Sunday, April 3, 2016

How Important Is a Large Moon to Life?

Other posts in this blog have talked about the utility of the moon in several aspects of originating and evolving life. One idea is that the moon is responsible for reducing the severity of the variation in orbital and planetary parameters, such as axial tilt and eccentricity. By slowing down these changes, there is time for evolutionary adaptation to the changes, and early life forms are not subjected to sudden ice ages or other large scale changes in the surface conditions on the planet.

Another idea is that the formation of the moon, via impact of the proto-Earth with a large planetoid, would do many things necessary for the origination of life. In that concept, the impact was responsible for seeding the atmosphere with organics, enough so the immiscible ones would form pools on the liquid water bodies, making a meniscus where something like a cell membrane could form. The same impact would leave a completely unsettled liquid core and solid outer shell, meaning there would be massive volcanism for long periods, which provide the energy source for the early chemotrophs.

Now comes a third idea. A recent new theory hypothesizes that the moon is the principal source of energy for the magnetosphere around the Earth. That magnetosphere serves to deflect the solar wind, which might serve to kill off much of the early life, if the magnetosphere was not there. Tidal effects of the moon on the liquid portion of the current core keep it rotating, somewhat non-uniformly, which is enough to generate the magnetic field. This idea can be extrapolated backwards in time, when the moon was much closer to the Earth, and the Earth's core was all liquid metal, without the solid core which formed later. This would indicate the magnetosphere was more powerful at early eras of life, meaning a stronger degree of protection ws being provided.

There are other speculations as to the roles that the moon played. One relates to Earth's atmosphere. The impact of the planetoid would certainly have served to heat it up, accelerating the dispersal of the lightest gases, specifically hydrogen, from it. Other gases, not as light as hydrogen, may have been lost to a higher degree resulting from the planetoid impact, but also from the tidal action of the moon shortly after formation, when it was in close orbit around Earth. The interaction of the gravitational force of the moon and the dynamics of atmospheric motion would have led to some of the upper atmosphere being out farther than otherwise, and this could contribute to the loss of the lighter gases. These gases, if present at many times the amount they are today, would have cut photon arrival at the surface, slowing down the evolutionary process by which photosynthesis replaced chemotrophy as the principal energy source of early life.

All these factors, and all the ones no one has thought of yet, indicate a large moon is instrumental in the origin and evolution of life, which implies that planets without a large moon would not have alien civilizations on them, and certainly not be the sources of star travelers. Exo-planetary astronomy is not yet able to detect moons, but if any moon might be detected, it would be one which has such a high ratio of mass to its planet such as the Earth-moon system does.

There are currently at least three methods of detecting exo-planets, all rather recent in development. The wobble method, by which the motion of the star, specifically the doppler change in the spectroscopic location of the most distinguishable lines, is good for large planets is close orbits. To get down to Earth-sized planets at distances like an AU, it would be good to have another order of magnitude sensitivity in the spectroscopes that do the detection. This means more photons, which means larger collecting apertures. This is certainly possible. To then go and look for a large moon would require another one or two orders of magnitude in photon count, to say nothing about the instrumental accuracies that would have to be developed.

The second method is the transit method, which means that the reduction in total photon flux that happens when a planet crosses between a star's surface and us, and it is also difficult to beat without more photons, meaning more collection area. To discriminate between a planet and a planet-moon combination requires more than simply photon counting. The total cross-section of the planet-moon stays constant, except for something of the nature of an eclipse of the moon, except seen by us not them, and only when the planet is making a transit. The slight fluctuations possible when the planet-moon system is entering the disc of the star or leaving it is again just a tiny change in the current signal. The transit method looks to be less likely to detect large moons than the wobble method.

The third method is direct imaging, which requires the light of the star to be blocked out very accurately, and the planet being imaged to be distant, rather than close in with the wobble method. Typically imaging a planet results in the planet occupying only one pixel at a time, and so there is no opportunity for doing a Galileo on an exo-planet until some way of extending the aperture of the observing equipment is found, something that could resolve the planet into more than one pixel, something like ten or more.

What comes first, the chicken or the egg? Or phrased more specifically, in a situation where one piece of research would motivate another, in either direction, and neither of the two are being pushed hard enough so that they are in the near future, where to press to see a resolution of the questions? If more geological work was done on the formation of the moon, and the condition of the Earth shortly after the impact, and for the folowing hundred million years, together with work on life origination under these unusual conditions, then there might be motivation to look for moons on Earth-like planets.

If, by some stroke of astronomical genius, it became possible to detect large moons on Earth-size planets, and it were found that there simply weren't any at all of them, and further there was no signature of life on all of the planets, this would imply that the moon has something to do with it, and Nobel prize seekers could focus on figuring out the mysteries of life origination on Earth-moon systems. Regrettably, it seems that neither of these two avenues is going to be explored while most of us are alive on the planet.

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