Where, when, and how life began are the ultimate scientific questions to which there are, as yet, no answers. But evolve we did, so another question is, from what? Comets and the Origin and Evolution of Life is an updated and extensively rewritten second edition of a 1997 conference volume. Since the publication of the original work, there have been theoretical and practical advances in this fast-moving field. For example, several cometary and asteroid sampling missions are either, like Rosetta and Hayabusa, in progress or, like Stardust, have delivered material back to Earth.
In the first chapter, Oró, Lazcano & Ehrenfreund provide an historical background, and pose the question central to the book: ‘Did comets play a role in the origin and early evolution of life on Earth?’. Varying views from the fifteen authors of the further eleven chapters follow, and definitive answers are just as elusive as they were a decade ago. While there is general agreement that comets delivered prebiotic organic materials and water to the early Earth, it now seems possible that the Earth's early organic inventory included both ‘external’ organics, and ‘internal’ organics synthesized in the atmosphere, or in the oceans. And while comets may have played a vital role in the origin of life, conversely they may also have been inimical to its development.
In Chapter 2, Delsemme addresses the complex problem of the origin of the Earth's atmosphere and oceans into which prebiotic organics were introduced. Maurette (Chapter 3) also reviews the evolution of the atmosphere, and considers the role of dust (micrometeorites) as probable cometary material delivered to the Earth. Advances in the remote search for organic macromolecules in comets, star-forming regions, and interstellar clouds is the subject of Chapter 4 by Huebner & Snyder. Greater computational power has allowed more accurate simulations of cometary impact, and Pierazzo & Chyba (Chapter 5) compare and contrast models of survivability of prebiotic organic matter during impact delivery to the Earth, Mars, the Moon, and Europa. In Chapter 6, Chyba & Hand consider the delivery of organics in the broader context of the Late Heavy Bombardment by comets 3.9 Ga ago.
The next three chapters by Zahnle & Sleep, Glikson, and Morrison, respectively, focus on the role of cometary impact in the destruction of life. They suggest that the sterilizing effects of giant impacts may have posed a severe challenge to the survival of early life on Earth. Glikson (Chapter 8) presents some tentative supporting evidence for this from 3.8–2.4 Ga rock sequences in the Pilbara (Western Australia) and the Kaapval (South Africa) cratons, while Morrison (Chapter 9) assesses the threat of cometary impacts with the Earth. In Chapter 10, Podolak & Prialnik model the conditions where liquid water, fundamental to life, may be released in comets.
Embodied in this volume is much of the current knowledge on the constitution of comets. Sadly, due to publication timing, while remote cometary observations from space missions are included (Kissel & Kreuger, Chapter 11; Kreuger & Kissel, Chapter 12), the early results from the study of the material returned to Earth from Comet Wild 2 by Stardust are not. Moreover, although data from carbonaceous chondrites are sprinkled throughout the book, there is no extended treatment of the work by organic chemists on these primitive Solar System materials as possible proxies for cometary material. Nevertheless, I highly recommend the book to all involved in Earth and Planetary sciences, and the text, which is clearly written and illustrated, is also accessible to a much wider readership.
