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Let's say that after decades of development there are now cities on Mars. Most of their facilities are located underground in order to protect them from radiation and small meteorites, as well as providing additional insulation from the Martian climate. However, buildings that require access to sunlight, such as agricultural facilities, would be left vulnerable.

So, are there any issues with using water reservoirs (with a clear top and bottom) as radiation shielding to protect the upper layers of the colony? And would there be problems with using that water for irrigating crops or drinking water?

Also, I should mention that the water is kept circulated and heated to stop it from freezing. Preventing it from bursting the pipes or the reservoir itself.

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    $\begingroup$ Given that martian dust storms block out the sun for weeks on end you'll need a backup lighting system and a source of power. $\endgroup$ – Donald Hobson Jun 3 '16 at 10:40
  • $\begingroup$ Very good point. They would probably have to rely on multiple power sources (such as solar and nuclear), and each complex would need some kind of communal battery storage (or generators) for emergencies. $\endgroup$ – D-Nate Jun 5 '16 at 6:31
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And would there be problems with using that water for irrigating crops or drinking water?

Of course. This would make radiation shield thinner every time someone takes a drink. You don't want that. If water is sufficiently pure, using it as a shielding this way will not make it radioactive on it's own, so there is no radiation risk for it's users.

Also, I should mention that the water is kept circulated and heated to stop it from freezing.

And when electricity goes off, it is a race. Can they fix it before water freezes, pipes burst, and colony is left without water? Or before crop dies and we are waiting for hunger death? I wouldn't like to live there.

to protect them from radiation and small meteorites

If you believe small meteorites will be a real threat, then you do not want to expose your water reserve to them.

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  • $\begingroup$ You make some incredibly good points. I suppose a Martian city would treat a habitation breach with the same urgency as we we do with firefighting and ambulance services. Plus, they would have to have a ton of redundant systems in place too. Your answer has given me a lot to think about, thanks! $\endgroup$ – D-Nate Jun 3 '16 at 9:03
  • $\begingroup$ @D-Nate Always glad to make stories a bit more believable :) With lack of lead ore and quartz sand, water shielding doesn't seem that bad. If, of course, there is water on Mars. But I bet your story assumes there is some ice. $\endgroup$ – Mołot Jun 3 '16 at 9:07
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No

There are no insurmountable problems with using water as your radiation shield.

In fact, several Martian Colony studies use this approach.

Radiation

The water won't be made noticeably more radioactive even if used as a radiation shield. This won't be a problem.

Furthermore, water would be almost the ideal shielding material for solar wind and cosmic ray protection. It would provide adequate protection from UV & XRays.

Reservoir

If you use water as your radiation shield, the shield will not be your only water reservoir. You'll have at least one other reservoir as storage for your other needs.

Temperature moderation

If you cycled your water through the agriculture dome, then you could also use it as a form of areothermal heating. Sink shafts deep underground, run closed-loop pipes into them, then pump water through them, and when the water comes back up, it will have been warmed by Mars' internal heat.

Meteorite protection

This system isn't nearly as good at protecting you from meteorites as it would protect you from radiation. However, a puncture of the dome would cause the water to simultaneously freeze and sublimate. The frozen portion would significantly slow the loss rate of water to the environment.

Of course, the system could include self-sealing materials to reduce the loss even more.

Summary

You could plausibly and scientifically use water in this capacity if you want. There are a few engineering issues with it though so if you chose to not use it, you could justify that too.

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