Tuesday, January 19, 2016

Fast Origination of Life

When we go looking for life, it would be nice to know where to preferentially look. If we had the resources, we could use the big eyes, with all the spectrographic marvels attached, for every star within some radius, just to be sure that we didn’t miss anything. But if resources are short, it would be good to figure out where the likely solo planets are hiding.

Suppose we use age as a clue. If life takes a billion years to originate, or two, there is no point in looking at solar systems which are younger than that. Stars which don’t have that long a lifetime can be eliminated immediately, as can those who evolve so quickly that a habitable zone, whatever that turns out to be, moves before life has a chance to take advantage of it. So, because we know that life takes a long time to originate, we can filter out solar systems. But, how do we know that?

We have one example here, Earth, and life took a long time to originate and to evolve to some detectable level. So, how do we conclude that it takes that long everywhere? Maybe we have some problem here that delayed our progress by a factor of two, or ten, or a hundred. Since we know so little definitively about the origin of life and all the stages it might have needed, it could be somewhat premature to say that life takes a long time. Earth’s life may have taken a long time, but on Planet X, who knows.

Let’s take the working hypothesis for the mechanism for the origination of life that this blog uses and see if it can be sped up. To recap, the first step is for some self-replicating molecule to form by chance and then just continue to increase its density, in the oceans of the planet. What controls the rates of this first step? If the molecule is, as hypothesized here, a couple or a few amino acids hooked together into a primitive DNA, then the formation of amino acids is a rate controller. What are the various ways that amino acids can be made?

This question has been the subject of numerous experiments. To encapsulate the results, they are that amino acids of a wide variety, along with other organic compounds, can be formed both in an ocean and in an atmosphere with some energy source, such as a lightning discharge, UV radiation, or volcanic heating. One condition seems to be that free oxygen not be present, as it oxidizes them. Oxygen was not present in the early Earth atmosphere, but was produced later as a byproduct of photosynthesis. The precursors involved included ammonia, methane, water vapor and carbon dioxide, and sometimes others. Nitrogen was a killer, as nitrites were formed which decomposed the amino acids. If iron and some carbonates were present, the nitrites were deactivated and the amino acids could form. Many other alternatives have not been tried, but the basic idea is that the right combination of gases and surface minerals, or ocean solutions, can produce amino acids.

The rate at which a self-replicating molecule forms would be proportional to the density of the precursors which form this molecule, to some power dependent on the number of precursors. The power is higher than one. A simple synthesis of two components varies as the product of the density of each, meaning that scaling the densities up together increase the rate as the square, for low densities. So, for fast origination of life, something which could dump more and more amino acids into the primitive world’s oceans and atmosphere would do the trick.

If UV can do it, then having a hotter star would be the lucky ticket. This is just the opposite of what was posted in the blog about hot stars. Since their lifetimes are short, there isn’t enough time for life to originate. But if the UV is extremely effective at producing amino acids or whatever other organic compounds are needed to generate the very first self-replicating molecule out of non-self-replicating molecules, then the short lifetime of the hotter stars might be mitigated.

Another known source of this production of amino acids is lightning. Under what conditions would there be more or less lightning in a young planet’s atmosphere? Fulminology is a well-developed, although incomplete, science, so the principal cause can be provided. Ice crystals forming in the atmosphere have a tendency to attract free electrons, becoming negatively charged, while drops do the opposite. If there is some convection, so that charged ice crystals separate from the oppositely charged water drops, then an electrostatic charge can develop and then when it reaches breakdown potential, a discharge can occur. So, the more clouds and rain that a planet has, with the right temperature of the air so ice can form at some band of altitude, the more lightning it would have.

Here is yet another problem with the determination of ‘habitable’ planets. Liquid water may certainly be present on a planet, but if the mean temperature is too high, in the upper end of the range for liquid water, there may be no ice formation and then no lightning, and then no amino acids raining down into the ocean, no self-replicating chemicals forming, and no life. So, habitability in the astronomers’ lingo may mean no origination of life. There is not much else that word does mean, except the presence of liquid water.

Volcanoes can induce lightning in some situations, and an underwater volcano can produce thermal currents which might also make amino acids. So, if the crust is such that vulcanism is present, that is yet another factor in finding where fast life origination might happen. What could cause a high degree of volcanism? Having a thin crust to the planet, along with much mantle convection. It might also be that the composition of minerals changes the degree to which the subterranean magna penetrates to the surface. The formation of large cracks in the crust would seem to promote it as well. Continental drift results in thinner areas, as well as a change of thickness with location, ocean crust being thinner. Does rotation rate cause more convection, or does the presence of a large moon have an effect? Is it both of these together, with the large moon causing more of a tidal influence on the crust than on the mantle, so that there is a bit of differential rotation which would lead to cracking and drift and convection? Perhaps we have come around again to thinking that a large moon is a good thing for life origination. Does that mean that a binary planet, two planets in orbit around one another with the mass ratio closer to one than here on Earth, would have more volcanism, therefore more organics in the ocean and hence earlier formation of life?

So, exposed iron and carbonate rock for the nitrite problem, lighting or a hot star or a large moon for the energy source, and we might have some candidates for early formation of life. Recall that the rate at which compounds form is some power of the density of the constituents, so doubling the constituents in the ocean may cut the time down by four or eight times. Tripling it – well, do the arithmetic yourself. The changes in life origination times could be dramatic. If it was possible to get ten times as much organics in the primordial ocean, and it was a three-way race to the self-replicating chemical, that’s a factor of a thousand. In case you are math-impaired, that reduces a billion years to a million. Unbelieveable!

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