"It's a multi-level calculation," Scalo said. "First you have to determine the spectrum of the source [flare star, supernova, or gamma-ray burst], then you must 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.

"The calculation presented today follows the paths of individual photons as they scatter off electrons bound in molecules and gradually lose energy until they are absorbed by atoms. The result shows just what fraction of the radiation reaches a planet's surface (as function of the intensity and energy of the source and the thickness of the planetary atmosphere)."

Today, Mars has a thin atmosphere -- about 100 times thinner than Earth's. More than 10 percent of the incident energy reaches its surface for photons with energies above about 100 kiloelectron volts (high energy X-rays and gamma-rays).

"Any organisms unprotected by sufficient solid or liquid shields should have been lethally irradiated by cosmic radiation sources many times in the last few billion years," David Smith said.

According to John Scalo, "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 interestingly different evolution than on Earth. Mutations are usually deleterious, but they 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 said.

"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," Scalo said, "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 gamma-ray bursts may cause long-lived changes indirectly by affecting planetary atmospheres.

Significant gamma-ray irradiation from supernova explosions are more frequent and have a much longer duration and may be capable of driving evolutionary effects directly. Both of these distant cosmic sources are capable of delivering atmospherically and biologically significant high-energy radiation jolts every hundred thousand or million years -- possibly hundreds or thousands of such events over the history of a planet.

This picture of sporadic zaps of radiation is quite different than when a planet is constantly bathed in radiation from its parent star. "Most stars in our galaxy aren't like the Sun," Scalo said.

"Most are red dwarfs." These stars have little ultraviolet radiation that can cause mutations, but they flare violently, mostly in X-rays. "Conventional wisdom said that planets orbiting these stars couldn't have atmospheres, that any atmosphere would freeze out because the planet's rotation would be tidally locked," Scalo said.

"More recent calculations show these planets can have atmospheres. What might life be like on a planet orbiting a red dwarf with powerful flares and continuous intense coronal X-rays? One possibility is that most of the biosphere would need to be underground or underwater; another is that the challenging mutational radiation environment would accelerate the evolution of life."

Future work will focus on the reprocessing of the lost gamma-ray and X-ray energy to ultraviolet radiation that can reach the ground. The high-energy photons lose energy to electrons that in turn excite atoms and molecules in the atmosphere. When those atoms de-excite, they can produce substantial ultraviolet radiation that can also affect the biosphere on the surface of the planet.

In this case, bursts of cosmic irradiation would be important even when there is a thick atmosphere (like Earth's) that will stop the original X-rays or gamma-rays. These jolts of irradiation can cause the formation of a "second ionosphere" at fairly low altitudes and disrupt a planet's atmospheric chemistry. Smith, Scalo, and Wheeler are adding these effects into their calculations.

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