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Regolith shields

Radiation and the atom · level 7, lesson 5 of 6

A question to start

Every kilogram of shielding launched from Earth costs a great deal. But on Mars there is a shield lying everywhere, free: the ground. How much of it would a home need?

The idea

Mars's loose surface soil and broken rock is called regolith. Piled over a home, it adds mass per area.

Suppose loose regolith has a density of about 1.5 g/cm³. Then each metre of it adds \(1.5 \times 100 = 150\) g/cm², almost nine times all of Mars's air.

But a thin layer can make things worse. Cosmic rays smashing nuclei in the soil make secondary neutrons. A 2022 study using Curiosity's measurements found that the dose peaks about 30 cm down, higher than at the surface. Only deeper does it fall. The study found that 1 to 1.6 m of regolith would keep a year's dose under 100 mSv.

Shielding above each cm² Mars, under 1 m of regolith Earth, in the open
Air 17 g 1,030 g
Regolith (supposing 1.5 g/cm³) 150 g none
Total about 170 g about 1,030 g
peak, 30 cm100 mSv a year1 to 1.6 msurfacedepthdose

The depths are to scale; the height of the curve is a sketch.

Worked examples

1. One metre. \(1.5 \times 100 = 150\) g/cm². With the air, about 170 g/cm²: still only a sixth of Earth's air.

2. A roof. Suppose a habitat roof of 10 m by 10 m is covered with 1.5 m of regolith. Volume \(= 100 \times 1.5 = 150\) m³. At 1,500 kg/m³, that is 225,000 kg, 225 tonnes of soil, to be dug and moved by machine.

3. As much as Earth's air. To match Earth's 1,030 g/cm² you would need \((1{,}030 - 17) / 1.5 \approx 675\) cm of regolith: nearly 7 m.

Try it

  1. What is regolith?
  2. Why can 30 cm of regolith give a higher dose than none?
  3. Find the mass per area of 2 m of regolith at 1.5 g/cm³.
  4. Why is regolith a cheaper shield than water brought from Earth?
Answers
  1. The loose soil and broken rock on the surface.
  2. Cosmic rays smash nuclei in the soil and make secondary neutrons; too thin a layer adds them without blocking enough.
  3. \(1.5 \times 200 = 300\) g/cm².
  4. It is already there; it does not have to be launched.

Common mistake

"Any cover is better than none." A thin layer of soil makes extra secondary radiation. A regolith shield must be thick enough, around a metre or more, to help.

Where it leads