[meteorite-list] Cosmic Rays Zap a Planet's Chances for Life

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 12 Nov 2013 15:09:29 -0800 (PST)
Message-ID: <201311122309.rACN9Uh8021274_at_zagami.jpl.nasa.gov>

http://www.astrobio.net/exclusive/5792/cosmic-rays-zap-a-planets-chances-for-life

Cosmic Rays Zap a Planet's Chances for Life
Charles Choi
Astrobiology Magazine
November 11, 2013

Mysterious cosmic rays constantly bombard Earth from outer space. Now
scientists find these energetic particles could limit where life as we
know it might exist on alien planets.

Cosmic rays continue to baffle scientists more than a century after they
were first discovered. These charged subatomic particles zip through space
at nearly the speed of light, a few strangely with energies up to 100
million times beyond what is possible from the most powerful particle
accelerator on Earth. Cosmic rays are believed to be atomic nuclei, with
the vast majority being protons, or hydrogen nuclei.

When cosmic rays hit Earth's atmosphere, they generate a shower of other
particles, including muons, which are essentially much heavier versions
of their cousin the electron. Some of these particles reach Earth's surface,
potentially damaging life on land and in the oceans - muons can even penetrate
hundreds of feet below a planet's surface.

Scientists investigated how cosmic rays might influence the habitability
of distant alien worlds. The hundreds of exoplanets astronomers have discovered
in the past two decades using ground and space telescopes have raised
the possibility that some might be home to extraterrestrial life. Interest
is especially focused on worlds in so-called habitable or Goldilocks zones,
which receive enough heat to possess surfaces that can keep water liquid
rather than freeze - on Earth, there is life virtually wherever there
is liquid water.
 
The investigators reasoned the level of radiation a planet receives helps
control its habitability. While a planet might see much fewer galactic
cosmic rays compared to the radiation from its star, the average energy
of cosmic rays is far higher than photons and protons from the star, making
them critical to focus on.

"If the radiation dose is too high, then life as we know it cannot exist,"
said study author Dimitra Atri, a physicist at the Blue Marble Space Institute
of Science, a nonprofit institute with a network of scientists across
the world.

The researchers concentrated on two factors that might influence the cosmic
ray dose a planet gets - the strength of its magnetic field, and the depth
of its atmosphere.

"I started thinking about this problem when I was thinking about Mars
and Earth, which are next-door neighbors, and how we have a thriving biosphere
here on Earth, while it's safe to say Mars does not have a thriving biosphere
on its surface. Why is that so?" Atri said. "The main factor is that Mars
has a high level of radiation - the atmosphere on Mars is almost negligible,
very, very small compared to Earth's, and it has no planetary magnetic
field, so it has no shielding from the cosmic rays found everywhere in
the galaxy. So I wondered what intermediate scenarios might be like, lying
between these two extremes."

The investigators simulated planets ranging from ones with no magnetic
field to ones as strong as Earth's, and worlds with atmospheres ranging
from as thick as Earth's to just a tenth as thick.

"We know the magnetic field around Earth protects us from these harmful
cosmic rays, and we thought magnetic fields were going to be the main
factor that controls the radiation dose to the surface," Atri said.

Unexpectedly, "we found the thickness of a planet's atmosphere is a much
more important factor in determining a planet's radiation dose," Atri
told Astrobiology Magazine. "If you took the Earth and you completely
removed the magnetic field, the radiation dose rate will increase by two,
which is a big increase, but it would still have very small effects, and
would not have any effects on us. However, if you keep the magnetic field
and decreased the atmosphere so it is a tenth as thick, the radiation
dose will increase by more than two orders of magnitude."

Planets around red dwarf stars are often thought of as prime targets for
the search of alien life, since these relatively dim stars are very common
in the universe, making up at least 80 percent of the total number of
stars. Theoretical calculations suggested planets in the habitable zones
close to red dwarfs are more likely to have weaker magnetic fields, especially
in the case of so-called super-Earths, large rocky planets up to 10 times
Earth's mass. Astrobiologists were concerned these weak magnetic fields
could make them poor candidates for life, but these new findings suggest
weak magnetic fields are less of a problem than they thought.

Future research can examine how increasing radiation affects the evolution
of life, Atri said. "Most studies of radiation's effects on life mostly
expose organisms to very high doses of radiation to see if they get killed
or not, but I think systematic studies that gradually increase the radiation
microbes receive could show how they evolve in environments that receive
a lot of cosmic rays," he said.

Atri and his colleagues B. Hariharan and Jean-Mathias Griessmeier detailed
their findings in the October issue of the journal Astrobiology.
Received on Tue 12 Nov 2013 06:09:29 PM PST