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

[Ron Baalke]

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.