Tuesday, June 7, 2016

Devil's Advocate on Origin of Life Theories

Suppose you were interested in tearing holes in some origin of life theory. Where would the weak points be? Let's attack the Early Origination aka Organic Ocean theory.

The theory supposes a certain condition for life to originate, specifically, that there were two types of oceans on the early earth, immiscible mutually, a water one and a mixed organic one. To have an organic ocean, there has to be lots of organic compounds that are immiscible with water. Is it realistic to assume that these could be produced? There may have been a lot of methane in the gas cloud that condenses, but methane is too volatile to condense in the LWZ (Liquid Water Zone). So is ethane. There is carbon dioxide are in the gas cloud, but what is the mechanism by which large quantities of heavier organics are made?

Lightning is one source, proven in the laboratory. Satellite data on the current Earth show there are about 3 million lighting strokes per day. Over a hundred million years, that is about 10^17 stokes. Each stroke has about 5000 MJ energy in it, which is about as much as 100 kg of gasoline. So if there was a 1% energy conversion of lighting energy into organics in the plasma that is created by the lighting and the surrounding hot gas, plus the shock, there might be 10^17 kilograms made. This is only enough for about 1 cm of ocean, so it would be necessary to assume that the early Earth had 10 or 100 times as much lightning as the current Earth. This is a bit of a stretch, but not a ridiculous amount.

The atmosphere on earth has 5 x 10^18 kilograms in it, and if we assume the early Earth had 100 times as much, that is, five times as much as Venus now has, that would be 5 x 10^20 kilograms. If 0.2% of this was heavier organics, that would be 10^18 kilograms, enough for a meter or so of organic ocean. Not too much of a stretch, but no grounds for the assumption exist.

Volcanoes are another source, and each volcano has enough energy so that 1 every ten days or so is the same as the current energy release of lightning, so there would need to be 1 a day somewhere on the planet for a 1 m deep ocean, or 10 to get a 10 m deep ocean. For the chaotic world of early Earth, not too much of a stretch.

Another cause is planetesimal impact. The energy release in the Chicxulub asteroid impact alone is about equal to the energy of a hundred million years of lighting. That is one asteroid, and over a hundred million years in the early Earth, there would have been a large number. These are visible on the face of the moon, which has preserved the records of some early impacts. If we assume there were a hundred thousand of them, this would be more than enough for a deep early ocean. There are about two hundred thousand craters on the moon over 1 km in size, and the Earth is a larger target than the moon.

One source which is not quantifiable is the proposed impact of Theia, the planetesimal which formed the moon. Because the masses are so much larger, there is no way to scale this. But the proto-Earth would have its crust ripped, and huge gushes of magma would have been released. How long the heating from the mantle or even the core would continue to be released is open to question. But a lot of energy is available here.

To sum up this first probe, it appears quite reasonable that asteroid bombardment would produce the amount of heavier organics needed for an organic ocean of sufficient depth, and lighting, volcanoes, and the formation of the moon would add in more. Not a deal-breaker. And we have not covered the conversion in the ocean of miscible organics into immiscible ones.

Another question is, given the existence of the organic ocean, is whether any ambiphilic compounds would form? Numbers on this are scarce, but the point to note is that the meniscus is the boundary between an ocean full of lipophilic molecules and an ocean full of hydrophilic molecules. Any energy-containing molecule at this interface that expended its excess energy on forming a compound with something from across the boundary would be forming an ambiphilic compound. Compounds with excess energy do not act like time bombs ticking and waiting to go off, but instead go about seeking a suitable partner, which is decided on the basis of polarity and geometry. If there are energy-rich compounds on the organic side which need a polar molecule to trigger their chemical reaction and the only place they might find it is at the meniscus. Similarly for energy-rich compounds on the water side. There seems to be no reason to assume ambiphilic molecules will be scarce in a dual ocean situation, when the processes which made the organics were all high temperature, rapid cooling ones, which could be conducive to producing energy-rich compounds.

The next question might be, given a meniscus with ambiphilic molecules in abundance, are there any that will form membranes? This is not a question of whether ambiphilic molecules would join together with intermolecular forces, in alignment, because we have the example of current biological molecules which do that to make up cells. These are unique molecules, however, and it is necessary to ask if there would be any such molecules that could be made up of the components available in the two oceans. Recall that solubility is dependent on like molecules having a mutual attraction. This is what causes surface tension and what decreases volatility. Any molecule whose pure solution has surface tension would attract itself. Any solution with a low volatility would have molecules which attract each other. This is not a substantial objection.

This question might be better phrased as, given a meniscus with ambiphilic molecules in abundance, are there any that will form membranes solely with identical molecules? This question is a red herring, as it is quite possible to have a mixture of molecules in the ambiphilic membrane, and still have all the subsequent steps of replicator formation follow. Perhaps it would even be easier.

Going any further in this series plunges us into chemistry questions that have not yet been posed or answered. So, it might suffice to say that the obvious devil's advocate questions do know knock out the early Earth, organic ocean hypothesis for the origin of life.

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