stickmaker | Joy of High Tech #49 (Reply)

The Joy of High Tech

Rodford Edmiston

Being the occasionally interesting ramblings of a major-league technophile.

Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. If you find a mistake, please let me know about it.

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 on comets and icy moons - decry this as pure folly. (Ice Pirates can get away with this because it's a farce. However, there have been some serious works which base their entire plot on absurd concept of aliens wanting Earth's water, usually written well after we learned how much water was in - as one example - Saturn's rings.) 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? (In 1952 Isaac Asimov's story The Martian Way pointed out the folly of this idea.)

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. Good old CHON (Carbon, Hydrogen, Nitrogen and Oxygen) is available in varying proportions in the atmospheres of gas giant planets. Asteroids contain 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, or regolith, on the Moon is rich in helium-three, which is easy (relatively speaking) to fuse, making it a valuable energy source.

The main problem with acquiring these resources is that they are dispersed. There hasn't been the gravitational and chemical differentiation on these smaller bodies which occurs on rocky planets. Though the Moon and larger asteroids have had some. Vesta and - especially - Ceres show signs of the former, and perhaps of the latter. 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 usually temperature dependent. Temperature in turn depends on several factors: Kinetic heating (from infalling material); Tidal heating (from the large-scale 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 look in the right places. 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. As well, large bodies have transport mechanisms 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 planetary surface from space, 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, the particular planet and the technology available.

I previously posted some of my thoughts about this to my LJ account. A professional geologist added the idea of 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 for smaller bodies than it can occur in now. There would also be a greater likelihood of bombardment by energetic particles and photons on bodies with little or no atmosphere or magnetosphere, which can alter chemical and physical properties. We could find that the largest asteroids are now-solid bodies made up of radial layers of differentiated materials, slowly cooling. 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. There is evidence from the Dawn probe that at least Ceres has experienced differentiation, and even a form of vulcanism. These activities may actually be ongoing, though at steadily reducing levels. 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. Some bodies of uranium ore in Oklo, Africa, were 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 in a glancing blow. Most of the incoming material was added to our planet, with much of the rest forming a ring around it. Most of the remainder of the ring was made of crust from the proto-Earth, knocked into space by the impact. 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. How much of the Moon might still be molten, and how active it remains are subjects heatedly argued.

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, this created 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, with the possibility of ejected water freezing as ice around the vent and forming cones. 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 recently found on Ceres; except that there mineral-rich water boils to the surface and evaporates, leaving the minerals.

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 exploitation here on Earth. On Enceladus and other bodies, rather than mining for gems or rare metals, settlers might have to tap similar veins for the relatively high concentrations of vital trace elements which are otherwise segregated in the core.

On Earth, limestone, granite and other metamorphic rocks are used both directly - for structure or decoration - and as 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, such as titanium, 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. This means that Venus had plate tectonics long enough for the conversion and transportation, 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?