This book provides a whistle-stop tour of the prospects for life elsewhere in the Universe. It is an exhilarating ride because, as the title makes clear, the authors take a very optimistic view of their subject matter. Indeed, they argue that we have already found evidence for life on Mars, and plausible evidence for life in a number of other solar system locations, extending from the clouds of Venus to the icy moons of the giant planets. This is certainly exciting stuff, and many of the arguments advanced for the prevalence of life in the solar system are well developed and highly stimulating. Some of them may indeed turn out to be correct. Unfortunately, on the basis of current knowledge, their strength is overstated - the sad truth is that we have not yet found extraterrestrial life (as the book's subtitle claims), and it is misleading to imply that we have.
Take Mars, for example. The authors provide a good summary of the results from Nasa's 1976 Viking missions - the only spacecraft to have landed on Mars actually equipped to detect life. Although the scientific consensus is that Viking did not detect life on Mars, the authors correctly point out that the actual experimental results were more ambiguous. This is a refreshing discussion because the full complexity of the Viking results is often overlooked. Nevertheless, it is too much to claim that "through the eyes of twenty-first century astrobiology, the Viking results make a strong case for active biology on Mars". Currently we just don't know enough about the chemistry of the top few centimetres of the Martian soil, which was the only part of Mars studied by Viking, to draw such a far-reaching conclusion. Of course, regardless of the Viking results, life may still exist elsewhere on the planet (perhaps deep in the crust), or may have existed in the past.
In support of the former possibility, the authors draw attention to the recently discovered presence of methane in the Martian atmosphere, and of the latter to the enigmatic structures found within the (somewhat infamous) Martian meteorite ALH84001. The problem is that, intriguing as these discoveries are, they lend themselves to plausible biological and non-biological interpretations in about equal measure.
For reasons well articulated in this book, it would be somewhat surprising if life never arose on Mars, but there is no royal road to obtaining a definitive answer - this will come only by exploring the planet in much more detail. The same applies to the other possible locations for life discussed in the book. The authors make good cases for the habitability of a number of solar system bodies, but actual evidence of habitation is lacking. Some statements certainly extrapolate much too far from current knowledge, for example that "photosynthetic microbes likely bathed in the sunlight in warm early oceans on Venus". For all we know this may once have been the case, but there is no solid evidence to support it; claims for oceans on early Venus are themselves controversial, never mind speculations regarding their hypothetical inhabitants!
Nevertheless, this is a thought-provoking book, well referenced to the primary literature, and I am glad that I read it. Above all else, it is a plea to keep an open mind with regard to the possibilities for life elsewhere, and in this I entirely agree with the authors. Obtaining actual, definitive evidence for extraterrestrial life, however, will require a much more robust programme of planetary exploration than has occurred to date. We are not there yet.
We Are Not Alone: Why We Have Already Found Extraterrestrial Life
By Dirk Schulze-Makuch and David Darling Oneworld Publications, 208pp, £12.99
ISBN 9781851687190 Published 25 March 2010