At this time (January 2006), exochemistry is very much in its infancy. To the best of my knowledge, the only lab working full time on processes suitable for space is at Pioneer Astronautics, althought their focus is on rocketry rather than the study of chemical processing per se. There are several other pioneering research groups working on processes for chemical production in space, and some have tested out processes in the laboratory. Most of this work has been done by geochemical researchers, because the main problems at first center around the difficulties of handling materials in the space environment (crushing and processing rock, etc.). Since the products needed at this early stage are simple, the chemistry of these early processes is likewise fairly basic, though some of the techniques used are anything but simple!

Methane/Oxygen Production on Mars

This is the first process to be seriously considered by NASA. Any viable large-scale return mission to Mars has to be able to refuel on Mars, necessitating chemical production of methane/oxygen fuel on Mars. The oxygen would also be useful for life support on manned missions.

Sabatier methanation

Most methods under consideration center around the overall process:

2 H₂O + CO₂ → CH₄ + 2 O₂ (Equation 1)

Water is available in small amounts in Martian soil, and carbon dioxide is the principle constituent of the rather thin atmosphere. Since this equation is the exact reverse of the high exothermic combustion of methane, a large amount of energy must be supplied. There are a variety of methods of achieving this, though many center around the (well-established) electrolysis of water (using solar electricity) to produce hydrogen:

2 H₂O → 2 H₂ + O₂ (Equation 2)

followed by reduction of carbon dioxide to produce methane and some oxygen:

4 H₂ + CO₂ → CH₄ + 2 H₂O (Equation 3)

Note that Equation 1 = (2 x Equation 2) + Equation 3.

The second step, known as Sabatier methanation, is in fact exothermic. An excellent method has been developed for achieving this in a microreactor at around 250 ^oC using a ruthenium catalyst. This paper (PDF, 54k) was presented by VanderWiel et al. at the AIChe 2000 Spring National Meeting, Atlanta, GA.

Related chemistry may be used to produce acetylene (C₂H₂) as a fuel in place of methane.

Electrochemical reduction of carbon dioxide

An alternative approach is to reduce carbon dioxide directly using electrochemical methods. This was reviewed in late 2005 by a SUNY Potsdam undergraduate student, Matthew Hudson, in his paper (PDF, 352k).

Oxygen Production on the Moon

For a base on the Moon, such as that proposed by China, the most important process is likely to be production of oxygen for life support and fuel. [The other important life support material, water, does occur in trace amounts on the Moon.] Oxygen must be obtained from minerals such as FeTiO₃ or Mg₂SiO₄, either directly or indirectly. A large number of possible processes have been suggested, of which the most widely supported are the following:

Pyrolysis of minerals

In this method, high vacuum and high temperatures (>1700 ^oC, typically from a solar furnace) are used to decompose minerals directly into oxygen and metals, e.g.

2 FeO(g) → Fe(g) + O₂

This method is attractive in that it is a simple, direct method, and it does not require materials from Earth (other than the equipment used). However this method is still in a relatively early stage of development.

Electrolysis of molten silicates

As with pyrolysis, this is a direct preparation of oxygen from common minerals. If an electric current is passed through various molten silicate rocks (at ca. 1500 ^oC), oxygen gas is produced. As with the pyrolysis method, this process is chemically simple and requires few materials from Earth. It has been demonstrated in a terrestrial laboratory, and it can use simpler equipment than the pyrolysis method. However, there are some problems to be considered, such as energy loss through unwanted reaction paths (e.g., oxidation of FeO in place of O₂ formation at the anode) and corrosion of electrode materials such as Pt (due to attack by O₂ at the high temperatures used).

Chemical reduction of ilmenite (FeTiO₃)

FeTiO₃ is moderately common on the Moon, and it represents a more energetically favorable source of oxygen than do commoner silicates such as olivine (Mg₂SiO₄). It may be reduced by hydrogen, carbon monoxide or methane at temperatures of around 1000^oC, lower than is needed for silicates but still high. For example, the following reaction has been demonstrated in a terrestrial laboratory:

FeTiO₃ + H₂ → Fe + TiO₂ + H₂O

followed by electrolysis of water:

2 H₂O → 2 H₂ + O₂

A related process involves the chemical reduction of a naturally occurring glassy mineral which like ilmenite contains the equivalent of FeO.

These processes involves more moderate temperatures than the other processes mentioned, but unlike the other processes it does require considerable benificiation (pre-processing) of the mineral. It also requires the use of materials that are rare on the moon (hydrogen is much less common than on Earth, while carbon is much more rare).

As yet, no one has devised a successful low temperature process, although clearly this would be desirable in a location without cheap energy. However the recovery of elemental oxygen from any mineral found on the Moon (or on Mars) necessarily takes a large amount of energy, since these minerals are very stable thermodynamically. The final choice of process will depend on a variety of factors- the size of the facility or base, the costs of bringing materials from Earth, location of suitable minerals and processing reagents (e.g., H₂O or carbon sources) and new developments in technology.

Manufacture of Metals

This topic has been less studied, but metals are likely to valuable materials for construction on the Moon and Mars when physical strength is necessary. Since iron (and to a lesser extent nickel) exist in the native state on the Moon (ca 0.5% of lunar soil), this might be used without significant chemical processing. Alternatively, purification of this free metal using a chemical process (such as the Mond process for Ni) might be used. There is not believed to be a significant amount of native metal on Mars, though we may expect to find some native metals on hydrogen rich moons such as Titan.

Many of the processes for lunar oxygen production discussed above also produce metals (such as Fe, Mg and Al) as by-products, as well as elemental silicon. On Mars, a separate process would be necessary, though this might in fact be similar to those used for lunar oxygen production.

Asteroids, including near-Earth asteroids (NEAs), are a rich source of metals, often in their native state. For rare, valuable metals such as Pt, Pd, and Rh, it may even turn out to be profitable to mine NEAs to supply Earth. At present we probably do not possess sufficient knowledge of these NEAs to be able to design a practicable process.

Other materials

For other materials, such as ceramics, polymers and organic compounds, virtually no processes have been developed or evaluated for their suitability in space.

General comments

It is worth remembering at this point that the criteria for a viable process in space are very different from those on Earth. On Earth, we work with a large feedstock of water and also biologically-derived resources, such as oxygen, oil and coal. The combustion of the fossil fuels provides cheap heat energy on Earth. By contrast, any heat generated in space may well require solar or geothermal power, making simple thermal processes less attractive in space. Solar powered electrical energy is relatively more attractive in some parts of space (e.g., the Moon) than it is on Earth. At first, simple processes are to be preferred over more complicated ones, since most of the chemical plant will need to be brought from Earth, and the plant will be more difficult to maintain.

At present, only a few key processes have been examined, often with little or no laboratory testing. In many papers on in situ resource utilization (ISRU), there is an implicit assumption that "we can adapt this terrestrial process, as long as we adapt the engineering for space." It is essential that the process is designed with space in mind, and that it is fully tested in the laboratory before a mission is planned. Writing from personal experience, I know that the scale-up (on Earth) of a well studied laboratory process frequently goes awry- sometimes disastrously- how much more likely is this in space?

In reality, it may be that processes will not be designed until a specific mission has been planned (e.g., a Mars living environment or an outpost on Europa). However there is a "chicken and egg" aspect to this, namely that the very feasibility of the whole project may rest on an exochemical process (as yet undeveloped)- causing the project to be cancelled because of uncertainty and the absence of the necessary technology. Even if a project is given the green light, it may be too late; new chemical phenomena (unknown on Earth) might be uncovered, rendering the whole project unworkable and discrediting the entire enterprise. It is essential that we begin studying possible exochemical processes now, so that we are ready when these processes are needed. Along the way, we will know doubt make new discoveries that will transform our understanding of chemistry itself.

Finally, we must not ignore the formidable engineering challenges facing the designer of a chemical plant in space, to be discussed in a separate section on process technology.

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Last updated 24th January, 2006.