GREAT DEBATES PART VI: ENCORE PERFORMANCE From Astrobiology Magazine 19 August 2002 Last month for the Astrobiology Magazine, astrobiologists debated the "Rare Earth" hypothesis. The debate series, separated into five separate installments, ran on July 15, 17, 22, 24 and 29. As a follow-up, debate participants Christopher McKay and Michael Meyer answer questions submitted by our readers. Q: What might constitute a life form? I'm assuming complex life must be carbon and water-based, but could they be based on liquid carbon dioxide or liquid oxygen as a solvent, and even methane or ammonia? Christopher McKay: We have only one example of life: life on Earth, life as we know it. We can hardly guess at what other types are possible. Carbon chemistry and water solvent seem particularly well suited for life, and our current detection strategies are based on this model. Looking for carbon/water life means that we need to look for planets with Earth-like conditions in terms of the pressure, temperature range, and, most importantly, with liquid water present. Liquid carbon dioxide and ammonia are both good solvents and theoretically could be the liquid of life. However we do not know of any planets with large amounts of either of these as liquids. Q: In your opinion, what is the single strongest piece of evidence that we are alone; that the Earth is a biological anomaly among the billions of planetary experiments that must have failed? Michael Meyer: The strongest evidence is a lack thereof--we don't have evidence of life beyond Earth. However, from what we think we know (and this seems to be the opinion of the debaters) microscopic life may be common. But, if your question is about intelligent life, the unknowns are tremendous. Out of approximately 10,000,000,000,000,000,000,000 stars, it seems reasonable that at least one other star is capable of harboring a planet sustaining complex life. If complex life is rare, space is so vast and inimical to life that we may never learn about our nearest neighbor. But, if there are many planetary experiments that did not fail, then it is just a matter of time before we learn that we are not alone. Q: Did life appear multiple times independently on Earth during different climatic epochs? Christopher McKay: All life on Earth can be mapped into one tree or web of life. This argues for a single genome of life, a common ancestor. We have no evidence that directly suggests that life appeared multiple times on Earth. The isotopic shift of carbon-13 with respect to carbon-12 that appears throughout the geological record is consistent with the shift that is due to present types of life. This isotopic shift is found in sediments as old as 3.8 billion years, suggesting a continuity of life between then and now. Q: What credibility does the astrobiology community attach to the panspermia hypothesis? Is there any evidence one way or another? Christopher McKay: The question of panspermia is attracting new interest due to recent research on the possibility of interplanetary and interstellar transfer of life. There's evidence that some grains of material in our solar system came from beyond our solar system. Recent experiments show that microorganisms can survive dormancy for long periods of time and under space conditions. We also now know that rocks can travel from Mars to Earth. Finally, there's evidence that life appeared very early on the Earth--about 3.8 billion years ago. Q: How important is the moon in the evolution of life on this planet? I read somewhere that tidal forces, planetary rotation, etc., caused or moderated by moon's gravitation, were essential to the formation of life here. Michael Meyer: The moon could have played many roles, and the relative importance of those roles depends on how things got started on Earth. Early on, the moon was much closer, and so tides must have been spectacular. If life evolved in splash zones, with the necessary chemistry taking place during the wetting and drying of the tidal cycle, the moon would have been critical in turning the crank on this prebiotic chemical engine. Perhaps more critical, the moon has helped stabilized obliquity variations of Earth, such that we do not experience occasional major changes in our inclination. Right now, the Earth is tilted at 23.5 degrees, giving us seasons. Much higher variation in obliquity would generate greater extremes between summer and winter, and might be considered a major impediment to the evolution of complex life--but who knows? Tidal forces have moderated planetary rotation, but I don't think that's particularly important - other than a lengthening of the diurnal cycle. Interestingly enough, Mars is thought to tilt as much as 40 degrees every tens to hundreds of thousands of years. This occasional excursion into high obliquity may be the cause of many flow features we see on Mars today. A large martian moon would have moderated those obliquity excursions. Additional information on this article is available at
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- RE: Great Debates Part 6: Encore Performance LARRY KLAES
- RE: Great Debates Part 6: Encore Performance Thomas Green
