Bulletproof bacteria. (Astrobiology).

About a decade ago, a scientific debate erupted over whether there were signs of life on a piece of rock that had been blasted from Mars and traveled to Earth. Today, few researchers believe the infamous rock ever bore microbes, but some are still testing whether a Mars-to-Earth transport of life is possible.

After firing bacteria-loaded projectiles into clay, Wayne Nicholson of the University of Arizona in Tucson and his colleagues argue that microbes could survive the extreme acceleration and shock forces experienced when a rock is blown into space by a major impact on a planetary surface.

Some investigators have concluded that microbes within rocks could survive the long transit between planets and the plunge through Earth's atmosphere, but there has been little focus on the initial launch into space, notes Nicholson. To reach a velocity high enough to escape Mars' gravitational pull, material on the planetary surface would undergo acceleration up to 3.4 million meters per second per second (m/s/s). That's about 35,000 times the force of Earth's gravity, or G. In contrast, shuttle astronauts experience up to 3 Gs during a launch.

To investigate the effects of this extreme acceleration on life, the researchers loaded two kinds of bacteria--the spore-forming Bacillius subtilis and the radiation-resistant Deinococcus radiodurans (SN: 12/12/98, p. 376)--into lead pellets and used an air rifle to fire the pellets into chilled modeling clay. The deceleration of some of the pellets reached 4.5 million m/s/s, and survival of the bacteria ranged from 40 to 100 percent. "We're not reducing viability by a lot," says Nicholson.

The investigators plan to use a large gas gun owned by NASA to fire extremely high-velocity projectiles into bacteria-covered concrete. They'll search the resulting debris, which should reach speeds close to that needed to escape Mars, for any microbial survivors. "This is the closest simulation we can get on Earth," says Nicholson.