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On the surface of Mars

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

A question to start

Once Curiosity landed, its detector kept going. Is the surface of Mars safer than space, or not much better?

The idea

On the floor of Gale Crater, from August 2012 to June 2013, Curiosity measured an average of 0.67 mSv a day. More than 95% of it came from cosmic rays; there were no big solar storms in that time.

That is about a third of the rate in deep space. The planet itself blocks the half of the sky below your feet, and the thin air stops a little more. But it is still far more than at home:

Natural radiation Mars (Gale Crater) Earth (Canada)
Per day 0.67 mSv about 0.005 mSv
Per Earth year about 245 mSv 1.8 mSv
deep spaceon Mars: half the sky

Worked examples

1. A year. \(0.67 \times 365 \approx 245\) mSv in an Earth year: \(245 \div 1.8 \approx 136\) times the Canadian natural dose.

2. Space against surface. \(0.67 \div 1.8 \approx 0.37\): the surface gets about a third of the deep-space rate.

3. A long stay. Suppose a crew stays 500 days. \(500 \times 0.67 = 335\) mSv.

Try it

  1. What dose did Curiosity measure each day on Mars?
  2. Give two reasons the surface of Mars gets less than deep space.
  3. How many days on Mars equal the worker limit for a year of 50 mSv?
  4. Is a year on Mars more or less than a year on the space station?
Answers
  1. About 0.67 mSv.
  2. The planet blocks the half of the sky below, and the thin air stops a little.
  3. \(50 \div 0.67 \approx 75\) days.
  4. More: about 245 mSv against 150.

Common mistake

"Mars has air, so it is as safe as Earth." Above each square metre, Mars has about 60 times less air than Earth, and there is no global magnetic field. It blocks only a little of the cosmic rays.

Where it leads