Radiation may affect planetary evolution.

• Position: Universe

Jolts of radiation from space may affect biological and atmospheric evolution on planets in the solar system and those orbiting other stars, according to calculations by a team of astronomers at The University of Texas at Austin. Bursts of radiation that can cause biological mutations, or even deliver lethal doses, can come from flares given off by the planet's parent star or from more-remote cosmic events (e.g., supernovae and gamma-ray bursts). The magnitude of the effect on life and evolution on a planet is related to how much protection the planet gets from its atmosphere. The researchers focused on the transmission of high-energy X-rays and gamma rays through planetary atmospheres.

"It's a multi-level calculation," says John Scalo. "First, you have to determine the spectrum of the source--flare star, supernova, or gamma-ray burst--then you [have to] calculate how the radiation propagates through and disrupts a planet's atmosphere. Then you follow the radiation down to the surface of the planet, even underwater, eventually calculating how strongly it interacts with cellular material." He says the calculation presented by the researchers "follows the paths of individual photos as they scatter off electrons bound in molecules and gradually lose energy until they are absorbed by atoms. The results show just what fraction of the radiation reaches a planet's surface, as a function of the intensity and energy of the source and the thickness of the planetary atmosphere."

For instance, Mars has an atmosphere about 100 times thinner than Earth's. More than 10% of the incident energy reaches its surface from photons with energies above about 100 kiloelectron volts (high-energy X-rays and gamma rays). "Any organism unprotected by sufficient solid or liquid shields should have been lethally irradiated by cosmic radiation sources many times in the last few billion years," suggests David Smith, now a graduate student at Harvard University.

Scalo...