see any life processes functioning without a physical intra-cell transport mechanism.

Titan is either pre-biotic annoxic, or liquid water-based 'living'. Ya just don't seem to be able to have it both ways.
This term 'methanogen' combines a bunch of processes emerging from liquid water based life (from Earth's present-day environment), with another bunch of hypothetical processes which would be physically at odds with those functions, specifically because of the non-solubility of its base chemistry (??)

Earth's methanogens (archaea) do their thing by:CO₂ + 4 H₂ → CH₄ + 2 H₂O ... so where is the residue H₂O on Titan's surface produced by the hypothetical methanogens ? Everyone agrees there isn't any surface water (and has never been measured).

Either one basis one's assumptions on Earth-life .. or one doesn't. McKay's mix/match inference approach, just results in inconsistency.

Regards

[Colin Robinson]

Thanks for that .. much appreciated.

I'm glad that it's of interest.

I hope I got all that right ..

I don't think that you did get all of it right...

It still seems to stand against McKay's statements about 'depletion of H2'. It isn't depleted. Its still equally 'abundant' at ground level as at higher altitudes (?!?!).

Look, in the very first paragraph of his introduction, Strobel mentions the following figures.

In the troposphere/tropopause region,
Voyager IRIS measurements yielded an inferred H2 mole fraction
of 0.00112 ± 0.00016... At very high altitudes the INMS found the H2 mixing ratio to be 0.00405 ± 0.0003

The figures 0.00112 and 0.00405 are different by a factor of four. Not equally abundant at all! Strobel then goes on to mention reasons why this difference is notable, and to consider what is going on.

Because you've missed this point, the rest of what you've said about Strobel's paper is basically beside the point.

Earth's methanogens (archaea) do their thing by:CO₂ + 4 H₂ → CH₄ + 2 H₂O ... so where is the residue H₂O on Titan's surface produced by the hypothetical methanogens ?

There is hardly any CO₂ in Titan's atmosphere. The hypothesis of McKay and others is that organisms on Titan do their thing by combining H₂ with compounds such as acetylene (C₂H₂) and ethane (C₂H₆). This reaction does not produce any H₂O.

The question is how is the volume of CH₄ sustained, given that it is constantly destroyed at altitude ? The obvious answers are either something at ground level produces it chemically by combining reversibly, the H₂ with C (requiring energy .. where does this come from? .. ie: catalysis, chemical reaction, etc), or it is leaking out of the ground, already formed ?

As Strobel mentions (page 7, under the heading "Concluding remarks"), the reaction between H₂ and the "less saturated, more complex hydrocarbons" (i.e. ethane, acetylene etc) is an "exothermic process". That means the reaction does not require energy, it releases energy.

It would, however, require a catalyst, certainly...

[Selfsim]

Look, in the very first paragraph of his introduction, Strobel mentions the following figures.

In the troposphere/tropopause region,
Voyager IRIS measurements yielded an inferred H2 mole fraction
of 0.00112 ± 0.00016... At very high altitudes the INMS found the H2 mixing ratio to be 0.00405 ± 0.0003

The figures 0.00112 and 0.00405 are different by a factor of four. Not equally abundant at all! Strobel then goes on to mention reasons why this difference is notable, and to consider what is going on.

Because you've missed this point, the rest of what you've said about Strobel's paper is basically beside the point.

Well, I didn't miss this point … and your judgement of 'beside the point' is purely an opinion, and doesn't change the query on the flawed assumptions used by McKay for hypothesising a 'methanogenic' Titanian lifeform.

The separation between the two measurement readings is 1000kms distance. Strobel concludes that either the readings are inconsistent, or their understanding of the CH₄ and H₂ chemistry is flawed. The Huygens and Cassini readings seem to support the uniform 0.001 figure, from the troposphere right down to the surface.

Strobel points out that there's a measured difference between the Cassini CIRS and the initial Voyager INMS measurements (in the troposphere and lower stratosphere) but:

The infrared experiments on Voyager and Cassini have consistently converged on the tropospheric H2 mole fraction to be 0.001 (Courtin et al., 1995, 2008; Samuelson et al., 1997; although Samuelson et al. (1981) initially derived 0.002 ± 0.001). The H₂S(0) probes the lower troposphere at ~10 km, whereas the S(1) samples the lower stratosphere at ~70 km, and no gradient in the H₂ mole fraction between these two regions has been reported or expected.

However:

The confirmation of the IR results by Niemann et al. (2010), coupled with the long lifetime of H₂ in the atmosphere, validates that the near surface H₂ mole fraction is 0.001 with an uncertainty of ~25%. Note that Courtin et al. (2008) find this value applies over the region 40°N to the south pole and hence is effectively the globally averaged magnitude.

So, we have a globally averaged near surface H₂ mole fraction which is the same as the Cassini and Voyager measured tropospheric figure of 0.001.

Whilst, it is true that:

Without any adjustment in either or both the CIRS and INMS H₂ mole fractions, they jointly require a downward flux of H₂ into Titan’s surface at a rate on the order of 10²⁸H₂ s^-1, comparable to the H₂ escape rate.

So, in other words, the downward flux in Strobel's model is about the same as the escape rate in the upper atmosphere, which effectively maintains the equilibrium mixing value of the H₂ mole fraction whilst fitting the INMS thermosphere data.

He then goes on to say:

In fact, McKay and Smith (2005) discuss this possibility in the speculative context of methanogenic life on Titan. They state ‘‘If life is consuming atmospheric hydrogen it will have a measurable effect on the hydrogen mixing ratio in the troposphere if the biological consumption is greater than 10⁸ cm^-2 s^-1”. From the calculations in Figs. 6 and 7, the Cassini-Huygens H₂ measurements imply that the downward H₂ surface flux is ~2 x 10¹⁰ cm^-2 s^-1, in substantial excess of their threshold value, but unlike their calculations the surface H₂ mole fraction is 0.001 in contrast to their off-scale value of much less than 10^-5.

So McKay etal's originally derived value, (supporting the 'methanogenic life' posit), was off by two orders of magnitude which would seem to be a huge amount, (compared with the 4 times difference between the two measurements when dealing with a posited widespread, surface dwelling lifeform model (??) )

The INMS figure was taken at such high altitudes, I doubt that it can be reasonably taken as a baseline for inferring 'depletion' at the surface. I mean, the H₂ is light, it floats, it escapes .. its relative concentrations at high altitudes will clearly be higher. MacKay modelled a variation in the mixing ratio in the troposphere .. and yet now he seems to be pointing to a thermosphere measurement as the baseline for inferring consumption of H₂ at the surface (??)

Doesn't make much sense to me ..
I guess the overall story will pan out eventually, but it looks like McKay's hypothesis is skating on very thin ice. I saw a lecture he gave on this exact Titan topic (2010 ?) and even then, he was basically saying Enceladus is now his focus for speculated life (not Titan) ... this overall approach seems very much like 'pin-the-tail-on-the-donkey' (??)

For me, most of what he comes up with, has some superficial basis in scientific physics and chemistry, but also contains a huge amount of sci-fi, which he frequently acknowledges, and even 'sends up' on occasions. This is not what I call 'quality' science. Speculation needs to be held accountable, if it is being portrayed as scientific speculation, and part of that accountability, is public acknowledgement of specific errors/overestimations of basic critical quantities.

The question is how is the volume of CH4 sustained, given that it is constantly destroyed at altitude ? The obvious answers are either something at ground level produces it chemically by combining reversibly, the H2 with C (requiring energy .. where does this come from? .. ie: catalysis, chemical reaction, etc), or it is leaking out of the ground, already formed ?

As Strobel mentions (page 7, under the heading "Concluding remarks"), the reaction between H₂ and the "less saturated, more complex hydrocarbons" (i.e. ethane, acetylene etc) is an "exothermic process". That means the reaction does not require energy, it releases energy.
It would, however, require a catalyst, certainly...

.. and, with this particular mechanism, the catalyst, provides the additional energy for getting over the energy barrier, thereby providing the 'additional energy' I mention in my quote above.

Overall, this is not a simple set of readings to explain (which seems to be acknowledged by all parties). I'm not necessarily saying Strobel's model is right, either. However, McKay's hypothetical Titanian life is really struggling to find any semblance of plausible supporting evidence. Circulation in the atmosphere and/or geochemistry between the atmosphere and liquids on the surface, or subsurface emissions are clearly the most obvious.

Overall knowledge of how all this works is still in its infancy .. even in Earth's case. Explaining Titan may well be beyond our abilities at present.

(Just for completeness: I might add that the previously discussed assumption that complex chemistry on other 'worlds' can be taken as inferring life, is what is under test here with Titan .. )

Regards

[Colin Robinson]

The Huygens and Cassini readings seem to support the uniform 0.001 figure, from the troposphere right down to the surface.

But the troposphere is the part of the atmosphere closest to the surface.

The INMS figure was taken at such high altitudes, I doubt that it can be reasonably taken as a baseline for inferring 'depletion' at the surface. I mean, the H₂ is light, it floats, it escapes .. its relative concentrations at high altitudes will clearly be higher.

The tendency of light gases to rise and heavy ones to fall is countered by diffusion -- the tendency of randomly moving molecules to spread out. Diffusion is something Strobel considers in detail in his paper.

So, in other words, the downward flux in Strobel's model is about the same as the escape rate in the upper atmosphere, which effectively maintains the equilibrium mixing value of the H₂ mole fraction whilst fitting the INMS thermosphere data.

If there is a downward flux, a net downward migration of H₂ molecules, logically the concentration of H₂ in the lower atmosphere increases steadily over time, until the downward flux stops happening...

Unless something is removing (depleting) H₂ from the lower atmosphere.

So McKay etal's originally derived value, (supporting the 'methanogenic life' posit), was off by two orders of magnitude which would seem to be a huge amount, (compared with the 4 times difference between the two measurements when dealing with a posited widespread, surface dwelling lifeform model (??) )

The "off-scale value" (as Strobel calls it) is one of a number of conceivable (not yet observed) effects which McKay and Smith argued might be observable if Titan had life. The most extreme hypothetical figure was based on the conjecture that microbes on Titan are as prolific and fast-metabolizing as those on Earth. We don't know that would be the case, nor has McKay ever claimed to know.

Nonetheless, if Strobel's findings about hydrogen diffusion are right, then something is depleting H₂. Perhaps not fast enough to win a gold medal in the microbial Olympics hydrogen depletion event, but fast enough to make a difference.

Organisms are not the only possible depleter of hydrogen, but they are a possibility which Strobel himself thought important enough to mention in the abstract of his paper, as well as in the paper itself.

The measurements imply that the downward H₂ surface flux is in substantial excess of the speculative threshold
value for methanogenic life consumption of H₂ …

Strobel's words, not McKay's...

I'm not necessarily saying Strobel's model is right, either.

It is quite possible that someone will publish a paper refuting Strobel's findings. As far as I know, that hasn't happened yet...

[Colin Robinson]

(Just for completeness: I might add that the previously discussed assumption that complex chemistry on other 'worlds' can be taken as inferring life, is what is under test here with Titan .. )

That sort of topic is one thing under test, yes. Fortunately Titan is much nearer than the exoplanets, so research about Titan doesn't have to be limited to chemistry. If it turns out that there really are complex catalytic agents there, their morphology can be studied by robotic devices with microscopes.

Whether there is life there or not, future robot missions to Titan are likely to bring a substantial scientific return. As a second instance of a world with complex and active organic chemistry, Saturn's largest moon may provide important clues about how abiogenesis happens; or, perhaps, in some cases, does not happen.

Regards
Colin