Why Exochemistry?
As chemists, we often have this sense that we have learned all of the basics, and we are just filling in a few gaps in our understanding. Even in organic chemistry- the younger sister of inorganic chemistry- we imagine that all of the basic functional groups were understood by 1900, physical organic chemistry by 1960, and Woodward, Corey and others had sorted out the basics of synthesis by 1975. OK, we're still finding refinements- palladium chemistry, ionic liquids, enzymatic methods- and we're only beginning to get to grips with asymmetric synthesis. But we know most of the basics, right?
Wrong! What we know is totally skewed towards one environment- that of Earth. All of our reactions assume a baseline of 298 K, 1.01 x 105 Pa of an oxygen/nitrogen/argon mixture with some water vapor, and one gee of gravity. We presume that water is common, as are hydrocarbons and their derivatives. In the same way that many of our ancestors thought of the universe as being centered on their own country, today many chemists think of Earth chemistry as being "normal" and anything outside of those conditions as being "abnormal." But why should 298 K be normal, rather than 98 K or 1298 K?
Our colleagues in other disciplines have embraced the exciting new environments found in space. Physicists have always explored the bizarre, pushing the boundaries of the physical realm to its limits, and they have naturally been at the forefront of space exploration. Engineers and material scientists have risen to the challenge of designing materials and machines that are able to withstand the rigors of space. Even those guys who just used to classify rocks all day, geologists, have explored the solar system with their NASA probes, and discovered ammonia ice, sulfuric acid rain and methane lakes. Now the field of "exobiology" is getting started, as people look for evidence of life on Mars, Europa and elsewhere. So where are the chemists?
The chemists have been involved, and played a vital role in developing our understanding of the space environment. Chemists have analyzed interstellar dust clouds, planetary atmospheres, and contributed greatly to our understanding of the many unusual chemical processes occurring in space, particularly those in our solar system. They have provided the basic foundation of how reactions occur under these unusual conditions. They have developed new specialties such as cosmochemistry and astrochemistry. So why do we need to create yet another field, exochemistry? The reason has almost as much to do with our perception as with our science.
Some might argue that we already understand chemistry under "unusual" conditions. After all, have we not worked with molten steel at 1900 K for centuries? Don't we routinely build semiconductor materials designed to work at 3 K? Haven't we studied how crystals grow in microgravity? Don't we already run reactions in laser beams, make artificial diamonds under enormous pressure, or perform organic syntheses at 200 K under argon? Surely any conditions that remain unexplored or unexplorable (on Earth) can be observed in space by cosmochemists and modeled using our best computational methods?
All of these are true, yet the Earth environment always remains our frame of reference. The steel is always designed to be used at 298 K, the organic syntheses are always warmed to the ubiquitous "room temperature" and worked up with water. We have never regarded 3 K as "hot" or 10-9 torr as "high pressure."
Consider the examples of butane and 2-butene, familiar to those taking introductory organic chemistry. We consider that 2-butene forms cis/trans "isomers" due to restricted rotation of the carbon-carbon double bond, whereas butane only forms "conformers" because of "free" rotation of the central carbon-carbon single bond. Yet on the surface of our neighbor Venus (at 740 K), the double bond of 2-butene would freely rotate, making the isomers behave like conformers! Meanwhile, on Neptune's moon Triton (at 28 K) we would be able to bottle up three "conformers" as separate isomers of butane- a pair of enantiomers (the gauche forms) and one other stereoisomer (the anti form). These would behave (on Triton) as different compounds with their own individual properties. Our chemical language- and thus our understanding- is surely inextricably bound to our home planet.
Also consider the reagents and solvents we use. We naturally favor things that are cheaply produced on Earth and are stable on Earth, ideally stable to oxygen and water. Yet there must be a countless number of potentially useful reagents, solvents and other materials which are simply not stable at STP. How many times have you ordered a bottle of t-butylpotassium (0.01 molar solution in liquid ethane), or run a reaction in liquid metallic hydrogen at 20,000 K and 4 million "atmospheres" pressure? We may be able to model or observe these things, but how can we claim to understand chemistry until we know how to perform reactions under such "exotic" conditions (which may be normal in other parts of the Solar System). And if we lack an understanding of our immediate neighborhood in space, how will we deal with the bizarre conditions found elsewhere in the universe?
Some might argue that we live on Earth alone, and so it is reasonable to focus our efforts on terrestrial chemistry. We've been able to land a man on the Moon without having to run a chemical plant on the Moon. Surely it would be impossible (or at least impossibly expensive) to build a chemical plant there anyway? What would be the point of it? Why bother with exochemistry- isn't it all just "pi in the sky?"
One answer is obvious if we consider the likely human presence in the Solar System in the coming century. There is little doubt that we will want to explore, study, build, and survive in many different corners of the Solar System. We will want to utilize the rich resources that lie there, we will want to establish bases in space with a permanent human presence. A basic energy analysis tells us that it is not feasible to carry all of our building materials and fuel from Earth, we have to use local resources as much as possible. Clearly a chemical "plant" built on Earth for use in space would have to be a small, compact automated system, very different from its terrestrial cousin. Those resources brought from Earth (at least initially), such as solar panels and chemical processing equipment, will have be used as efficiently as possible to maximize their value to the mission. This is where we hit a problem- we know virtually nothing about efficient chemical processing in space. By efficient, I mean optimized for the environment in which it is to be run. If we want to run a process on (say) the surface of the moon Titan (largely methane and ammonia ice), how would a native-born "Titanian" process chemist design the process (given the restrictions of limited equipment from Earth)? Surely such a chemist would come up with a process very different from what we would consider normal, yet it would almost certainly make far better use of the local resources and conditions available.
The second reason for studying exochemistry is more academic, perhaps philosophical- namely that it provides a fresh challenge that will transform our understanding of chemistry, and perhaps even how we see ourselves. As I see it, the chemical community has a cozy feeling that they have explored most of the land of chemistry. In fact, we have never left the valley- beyond the familiar foothills lies a massive Himalayan mountain range, as yet unexplored, that is exochemistry. It is a daunting prospect, to those of us (such as myself!) that have grown comfortable at 298 K, one gee and one atmosphere, but up there in the "mountains" are new possibilities for chemistry and chemical phenomena beyond our ability to imagine. We should go there and explore.