Geothermal power has some advantages that should exist, in certain situations, for alien civilizations. It is a fairly simply technology, designed to extract energy from the heat generated inside larger planets from gravitational collapse and impact. Because heat conduction is so slow, this energy can exist in extractable amounts for billions of years. It is one of the few ways of extracting gravitational energy, another being tidal power.
On Earth, it is used very little, as other sources, nuclear, hydroelectric, fossil fuels, wind and even solar usually produce more high-value energy at lower costs. For making electricity, these sources are more efficient. The majority of uses of geothermal power are for heating, as the energy it produces is simply low temperature heat.
The technology just involves drilling down into the Earth and extracting the heat. Usually heat transfer with a liquid is used, as liquids have a higher heat capacity than gases, and conduction is even worse. On Earth, in some areas with volcanic conditions, where hotter mantle material is closer to the surface and forcing more heat upward, it can be readily used. Iceland is everyone’s well-known example. There, by drilling down a half to one and a half kilometers, temperatures in the region of 200 to 400°C, which can produce steam at the bottom of the pipe, which then rises and turns a turbine or serves directly to heat something. Drilling and inserting pipe, even pipes with good insulation, is not difficult compared with other contemporary technology, and so this might be expected to be used by any alien civilization that has the heat source available.
Iceland itself is a volcanic island, and still has volcanic activity on and near it, resulting in, besides volcanoes, hot springs, warm lagoons, and geysers. On an alien planet, with these features widespread, it would be possible for them to dispense with fossil fuels. There is an obvious question as to whether geothermal power could provide the same transitional energy, taking an alien civilization from biomass, flowing water and wind power to nuclear power of the fission and fusion varieties. If that is the case, then this would be a separate line of development, differing significantly from those worlds which might be analogs of Earth’s development.
In an Earth analog developmental path, biomass is used for a long period, until coal is discovered and used as an improved power source. The higher energy density in coal allows machinery to be invented and powered, including mobile machinery. Coal deposits near the surface in England were one factor in why England initiated the industrial revolution here on Earth. In an alien world without coal deposits like this, or something equivalent such as large tar deposits, would it be possible for their technology to develop industry using geothermal power? Geothermal power is not mobile, and so the great advantages of coal in powering locomotives and hauling large quantities of materials, say from mines to processing plants, would not exist. This would mean the population would have to concentrate around the geothermal sites, if they were going to take advantage of the power.
Instead of watermills and windmills, it might be expected that sources of steam would be used to power mills, in some method or other. This might lead to the invention of some machinery, but there would be much less use of it, both because of the lack of mobility but also because of the lack of metals from mining, which on Earth was greatly facilitated by coal and later other fossil fuels. Metals were available as far back as the stone age on Earth, so they would be available on alien planets, if the right ore deposits were available. However, smelting takes advantage of the concentrated heat that can be generated by coal. Blacksmiths from the Roman era and even before used bellows to concentrate heat from hardwood fires and produce iron and steel, but there is a quantitative difference in the amount that can be produced.
With no coal for the first railroads, would there be any industrial revolution on an alien world that depended on geothermal power? Iron would be much more expensive, meaning machinery would be invented much more slowly and would not propagate as fast, meaning that progress would be very much slower than in the rapid pace of Earth’s industrial revolution. Electricity would be invented, but perhaps centuries later. Once electrical machines were invented by alien equivalents of Moritz Jacobi and Werner Siemens, the inventors of the ac and dc motors, and many other equivalents and accessories needed for an electrical power industry, along with generators, transformers and so on, the alien civilization should be able to develop some sort of mobility other than by animals and wood-fired engines. Because this type of transportation is much less efficient than fossil fueled transportation, the rate of progress would remain much slower than we experienced here on Earth.
Slower, but not impossible, is the diagnosis of the various technology steps that a non-fossil fuel world would experience with an abundant source of geothermal power. What kinds of worlds might have this?
There is a relationship between the presence of heat sources just below the surface of a planet and the surface temperature. If it is too hot, life cannot evolve in the same way that it did on Earth. Carbon-based life needs a fairly narrow range of temperatures to exist, and an even narrower one to evolve into multicellular creatures. It is not clear if there is even a narrower range in which large differentiated creatures can exist. So, a planet with a generally hotter core might prevent this. Is it possible that the phenomena that exists in Iceland could exist on an alien planet in many more places, but not so much as to heat the surface by more than a few degrees? Iceland is located on a boundary between two tectonic plates, where molten magma can rise to the surface more easily. Hawaii does the same, but the volume is less there and there are only few places where geothermal energy can be tapped as readily as in Iceland.
Suppose the chemical distribution of elements was somewhat different, and there were less lighter elements and the crust slightly thinner on some planet. The thickness of the planet’s crust cannot be much less than Earth’s, or heat would pass through too fast. Thus, a bit less light elements might do the trick. Then there might be many locations like Iceland, and industrial development could occur, only over millennia rather than centuries.
Another consideration is that there would have to be no fossil fuels available to the civilization, as these would displace any use of geothermal power and more accurately align the development there with that of Earth. The two theories for fossil fuel origination are the biotic one, in which vegetation is buried and becomes heated, transforming the remains into mostly carbon chains, and the abiotic one, in which carbon chains form and chemically separate, just as most other minerals separate into segregated ore bodies. However, it doesn’t have to be that there are no fossil fuels on the planet, but just that they are not available. This could mean no coincidence of shallow burial of coal deposits and alien civilization areas. With the biotic theory of origination of fossil fuels, this does not seem likely on a planet which has enough life to generate eventually intelligent aliens. So this may be the actual show-stopper on the geothermal variant of alien civilization. It certainly needs to be thought through more deeply.
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