Radiation may affect planetary evolution - Universe

USA Today (Society for the Advancement of Education), June, 2003

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 points out that "It may have been safe on Mars during the first few billion years, when the planet had a much-thicker atmosphere. But today, and probably for the past billion years or so according to current climate evolution models, the planet has had little protection from high-energy radiation. When the atmosphere thinned, any life on the surface was exposed to high-energy radiation from exceptionally strong solar flares and occasional stronger bursts from different astronomical sources throughout the galaxy."

The radiation need not be lethal, but may instead induce episodes of intense mutational damage and error-prone repair, leading to intriguingly different evolution than what is found on Earth. Mutations are usually deleterious, but they also provide the diversity necessary to drive evolution. "Radiation bursts may spur evolution by intermittently enlarging the genomic diversity upon which natural selection is believed to operate," Scalo explains. "As an example, chemical pathways adapted to a rapidly fluctuating radiation environment might result in organisms whose signatures of biological activity may be very different from those of terrestrial organisms. Gamma-ray bursts only last 10 seconds or so. The mutations they cause are unlikely to produce direct evolutionary effects."

Exposure to gamma-ray bursts will tend to sterilize life on the exposed side of the planet that is not protected under enough rock or water. However, they may cause long-lived changes indirectly by affecting planetary atmospheres. Significant gamma-ray irradiation from supernova explosions are more frequent, have a much-longer duration, and may be capable of driving evolutionary effects directly.