Answer to Question #160 Submitted to "Ask the Experts"

Q

I was recently at a tourist attraction, namely the Kennedy Space Center, and it appeared to me that the discussion of radiation on other planets had a negative connotation. What are the radiation characteristics on other planets in our solar system, particularly the planets that could be habitable?

A

The planets in our solar system that our technology would allow us to live on right now are probably limited to Mars and maybe some of the Jovian and Saturnian moons.

With that in mind, let me explain a little about radiation in space. The primary radiation sources in space consist of charged particles from solar wind (protons) and cosmic radiation consisting of heavier particles (e.g. iron nuclei) from other galaxies. Cosmic radiation is relatively constant, but the intensity of solar wind decreases as the square of distance from the sun (or whatever star you happen to be near). Space radiation exposure on the surface of a planet consists of the solar wind or cosmic radiation that makes it to the surface, plus neutrons and gamma-ray photons that are released when the particles interact with the planet's atmosphere and crust. We are protected from space radiation on earth by our atmosphere and by the earth's magnetic field. The atmosphere absorbs solar wind and cosmic radiation, as well as the neutrons and most gamma rays that are produced. The magnetic field diverts many particles to our poles, which creates the aurora borealis. Mars has little atmosphere and no magnetic field, to speak of, so the dose from charged particles anywhere on the surface is a concern. There's just enough atmosphere to result in a dosimetrically significant neutron field, both from charged-particle interaction with the atmosphere (which isn't thick enough to absorb the neutrons before they reach the surface) and with materials in the crust (which scatters some of the neutrons back toward the surface). I presume that x- and gamma-ray photons are also a concern because of the lack of atmospheric shielding. The Jovian and Saturnian moons are believed to be similar to Mars, in that they have little or no atmosphere and not much magnetic field (with the possible exception of satellites like Io, which is close enough to Jupiter that tidal forces may melt the core and generate a magnetic field). One would presume that the radiation environment on these moons is similar to that on Mars, except that the solar wind would be greatly reduced by consequence of the much greater distance from the sun.

In addition, here's a convenient reference that discusses radiation issues associated with the Mars mission: Radiation and the Human Mars Mission If you would like to discuss the information given in the URL above please contact me using the link below.

James S. Bogard, PhD, CHP
Oak Ridge National Laboratory
Health Physics and Radiological Protection Program Leader