J. Brit. Astron. Assoc., 107, 4, 1997, p.221
Springer-Verlag, 1996. ISBN 0-387-94650-0. Pp xiii + 296, £23.00 (hbk).
reviewed by Mark E. Bailey
The idea that comets are associated with the construction – or at least the repair – of life has a long history, stretching back at least to the time of Isaac Newton. However, recent astronomical ideas, for example the notion of the accretion of the Earth from planetesimals formed within the orbit of Mars or of a giant single impact for the origin of the Earth-Moon system, consistently predict a volatile-depleted young Earth, raising the question: whence came the observed volatiles – water and organics? We are increasingly aware that comet and asteroid impacts have played an important part in Earth history, that comets are largely frozen water, and that liquid water appears to be a prerequisite for life. It is only a small step to argue therefore that comets may have played a key role in the origin and evolution of life.
This slim volume, which has its origin in a conference held in 1991 and in subsequent research up to about 1995, provides a superb introduction to this wide-ranging field. The work is presented in the form of a dozen review papers, involving 19 authors each of whom are distinguished researchers in their own right. The first few chapters encompass topics ranging from the accretion of organics and the organic chemistry expected to take place in and around comets, to numerical modelling of cometary impacts and their probably catastrophic consequences for life on Earth, however it began. The discussion continues with reviews of the lunar late heavy bombardment and its implications for understanding the terrestrial impact rate over time, and in turn for the frequency of events leading to global sterilisation, up to roughly 3.8 billion years ago.
The sudden, almost instantaneous onset of life as soon as the Earth provided an hospitable environment for living creatures, probably 3.8 billion years ago, contrasts sharply with the snail's pace of evolution for the following 3 billion years – 75% of Earth's history. Why this should be, and what triggered the Cambrian explosion of life from 533 to 525 million years ago, emerges as one of life's great mysteries. Following the development of complexity, including the plant and animal kingdoms, life on Earth apparently became much less robust and species became subject to mass extinctions, occurring roughly every 30 million years right up to the present. Again comets are implicated, via periodic perturbations of the Oort cloud arising from the changing position of the Sun in its orbit around the Galaxy. The book concludes with chapters on the contemporary impact hazard to life, the possibility that liquid water could exist within comets, that life might have originated independently in comets (the panspermia hypothesis), and the rôle played by the international space programme in advancing current research.
The book is intended as a graduate-level text, but the quality of exposition and the admirably uniform level of explanation, including ample references, make this a quite exceptional volume. As well as being a primary source, it could serve as background reading for an undergraduate planetary science course, or as a fairly digestible entry into the literature for non-specialists seeking merely to keep up to date with the latest ideas. A number of contributions provide models of physical insight, giving fresh back-of-the-envelope calculations to illustrate the application of a general physical principle or a trend derived from more detailed arguments, but on the whole the topics are presented with a minimum of mathematical formalism. Each chapter is a jewel, and taken together they provide a sparkling reflection of a complex interdisciplinary field.
At 23.00 pounds the book is a bargain: it is suitable for a wide readership and will undoubtedly serve as a springboard for further research. The editors and authors deserve to be congratulated.
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