Impossible Extinction: Natural Catastrophes and the Supremacy of the Microbial World

  • Charles C. Cockell
Cambridge University Press: 2003. 192 pp. £18.95, $28

One of the great intellectual achievements of the past millennium was realizing that the Sun is a star and the Earth is a planet. Over 400 years, this idea has gone from an excuse to burn the astronomer Giordano Bruno at the stake to a banality reserved for trick questions such as: “What is the nearest star to the Earth?” Reaction to the rank odour of biblical catastrophism and creationism has prejudiced serious consideration of extraterrestrial affairs in geology and biology. This has now changed, the main wake-up call being the discovery that an asteroid impact wiped out a vast number of species, including the dinosaurs, 65 million years ago.

A chief beneficiary of this change is astrobiology, the science of planetary habitability, including the effects of extraterrestrial processes on the evolution of life on Earth, which has become a hot topic. Charles Cockell, Peter Ward and Donald Brownlee add to the list of popular books on this subject. Both books focus on global sterilization and the finite tenure of life on a habitable planet, but Cockell concentrates on microbes, whereas Ward and Brownlee look at animals.

These treatments bring different emphases: that it is quite hard to sterilize all the microbes on a planet, and that the days of animals are numbered by a few hundred million years. But the authors agree on the salient conclusions of astrophysics, geology and biology. The Solar System formed about 4.5 billion years ago. Microbes inhabited Earth by the time of the first good geological record, 3.5–3.8 billion years ago, perhaps even earlier. Our Sun's useful life is limited: its luminosity has already increased by 30%, and will continue to increase, until, 7 billion years from now, the red-giant stage will finally incinerate the Earth. In addition, the tectonic processes that continually renew the Earth's surface are on the wane.

I found Ward and Brownlee's book the easier to read. Its organization around the various vicissitudes that await life works well, using an analogy to the progressive failure of various organ systems of an aged person. We can expect that as the Sun waxes, the weathering of rocks will reduce atmospheric CO2 to low levels, shutting down plant life and leaving little food or oxygen for animals. Still later, the greenhouse will become hotter, ending with either the loss of the ocean's hydrogen to space or a runaway period of oven-like conditions at the surface. The authors correctly note that the former possibility leaves us with Dune — a hot, dry planet that is still habitable by microbes, and perhaps by animals near the poles — whereas the latter leaves the surface sterile. In the meantime, the continents may group together, fouling the global climate, or plate tectonics may shut down, leaving us, as in the film Waterworld, with a water-covered planet from which all topography has been eroded.

In contrast, I found the organization of Cockell's book — on a trip of the Solar System once around the galaxy — rather contrived. As he admits, galactic effects on the Earth have been, and probably will remain, small. Any effect has more to do with our position relative to the galactic plane and to the spiral arms with giant molecular clouds and young stars than to absolute position relative to distant galaxies. This quibble aside, the book is worth reading. A highlight is the discussion of why we should not generalize too much from the actual state of the Earth. Cockell focuses on the presence of our large Moon. I might add that intelligent deep-sea organisms could probably come up with many reasons why intelligence could not arise on land, and that organisms circling a star ejected from its galaxy might think the plane of the galaxy to be an unattractive place.

Both books have the obligatory and entertaining discussion about how a high-tech civilization could avoid or at least delay its doom. We all like to feel that our distant descendants, or at least our more distant microbial relatives, will survive. It is easy to come up with suggestions to be filed away for 100 million years. Importantly, both books show that the Earth is not a truly safe place even now. There are hazards that we can do something about: global warming by the burning of fossil fuels; the more distant threat of advancing glaciers; and the ever-present danger of impacts from asteroids and comets. Science provides both the technology to cause trouble for ourselves and the knowledge to foresee and avoid that trouble.

The Life and Death of Planet Earth: How the New Science of Astrobiology Charts the Ultimate Fate of our World

  • Peter D. Ward &
  • Donald Brownlee
Piatkus Books/Time Books: 2003. 256 pp. £16.99/$25

Astrobiology derives from several fields that until recently had little interaction. An inevitable result is that it is difficult for one or even two authors to cover fully this rapidly evolving field. Specialists will quibble with various details and prefer more treatment of their own fields; my own include the effects of global tectonics, geochemical cycles and large asteroid impacts on early life. More bothersome is the fact that both books contain some errors, so care is needed in using them as secondary sources. For example, Ward and Brownlee give the formation rate of continents as 650–1,300 cubic kilometres per year, a factor of about 1,000 too high. And it seems unlikely that global biomass drops by several thousandfold in an ice age. In the other book, Cockell's Earth has a mainly iron mantle, and his surface of Venus is younger than 250 million years and still active, rather than all about 500 million years old.

Having said that, I recommend both books to the lay reader. They can also be quickly read by a professional astrobiologist and do provide insight. And they would serve as supplemental reading (but not as primary texts) for a non-major class on planetary habitability or astrobiology.