The origins of life - the nature of the transition from inanimate to animate chemistry - is one of the major mysteries of astrobiology. The first of the three theme-questions in astrobiology - Where did we come from? - deals in part with origins, whether the process took place on the ancient Earth or elsewhere. One perspective suggests that chemical interactions between water and various minerals might have been important.
This brief discussion is based on a comprehensive review by Martin Schoonen, professor of geochemistry at Stony Brook, and his colleagues. Their paper [A Perspective on the Role of Minerals in Prebiotic Synthesis], just published by the Swedish Academy of Sciences, can be found here
. Their work was supported in part by the NAI.
V.M. Goldschmidt, one of the founders of the field of geochemistry, suggested in 1947 that chemical reactions catalyzed by minerals might have played a role in the prebiotic chemistry that led to the origin of life. In the half-century since Goldschmidt's death, many of his ideas have been corroborated and extended. Minerals may have played a role in both the formation of simple organic molecules such as formaldehyde and, possibly, in the formation of molecules as complex as RNA, through reactions mediated by clay minerals. One of the major tasks is to identify which minerals might have been abundant on the early Earth, 4 billion years ago. Other questions concern the state of the primitive ocean: its temperature, salinity, and acidity.
The most attractive candidates are minerals that could have acted as catalysts, stimulating chemical reactions without themselves being consumed in the process. These reactions must have taken place where the minerals come in contact with the seawater. Experiments have suggested that it is often not the bulk chemistry of the minerals that is important, but conditions right at the water interface. The presence of surface impurities and of defects at the molecular level may have been critical for some of the reactions. Most of these reactions involved trace materials dissolved in the oceans interacting with mineral surfaces on the seafloor. It is not certain that there was any exposed land prior to 4 billion years ago, although this remains an open issue.
In this review paper, these authors note that we have no hard evidence of conditions on the Earth more than 4 billion years ago. The much greater heat flow from the interior probably created numerous submarine volcanoes and may have resulted in oceans considerably hotter than at present. Paradoxically, the lower luminosity of the Sun might simultaneously have led to a frozen crust of ice, especially as the internal volcanism gradually subsided.
The composition of the earliest submarine crust would have been influenced by the cycle of heating and eruptions of material. Perhaps the 4.2-billion-year-old zircon crystals found in the oldest sedimentary rocks are relics of this very early crust. Eventually the declining temperatures led to increased viscosity in the mantle and stronger coupling with the crust, leading to the plate tectonics we have today. Because we know so little about early geology, questions about which minerals were abundant cannot be answered with confidence.
Iron oxides and hydroxides have played a major part in the changing chemistry of the crust and ocean. Iron is the most abundant reactive metal on modern Earth, and its chemistry has varied over time as the atmosphere shifted from a reducing to an oxidizing state. In the early ocean, reduction by ferrous iron of dissolved gases such as carbon dioxide and carbon monoxide could have led to the formation precursors of life, such as ammonia and formaldehyde. Additional traces of oxygen and nitrites lead to more complex reactions. These authors also discuss the role of metal sulfides, which must also have been abundant on the early Earth.
In concluding their review, Schoonen and his colleagues emphasize the complexity of the chemical systems that may have played a role in prebiotic synthesis of complex molecules. Factors ranging from the microscale of individual crystals to the macroscale of global volcanism may have been important. Their final paragraph states: "Given the vast number of possible combinations of minerals and prebiotic reactions, it is crucial to develop a capability to predict the potential catalytic effect a mineral may have on a particularly reaction. We believe that progress can be made toward this goal by integrating molecular-scale observations, molecular theoretical models, and carefully designed experimental rate studies." Clearly there is a great deal more work to be done!