It’s looking more and more likely that Mars might already be inhabited—by Martians. Very tiny ones. Conley’s office serves to prevent NASA from doing to Martians what European explorers did to Native Americans with smallpox. Because Mars lacks Earth’s history of abundant life, it has that much more raw material for Earth’s bacterial stowaways to devour—should any of them, say, come into contact with water, find a niche they can survive in, and start to reproduce. “The whole planet is a dinner plate for these organisms. They will eat Mars.”

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