AUSTIN, Texas — Calculations by a team of astronomers at The University
of Texas at Austin show that jolts of radiation from space may affect
biological and atmospheric evolution on planets in our own solar system
and those orbiting other stars. The work by David Smith (a former
UT-Austin undergraduate, now a graduate student at Harvard University)
and UT-Austin astronomers John Scalo and J. Craig Wheeler is presented
today at the American Astronomical Society meeting in Washington, D.C.

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 it gets from its atmosphere. The work presented
today concentrates on the transmission of high-energy X-rays and gamma-
rays through planetary atmospheres.

"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

g (164KB)]
Mars: Atmospheric transmission of high energy photons as a function of time.