A wide range of conditions and reaction mechanisms can generate the spectrum of compounds observed in meteorites and dust particles. Isotopic evidence (especially enrichments in ¹⁵N and deuterium) supports the idea that at least half of the insoluble organic matter in carbonaceous meteorites originated from sources that existed before the Solar System formed^{3, 4}. In contrast, soluble organic compounds in these meteorites, including amino acids, are generally thought to have been produced soon after the formation of the Solar System, by the action of aqueous fluids on the asteroids that gave rise to the meteorites (Fig. 1)⁵. In this scenario, the carbon, nitrogen and hydrogen that went into forming the amino acids found in the well-studied Murchison meteorite^6-8 would have been carried by reactive precursors from the interstellar medium, but the amino acids themselves would have formed on the meteorite's parent body.

Figure 1 Is it possible that the amino acids found in meteorites originated in the interstellar medium, as recent laboratory experiments suggest? Or were they synthesized on the asteroid parent bodies of the meteorites by the action of aqueous fluids? This picture shows a fluid inclusion in salt crystals found in a meteorite that fell in Monahans, Texas, in 1998, and may be a rare sample of water from an asteroid.

Results from the Tagish Lake meteorite⁹, which fell to Earth in 2000, make the picture more complicated still. Although it contains evidence of aqueous alteration, and although the bulk abundance of organic carbon is similar to that of the Murchison meteorite, concentrations of amino acids in the Tagish Lake meteorite are around a thousand times lower than those in Murchison.

Now two groups^{1, 2} report amino-acid synthesis in experiments in which ice mixtures thought to be representative of the interstellar medium were subjected to ultraviolet radiation at temperatures below 15 K. Bernstein et al.¹ (page 401) report the synthesis of three amino acids (glycine, serine and alanine) in a mixture of water, methanol, ammonia and hydrogen cyanide, where the ratio of the components was 20:2:1:1, respectively. Muñoz Caro et al.² (page 403) report the synthesis of 16 amino acids and several other compounds in their 2:1:1:1:1 mixture of water, methanol, ammonia, carbon monoxide and carbon dioxide.

Assuming that conditions are comparable between the two experiments, the difference in their results demonstrates the profound effect that bulk composition can have on chemical synthesis. The water-rich experiment reported by Bernstein et al. produced only a few compounds. But the water-poor experiment of Muñoz Caro et al. generated a host of amino acids, some carrying not just one but two amino groups. The relative yields of mono-amino acids in this latter experiment show a reasonable correlation with the relative abundances found in extracts¹⁰ from the Murchison meteorite. But the meteorite shows no evidence of di-amino acids such as those seen by Muñoz Caro and colleagues.

Of course, precisely matching meteorite compositions is not the goal of these experiments. Nor has it been the goal of dozens of other successful synthesis experiments, be they gas-phase reactions driven by spark discharges, ultraviolet radiation, shock waves, -rays or heat, or aqueous synthesis from a variety of starting materials, including hydrogen cyanide, formaldehyde, methane, carbon monoxide, oxalic acid, carbides or mixtures of these compounds. It appears that nearly every experimental scenario produces organic compounds of some form, which could hardly be more frustrating when trying to unravel what actually produced the organic inventory of the Solar System. Perhaps it is time to stop this seemingly endless series of phenomenological experiments and to concentrate instead on quantifying reaction rates, the relative stabilities of synthesized compounds and the effects of variables such as temperature and pressure.

What about the big picture? Given the truly diverse array of energy sources that may initiate synthesis, complex organic compounds should be expected in any sector of the Solar System that is rich in volatile elements. This is especially true for the outer parts of the Solar System, which host most of the light elements that did not go into making the Sun. On Earth, the hydrosphere (water, including ice and water vapour) accounts for 0.0013% of the planetary volume. But water makes up more than 15% of the volume of Europa, and more than 65% of that of Ganymede — two of Jupiter's largest moons. Even the drier carbonaceous meteorites show evidence of a water-rich past on their parent bodies; some have organic carbon contents that rival those of petroleum source rocks on Earth. So it is plausible to suggest that most organic compounds in our Solar System are extraterrestrial — and abiological — in origin.

Does this tell us much about the origin of life? Well, you can study geology for a living, but knowing how different rocks form doesn't tell you which lumps of rock will become Teotihuacán, the Taj Mahal or Tony's Tavern. Studying the chemical building-blocks of life shows that they are ubiquitous and can exist in the absence of life. Indeed, inferred cosmic abundances of these building-blocks from abiological sources greatly exceed those from living organisms. Accepting that fact, it follows that process- driven investigations into the emergence of life may need to be cast in a different way, which takes into account the materials involved but is not directly tied to them. This, I believe, is a major challenge for the fledgling field of astrobiology.