Wednesday, June 17, 2020

Heavy Elements in Galaxies

One question relating to the geological separation of useful mineral ores on exo-planets, something critical for an alien species to develop technology and socially evolve into an alien civilization, is about the distribution of heavy elements around the Milky Way. If a exo-solar system evolves from a gas cloud with very little heavy elements, above neon for example, it might evolve life on a suitable origin planet in that solar system, but the aliens, after becoming intelligent, wouldn't find the metals they need to go from a stone age to a bronze age, and they would never develop an advanced civilization. Thus, in order for us to have visitors from a particular exo-solar system, it has to have formed out of the same set of materials in the gas cloud, approximately, as Earth did, or maybe one which was richer in heavy elements.

These heavy elements are thought to be produced in supernovas, of which there are multiple kinds. Stars are nuclear ovens, gaining energy from nuclear fusion, which produces the elements above helium. Larger stars burn nuclei up to the nucleus with the least energy per nucleon, iron-56, but the kinetic process of showering nucleons into nuclei produces a wide distribution, centered around iron. Other phenomena produce heavy elements, and may produce a different distribution than burning in stellar cores. One example is the merger of two stars, in particular, neutron stars. So there can possibly be multiple sources of heavy elements, but they all involve stellar fusion processes or stellar disruption processes.

There are very few observable supernova in our galaxy, and probably very few stellar mergers as well, down in the number of a few per century. This rate cannot have produced all the heavy elements we see today, so the rate of production must have been much higher in the early galaxy.

Galaxies may form from the condensation of gas clouds of appropriate size, and as they condense, there are fluctuations in density leading to places where individual stars can form. As the enormous, galaxy-sized gas cloud condenses, if the density is relatively large compared to our current location, large stars will form as opposed to small ones. Large stars invariably turn into supernovas, and the largest of them might even totally explode, rather than just the outer layers exploding. The center of the star will be almost all heavy elements, with iron as the center of the distribution of elements, and larger stars may be more likely to have completed more of the fusion, so the central iron-dominated core will be a larger fraction of the total stellar mass.

This means that during the first phase of galactic evolution, long before the disk evolves to carry away the angular momentum of the cloud, the gas will be large homogeneous, or at least homogenous in spheroidal layers. The disk will form from the outermost layers of the galactic gas cloud, and thus we might expect that the disk will be fairly homogeneous with respect to the amount of heavy metals that exist in the disk and spiral arms. Thus, to a very coarse first assessment, solar systems close to ours might be expected to have the same distribution of isotopes and therefore elements. So, unless we want to think of stellar travelers coming from distant parts of the galaxy, the initial fund of elements should be sufficient on origin-type planets to allow any civilization which develops to get past the stone age, and move onward to industrial development and past that, provided that the geological separation processes on their exo-planet were sufficient to allow the useful elements to collect into bubbles within the molten core, and drift out to the crust and condense there into a solid.

The crust of an approximately Earth-sized planet does not have to be stable. Lying just underneath it is a hot molten layer, which may be in motion relative to the crust. Why? Because tidal pulls on the crust and on the molten layer are different, and induce a differential motion. Tide does not affect different materials the same, and a molten layer might move differently underneath a frozen crust. The crust might be flexed, and molten material leak upward, in what is called a basalt flood, if it is large and spread over an area, or a volcano, if the leak is confined to just a crack in the crust.

It would seem that a moon, during its early days of being much closer to the planet, had yet another task to perform that would be useful to an alien species which would arise much, much later. It causes a mixing of materials between the upper part of the below crust layers and the crust layers. If the two of these are each filled with different ores, the upper surface, where alien miners might get to it, would have an even better mixture of elements than there would be on a planet without a large moon initially close into the planet.

Often solid materials are more dense that liquid ones, and thus the crust, if it breaks into fragments, might be denser than the upper part of the layer below it, which might be called the mantle as it is on Earth. Then any cracking of the crust would allow part of it to sink down slighly, providing an opening for mantle materials to move upwards, and cool. There would be a balance between these materials cooling and becoming more dense, and the pressure inherent in the mantle both pressing them upward and condensing them to higher density.

The iron core would be largely elemental, but the condensing minerals would be combinations of metals and anions of various kinds, as there would be plenty of these elements in the initial cloud as well. The proto-planet would have elemental carbon and oxygen, which might combine to form a carbonate with some metal. And so on for all the other types of compounds found in ores. It might even be that the gas cloud, which has some percentage of dust mixed in it, already has some beginning compounds, and these partially remain intact during all the condensation and heating phase of planetary formation.

It would seem that the best way to explore our local galactic neighborhood for planets containing life and also alien civilizations would be to improve our telescopes and other detectors, and look for an Earth sized planet, located in a stable orbit relative to the other planets, and with a large moon locked into a orbit around it. Of course the stable orbit must be in the liquid water zone, have some axial tilt, and not be in too elliptical an orbit, which may be implied by the stability of the orbit, unless there were no large planets in the solar system.

This tangentially raises another interesting question for our exo-planet astronomers: are there any solar systems which have only one planet? Or is this an impossibility due to some feature of the mechanism of planetary formation? We on Earth have detected only one planet in most of the solar systems we have so far discovered, but that is not the same thing. It would be fascinating to find out there were many like this, with one planet only. This revelation would mean that we have less guidance from our home solar system toward understanding what goes on in other ones.

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