Places: Possible sites for exochemical processing
Where might we want to make chemicals, other than on Earth? The Moon and Mars, as our nearest habitable neighbors, are the main locations being studied at present. In the long term other more challenging locations such as Titan or Triton may become important.
In the International Space Station (ISS), a microgravity environment is present which cannot be matched on Earth. Already some basic phenomena have been studied, such as crystal growth, fires and explosions, as well as the behavior of bubbles and drops. It has been suggested that there may be advantages in making certain materials in this environment, such as protein crystals and single crystals of semiconductors (for making "chips"). Clearly there are no natural material resources present, so chemicals will not normally need to be made on the station. However, the low gravity environment of the Space Station may well prove to be a useful testing area for low gravity processes intended for use on distant planets or moons.
On the Moon, our nearest neighbor is an obvious staging post for the exploration of space. It requires much less energy to lift materials into space from the Moon, making the Moon an attractive source of materials for use in space. Carbon and hydrogen are present in much lower amounts than on Earth, so organic compounds (e.g., plastics) are not suitable for lunar production. However, silicon and oxygen are abundant (though no oxygen is present as breathable O2) , as are metals such as iron, aluminium and titanium. Any humans living on the Moon would need a production system for water and oxygen as O2) and there are already several systems under discussion.
Our neighbor Mars has the most similar environment to Earth of all the planets. Even Mars, though, is an inhospitable place for human beings, far worse than anywhere on the surface of Earth. The atmosphere is very thin (less than 1% of the pressure of Earth's), and is composed principally of carbon dioxide. Human beings would need spacesuits, as well as specially prepared supplies of oxygenand food. Thus a permanent human presence on Mars would necessitate the exochemical production of oxygen as O2, and perhaps other materials such as chemical rocket fuels such as methane (CH4) or toluene (C6H5CH3)and O2). Mars also has two small moons, Phobos and Deimos, that have been considered by some to be good sources of raw materials (such as silicon and oxygen) because of their low escape velocities (hence low energy removal to space).
Venus is our other planetary neighbor, but there the environment is extremely hostile to human beings. The atmosphere of Venus has a pressure ninety times that of Earth's, more like an ocean than an atmosphere! The composition is (like on Mars) principally carbon dioxide, but highly corrosive materials are also present such as droplets of concentrated sulfuric acid. A runaway "greenhouse effect" causes the surface temperature of Venus to be a scorching 500oC or so (750 K), hot enough to melt lead- no ordinary spacesuit could protect against such conditions. Water and hydrogen compounds are probably much rarer than on Earth, though the crust is composed of familiar elements such as silicon, aluminum and oxygen. Humans are unlikely to want to visit Venus, but it is possible that we will find some valuable materials on Venus that can be extracted remotely by machine.
Mercury is the closest planet to the Sun, and also one of the smallest. Mercury has essentially no atmosphere, leading to vast temperature differences between night and day, or even light and shade. With the Sun shining from so nearby, temperatures can reach around 700 K (over 400oC), and yet there are places in permanent shade (at the poles) where it never exceeds 100 K (-170oC). This allows something highly unexpected so close to the Sun and at such low pressures- ice (probably H2O and possibly CO2)! Mercury may be an attractive site (and/or raw material source) for a solar energy station, and the presence of water ice may make a human base feasible, shielded from the Sun's powerful radiation.
Near Earth Objects (NEOs), consist of asteroids and comets that orbit fairly close to the Earth's orbit. Of these the Near Earth Asteroids, including Earth "Trojans", offer a very attractive material resource requiring little energy to transport (see John Lewis' book, Mining the Sky, for more details). The majority are silicate rocks similar to the surface of the Moon. About one-fifth are rich in volatile compounds of carbon, hydrogen, oxygen and nitrogen, useful for supplying rocket fuel and life support materials (water, O2). Another 3% are almost pure metal, mainly iron, nickel and cobalt, useful for construction. Some contain valuable rare metals such as the platinum metals (useful as chemical catalysts) as well as gallium, germanium and arsenic (for semiconductor "chips"). The NEAs are only a small fraction of the asteroids in the Solar System- most are in the "main belt" (between Mars and Jupiter) or belong to the group called "Trojans", 60o ahead of or behind Jupiter. Still others lie beyond Saturn or in the Kuiper belt (beyond Neptune).
Jupiter is the largest planet in the Solar System, but its this makes it a challenging place to visit because its large size causes it to have extremely high internal temperatures, pressures and magnetic fields. Its high gravity makes it energetically difficult to remove material from Jupiter. Jupiter might possibly be used as the source of a useful fuel for nuclear fusion, helium-3. This isotope is virtually unknown on Earth but makes up about 20 parts per million of Jupiter's atmosphere (in terms of molecular abundance).
The satellites or moons of Jupiter are of great interest as possible bases for exploration of the outer Solar System. The four largest, Io, Europa, Ganymede and Callisto (known as the Galilean satellites) are comparable in size to the Moon, or larger. Io is dominated by huge volcanoes, induced by tidal forces from Jupiter. It may be rather hazardous to plan a base there until Io is fully understood, but if this is possible its surface is very rich in sulfur. Europa probably experiences a much milder form of vulcanism, sufficient to continually remake the surface with a fresh layer of water ice. Some scientists believe that water oceans may exist below the icy surface. Ganymede, the largest satellite in the Solar System (diameter 5280 km), has dark and light areas on its surface. The light areas are similar to Europa's ice, while the dark areas look to be composed of a useful combination of clays and thorins (organic compounds of C, H, O, N) with some carbon dioxide and water ices. The surface of Callisto is probably similar to the dark areas of Ganymede; however its core is believed to be largely undifferentiated (i.e., it has not melted and separated into a core and mantle), and this may mean that the surface is richer in metals than most planets and moons, making Callisto an attractive source of raw materials. There are also several smaller moons including Amalthea.
Saturn is another giant planet whose sheer size limits its viability as a material source, although the rings might provide a useful source of H2O. Only one of Saturn's moons, Titan, is comparable in size with the Galilean satellites of Jupiter. Titan is one of the most fascinating places in the Solar System, since it has an atmosphere of nitrogen (94%) and hydrocarbons (6%, mainly CH4) comparable in pressure to that of Earth's atmosphere. It was visited for the first time by a space probe, the Huygens probe, in January 2005. Preliminary findings suggest the present of a surface (probably of hydrocarbons) with strewn with water-ice "rocks", and a large methane ocean. Titan is the main target for the Cassini-Huygens mission, due to reach Titan by late 2004. The planet is extremely rich in valuable organic compounds, which might provide material for growing plants as well as for plastics and fibres. However the presence of " methane oceans" remains disputed until Cassini-Huygens can resolve the issue. The surface is also believed to be rich in ammonia and water ice, all at a temperature of around 80-100 K. Density measurements indicate that the inside of Titan must also contain a lot of dense material, presumably silicates. As a large moon rich in resources, Titan is an obvious site for a base from which to explore the outer Solar System. The other moons of Saturn are much smaller, and are believed to be composed mainly of water ice, with less rocky material present.
Uranus, though smaller than Jupiter and Saturn, is also a giant planet with an atmosphere composed mainly of hydrogen, with a little methane, with other materials such as ammonia and water in the lower atmosphere. The moons of Uranus, though small, would provide a much more accessible source of materials . These moons are similar in size to those of Saturn excluding Titan, and they are also similar in composition ( H2O, NH3, CH4), except that more rocky material is present. Neptune is similar in many ways to Uranus in size and composition, but Neptune does have one moon comparable in size to the Galilean satellites, namely Triton. Triton is in many ways like a very cold (38 K) version of Titan, with a surface believed to contain much solid nitrogen and methane , as well as probably water and ammonia ice and an extremely thin atmosphere. Triton would therefore be a natural, if rather cold, choice for locating a distant research base. The final planet Pluto is smaller than the Moon and is believed by many to be simply a deviant icy satellite or perhaps an object from the nearby Kuiper belt. Our knowledge of Pluto is necessarily sketchy at this time, but it is believed to be a mixture of silicates and ices such as H2O, N2 and CO. Pluto also has a medium-sized moon, Charon.
Beyond the Kuiper belt and the edge of the Solar System, we know very little about what may be there. At present the nearest star, Alpha Centauri, lies well beyond our grasp and thus we must (regrettably) limit these speculations to the environs of our Solar System.
Last updated 6th February 2005.