cosmic ray protection from Martian atmosphere? ...Nope

[JoeStrout]

!My older son hangs out on the KSP forums a lot, and told me about a claim he'd read there that the Martian atmosphere adds up to something on the order of 10 tons per square meter.

If so, then you would get substantial protection against cosmic rays just from the mass of the atmosphere.  I've been going around claiming that cosmic radiation is a serious problem on Mars, so this surprised me quite a bit.

So today, while preparing for my New Worlds 2015 talk, I decided to investigate.  Using NASA's Mars Fact Sheet, and a bit of math, it's easy to divide the total mass of the atmosphere by the surface area:

2.5*10^16 kg / (144.8*10^6 km^2): 172.7 kg / m^2

Hmm... Nope!  172 kg per square meter isn't enough to make the cosmic rays even notice.  It'll help against solar particles, maybe, but cosmic rays will zip right through.

Incidentally, here's the same calculation done for Earth:

5.1*10^18 kg / (510.1*10^6 km^2): 9998 kg / m^2

So down here, we really do have 10 tons per square meter over our heads.  And, a strong magnetic field too!

Moral of the story: if you're going to live on Mars, be prepared to stay underground (or under a very thick dome) most of the time.  Significant surface time would result in infertility, cancer, and death.  (Sounds like a Surgeon General's warning, doesn't it?)

[William]

(09-29-2015, 10:52 AM)JoeStrout Wrote: Moral of the story: if you're going to live on Mars, be prepared to stay underground (or under a very thick dome) most of the time.  Significant surface time would result in infertility, cancer, and death.  (Sounds like a Surgeon General's warning, doesn't it?)

Or live under ~9 m. of water, or ~8 km. of air @ earth sea level pressure. Perhaps larger values, since Mars has no magnetic field and Earth's magnetic field also provides some insulation (how much?).

[JoeStrout]

(09-29-2015, 12:13 PM)William Wrote: Or live under ~9 m. of water, or ~8 km. of air @ earth sea level pressure. Perhaps larger values, since Mars has no magnetic field and Earth's magnetic field also provides some insulation (how much?).

Quite a lot.  I don't have the figures handy, but if you have High Frontier, you can explore this for yourself: compare radiation in LEO (which is well within the magnetosphere) to radiation in High Earth Orbit (outside the magnetosphere).  It makes quite a difference.

But yeah, nine meters of water would do the trick too!

[JoeP]

Mars has such a thin atmosphere, but I think the images of it look so earth-like, for examples desert and arid places here at home, that it inspires us greatly as a potential second home.

The radiation problem is such a serious issue though. Your comments about the Moon being a better place in many regards is well taken.

I wonder about Titan's prospects. It seems to me that such a thick atmosphere and a pressure that is acceptable for us, one would only need an electrically heated suit or even really excellent clothing and an oxygen mask helmet to survive there. How is the radiation at Titan?

[JoeStrout]

Titan, hey, that's a really good question! Titan's atmosphere masses over 7 times as much as Earth's per square meter. So yeah, that's substantial protection against cosmic rays — I think you'd be fine as far as that goes.

I'd sure be nervous about an oxygen mask there, though... one spark and it's going to go FOOM!

[William]

(09-29-2015, 01:16 PM)JoeStrout Wrote: Quite a lot.  I don't have the figures handy, but if you have High Frontier, you can explore this for yourself: compare radiation in LEO (which is well within the magnetosphere) to radiation in High Earth Orbit (outside the magnetosphere).  It makes quite a difference.

But yeah, nine meters of water would do the trick too!

Well according to the paper below in fact it would require less water not more, 6-7 meters of water in High Earth orbit, so that would work on Mars.

After posting, I thought that non-opaque solid materials, in particular quartz glass, could be used instead of water. No potential problem of leakage.

http://www.nss.org/settlement/space/RadiationPaper.pdf

[antred]

Then there's also the low gravity. That's not at all good for people if they're going to stay for an extended period of time. Mars is not a very reasonable target for colonization.

[JoeStrout]

That's true. We don't know what the effect of living (and raising children) at 1/3 G would be, but it seems premature to assume everything will work out fine.

I expect Mars to be a very popular tourist destination, though. Maybe people who have to live there (because they work at the tourist or research facilities, for example) can live in underground track-supported centrifuge towns... like orbital colonies, but below ground.

[antred]

Hmm, but how would you position such a centrifuge in an environment that already has a low but not negligible gravity component? If you make it spin around a vertical axis then people we still get pulled "down" at 0.38 G. If you make it spin around a horizontal axis, then "gravity" as felt by people living in the thing would vary from 1 G - 0.38 G = 0.62 G at the top to 1 G + 0.38 G = 1.38 G at the bottom of the centrifuge. Or have I completely misread your comment?

[hanelyp]

The long term effect of fractional G is a point where we really don't have enough data.