Wednesday, June 27, 2012

My Little Planet: Interfaces are Magic

Okay, I've finished Thomas Gold's "The Deep Hot Biosphere" and have decided that I'm not quite ready to shout 'Eureka!' and say that this will be the continental drift of the 21st century.

Overall the chemistry seems pretty solid, and the model makes sense in terms of the likely process of planetary evolution, but I need to wander off and check some more recent primary references. Most intriguing is the information about helium and heavy metal distribution in hydrocarbons, which really doesn't seem to have any good explanation in terms of the traditional biotic origin theory. On the other hand I've thought of a perfectly good alternate explanation for the lack of isotopic drift in oceanic carbonate deposits over geological time – I'm sure you can do the same, so I won't tell you now. And Gold's explanation of methane clathrates doesn't seem to square with our current understanding of their distribution. So as I said, I have decided I'm not quite ready to jump on the bandwagon.

(Note the extreme sketchiness of my treatment here – this is because I want you to read the book yourself so we can talk about it, rather than spend my time making an exhaustive book report.)


What I really want to do is to step back a little and discuss the implications of Gold's model for the origins of life in a more focussed way than he does in his Chapter 9. I say step back a little, since I'm not going to assume that Gold's model of upwelling of abiotic hydrocarbons is occurring on Earth now, or necessarily occurred in the past. What does seem plausible – so plausible that it is certain to have happened many, many, many times – is that such a model system has arisen on rocky planets somewhere. We know the clouds of junk available for making planets can contain a lot of carbonaceous material. The cold aggregation of an Earth-sized planet containing a lot of this material is sure to happen sometimes. The timeframe for outgassing of this carbonaceous material is sure to have extended over very long time periods on some worlds. As Gold postulates, it is sure that on some worlds there will have been a persistent water-rich layer on top of this hydrocarbon layer.

This plausible persistent environment is exciting. It is by far the best postulated locale for the development of life, or of pre-life, that I have come across.

Remember what we need:

We need to assemble a collection of complex molecules in something with an edge to it, so the collection can get more complicated inside that surface; we need a flux of energy into the system; and we need a flux of matter in and out of the system:a proto-proto-proto-proto-metabolism of some kind.

This all seems far more likely in the upwelling hydrocarbon system than any other possible system I have read about.

1. The upward flow of hydrocarbon material provides a source of chemical energy, a steady replenishment of raw material ('food') and a means of disposal of superfluous matter ('waste'). This is not going to be the case at all in the traditional 'small tidal pool' or a homogeneous cloud of dense molecular gas. The energy flux is going to be far more useful than the heat gradient one might get a large body of water, or in a small dense body like a comet, and the mass flux will be more persistent than any mass flux driven by these heat gradients.

2. As Gold stresses, and as he shows in a splendid figure from a paper by one of his Soviet precursors , complex molecules are much more stable at very high pressures. This means the necessary molecular complexity is much more likely to be present under such conditions.

3. Furthermore, the heterogeneous environment of the deep underground is fantastically more suited for complexification than something like an ocean, which is a pretty well-mixed homogeneous fluid. There are pores of different sizes; there are interfaces everywhere, with different adsorption characteristics on different minerals; there are thermal and chemical gradients. It is a chromatography column on a planetary scale providing an incredible opportunity for sorting molecules into a gazillion different microenvironments. There is nothing like this in any of those other possible environments for pre-biotic evolution. Gold makes much of the greater volume available for experimenting with chemistry underground than in the ocean, but does not mention this incredible advantage in terms of its partitioning into innumerable separate experiments.

4. At this point Gold wanders off the point and wastes the rest of Chapter 9 talking about numbers with large powers of ten – as people who talk about the origin of life are wont to do – and about autocatalytic reactions. You should know by now what I think about autocatalytic reactions and their irrelevance to the  more important questions of the origin of life, so I won't beat that dead horse just now.

What Gold could have done instead is consider some other implication of one other part of his model. He postulates that a layer dominated by hydrocarbons lies deep in the crust, beneath a higher layer dominated by aqueous solutions. What happens when they meet one another?

4a. Well, they don't mix homogeneously. We have proverbs about that.

4b. The water layer is relatively full of oxidants, so some of the large hydrocarbon molecules are partially oxidised. This can make them surface-active. Which means they will want to stay at the interface between the hydrocarbon and aqueous phases.

4c. As more surface-active material is generated, it will generate more interface for itself to sit at. That's what surfactants do. The interface will grow more complicated and interesting.

4d. As more surface-active material is generated, it will self-assemble into interesting structures. That's another thing surfactants do. Among the self-assembled structures they are likely to form are vesicles, the basis for all biological compartmentalisation we know about.

Voila, we have a persistent zone of CHO(N)-rich molecules and plausible proto-proto-cellular menbranes at the interface between the hydrocarbon world and the water world, eminently susceptible to further complexification!

This gives a strong pointer towards where we should be seeking the chemical building blocks of pre-pre-life: What do we get in reactions of hydrocarbon mixtures and oxidising aqueous solutions at very high pressures? And then, what do the phase diagrams of surfactant molecules formed in this way look like at the same very high pressures?

Even if Gold's theory turns out to be bogus as far as Earth is concerned, I am sure that it happened somewhere, and if it did, it could give rise to persistent complexifiable chemical systems: systems which, being embedded in very large lumps of matter, would be far more likely to survive fortuitous transport from one solar system to another than analogous systems somehow arising in gas clouds, planetary surfaces, or the interior of small cold bodies. So that's why I'm excited. :)

Edit June 30th: Also, this.


Marco said...

I am not sure what I want to say about it all, except that perhaps I went through the same process of thinking that abiogenic hydrocarbon generation was a crucially important theory in regards to abiogenesis. Unfortunately, we have different premises in regards to abiogenesis that I am not sure we can overlook, and I feel that reading the book would not make much difference.

Chris Fellows said...

So, what's wrong with my premises? We can't be *that* orthogonal in our thinking. You know I don't hunger and thirst to be in error...

Marco said...

Just the premise that life eats prelife destroying the evidence, for one is one I cannot accept. Our ideas on pre-life had come to quite an impasse, from memory. However, a planet is, for all intents and purpose, a huge comet. Replace my overall "comet origin" theory with "planet origin" and I could be close to agreeing on some points.

However, I still believe that material exchange between astronomical bodies is an essential aspect of abiogenesis, rather than just a way for successful end result of life to be distributed. Frequent dispersal of matter from a deep within the crust with the gravity well of the planet to escape from doesn't gel with that.

Chris Fellows said...

Okay, I can buy that... being a bit more agnostic on my 'life eating pre-life' assertion, I would say that the drastically different conditions of up here vs. down there would make it more likely that pre-life could survive down there, with only waste products drifting up for life to use, so it would be the ideal place to look for surviving pre-life.

And... I would suggest that a smaller body than Earth, which would not heat up so much inside post-aggregation, and would be more susceptible to being torn apart by chance gravitational encounters and dispersed, is a more likely candidate for a ThomasGoldWorldTM. So yes, it is just a large big comet, but with a better mechanism for energy and mass fluxes!

Marco said...

I was thinking a Centaur sized body could be the Goldilocks size. Not too big not too small not too hot not too cold. I do get the point.

I've got a hunch that it might be a tad easier than that. Flying into the "holes" of comets might get deep into the comet, or even drilling down a few hundred metres is plausible within our lifetime, and whether it is a fragment of a bigger body where the chemistry happened or happened in situ as part of a pseudo life process would be a question for the future.