Sunday, December 6, 2015

Life Originating on a Satellite

Here on Earth we have speculated about some form of life originating on a satellite of a gas giant, perhaps in a subsurface ocean heated by tidal effects. But in our solar system there is no satellite large enough to originate intelligent life. In other solar systems, this may be quite possible.

The ratio of satellite mass to planet mass varies a great deal in our own solar system, which demonstrates that it can happen and is likely to happen in other solar systems. In order to generate intelligent life, in the way that it happened here on Earth, a satellite of Earth-like size is needed, and the planet it orbits has to be near the habitable zone of the star.

It would be expected that the planet around which an Earth-sized satellite was orbiting would be large. Planets can range to masses more than ten times Jupiter’s mass. Going much beyond that raises the possibility of thermonuclear fusion in the object, which would then be called a star, and the system a binary star. Having a planet around a brown dwarf in a binary system has yet other differences from a star-planet-satellite trio, and it deserves a separate post or two or ten. For an example, assume the planet has about ten times Jupiter’s mass, and the satellite has about Earth’s.

There are many major differences between a planet and a satellite of the same size and composition. One is in the source of heating. Gas giant planets by definition do not have thermonuclear fusion, even of deuterium, the easiest isotope to fuse, but they do have heating from gravitational contraction. A gas giant of ten times Jupiter’s mass would be radiating this heat for a long time, and any satellite around it would be receiving some infrared radiation from the planet. There is also gravitational heating from the variations in gravitational pull on the satellite, depending on how much it deviated from a circular orbit in the planet’s equatorial plane. The star might be contributing a bit of gravitational heating as well, depending on its size and the distance from it to the planet. The smaller the star, the closer the orbit, and the larger the stellar tidal forces.

It is not necessary to presume that a large planet would be orbiting a large star. There are examples of all types of mismatches between the masses of binary stars, and there does not seem to be any reason why a large planet could not form around a small star. The question arises about the longevity of the orbit, if there are more than one large planet in the solar system. Let’s assume there is only one, and the other planets are small.

A likely situation is that the satellite is tidally locked onto its planet. This is vastly more mild in effects than for a tidally locked planet around a star. The solar heating is still diurnal at everywhere on the satellite except at the poles, assuming that there is some alignment between the orbit of the planet, the orbit of the satellite and the axial tilt of the satellite relative to its orbit. Unusual situations are very surely possible, but with tidal locking, axial tilt should be small.

With a large planet and a large satellite, the orbital period may be long, meaning the day would be long. The temperature differences that occur during a day would be greater than that for a quickly rotating planet. The effect of the eclipsing of the planet during part of the day would likely not be significant, providing there was an alignment between the orbit of the planet and the satellite. As an example, Titan is eclipsed by Saturn once for four days during a fifteen year period. Obviously this amount could have a wide variation on exoplanets.

If the planet, via thermal radiation and tidal forcing, is providing a significant part of the heating to the satellite, the star is providing less by way of photons that can be used in photosynthesis. Furthermore, the heating would not be as concentrated diurnally, as there are in effect, two days for the satellite, one from the planet and one from the star. This means less temperature variation during a day. If tidal forcing is providing some heating as well, the overall diurnal variation would be less yet.

There could be more tectonic activity, owing to the strength of the gravitational field variation on the satellite, meaning perhaps more sources of outflows in an ocean where chemotrophs could both originate and thrive. Possibly this would mean that the time needed for life to originate would be shorter, other things being equal. Lower photon intensity would mean less impetus to switch to photosynthesis, so the time shortened in achieving multicellular organisms via chemotrophy might be lost due to a slower emergence of effective photosynthesis.

Life out of the ocean would depend on the ability of the planet to hold onto an atmosphere. There would be more tidal forcing on the atmosphere, and more opportunity for atoms and molecules in the upper atmosphere to escape, owing to the pull of the planet. Whether this is significant depends on the closeness of the satellite to the planet.

Once land creatures have evolved into existence, intelligence may be predicated on a particular environment to exist, such as a forest, where there would be opportunities for both grasping appendages and materials for early tool use. Trees evolve in a competition for solar photons, as near the ground there is much competition. With a lower proportion of planetary temperature maintained by heating from solar photons, this might take longer to evolve.

If the day is significantly longer, this means that plant organisms have to rely on energy storage for longer periods of darkness, meaning that there would be a higher ratio of roots and stalks to leaves, which might compensate for the lower amount of solar photons. On the other hand, less photons may drive more organisms for height, which coupled with the need for more storage mass, trees might evolve into existence faster, once they had come into existence.

This same need for energy storage during the night hits animals. The development of fast-moving animals might be inhibited by the need to carry more weight for energy storage, and something like hibernation occurring every evening, rather than only during a season caused by orbital variation or axial tilt. This might inhibit the development of intelligence.

If the planet had some eccentricity in its orbit, so that there would be annual effects as well, the combination of long days comparable to the length of a season might make the overall variation on the planet larger than otherwise. Just exactly how animals would evolve to cope with this is not immediately obvious, but it could affect the development of intelligence. Some more thought is necessary on this topic.

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