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Colin Robinson
2012-Jul-26, 12:08 AM
Latest news from Titan

* Surface methane lakes not only at the poles
Tropical lakes on Saturn moon Maggie McKee at Nature.com (http://www.nature.com/news/tropical-lakes-on-saturn-moon-could-expand-options-for-life-1.10824)
* Tidal bulging of Titan's bulk shape -- evidence for subsurface water ocean
Tides turn on Titan Sid Perkins at Nature.com (http://www.nature.com/news/tides-turn-on-titan-1.10917)
* Enigmatic surface erosion -- spindly river valleys, resembling those in areas on Earth recently affected by volcanos, glaciers.
River Networks on Saturn's Largest Moon at Science Daily (http://www.sciencedaily.com/releases/2012/07/120720103543.htm)

To sum up -- seems to be lots more liquid that previously known, more stuff moving about, recent re-shaping of surface...

I am reminded of David Grinspoon's "living worlds hypothesis": the view that a planet or moon which is geologically and meteorologically alive is more likely (than a less active world) to be alive biologically as well. There are a number of reasons this makes sense. The simplest reason is that geological and meteorological activity involves both liquids and flows of energy -- both important for life. There are also subtler arguments, to do with the way organisms and their environment shape one another (co-evolve)...

We still don't know whether there is life on Saturn's biggest moon. The question, for those interested in life beyond Earth, is whether Titan should get a closer look?

primummobile
2012-Jul-26, 11:13 AM
Latest news from Titan

* Surface methane lakes not only at the poles
Tropical lakes on Saturn moon Maggie McKee at Nature.com (http://www.nature.com/news/tropical-lakes-on-saturn-moon-could-expand-options-for-life-1.10824)
* Tidal bulging of Titan's bulk shape -- evidence for subsurface water ocean
Tides turn on Titan Sid Perkins at Nature.com (http://www.nature.com/news/tides-turn-on-titan-1.10917)
* Enigmatic surface erosion -- spindly river valleys, resembling those in areas on Earth recently affected by volcanos, glaciers.
River Networks on Saturn's Largest Moon at Science Daily (http://www.sciencedaily.com/releases/2012/07/120720103543.htm)

To sum up -- seems to be lots more liquid that previously known, more stuff moving about, recent re-shaping of surface...

I am reminded of David Grinspoon's "living worlds hypothesis": the view that a planet or moon which is geologically and meteorologically alive is more likely (than a less active world) to be alive biologically as well. There are a number of reasons this makes sense. The simplest reason is that geological and meteorological activity involves both liquids and flows of energy -- both important for life. There are also subtler arguments, to do with the way organisms and their environment shape one another (co-evolve)...

We still don't know whether there is life on Saturn's biggest moon. The question, for those interested in life beyond Earth, is whether Titan should get a closer look?


I think it should get a closer look just because it's there. But why do they assume that the global ocean is made from water?

Noclevername
2012-Jul-26, 11:21 AM
I think it should get a closer look just because it's there. But why do they assume that the global ocean is made from water?

Per wikipedia,

Based on its bulk density of 1.88 g/cm3, Titan's bulk composition is half water ice and half rocky material. {snip}
Titan is likely differentiated into several layers with a 3,400 km rocky center surrounded by several layers composed of different crystal forms of ice.[22] Its interior may still be hot and there may be a liquid layer consisting of a "magma" composed of water and ammonia between the ice Ih crust and deeper ice layers made of high-pressure forms of ice. The presence of ammonia allows water to remain liquid even at temperatures as low as 176 K (−97 °C) (for eutectic mixture with water).[23] Evidence for such an ocean has recently been uncovered by the Cassini probe in the form of natural extremely-low-frequency (ELF) radio waves in Titan's atmosphere. Titan's surface is thought to be a poor reflector of ELF waves, so they may instead be reflecting off the liquid–ice boundary of a subsurface ocean.[24] Surface features were observed by the Cassini spacecraft to systematically shift by up to 30 km between October 2005 and May 2007, which suggests that the crust is decoupled from the interior, and provides additional evidence for an interior liquid layer.[25]

primummobile
2012-Jul-26, 11:55 AM
Per wikipedia,

Wow. Guess I could have looked that up!

mutleyeng
2012-Jul-26, 02:39 PM
I dont think there is a question about it being worth a look.
Unfortunately we have to prioritize.
Best as I can tell the astrobiologist hit list is
Mars (easiest rather than most likely)
Enceladus
Europa
Titan

Colin Robinson
2012-Jul-26, 11:16 PM
I think it should get a closer look just because it's there. But why do they assume that the global ocean is made from water?


Per wikipedia,

Yes, it seems to be mainstream science that Titan's bulk composition is largely H2O. The question has been how much if any of the H2O is liquid rather than solid, and whether subsurface materials ever find their way to the surface thru cryovolcanism.

The Wikipedia article on Titan mentions arguments against existence of a subsurface ocean, developed by planetary geologist Jeffrey Moore. Moore argued in 2008 that Titan's interior is too cold for water to behave like lava, so the H2O stays solid, and there are no current or recent cryovolcanos.

Since then Cassini has found a feature named Sotra Facula, which looks like a cryovolcano, but hasn't necessarily removed all doubts. The recent findings about tidal flexing are a further argument against Moore's model...

Colin Robinson
2012-Jul-26, 11:56 PM
I dont think there is a question about it being worth a look.
Unfortunately we have to prioritize.
Best as I can tell the astrobiologist hit list is
Mars (easiest rather than most likely)
Enceladus
Europa
Titan

Each of these places has one or two features that make it interesting to astrobiologists.

However, Titan has a whole great list of interesting features: the organic chemistry taking place in the upper atmosphere, the methane lakes and rivers, water behaving like lava... Not to mention the evidence for stuff decomposing at or near the surface.

How can it belong at the bottom of your list?

mutleyeng
2012-Jul-27, 12:51 AM
The order of my list is based on my assessment of priority I have heard from the astrobiologists. Chris mckay put it in that order, ive heard someone else swap a couple around, but 2 and 3 are always mentioned when asked where they want to go look. It dosnt mean minds cant have changed of course - but I can only base it on what i have heard.
I guess the others are thought could have conditions where life as we know it could exist. Titan would be something radically different.

Selfsim
2012-Jul-27, 01:34 AM
Yes, it seems to be mainstream science that Titan's bulk composition is largely H2O. The question has been how much if any of the H2O is liquid rather than solid, and whether subsurface materials ever find their way to the surface thru cryovolcanism.

The Wikipedia article on Titan mentions arguments against existence of a subsurface ocean, developed by planetary geologist Jeffrey Moore. Moore argued in 2008 that Titan's interior is too cold for water to behave like lava, so the H2O stays solid, and there are no current or recent cryovolcanos.

Since then Cassini has found a feature named Sotra Facula, which looks like a cryovolcano, but hasn't necessarily removed all doubts. The recent findings about tidal flexing are a further argument against Moore's model...I'm back for a fleeting comment:
The first linked article by Maggie McKee says:

Besides Earth, Titan is the only solid object in the Solar System to circulate liquids in a cycle of rain and evaporation, although on Titan the process is driven by methane rather than water.It goes on to speak of liquid methane forming liquid hydrocarbon 'lakes and seas' (ie: NASA's: Lunine and Griffith).

The second linked article quotes Lunine who speaks of tidal flexing, but again, he is only talking about liquid hydrocarbons … not liquid water.

The third article speaks only of liquid hydrocarbons carving out geological structures, similar to what water does on Earth. There is no evidence cited about liquid water.

As far as I know, there has never been any water directly detected on Titan. There was a reflectometry measurement done on a rock on the surface (by Huygens) which gave light reflection measurements consistent with ice … this is not evidence of water. There were no spectroscopic readings of water by Huygens, either.. as a matter of fact, absence of water in the spectra taken, requires further explanation.

AFAIK, the only 'evidence' of water is: (i) the planet density figure calculation, and; (ii) other indirect readings, which can sometimes infer the presence of water, but these do not rule out other explanations either. When last I looked, planetary density calculations do not constrain the conclusion to only water/rock mixtures. In the case of Titan, there still exists the possibility of the presence of subsurface hydrocarbons (liquid and/or clathrate-like compounds).

There is nothing mainstream about water existing on Titan. The problem seems to be the high frequency usage of the terms: 'rivers, lakes, seas and oceans', all of which conjure up the Earth-biased view of these being caused by water. This is mainly journo spin and, perhaps, frequently misinterpreted Astrobiologist speculations.

There is lots of spectroscopically confirmed water ice on other moons/rings, within the vicinity of Titan (eg: Enceladus), but this does not mean that Titan retains any significant water, in any particular state.

Regards

Colin Robinson
2012-Jul-27, 01:36 AM
The order of my list is based on my assessment of priority I have heard from the astrobiologists. Chris mckay put it in that order, ive heard someone else swap a couple around, but 2 and 3 are always mentioned when asked where they want to go look. It dosnt mean minds cant have changed of course - but I can only base it on what i have heard.

At the Astrobiology Science Conference 2010, they had a debate on the topic "Titan versus Europa - Potential for Astrobiology".


I guess the others are thought could have conditions where life as we know it could exist. Titan would be something radically different.

Yes, that has been the argument. But it's based on the premise that Europa and Enceladus show more evidence of subsurface liquid water, compared to Titan. If Titan had no liquid water, then yes, any life there would have to be rather different from Earth life, in that all living things on Earth use liquid water as their solvent. But now evidence is increasing that Titan actually does have liquid water beneath its surface, why would life there need to be to be any more radically different than life on/in Europa or Enceladus?

Colin Robinson
2012-Jul-27, 03:01 AM
I'm back for a fleeting comment:
The first linked article by Maggie McKee says:
It goes on to speak of liquid methane forming liquid hydrocarbon 'lakes and seas' (ie: NASA's: Lunine and Griffith).

The Maggie McKee article, linked from the opening post, is about liquid hydrocarbons, yes. Did anyone suggest otherwise?


The second linked article quotes Lunine who speaks of tidal flexing, but again, he is only talking about liquid hydrocarbons … not liquid water.

The second linked article, from Nature.com, is Sid Perkins' summary of very recent work by Luciano Iess, a planetary scientist at the Sapienza University of Rome. The article states:


Data gathered by NASA’s Cassini probe as it repeatedly swept past Titan, Saturn’s largest moon, offers the best evidence yet that the smog-swaddled satellite has a substantial ocean of water sloshing beneath a thick icy crust.

The third linked article makes the following comparison between terrain on Titan and on Earth.


Black compared Titan's images with recently renewed landscapes on Earth, including volcanic terrain on the island of Kauai and recently glaciated landscapes in North America. The river networks in those locations are similar in form to those on Titan, suggesting that geologic processes may have reshaped the moon's icy surface in the recent past.

No-one doubts that Titan's surface has been eroded by methane rivers. However, that does not explain the similarities with Kauai volcanic terrain and with North American recently glaciated terrain. Geologically recent cryovolcanism (H2O behaving like a cross between a lava flow and a glacier) is a likely explanation...


When last I looked, planetary density calculations do not constrain the conclusion to only water/rock mixtures. In the case of Titan, there still exists the possibility of the presence of subsurface hydrocarbons (liquid and/or clathrate-like compounds).

Do you mean compounds like methane clathrate, which consists largely of H2O? If not, what specifically do you mean?

John Jaksich
2012-Jul-27, 04:46 AM
I am unsure if anyone has mentioned it, but subsurface methane and water--> may result from physco-chemical processes rather than a biological role----> water under the subsurface may take part in a "pseudo" Fisher-Tropsch synthesis with Carbonates under the surface. So, in short, hydrocarbon lakes are formed through a seepage of "hydrocarbons" from under the "hard" crust. They "may" subsequently solidify or become "rain" --based upon so-called local conditions.

My personal readings tell me that some of this is good science---but much more needs to be done so it may be confirmed or thrown out as "wrong".

Selfsim
2012-Jul-27, 04:51 AM
The Maggie McKee article, linked from the opening post, is about liquid hydrocarbons, yes. Did anyone suggest otherwise?No. But your question is premature as it presumes the presence of sub-surface water … which is far from a 'given' piece of 'mainstream science' ...

The question has been how much if any of the H2O is liquid rather than solid, and whether subsurface materials ever find their way to the surface thru cryovolcanism.
...


The second linked article, from Nature.com, is Sid Perkins' summary of very recent work by Luciano Iess, a planetary scientist at the Sapienza University of Rome. The article states:Data gathered by NASA’s Cassini probe as it repeatedly swept past Titan, Saturn’s largest moon, offers the best evidence yet that the smog-swaddled satellite has a substantial ocean of water sloshing beneath a thick icy crust.This is what Sid Perkins is saying … not what the scientists who did the study are saying.


No-one doubts that Titan's surface has been eroded by methane rivers. However, that does not explain the similarities with Kauai volcanic terrain and with North American recently glaciated terrain. Geologically recent cryovolcanism (H2O behaving like a cross between a lava flow and a glacier) is a likely explanation…The term 'Cryovolcanism' (http://en.wikipedia.org/wiki/Cryovolcanism) is not limited exclusively to water flows, and it in no way can be used to infer that liquid water has caused the erosion. Anything liquid under those conditions, could have caused the erosion … and does.


Do you mean compounds like methane clathrate, which consists largely of H2O? If not, what specifically do you mean?No .. I mean what I said … Clathrate-like compounds. (http://en.wikipedia.org/wiki/Clathrate)

A clathrate hydrate, in particular, is a special type of gas hydrate in which a lattice of water molecules encloses molecules of a trapped gas. .. that's not the only clathrate compound.

Colin .. I'm not going to argue further with you about this.
Titan is an interesting moon, and is a good candidate to study in order to find out how other compounds behave on a moon having 0.14g, -180oC, and an active atmosphere.

John Jaksich
2012-Jul-27, 05:50 AM
I thought I would throw this into the fray---->

Here is the latest proposed (schematic) model for Titan: (Credit for schematic is : A. Tavani---NASA)

Selfsim
2012-Jul-27, 06:38 AM
I thought I would throw this into the fray---->

Here is the latest proposed (schematic) model for Titan: (Credit for schematic is : A. Tavani---NASA)
Hi John;
Is there more to this than an artist's graphic ? Who/what is A. Tavani ?
(References please ?).
Cheers

Colin Robinson
2012-Jul-27, 07:15 AM
This is what Sid Perkins is saying … not what the scientists who did the study are saying.

OK, here is a link to a NASA press release, with a direct quote from the scientist who led the study
Cassini Finds Likely Subsurface Ocean on Saturn Moon (http://www.jpl.nasa.gov/news/news.cfm?release=2012-190)


Data from NASA's Cassini spacecraft have revealed Saturn's moon Titan likely harbors a layer of liquid water under its ice shell...
"Cassini's detection of large tides on Titan leads to the almost inescapable conclusion that there is a hidden ocean at depth," said Luciano Iess, the paper's lead author and a Cassini team member at the Sapienza University of Rome, Italy. "The search for water is an important goal in solar system exploration, and now we've spotted another place where it is abundant."


Colin .. I'm not going to argue further with you about this.

Selfsim, if you wanted to demonstrate that Titan doesn't have subsurface liquid water, it's not me you'd need to argue with, it's Luciano Iess and his colleagues...

Colin Robinson
2012-Jul-27, 07:47 AM
I am unsure if anyone has mentioned it, but subsurface methane and water--> may result from physco-chemical processes rather than a biological role----> water under the subsurface may take part in a "pseudo" Fisher-Tropsch synthesis with Carbonates under the surface. So, in short, hydrocarbon lakes are formed through a seepage of "hydrocarbons" from under the "hard" crust. They "may" subsequently solidify or become "rain" --based upon so-called local conditions.

My personal readings tell me that some of this is good science---but much more needs to be done so it may be confirmed or thrown out as "wrong".

John, I'd basically agree with you that methane could be produced by chemical reactions beneath the surface, and later seep up into the surface lakes and the atmosphere... Or maybe no chemical reaction was needed, because there has been methane in some form inside Titan since Titan was formed.

This is not necessarily an argument against Titan having biology, though.

The crucial question is not how the methane got there, but what happens after photolysis reactions in the upper atmosphere turn some of the methane into other, more complex organic compounds. Does something happen that breaks down (decomposes) those more complex organics back into methane again? There is some reason to think so -- to begin with, the first thing Huygens found when it landed, was much less ethane on Titan's surface than predicted by a model without any decomposition...

It is also true that break-down of complex organics, if it is in fact occurring, could be due to a non-living catalyst. It is difficult to distinguish on chemical grounds between an organism and a catalyst, because, looked at from a chemical point of view, metabolism is catalysis.

Nonetheless, if you are looking to find live microbes, a mass of decomposing organic material is the logical place to look.

John Jaksich
2012-Jul-27, 11:11 AM
Hi John;
Is there more to this than an artist's graphic ? Who/what is A. Tavani ?
(References please ?).
Cheers


As with all NASA images--it falls within the public domain. I don't personally know the illustrator, but it is (?) safe to assume that he/she works for NASA.

The article and associated website are:


http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20120628.html


Further information can be found at:


http://www.nasa.gov/cassini




or




http://saturn.jpl.nasa.gov/

mutleyeng
2012-Jul-27, 01:34 PM
Yes, that has been the argument. But it's based on the premise that Europa and Enceladus show more evidence of subsurface liquid water, compared to Titan. If Titan had no liquid water, then yes, any life there would have to be rather different from Earth life, in that all living things on Earth use liquid water as their solvent. But now evidence is increasing that Titan actually does have liquid water beneath its surface, why would life there need to be to be any more radically different than life on/in Europa or Enceladus?

All i can say is should the mainstream consensus consider this to be the case, then sure, Titan will have a stronger case. Practicality also comes into the equation though. Enceladus has what seems to me to be a huge advantage that it is venting its guts from dirty great cracks in the ice. That makes it a more likely target than Europa for me. So far as Titan is concerned, if the case is made then I am all for it

Colin Robinson
2012-Jul-27, 08:59 PM
All i can say is should the mainstream consensus consider this to be the case, then sure, Titan will have a stronger case.

Luciano Iess' findings about the tidal flexing and the subsurface water ocean have been published in the mainstream journal Science.


Practicality also comes into the equation though. Enceladus has what seems to me to be a huge advantage that it is venting its guts from dirty great cracks in the ice. That makes it a more likely target than Europa for me.

Yes, Enceladus still offers us that advantage -- it has the most accessible body of liquid water beyond Earth...

mutleyeng
2012-Jul-27, 09:45 PM
Luciano Iess' findings about the tidal flexing and the subsurface water ocean have been published in the mainstream journal Science.
.

Thats not the same thing as being the mainstream consensus. Time will tell on that

Selfsim
2012-Jul-27, 11:11 PM
What we have here, (with Iess et al), is a team which has developed a working model. I have no problems in reviewing a working model with a critical eye .. and in this case I don't care what the qualifications are, of those who developed it. In this case, the issue is: 'Where/what is the evidence ?'

At present, the evidence for an internal liquid water ocean is circumstantial. Having said this, the development of a model which does have a liquid water ocean, is as good as any other circumstantially based model constrained by the same physical parameters measured by Cassini, Hubble, Huygens, Keck, ESA scopes etc.

IMO, a working planetary model, which targets an explanation for the makeup of a remote moon, does not justify the use of definitive terms implying that it represents reality. I feel strongly suspicious that Iess' model serves the sole purpose of swaying opinion in the direction that would result in the further exploration of Titan, and I don't buy it.

In the absence of independently verifable, direct spectroscopic measurements of water at or on the surface, its scientific implications as far as prebiotic Earth-like life are concerned, are zilch. Even if this were detected, it still means zilch scientifically (life-wise), but data gathering via exploratory probes, is the only way we can move the issue along, so it makes sense to proceed with probe missions in the future.

Having said all of this, it also seems strange that the heated Huygens probe, did not result in the vaporisation of, and subsequent detection of, surface cryovolcanic by-products (such as the hypothesised water). The spectroscopic data retrieved from Huygens, showed absolutely no signs of the presence of surface water, in any detectable quantities (!).

If interested in the scientific side, for a good summary on how the various environmental cycles are thought to work on Titan, which fairly denotes the various unknowns, see here. (http://lasp.colorado.edu/~bagenal/3720/CLASS23/23Titan.html) (Note that spectroscopically measurable water vapour exists in the stratosphere .. but that's not surprising, given the amount of water being ejected by Enceladus, and other Cassini-detected water within the general vicinity).

Cheers

Noclevername
2012-Jul-27, 11:23 PM
I feel strongly suspicious that Iess' model serves the sole purpose of swaying opinion in the direction that would result in the further exploration of Titan, and I don't buy it.

And how are we going to find evidence for/against anything without further exploration?

Selfsim
2012-Jul-27, 11:57 PM
And how are we going to find evidence for/against anything without further exploration?Titan is an interesting moon, and is a good candidate to study in order to find out how other compounds behave on a moon having 0.14g, -180oC, and an active atmosphere.
...
… so it makes sense to proceed with probe missions in the future.I agree with the exploration of Titan to gather more data ... (and always have) !!
Regards

Colin Robinson
2012-Jul-28, 06:39 AM
At present, the evidence for an internal liquid water ocean is circumstantial. Having said this, the development of a model which does have a liquid water ocean, is as good as any other circumstantially based model constrained by the same physical parameters measured by Cassini, Hubble, Huygens, Keck, ESA scopes etc.

When you say "any other model"... Is there some "other model" that fits the observed facts just as well?

You've repeatedly suggested that Titan may not have water in any form, liquid or solid, except for a bit of atmospheric vapor due to some external source.

What I'd like to know, is whether anyone has developed a radically waterless model of Titan's internal structure – a model with neither liquid water nor water-ammonia mixtures, nor water ice, nor clathrates based on water ice? Is there published scientific literature that sets out such a model? If so, when was it published, and by who?

Selfsim
2012-Jul-28, 08:27 AM
Well, here's an interesting version of the relevant press release (http://www.zmescience.com/research/studies/titan-subsurface-ocean-of-water-23323/) .. I really wish I could get a copy of the Iess paper to find out exactly the logic leading to the conclusion of a specifically water ocean .. unfortunately its behind a paywall.
In the meantime he's an interesting quote by Lunine (from the above release ...):

Spacecraft data show Saturn creates solid tides approximately 30 feet (10 meters) in height, which suggests Titan is not made entirely of solid rocky material; in fact, “[...]the density of Titan is consistent with a body that is about 60% rock and 40% water,” says planetary scientist Jonathan Lunine, of Cornell University, one of the co-authors of the paper. “The ocean must be fairly thick — a few hundred kilometers. And it must lie relatively close to the surface — beginning no farther down than 100 kilometers.”

Of course, it doesn’t necessarily have to be water; computations so far show the presence of a subterranian liquid. For all we know it could very well be covered in methane, however water is the most solid bet.Notice how now its a 'bet' that its water ?? ...


“It’s a matter of what’s abundant in the solar system,” Lunine says. “Water is far and away one of the most abundant substances. Europa, as well its sister moons Ganymede and Callisto are covered with water ice like Titan. Rock and metals are similarly plentiful. All this hangs together to suggest what the interior of Titan is like.”So, now its suddenly become generalised ... and non-specific to Titan ...

Life on Titan

Water, in liquid form, is the first thing scientists are looking for when investigation for signs of extraterrestrial life.So, ET searchers may just have 'water-the-brain', eh ?
In this case, on Titan the presence of a subsurface layer of liquid water is not itself an indicator for life. Scientists think life is more likely to arise when liquid water is in contact with rock, and these measurements cannot tell whether the ocean bottom is made up of rock or ice. The results have a bigger implication for the mystery of methane replenishment on Titan.So, as I've said ... a water core is not compulsory !! :)

I accept that the model they've created might be consistent ... I'd prefer to read their paper before commenting further on that aspect ... but a consistent model doesn't necessarily provide strong ties back into the real-world (in this case, the real Titan).

It has only been suggested that clathrate hydrates may trap missing sequestered carbon beneath the surface .. but that's only a suggestion. There are other explanations and mechanisms which can also explain how this can happen ... and could also account for the tidal flexing/movements ... and, perhaps, the overall planetary density figure.

This just doesn't look like a slam-dunk to me !

Cheers

Colin Robinson
2012-Jul-28, 10:10 AM
Well, here's an interesting version of the relevant press release (http://www.zmescience.com/research/studies/titan-subsurface-ocean-of-water-23323/) .. I really wish I could get a copy of the Iess paper to find out exactly the logic leading to the conclusion of a specifically water ocean .. unfortunately its behind a paywall.
In the meantime he's an interesting quote by Lunine (from the above release ...):
Notice how now its a 'bet' that its water ?? ...

Well, it is not exactly "a version of the press release", it's an article partially based on the press release. The journalist's name is given as Tibi Pulu. And most of the parts which you've bolded and underlined from your "quote by Lunine", including the word "bet", are actually the journalist's words rather than Lunine's.

True, it was Lunine who said: "All this hangs together to suggest what the interior of Titan is like."

Theories about the insides of any planet or moon are naturally going to be based on indirect evidence, unless there is actually stuff gushing out of the surface, as on Enceladus. Europa, on the other hand, does not have stuff gushing out like that. Therefore, even if a subsurface ocean in Europa is considered highly probable, its existence and composition are less certain than in the case of Enceladus.

The point that the bottom of Titan's subsurface ocean may consist either of rock or of ice, fits with Lunine's statement that "the density of Titan is consistent with a body that is about 60% rock and 40% water".


So, now its suddenly become generalised ... and non-specific to Titan ...

Lunine's point about the overall abundance of water in the solar system will only seem relevant to those who think the whole solar system emerged from a single accretion disk. But where is the planetary scientist who thinks otherwise?


There are other explanations and mechanisms which can also explain how this can happen ... and could also account for the tidal flexing/movements ... and, perhaps, the overall planetary density figure.

It is easy to talk vaguely about "other explanations and mechanisms"... But where in the scientific literature are these "other explanations" to be found?

Selfsim
2012-Jul-31, 07:30 AM
When you say "any other model"... Is there some "other model" that fits the observed facts just as well?

You've repeatedly suggested that Titan may not have water in any form, liquid or solid, except for a bit of atmospheric vapor due to some external source.

What I'd like to know, is whether anyone has developed a radically waterless model of Titan's internal structure – a model with neither liquid water nor water-ammonia mixtures, nor water ice, nor clathrates based on water ice? Is there published scientific literature that sets out such a model? If so, when was it published, and by who?Not that I'm aware of .. but so what ? It has been suggested that the martian gullies in the southern hemisphere of Mars, might have been formed by liquid carbon dioxide flows, rather than water .. which kind of demonstrates another geological mechanism not familiar to us, which might be at play in the different physical environment there.

In the case of Titan, it has also been suggested that a reservoir of carbon in its original form (organic compounds, carbon dioxide and grains) that is still being converted to methane, might exist far below the surface ('The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe'. Niemann, H. B. et al. Nature 438, 779–784 (2005)). This particular idea is not favoured over the clathrate mechanism I might add, (due a lack of supporting evidence of other detailed mechanisms), but it hasn't been ruled out either.

I do not question the abundance of water ice at/around the Saturnian environment .. I also don't have a problem with clathrates being suggested as a plausible mechanism involved in the tidal flexing of Titan. The formation of clathrate hydrates in the 'feeding' zones of the ice giants depends on the efficiency of the trapping of volatiles by microscopic icy grains. Whether this is applicable on a wide scale in the case of Titan, is unknown. What I do question is the strong declaration that Iess' story is real, exists for certain, and that all other alternatives have been ruled out. We simply don't have sufficient data to do that. Having a consistent solar system planetary formation theory, which makes a testable prediction of deformable methane based clarates residing at the core of Titan, is also not an issue.

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. If we dwell too heavily on things observed on Earth, (.. like clathrates ... which formed because of physical conditions applicable on earth or its location and/or physical environment within the Solar nebula), I don't see how we'd ever see these other mechanisms and further scientific knowledge (?)

Another example: (and somewhat off-topic), just yesterday, I was reading about giant avalanches on Iapetus, whose runoff distances are out of proportion with the heights of the material falls. Frictional forces between the particles in a low gravity, extremely cold environment have formed the basis of a testable hypothesis. 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.

Cheers

Colin Robinson
2012-Jul-31, 11:19 PM
When you say "any other model"... Is there some "other model" that fits the observed facts just as well?

You've repeatedly suggested that Titan may not have water in any form, liquid or solid, except for a bit of atmospheric vapor due to some external source.

What I'd like to know, is whether anyone has developed a radically waterless model of Titan's internal structure – a model with neither liquid water nor water-ammonia mixtures, nor water ice, nor clathrates based on water ice? Is there published scientific literature that sets out such a model? If so, when was it published, and by who?Not that I'm aware of .. but so what ?

Let's backtrack a little...

A few days ago, I wrote:


Yes, it seems to be mainstream science that Titan's bulk composition is largely H2O. The question has been how much if any of the H2O is liquid rather than solid, and whether subsurface materials ever find their way to the surface thru cryovolcanism.

To which you objected


There is nothing mainstream about water existing on Titan.

If you had been able to back up your objection, by showing me even one scientific publication which argues for a bulk composition of Titan without large amounts of water (in solid and/or liquid form)... then I would have had to revise what I said about the mainstream scientific view.


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 agree that when studying other planets/moons, we need to be open-minded about unfamiliar forms of geology.

And unfamiliar forms of life?

Selfsim
2012-Aug-05, 02:21 AM
I agree that when studying other planets, we need to be open-minded about unfamiliar forms of geology.One of the more interesting discussions might be about the methane present there .. that report I linked to says: (http://www.nature.com/nature/journal/v438/n7069/full/nature04122.html)

The value of 12C/13C in methane provides no support for suggestions of an active biota on Titan. It takes less energy to form a chemical bond between two 12C atoms than between 12C and 13C, so complex organic molecules associated with biological processes on earth show an enrichment in 12C—the 12C/13C ratio is greater than the Pee Dee Belemnite (PDB) inorganic standard value of 89.9, and could be as high as 95. Therefore the assumption that such enrichment will occur in carbon-based non-terrestrial biology seems reasonable.

We do not find this enrichment in the methane of Titan. Instead, a geological source for methane, with a possible clathrate reservoir as storage in the interior of Titan is favoured. .. 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 ?
Ie: in spite of the 'complex chemistry' of Titan, and in spite of theorised subsurface ammonia-water clathrates (and in spite of there being no measured water in the atmosphere or the surface), the measured methane also exhibits no sign of biogenic properties ...

They go on …


As occurs on Earth, serpentinization that releases hydrogen from water while oxidizing iron- or magnesium-bearing minerals could produce methane through a Fischer–Tropsch reaction of the H2 with CO2, or reduction of carbon grains in the crustal rocks in the interior.… at least they are considering other than subsurface water-based clathrates, as a release mechanism for the methane, I suppose ...

And unfamiliar forms of life?Now what might they be ?

Colin Robinson
2012-Aug-05, 10:21 PM
.. 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 (http://www.ciclops.org/news/making_sense.php?id=6431&js=1)

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
2012-Aug-06, 01:25 AM
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 (http://www.ciclops.org/news/making_sense.php?id=6431&js=1)

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
2012-Aug-06, 01:32 AM
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
2012-Aug-06, 01:50 AM
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
2012-Aug-06, 01:57 AM
"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
2012-Aug-06, 02:12 AM
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
2012-Aug-06, 04:16 AM
I find it difficult to reconcile the measured readings with the intent of the article.

According to the GCMS Huygens readings (Fig 1) (http://www.nature.com/nature/journal/v438/n7069/fig_tab/nature04122_F1.html) , H2 had a count readings of:

~ 104.1 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 ppmFootnote 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
2012-Aug-07, 12:29 AM
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
2012-Aug-07, 02:33 AM
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
2012-Aug-08, 03:55 AM
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) (http://www.ncbi.nlm.nih.gov/pubmed/12596809)

Selfsim
2012-Aug-08, 09:58 AM
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
2012-Aug-09, 02:06 AM
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 (http://astrobiology.jhu.edu/wp-content/uploads/2010/06/Icarus-2010-Strobel.pdf)


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 (http://en.wikipedia.org/wiki/Great_Oxygenation_Event).

Selfsim
2012-Aug-10, 09:46 AM
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 (http://astrobiology.jhu.edu/wp-content/uploads/2010/06/Icarus-2010-Strobel.pdf)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
2012-Aug-10, 06:49 PM
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
2012-Aug-11, 10:54 PM
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 ± 0.0003The 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 (http://www.youtube.com/watch?v=mDUnLArk-dw) 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
2012-Aug-12, 02:31 AM
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
2012-Aug-12, 06:53 AM
(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