This book is a little like "Here be Dragons," (Koerner, LeVay, Oxford University Press, 2000) and "Rare Earth," (Ward, Brownlee, Copernicus, 2000) except that it is more focused and specific than the others. While Koerner, LeVay, Ward, and Brownlee consider the possibility that life exists outside the solar system, Walter limits the scope of his book to the question of whether microbes exist, or once existed, on Mars.
Life outside of earth has never been unambiguously observed and verified. Consequently, discussions about the possibility of life beyond earth inevitably begin with thoughts about how life originated here. There seems to be an emerging sense that life is the result of a universe that is naturally self-organizing (Stuart Kauffman is in this camp. See his book "At Home in the Universe, Oxford University Press, 1995). According to this point of view, life is all but certain to arise on any planet having the basic chemicals and physical conditions found on earth 4 billion years ago. Given this hypothesis - that life arises quickly and naturally in the proper environment - it's natural to ask if any other planets in the solar system have (or had) the necessary ingredients. If they did, we should look to see if life evolved there. Since there is growing evidence that Mars had a distant past with some of these conditions, it seems more and more important that we look for life on Mars. Finding evidence of life there would buttress the concept that life readily evolves given the proper environment. Obviously, if that's the case, it holds enormous consequences for modern science.
Walter has a nice chapter on the tree of life, and describes recent information showing that "all the lowest branches of the tree are occupied by hyperthermophiles." The discovery that life exists on earth under extreme conditions (like those of deep-sea thermal vents) has increased the hope among scientists that it might also have evolved and flourished on Mars many thousands of millions of years ago. He also shows how genetic transfer between species happens today, and was probably common among our earliest ancestors, so that the whole concept of a "tree of life" becomes somewhat tangled during the earliest stages of the evolution of life. Instead of a tree, the topology looks more like a web, with the roots of the tree (consisting of Bacteria, Eucarya, and Archaea) rising out of this web.
The expectation of finding evidence of life on Mars depends on the type of environment that Mars supported in the distant past, and the circumstances under which life arose on earth. It also depends on how easy it is to ascertain the evidence of fossilized ancient microbial life. It turns out that identifying evidence of microbes in very old rocks is a pretty hard thing to do. To illustrate this, Walter describes the difficulty of identifying stromatolites in ancient rocks. This was new information for me, and a real insight into the nuts and bolts of making these sorts of identifications. I'd thought that stromatolites were easy to identify, but in the very oldest rocks, they're not. When identifying stromatolites in rocks 3000 million years old, there can be (and often is) a great deal of controversy regarding the conclusion. Walter's point in making this so clear is that stromatolites are large colonies of microbes, yet even they are not unambiguously identified in the oldest rocks. The problem of identifying evidence for individual microbes in rocks 3000 to 3500 million years old is even tougher. The point being that even with Martian rocks in our hands, it's not going to be easy to affirmatively state whether there is evidence of ancient life on Mars.
To drill the point home, Walter points to the fact that we do have chunks of Martian rocks on hand, in the form of bits and pieces that have been blasted off the Martian surface by meteorite impacts. Walter describes in detail the scientific examination of some of these rocks, and one, in particular, identified as ALH84001. This meteorite made world news when a team of scientists reported finding evidence of ancient microbes buried inside it. Walter describes the initial reports, the objections, and the eventual state of limbo in which these conclusions came to rest. This helps set the tone for expectations regarding the difficulty against which such analysis will proceed even when we manage to return samples from the Martian surface using spacecraft.
In describing how scientists make conclusion about the presence of microbes in ancient rocks, Walter does a real service by illustrating the importance of convergent evidence. Identifying ancient microbes involves more than one type of observation. It involves many types of converging data, including visible observations of deposits in rocks, the types of rocks involved, and things like carbon isotope ratios (not to be confused with carbon 14, which decays far to quickly for analysis in 3000-million-year-old rocks). Along these lines, I noticed a recent article in Photonics Spectra (May 2001) describing the use of Raman imaging to identify microfossils - another tool, in the search for the ancient life on earth, and possibly on Mars.
The book ends with some very informative discussions about proposals for future landing sites on Mars, for sample analysis and/or return.
This is a very informative book, with useful insights into the way science works, complete with several pages of color plates, a useable index, and short list of further reading material. If you are interested in what NASA does, and how the scientific search for life on Mars is (and will be) carried out, I think you will like it. I certainly did.
