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.

Geological Separation on Exo-Planets

In order for an alien species to proceed upward through the various stages of technological development, finally arriving at the top level, asymptotic technology, where it might start a starflight project, it has to have access to resources of many types. Energy sources are of course on the list, as without abundant easy-to-obtain sources of energy, the aliens cannot move into the industrial phase of development. Without large areas of fertile soils, they cannot even get far into the agricultural phase, and are forced to languish in the stone age until they become extinct.

There are more. The industrial era needs some mineral resources, such as iron and other metals, and as the age progresses, more and more elements and compounds are needed. The history of technology on Earth might be written as a history of materials and their availability, and it is the same for any alien species on an exo-planet. For example, one cannot have the massive computational capability needed to move into the artificial intelligence phase unless there are the unique materials needed for processors and memories, as well as other electronic components. On Earth, we started with vacuum tubes, which only require some glass, tungsten, copper and maybe a few more. But one cannot get far into heavy duty computation without the invention and deployment of transistors.

Where do all these materials come from? Some are directly obtained from mining, and others are produced from mined ores and their derivates. Hydrocarbons have to be included as a mined material, as many products include hydrocarbon derivates. Would these all be available on every exo-planet?

Not all dust clouds in the galaxy are equal. Before a star condenses and forms a system of exo-planets, it receives the residue from some supernova explosions, which are the accepted generator of higher atomic number elements. A huge tsunami of neutrons comes rushing out of the stellar implosion, and these build up existing elements to ones higher in atomic number. A particular gas cloud, prior to condensing to a star and a planetary disk, might have had a large number of large supernova and therefore be very rich in elements, or it might have not been so fortunate, and the star condenses with a planetary ring having little iron and the whole slew of other useful elements in it. This means the planets cannot have rich resources for any alien species which develops intelligence on one of them. It is not clear why an alien species could not develop on such a planet, so it could be what we call an origin planet, but it is one which will never have an alien civilization that could build a starship to come and visit Earth.

We should do some surveys, if we haven't already, and see if the galaxy around us is filled with very rich-in-resources clouds or if there are some that are and some that are not. That is one piece of astronomy which would help answer the resource availability question, but it is not the only one needed.

The other half of this question involves the accessibility of resources. Suppose we have a planet which condensed from the inner part of the disk where there were lots of resources, and the free hydrogen and helium all escaped, leaving a planet like proto-Earth. Does geological separation into the crust automatically follow? The planet upon condensing would be molten, from the huge release of gravitational energy, and it would be radiating its energy outwards as heat, gradually cooling. The outer surface of the molten droplet would get cooler faster, as the cooling happens faster than the conduction of heat from the interior. So a crust forms, but does it have separated ores? Ores need to be separated to a large degree, or they are inaccessible to the aliens.

If we had, on Earth, exactly the same set of elements in the crust, except they were not separated out but the crust was fairly homogeneous with a little of this and a little of that, in roughly the same proportions, everywhere, there would be no use in mining. There would be no point in searching all over the planet for some concentrated source of some industially important material, as it would be everywhere in tiny concetrations and nowhere in large concentration. Thus geological separation of various ores is a critical and mandatory requirement for the development of an advanced alien civilization.

We have one example to examine: Earth. We need to know if Earth is unique or ordinary, as far as geological separation is concerned. There can certainly be all kinds of degrees of this, so ordinary covers a huge plethora of types. There could be an exo-planet, with an even higher degree of separation, so at different points on the surface of the crust, there would be mountains of cobalt ore, or mountains of germanium ore, and more and more. Or it could be that an exo-planet has the same ores as Earth, but they are just smaller in amount, and harder to obtain. There is a question of the cost of accessing these ores. They produce some benefit to the alien society, at whatever stage in technology development it has reached, and if the benefits are small compared to the cost of mining, processing, refining and transporting them, they would not be mined. The society would not have them around to develop new applications and new technological uses, and therefore new technology. With costs of obtaining resources prohibitive, it is just as bad as if the primordial gas cloud was less rich.

Do we understand the process of geological separation of ores, quantitatively, so that we can compute some estimates of the existence of large, low-cost deposits on other exo-planets? When condensation happens, everything is mixed together, and immiscibility in the molten drop, perhaps mostly of iron and those elements which mix well with it, will lead to a separation. The ores which separate out, condensing somewhere in the molten planet, and which have density lower than that of the drop itself, will rise up to the crust, where cooling is taking place. These bubbles of molten ore might reach the crust anywhere, so the crust could have any type of ore anywhere. How big do the bubbles, which are concentrated in a few elements, specifically metals, with some carbonate or sulfate or other anion attached, get? The ones which are lower in density move upwards faster, but do they have time to grow larger? The slower the rise to the crust, the longer the time for a bubble of ore to grow. Several ores might be tangled together, leading to a mixed ore region, but that might actually help in the cost of accessing them. If the crust cools too fast, they don't rise up to near the surface, but are stuck below where they are too deep to practically dig out. What would keep a proto-planet from cooling to fast? Tidal friction from a large moon, in close.

The Earth, as far as we can tell now, is unique in that its moon is a large mass fraction compare to other satellite-to-planet ratios. Did the tidal heating from the moon, shortly after it was formed in a planetesimal impact on the proto-Earth, keep the crust hotter and thinner so that ores could form in large volumes more easily? If this is so, there might not be only one reason why a large moon is necessary for an advanced alien civilization but two: life originates with the moon's influence and ores form in larger quantities with the moon's influence. What an astronomical coincidence...