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Ultraviolet on Mars

Radiation and the atom · level 2, lesson 6 of 6

A question to start

Mars is farther from the Sun than Earth, so its sunlight is weaker. Yet scientists say the ultraviolet on the ground there is harsh. How can that be?

The idea

Two things shield Earth's ground from ultraviolet: a thick blanket of air, and the ozone layer in it. Mars has little of either.

  • Mars's air is very thin, about a sixtieth as much as Earth's above each square metre.
  • Mars has only a little ozone, and most of it gathers over the winter pole. Water vapour destroys ozone, and over the winter pole the water vapour has frozen out of the air.

So although the sunlight is weaker, much more of its ultraviolet reaches the ground.

Shields against ultraviolet Mars Earth
Air above each square metre about 170 kg about 10,300 kg
Ozone layer only a little, mostly over the winter pole all round the planet
Harmful ultraviolet at the ground harsh mostly blocked

That is one reason scientists think any tiny living things on Mars would have to live underground, shielded from the ultraviolet. Astronauts would need suits and visors that block it.

Earth: ozone stops mostMars: most gets through

Worked examples

1. Air above you. On Earth, about 10,300 kg of air sits above each square metre. On Mars, about 170 kg: around 60 times less.

2. Where the ozone is. In winter, water vapour freezes out of the air over Mars's pole, so ozone can build up there. Elsewhere, water vapour keeps destroying it.

3. Hiding from it. Even a thin layer of soil blocks ultraviolet. Tiny life, if it exists, could shelter just under the surface.

Try it

  1. What two things shield Earth's ground from ultraviolet?
  2. Why does Mars have so little ozone in most places?
  3. Mars's sunlight is weaker. Why is its ultraviolet still harsh on the ground?
  4. Where might tiny living things on Mars shelter from ultraviolet?
Answers
  1. A thick blanket of air and the ozone layer.
  2. Water vapour destroys ozone; only over the winter pole, where the water vapour freezes out, can it build up.
  3. There is so little air and ozone that much more of it reaches the ground.
  4. Underground.

Common mistake

"Farther from the Sun means safer from ultraviolet." The Sun's light is weaker at Mars, but with no real shield, the ground there gets more of the harmful kinds than Earth's ground does.

Where it leads