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How deep to build

Radiation and the atom · level 7, lesson 6 of 6

A question to start

A crew will live on Mars for 500 days. They must go outside to work, but they will sleep and eat indoors. How should their home be built to keep the dose down?

The idea

Put the pieces together.

  1. The surface dose. Curiosity measured about 0.67 mSv a day, about 245 mSv a year.
  2. A regolith roof. The 2022 study found that 1 to 1.6 m of regolith keeps the dose under 100 mSv a year. Thinner than about a metre is not worth it, because of the peak near 30 cm.
  3. A storm shelter. For solar outbursts, a small room with extra water or regolith around it.
  4. Hydrogen. Water tanks, food and plastic placed in the walls add more shielding. Ground with ice in it would shield better than dry soil, since ice is rich in hydrogen.
  5. Natural roofs. The study's authors point to caves and lava tubes, whose rock roofs could be many metres thick.

Deeper is safer, but every extra metre of roof means more digging and moving. The engineering choice is a balance.

Home on Mars for 500 days Mars, under 1.5 m of regolith Earth, at home in Canada
Dose indoors under about 0.27 mSv a day about 0.005 mSv a day
1.5 m of regolithliving spacestorm shelter,water wallsground

Worked examples

1. Indoors. Under 100 mSv a year means under \(100 / 365 \approx 0.27\) mSv a day.

2. A 500-day stay. Suppose the crew spends 80% of the time indoors and 20% outside. Indoors: \(0.8 \times 500 \times 0.27 = 108\) mSv. Outside: \(0.2 \times 500 \times 0.67 = 67\) mSv. Total about 175 mSv, against 335 mSv for the whole stay out on the surface.

3. More time outside. If they spend 40% outside, indoors gives \(0.6 \times 500 \times 0.27 = 81\) mSv and outside \(0.4 \times 500 \times 0.67 = 134\) mSv: 215 mSv in all. Time outdoors costs the most dose.

Try it

  1. Why is a 30 cm roof of regolith a poor choice?
  2. Find the dose for 500 days spent all indoors under the shield, at 0.27 mSv a day.
  3. Why would ice-rich ground make a good place to build?
  4. Name two ways to cut the crew's dose besides a thicker roof.
Answers
  1. The dose peaks about 30 cm down, because of secondary neutrons.
  2. \(500 \times 0.27 = 135\) mSv.
  3. Ice is rich in hydrogen, which shields best.
  4. Any two of: less time outside, a storm shelter, water and supplies in the walls, a cave or lava tube.

Common mistake

Planning only the roof. A crew's dose depends on where they spend their hours. Time outside, at the full surface dose, can add as much as the hours indoors.

Where it leads