Could life on Earth have originated on Mars? over the past two decades that question has left the pages of science fiction and entered the mainstream of empirical science. Planetary scientists have found that rocks from Mars do make their way to Earth; in fact, we estimate that a ton of Martian material strikes our planet every year. Microorganisms might have come along for the ride. The impacts that launched these rocks into Earth-bound trajectories were violent, high-pressure events, but experiments show that certain species would survive. On passing through Earth’s atmosphere, Martian meteoroids are heated only a few millimeters in from their surfaces, so any microbes deeper inside would not burn up [see “Did Life Come from Another World?” by David Warmflash and Benjamin Weiss; Scientific American, November 2005].