Roughly a year ago, there was a TV show about colonizing Mars; National Geographic channel I think. The habitats were underground to avoid the Sun's radiation.

So I'm wondering: does the radiation come directly (i.e. line-of-sight) from the Sun? If so, then in the upper or lower latitudes you could just make your camp in the shadow of a cliff.

On the other hand, if the radiation comes from all directions, then you better go underground.

  • $\begingroup$ 5 minutes on google led me to the cosmic ray wiki which says that cosmic rays also produce more cosmic rays when they hit atmospheric molecules which then scatters in all directions. Also, the sun moves around in the sky and the sun is not the only source of cosmic rays. $\endgroup$
    – DKNguyen
    Apr 17, 2021 at 4:18
  • $\begingroup$ Yes. Radiation certainly comes from all directions too. GCRs (Galactic Cosmic Rays) are high energy atomic nuclei that originate from outside the solar system. The flux depends on the Sun's solar cycle as it effects the interplanetary magnetic field. $\endgroup$
    – Star Man
    Apr 17, 2021 at 4:18
  • $\begingroup$ @StarMan It does beg the question how much cosmic rays doesn't come from the sun though, because if you use some comparison such as starlight vs sunlight, it would seem that it would be a lot less; perhaps to the point where it alone wouldn't be such a problem unless radiation is a lot more potent than one feel it might be. OP might also be interested in space.stackexchange.com/questions/6060/… $\endgroup$
    – DKNguyen
    Apr 17, 2021 at 4:23
  • $\begingroup$ @DKNguyen Solar cosmic rays (also called SEPs) are mostly emitted during solar storms. GCRs however are always constant. So I'd guess that most cosmic rays are GCRs. $\endgroup$
    – Star Man
    Apr 17, 2021 at 4:29
  • $\begingroup$ @StarMan Doesn't the solar wind count as cosmic rays? $\endgroup$
    – DKNguyen
    Apr 17, 2021 at 4:31

1 Answer 1


You have several kinds of radiation to deal with:

  1. Sunlight. Sunlight has a fair amount of UV. On Earth most of this is blocked by the ozone layer. Only a fairly small percentage gets through. Hence all the fuss and feathers about chloro-fluoro carbons (freon and friends) breaking down the ozone layer. On Mars you don't have enough free oxygen to form an effective ozone layer. Even with the dimmer sunlight, going outside without UV protection will be hazardous. UV also degrades many plastics quickly, leading to maintenance issues.

  2. Solar wind. The sun emits a continuous stream of ionized gas -- mostly electrons and protons. On Earth these are intercepted by the earth's magnetic field. When the solar wind is more brisk, the particles rattling around the magnetic field can hit the atmosphere at the poles causing the aurora. During a pole reversal the earth is without an effective magnetic field for some unknown period of time. Recent evidence from New Zealand suggests significant ecological issues (die offs)

Solar wind ultimately causes the aurora, when energetic protons and electrons hit the atmosphere. On earth aurora effects range from 80 to 600 km above the surface. Very roughly the earth's atmosphee halves in density every 18,000 feet. So 80 km = 240,000 feet. About 1/8000 of earth surface pressure. So about 80 times the pressure on Mars.

My first guess then, would be that while solar wind is making off with Mars's atmosphere, there isn't much effect at the surface.

  1. Cosmic rays. Most of these come slamming into the upper atmosphere, each one producing a cascade of other particles. These are actually more dangerous with an atmospheric shield if the shield is thin enough that the tertiary cascade particles reach the surface. Cosmic rays are relatively rare.

The solar wind is the big problem, especially when the sun flares. A permanent terraformed Mars ideally would have a superconductor cable running around the equator with about 40 million A flowing through it. This would create a magnetic field similar to Earths. (Check the 40 MA figure. I did it once before and was surprised by how modest it was.)

  • $\begingroup$ It sounds like solar wind is not line-of-sight. But as the ionized gas hits the (admittedly thin) Martian atmosphere, wouldn't oblique directions lose most of their energy? Maybe the shadow of a cliff still makes sense. $\endgroup$
    – Daniel
    Apr 19, 2021 at 5:11

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