Monday, August 1, 2016

Planets and Satellite Formation

There are seven natural satellites in our solar system larger than 2000 km in diameter. Where did they congeal? They could be from the vicinity of the planet itself, at a Lagrangian point in the planet's orbit, or in orbit around the sun. All three are reasonable points of condensation. It might be possible that one planet's satellite was lost to that planet and later captured by another, but it seems at first glance the probability of that would be very small.

One of the details of the organic oceans hypothesis for the origination of life is that Earth's moon was formed at a Lagrangian point and was later captured. The original moon may not have been the same size as the current moon, as there was an impact and mass exchange between the proto-Earth and the proto-moon, resulting in the current mass ratio. If some or all of the other large satellites were formed in this manner, perhaps something could be learned about the process by examining all of them, rather than only the case of Earth. The name Theia is used for the proto-moon.

Capturing a satellite that formed co-orbitally with a planet should be much easier than capturing one from a solar orbit. Solar orbits are largely formed in stable locations, and in Earth's solar system, with two giant planets setting up the resonances, getting out of these orbits might require some unique phenomenon. Lagrangian points are also points of resonance, but stability there might be adversely affected by the presence of the other planets. Jupiter is about four to six AU away from a point in Earth's orbit and its mass is three hundred times that of Earth, meaning the gravitational effect of Jupiter is greater than that of Earth on an object at one of Earth's Lagrangian points. This is, of course, irrelevant, as the mass of Jupiter at the time of formation of Theia may have been much less, as it could have been still in the process of forming, and much of its mass could still have been spread around its current orbit. A ring of mass does have a gravitational effect, but not as much as a concentrated planet. There would have had to be some concentration of mass, in order to set up the resonant locations where the other planets would condense their rings of mass and then the planets themselves, but it would not have had to have been the whole mass by that time. The gradual accretion of matter by the proto-Jupiter may have been one of the factors that caused the migration of Theia into impact with the proto-Earth.

What about the other six of the largest moons? They all orbit planets with deep, dense gas atmospheres. Impact is quite different for a large moon impacting a planet with a thick atmosphere. Instead of having to hit the core and disrupt the crust or mantle or whatever the equivalent is on these gas giants, it could come through the atmosphere at a slow speed, and be captured. The speed would be very slow as planets in the same orbit have the same orbital speed, so the only difference in speed would be that caused by the freefall into the planet's gravitational well. That certainly is enough to make the contact spectacular, but it would only take a small reduction in speed via atmospheric friction to reduce the relative velocity of the satellite down to a highly eccentric but captured planetary orbit. After this interaction, more entries into the atmosphere would further reduce the speed and relative angular momentum, allowing eventual circularization of the orbit, aided by tidal interactions. Thus, capture by a gas giant of a Lagrangian-origin moon seems quite feasible.

This also means that the precise depth of maximum entry of the moon on its first encounter is not critical, but almost anything deep enough to draw off some angular momentum would be enough for a capture. This is in contrast to the Earth, which may have had an atmosphere much larger than today's, perhaps a few times more than Venus has today, but this would be nowhere near enough to cause much drag on the approaching Theia. There would have to be an impact, glancing at the least, to do that.

A glancing impact would tend to equalize rotation rates. This would likely mean Theia would slow down, and to maintain the total amount of angular momentum, it would seem that it would have to leave the joint blob of mass with more than it brought in. So the moon is particularly large.

This means that on other solar systems, ones which have not been disrupted by a stellar encounter, there should be numbers of large satellites on gas giants, but few or none on rocky inner planets. This theory therefore reduces the number of solar systems with origin planets. The reasoning goes like this. Organic oceans are necessary for the early origination of life, and a Theia-like impact is the most prolific means of making them. Some might be made by asteroid impact, but the amount would be less, meaning less chance for life to originate. The atmosphere would also be much thicker, making it harder for life to evolve, and much harder to replace the carbon dioxide with oxygen. Less photons would get through the atmosphere and there is much more carbon dioxide to begin with. The moon also may have some beneficial effects on later stages of evolution.

So, if we go to look for life-containing solar systems, using a giant telescope able to see not only planets, but satellites, it will be a clear indication of a promising planet for life if it is located on the inner portion of the liquid water zone, and it has a very large moon. There should also be a minimal atmosphere, such as we have here on Earth. If it does turn out that this is rare, as it may be, then life itself is rare and originates very seldom in the galaxy. This might be the most striking answer to the puzzle of why there are no alien visitors here.

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