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I am a Mars enthusiast, and lately have been wondering how safe Mars would be to live on after it hypothetically has been terraformed into a pseudo Earth. Obviously, the bigger health hazard would be the potentially high levels of radiation and cosmic rays from the sun. While Earth has a robust magnetic field, which shields the surface from most radiation, a terraformed Mars would not have one. But, assuming a terraformed Mars had a pressure of 1 atm at sea level, the atmosphere presumably would offer some sort of protection.

This question is partly a follow up to a previous question discussing radiation levels on Mars given its current tenuous atmosphere.

I am hoping to find out the following two things:

  • How much radiation would reach the surface of a terraformed Mars
  • How would this compare to the total radiation on the surface of Earth, minus the effects of both the relatively thick atmosphere and the magentic field, and
  • How safe would it be for human, plant, and aquatic life

Regarding the last bullet point, presumably most ocean life, which exists at an average depth of some meters, would be completely safe, assuming Mars also had an atmosphere of 1 atm at sea level.

One reason I am asking this question is as a reality/sanity check. There is a lot of hype of colonizing and terraforming Mars. But if the resulting planet would still be hostile to life, there would not seem to be much point in doing it.

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  • $\begingroup$ A long time ago, Mars had an atmosphere, but it was not able to hold it. When Mars is given an artificial atmosphere, it will loose it again. The problem is to add gas faster than it is lost. $\endgroup$ – Uwe Aug 24 '18 at 14:43
  • $\begingroup$ I think you're going to need a magnetosphere to block harmful radiation, and to keep the solar wind from stripping away your atmosphere. $\endgroup$ – Don Branson Aug 24 '18 at 14:52
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    $\begingroup$ @Uwe True, but my understanding is that the solar wind would take tens of thousands of years, or longer, to strip away the atmosphere. If being exposed on the surface would kill us in our lifetime, then this is the more immediate problem. $\endgroup$ – Tim Biegeleisen Aug 24 '18 at 14:52
  • $\begingroup$ Im not sure there is a moderately reasonable terraforming theory that doesnt attempt to fix the magnetosphere so this question is kind of moot. $\endgroup$ – anon Aug 24 '18 at 22:48
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The level of radiation on the surface would actually be lower than that on Earth. The lack of magnetosphere makes it harder to retain the atmosphere, but the atmosphere will stop pretty much all harmful radiation from reaching the surface.

The reason it will actually be lower is that, due to the weaker gravity, the atmosphere of Mars will actually need to be thicker than that on Earth to give the same surface pressure, so it will absorb more. Exactly how much depends on what the atmosphere is made up of, and how hot it is, but you should be better protected than Earth, as well as being farther from the Sun. There will presumably be enough oxygen to form an ozone layer, blocking UY.

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  • $\begingroup$ Even the current atmosphere provides shielding equivalent to or better than Earth's magnetosphere provides for the ISS: jpl.nasa.gov/spaceimages/details.php?id=PIA03480 $\endgroup$ – Christopher James Huff Aug 24 '18 at 23:14
  • $\begingroup$ I had in mind that "radiation" includes not just photons, but also cosmic rays. My question was trying to get at how much of the total protection comes from the atmosphere and how much from the magnetic field, and therefore how safe would Mars be with just 1 atm at sea level. $\endgroup$ – Tim Biegeleisen Aug 25 '18 at 6:05
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    $\begingroup$ @TimBiegeleisen: Particle radiation is exactly what sheer thickness of atmosphere is effective against. Solar UV is an exception in that a breathable atmosphere is mostly transparent to it, but a terraformed atmosphere would have oxygen to form ozone, which does provide UV protection. $\endgroup$ – Christopher James Huff Aug 25 '18 at 14:18
  • $\begingroup$ @ChristopherJamesHuff Then I'm happy to accept this answer if you can also let me know then why a magnetic field has any importance in radiation prevention whatsoever. From memories of physics lectures, I thought that Earth's magnetic field created a bottle, which then trapped bad incoming particles. $\endgroup$ – Tim Biegeleisen Aug 25 '18 at 16:39
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    $\begingroup$ Earth's magnetic field protects the atmosphere from radiation, which helps stop it being lost to space, and protects satellites in Low Earth Orbit, but even without it little radiation would make it down to the surface. $\endgroup$ – Steve Linton Aug 25 '18 at 16:42

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