Life in Space

Extraordinary claims require extraordinary proofs. That observation from Scottish philosopher David Hume applies nowhere better than to speculations about the origin and evolution of life and whether it exists elsewhere in the universe. There is, after all, only a single example to judge by: life on Earth. The relatively new discipline of astrobiology nevertheless takes up those matters from a strictly scientific viewpoint, and Lucas John Mix explains how it does so in Life in Space: Astrobiology for Everyone. That subtitle seems tongue-in-cheek, given the complexities of the science involved, until one realizes that, even if astrobiology is not really for everyone, its findings will be. As Mix patiently, clearly, and humbly argues, how people define life influences how they look for life, and how people look for life reflects humanity’s deepest assumptions and aspirations. Mix possesses a notable ability to guide readers, if not to an outright understanding of astrobiology’s aims and methods, at least to a constructive appreciation.

Astrobiology studies life by drawing on many scientific disciplinesincluding biology, physics, astronomy, geology, oceanography, paleontology, and climate sciencein order to treat it as an Earth-wide phenomenon. Astrobiologists hope thereby to learn where they might best look for life on other planets. This project is less straightforward than it sounds. Mix first sets out the assumptions behind the discussion and then methodically surveys the contributions of science.

Among those assumptions is the belief that reductionism is a useful method of investigation. Reductionism holds that simple phenomena give rise to more complicated phenomena. It works very well for physics and chemistry, which have posited basic laws and forces, but less well for biology. Still, Mix points out, when regarded as one method rather than as an end in itself, reductionism provides insights useful to assembling a coherent picture of life.

Another assumption of astrobiology is that it is possible to define life clearly. There are many approaches to formulating such a definition. Mix discusses five approaches and shows the weaknesses of each. These five include the “pornographic” definition of life (“I can’t define it, but I know it when I see it”), the biochemical definition, the antientropic definition, the replication definition, and the evolutionary definition.

Yet another crucial set of assumptions is that the universe is well-ordered and that humans are capable of understanding it. In the first five chapters of Life in Space, Mix ably argues that science (and philosophy) have much of value to say about life. Thus, he asserts, astrobiologists should proceed with cautious confidence.

It is in chapter six, “Life in the Cosmos,” that Mix grows specific. It and following chapters rely on physical laws and, especially, chemical formulae to make their case. Readers with no knowledge of science, or patience with it, will likely founder, but a high-school-level acquaintance with astronomy, chemistry, and physics easily suffices to follow Mix’s argument. The effort will be rewarded not only with insights but, as important, with the assurance that even the most abstruse and seemingly intractable matters are accessible to an open mind.

In short order, Mix lists the circumstances of life on Earth: It needs a watery environment, is based on carbon, functions through reduction-oxidation (redox) chemistry and proton gradients, requires a few basic elements in sufficient quantity (carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur), and subsists on an influx of energy (mostly sunlight, but in some cases geochemical reactions). Given these terrestrial (curiously, astrobiologists use “terrean”) specifications, scientists logically can start their search for life elsewhere by locating places that seem to have the same characteristics. “In short,” Mix concludes, “we are looking for a rocky planet or moon, close, but not too close to a star”: A rocky planet (as opposed to a gas giant) will have the right variety of chemicals. It must be close but not too close to a star so that water can exist there as a liquid.

This formulation helps narrow astrobiologists’ search somewhat. As it turns out, two very good candidates for their search exist in Earth’s solar system: Mars and...

(The entire section is 1800 words.)