Tuesday, April 26, 2016

The Right Amount of Atmosphere

The atmosphere has a great many effects on life on a planet, probably more than anyone has enumerated. If we are Earth are going to make a great effort to locate some life on exo-planets outside our solar system, we should carefully think through signatures that we can find from here on Earth or perhaps from some telescope or other instrument located at a convenient point in our solar system.

It happens that atmospheres are easy things to see, compared with oceans and land surfaces, or even details of them. Light passes through an atmosphere, and so if an exo-planet transits its star, it should be possible with some fantastic instrumentation, to figure out some details of the atmosphere. This is not something we can do today, but we might be able to design an instrument that in the near future could do this. Going on further, if we decide to build a large observatory, space-bound, for looking at exo-planets before we send any probe there, we might be able to deduce some details about the atmosphere from within our own solar system. If you follow the development of astronomy, you know that every year it seems some brilliant astronomer has invented an outlandish gadget that measures something nobody else ever did or maybe nobody else ever thought possible. The rate of innovation in astronomy is impressive. Recall exo-planets were totally unknown and unobserved not long ago.

Observing the atmosphere of distant planets will be a glorious success, but it be more of a success if there is a direct correlation that can be found from the atmosphere data that is detected and the probability or certainty that life is there and has managed to climb up to some stage. Recall, as in other discussions, it is home planets for life that are the important issues. A successful alien civilization, mastering star travel, would be able to create an underground habitat on a planet with no atmosphere. This would be undetectable, although I have to say that some brilliant astronomer may someday prove me wrong on this. However, the main point is that the origination of life needs some conditions on the atmosphere, and there are some steps in the evolution of life and the development of an alien civilization that are affected by the atmosphere.

Start with the simplest thing: quantity. How much of the best possible type of atmosphere is needed? What is the least amount that allows life to originate? If there was some agreement on how life originates it would be easy to backtrack to the effect of the atmosphere. Let’s use as an example, the life origination theory promulgated most recently in this blog: organic oceans after the formation of the moon via impact. The atmosphere after the impact is going to be very hot, and water would be vapor. So asking about the quantity of atmosphere includes, in this theory, the amount of surface water there would be. When the atmosphere cools off, and water forms the oceans, the amount of atmosphere directly translates into the amount of oceans covering the surface of the planet. Life in this theory is probabilistic, and having half as much ocean area means that the rate of life origination would be halved.

In the theory, life has to originate before the organic pools of immiscible liquids disappear via one mechanism or another. So it is a race for life to originate while the conditions make it possible and these conditions disappearing. If there were less ocean area, life might lose the race, or in other words, the probability of life making through the very first stage could drop from a near certainty, with lots of oceans, to near zero, with only some small ones.

Does the quantity of atmosphere have any negatives if things go the other way, and there is, in the primordial atmosphere after the impact that forms a moon, lots of water. If water totally covers the planet, or leaves only a few high mountain areas protruding from the oceans, does life do well? Not if intelligent life is sought. It would seem that the origination of intelligent life underwater takes some very special conditions, and so without lots of land surface, land organisms are not going to be coming up with the necessary mutations fast enough to make intelligent life before some peril strikes, like stellar death.

So, the amount of atmosphere left on the planet after a violent impact of just the right kind to form a moon has a middle range in which things are best suited for intelligent life to develop. It never gets started with too little, and meets a roadblock if too much.

Perhaps this range of quantity of atmosphere can be shrunk by other factors. The photosynthesis transition, where biological organisms pick up a more powerful energy source and the evolutionary floodgates open wider, might not happen if there are few photons, or photons from the wrong part of the spectrum, reaching the top few meters of water. This might happen with cloud cover of any sort. The amount of cloud cover from water clouds on a planet where there are large bodies of water is not going to eliminate photons. Before that happens, the water in the atmosphere will rain out and evaporation will not replace it fully. This is a temperature question, and is also affected by surface winds.

If the amount of atmosphere is too small, there will not be a blocking of destructive UV photons traveling down to the surface. This will not eliminate underwater life, but might slow down the development of photosynthesis. It would also eliminate the existence of land organisms, no matter how much land is available for them to occupy. So, this may provide an improved lower limit for the amount of atmosphere.

Timing plays a role as well, as there is a feedback between life developing and the composition of the atmosphere. Oxygen seems to be only producible and maintainable in an atmosphere when large amounts of photosynthesis are going on. In the early time, carbon dioxide in the atmosphere might serve as a resource for underwater life. Too little, and the odds turn against life forming and surviving the inevitable changes in the planet. Too much may not do anything strongly negative, except for affecting the temperature of the planet. So, there is an interaction between atmospheric quantity during the pre-oxygen atmosphere and the planet’s temperature. If the star-planet distance is a bit on the long side, and only the large amount of carbon dioxide in the atmosphere keeps it in the LWZ, the liquid water zone, having life eat up the atmosphere’s carbon dioxide bit by bit might trigger some climate shift to a frozen planet, which would eliminate life.

There seem to be multiple interactions that need to be laid out in detail, probably quantitatively, to get an answer to the quantity of atmosphere range that is important for life origination and survival. Oh well, another entry on the to-do list.

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