Monday, April 24, 2017

Water Planets

It is relatively easy to compare the mass of the atmospheres of Venus and Earth and ask why is the atmosphere of Venus so much more massive than that of Earth. A simple answer, that it came from some peculiarity of the formation of the planets, allows much speculation, but there are other ways of looking at the question. Earth has an ocean, Venus does not. The comparison of the surface fluids of Earth and Venus goes the other way. The surface fluid mass on Earth is about three times that of Venus. So, perhaps the correct question might be: “Why is the surface fluid mass of Earth so much larger than that of Venus?” The speculative answers as to where the missing atmosphere of Earth is would be no longer applicable.

If the mean surface temperature of Earth were quickly to go up, not to that of Venus, but something high enough so that the whole surface was above the boiling point of water, then the mass of the atmosphere of Earth would be about three times that of Venus, instead of a small percentage. Venus’ atmosphere is almost all carbon dioxide and nitrogen, and Earth’s would be almost all water. The perspective changes. Instead of asking about the mass difference of the atmosphere’s, the most striking question is about the chemical compositions. Where did all the carbon dioxide go from Earth’s atmosphere, and where did all of Venus’ water go? The two planets formed out of the same original cloud of gas, flattened into a ring and rotating about the newly forming sun. They aren’t all that different in distance from the center of the solar system. They aren’t very different in mass. What caused this?

If we want to understand upon which planets life could form, and where it might evolve into star-faring aliens, it would be certainly important to understand how an atmosphere that could support life would form, and how it would transform over the lifetime of the planet. It is apparently much more complicated than might be first imagined.

Suppose the early cloud that formed the inner planets had a slightly different composition, with less of a percentage of heavier elements, those that form the huge core of a planet, and more of a percentage of lighter elements, which might form an atmosphere. If this happened, there might be more of an atmosphere. A solar system with this arrangement might have a rocky planet with more water, among other constituents, in its initial atmosphere, and if the temperature dropped below the boiling point of water, it would condense. Most of the water would condense out of the atmosphere, as the vapor pressure of water is not very high at temperatures well below boiling.

If the mass of water on this alien planet were five times at much as on Earth, and the planet was about the size of Earth, there would be so much water that a mountain range, as high as Everest, would be completely covered. There would be no dry land. This obviously is a major impediment to evolution leading to intelligent aliens. The mass of the oceans are about 0.025% of the mass of the whole planet, and so raising that to 0.125% would do the trick. This is not a large fraction of the mass, meaning there does not seem to be any need for some exotic process to get all the water to the planet. So, one question that pokes up is: “What is the water fraction in distant solar systems?” If it is too large in the region where rocky planets form, we have a water world, with no dry land. Even on a planet which had only a few small islands, like the tips of the Himalayas, it would seem to be unlikely for an intelligent alien species to evolve.

The fraction of mass throughout the universe that is heavier elements is small. It is mostly hydrogen, plus some helium. Somehow this ratio is inverted in the region where rocky planets form. Whatever the totality of processes that do this are, it would seem that a factor of five or ten in the ratio of water is not unlikely. So, water worlds may be common instead of worlds like Earth, where we have almost 30% of the surface dry.

A cloud of gas that is about to form a star and solar system might have enough heavier elements to make rocky planets in it, but how do they get concentrated? If the cloud is rather homogeneous to start, this means that the concentration operation has to get completed either during the initial formation of the central mass or during the time where there is a disk of gas forming into planets or preplanetary clumps. During both of these eras, the gravitational attraction of the protostar causes a migration of heavier elements and molecules toward it. If there is enough time for these twin processes to come to completion, then the ratio of water to heavier elements would reflect the mass of the protostar and the overall composition ratio of the primordial gas cloud. The mass of the protostar is again a reflection of the total mass and volumetric density of the gas cloud. The original composition ratio reflects the history of the cloud, and, according to the current theories of the formation of heavier elements, how many supernovas went off in the vicinity of it during its life in the galaxy.

Both of these go in the same direction. In regions of the galaxy where the density of gas is larger, larger stars would form and more gravitational segregation of heavier elements is possible. In the same region, more supernovas of type II, the most common type in the earlier galaxy, massive stars which explode at a pre-ordained point in their history, should form and contribute heavier elements. Thus, in denser parts of the galaxy there might be dry worlds and in less dense parts, water worlds. In between, worlds which can form alien civilizations that might venture out into interstellar space.

The total mass density of the galaxy is highest in the central core, still high in the bulge, and less in the disk, dropping off as the distance from the galactic center increases. Spiral waves pass through this, changing the density up and down as they pass, but they do not affect the time-averaged density. So, if we want to find alien civilizations, a band of galactic disk about the same distance from the galactic center as Earth is might be a good place to look, if only because that is where partially wet and partially dry rocky planets might be more likely to form. It also means that would be a good place to hunt for planets to colonize, if Earth ever reached that capability and had the desire to do so.

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