Being the occasionally interesting ramblings of a major-league technophile.
Why We Need Planets
It is a bit of a trope in SF - including some very bad stories - that aliens land on Earth to steal its resources. People aware of such things as the composition of lunar regolith and asteroids - as well as the fact that there's lots of water on those bodies as well as comets and icy moons - decry this as pure folly. (Ice Pirates can get away with this because it's a farce, but there have been some serious works which base their entire plot on absurd concept of aliens wanting Earth's water.) Why drop down into a deep gravity well, grab something, then climb back out, when the same resources are available for far less work elsewhere?
The thing is, there are resources on planets which aren't available for far less work in space. Or, in some cases, at all.
Much is available in space. Light elements - such as carbon, nitrogen, oxygen, hydrogen and so forth - are plentiful, both in the interplanetary dust and in asteroids and comets. Asteroids also contain small but significant quantities of heavy metals, valuable both intrinsically (as conductors, for reflective coatings, etc.) and extrinsically, due to historical significance. (Gold! Gold! Gold! Eheh... 'scuse me...) The soil on the Moon is rich in helium-three, which is easy (relatively speaking) to fuse, making it a valuable energy source. Mining regolith or asteroid crust should be easy. The material is loose, barely consolidated if at all. Just rake it up and sort out what you want. However, while these sources contain many raw materials necessary for life or profit, there are other things which are necessary, useful or simply desirable, which cannot be found on small bodies.
Geological processes on large bodies - such as differentiation and metamorphosis - take those raw materials and segregate, concentrate and physically and chemically alter them. Think of gravitational differentiation as a radial centrifuge, pulling everything in a body towards the center of mass, in the process separating them by density. The more fluid the materials, the better this works, of course. Fluidity is temperature dependent. Temperature in turn depends on several factors: Kinetic heating (from infalling material); Tidal heating (from the flexing which takes place due to tides); Radiative heating (from incoming light and heat, usually from being close to a star); and Radioactive heating.
Gravitational differentiation concentrates materials according to density. Other forms of differentiation concentrate materials by different mechanisms. This means that on bodies which are big enough for these processes to take place, normally rare substances can be found in large amounts if you know where to look. Differentiation not only concentrates rare materials where they can be easily gathered, but also allows chemical processes to occur on differentiated bodies which would be extremely rare to impossible in space. (Remember how happy scientists were to find minerals on Mars which require water to form?) In addition to the chemistry, physical and mechanical processes only found on planets are necessary to produce certain minerals. Additionally, large bodies have transport processes which move things around, often selectively. (Diamonds only form deep underground. Most diamonds humans find are brought to or near the surface by volcanic processes.)
Gold has many technical uses, besides its purely decorative value. So do all the noble metals. Under what circumstances would it be more economical to drop down to a surface, mine the metal or high-quality ore (which would presumably be processed on the planet to further concentrate the target material) then boost back out, instead of processing regolith, or whatever? Is the easiest way to mine gold actually on a planet, where you have large veins forming in rock? Or is it by processing huge amounts of asteroidal material to recover the faint traces distributed throughout? The answer depends on both the particular asteroid and the technology available.
I posted my thoughts about this to my LJ account. A professional geologist added geothermal processes, which combine heat and water to produce materials not available otherwise. She also pointed out that early in the formation of the solar system, the higher proportion of radioactive materials would allow gravitational differentiation on smaller bodies than can occur in now. There would also be a greater likelihood of bombardment by energetic particles and photons, which can alter physical properties. We could find that the largest asteroids are solid bodies made up of radial layers of differentiated materials, slowly cooling, thanks to this effect. There is evidence that at least Ceres has experienced differentiation. If you have ever seen a large jawbreaker cut in half, you have the general idea of the effect, though with jawbreakers the process is one of accretion during multiple baths in different candy formulations. Depending on the amount of kinetic heating from impacts during formation you might not even need radioactive heating for differentiation to occur on asteroids and moons.
A minor digression, here. Time alters the composition of materials in a body through the decay of radioactive isotopes. Uranium ore in Oklo, Africa, was discovered several years ago to be oddly depleted. Studying the veins of ore, it was determined that in the distant past the higher natural concentration of the shorter-lived uranium isotopes back then plus moderation by ground water had created a natural fission reactor! The topic is fascinating, and may be examined further in another JOHT.
Our Moon most likely formed when a Mars-sized body impacted the young Earth. Most of the material was added to our planet, with much of the rest forming a ring around it. This ring coalesced into the Moon in a process which probably left that body molten throughout for millions of years. The lightest volatiles were driven off, baked out of the forming rocks. Dense materials settled toward the core, leaving light but non-volatile materials on the surface.
The Moon is different from a typical asteroid in more than size. Besides the separation which took place due to this formative heating there were also the tidal effects of having the Earth so close. The distance between Moon and Earth was much smaller in those early days, amplifying the effect. One result is that the Moon is lop-sided, with masscons (mass concentrations) located deep under the surface in several areas. Further complicating things, late, heavy bombardments broke through the light crust and allowed dense magma from below to well up, among other effects creating the famous lunar maria.
One effect which could occur on other bodies which is rare as a natural process on Earth is distillation. There is considerable evidence for ice in permanently shaded craters around the Moon's poles. The most likely source is cometary water. Comets would impact the Moon, the water vaporizing into the vacuum, with some freezing out in those shadowed craters.
This brings up some interesting speculation on geochemical processes which take place on the moons of gas giants. Many of these are heated by tidal effects. They start with a different mix of ingredients than you would have on rocky, inner bodies and then bake for billions of years. What might you get? We know Saturn's moon Enceladus has geysers of mineralized water. Ice is a solid mineral there, but tidal heating can melt it deep under ground. The resulting effect is more like a volcano than a geyser. There is a very good chance that close-up images of that moon's surface will show conical formations of ice as the sources of these eruptions. Much like those more recently found on Ceres. The water has probably brought minerals to the surface with it. Volcanos on Earth can bring up enough sulfur to creates lakes of concentrated sulfuric acid, besides the diamonds mentioned above. Imagine mining the side of an ice volcano for veins of precious gems which can't even exist at room temperature.
Remember that some elements will be better concentrated on geologically active planetary bodies due to geologic processes. That's the basis of our ore deposit exploration here on Earth. Rather than mining for precious exotic gems, settlers might have to tap those veins on Enceladus for the relatively high concentrations of trace elements which are otherwise segregated in the core.
On Earth, limestone, granite and other metamorphic rocks are used for things besides sources of raw ingredients. However, we wouldn't likely need metamorphic rock in space for structural uses. The Moon's soil is rich in aluminum and some other light metals, and we already know how to make lunarcrete from the regolith. Asteroids would provide similar resources. I could easily see marble, granite and even limestone becoming luxury items, though, used to decorate executive offices in space. (I'm talking about really thin veneers, here. :-)
Besides providing potential resources, the presence of certain minerals tells us things about a body's past. For example, The ESA's Venus Express has found evidence of granite on the shrouded planet. If this proves out, that is significant. Before, it was thought that Venus' plate tectonic system was stillborn. However, to create granite you start with basaltic rock, bury it deep under the ground with enough water, leave it for a while, then bring it back to the surface where it can be found and quarried. This means that Venus had plate tectonics long enough for the conversion, and enough water to do the job. If there actually is granite there.
A large part of the answer to the question of whether going to the surface of a large planet is worthwhile depends on how easy access is. If the planet has a beanstalk (space elevator) things become much easier. If the miners are still limited to rockets - even very good rockets - the justification for the trip becomes much more difficult.
However, there is one thing we know is freely available on Earth which we haven't found anywhere else: Life.
How
much would you pay to bring a small package of grass seeds and some
potting soil to your Moon base?
This document is Copyright 2019 Rodford Edmiston Smith. Anyone wishing to repost it must have permission from the author, who can be reached at: stickmaker@usa.net