"I (and most biologists) would (and do) agree with some of his statements. For example, the likelihood of life on Mars is small, and the likelihood of more advanced, mammalian-like life is even smaller. Moreover, the likelihood that a Mars organism pathogenic to Earth life exists and that, if it did, it could compete with highly adapted and competitive life on Earth is very small. None of this is new-it is comprehensively reviewed and discussed (with proper citations) in a National Research Council report ("Mars Sample Return: Issues and Recommendations," Washington, DC: National Academy Press, 1997). This report, produced by actual experts, visits virtually all of the issues brought up by Zubrin and is, in fact, in general agreement with many of his statements. The problem is that Zubrin selectively ignores other facts and takes things out of context. For example, the following cannot be ignored:"

"This is an interesting statement from someone who, according to my brief research, has no credentials in biology, microbiology, evolution, epidemiology, or pathology. Not that such credentials are prerequisite to speaking up for the right thing, but in the absence of supporting citations, one wonders what expertise bolsters such a strong stand."

See http://planetary.org/html/news/opinions/safescience.html for further details