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Exposure ages

Radiation and the atom · level 8, lesson 6 of 6

A question to start

Cumberland formed about four billion years ago. But how long has it been lying near the surface, out in the cosmic rays? The same oven run gave a second answer.

The idea

Cosmic rays reach only about an arm's length into rock. There, they smash nuclei and leave behind rare gases, such as helium-3, neon-21 and argon-36, that build up in the rock a little each year. Below that depth, the rock is shielded and almost none is made.

So the amount of these gases measures how long the rock has been near the surface, its exposure age:

\[\text{exposure age} = \frac{\text{amount of gas made by cosmic rays}}{\text{amount made each year}}.\]

For Cumberland, three different gases gave exposure ages of 60 to 100 million years. So for nearly all of its four billion years, the rock was buried. Then the layers above were stripped away, quite recently, by windblown sand wearing back a cliff face.

This matters for the search for life. Cosmic rays also break down organic chemicals in rock. NASA estimated that at the depth of Curiosity's drill, 5 cm, they could be cut up to a thousandfold in about 650 million years. Rocks with young exposure ages are the best places to look.

cosmic raysexposed for a long timejust uncovered: youngwindblown sand

The pale layers are the top arm's length, where cosmic rays make gas.

Worked examples

1. A gas clock. Suppose cosmic rays at that depth make 25 units of neon-21 in a gram of rock every million years, and a gram holds 2,000 units. Exposure age \(= 2{,}000 / 25 = 80\) million years.

2. Two clocks in one rock. Cumberland: formed about 4,200 million years ago; near the surface for about 80 million. \(80 / 4{,}200 \approx 2\%\) of its life spent exposed.

3. Organics. Using NASA's upper estimate, a thousandfold drop is about ten halvings (\(2^{10} = 1{,}024\)) in 650 million years: one halving every 65 million years. In 80 million years, \((\tfrac12)^{80/65} \approx 0.43\), so about 40% would remain.

Try it

  1. Why are the gases made only near the surface?
  2. A rock holds 1,500 units of helium-3 from cosmic rays, made at 30 units a million years. Find its exposure age.
  3. Why are rocks with young exposure ages the best places to look for organic chemicals?
  4. How is an exposure age different from the age found by the potassium-argon clock?
Answers
  1. Cosmic rays reach only about an arm's length into rock.
  2. \(1{,}500 / 30 = 50\) million years.
  3. Cosmic rays have had less time to break down the organic chemicals in them.
  4. Potassium-argon tells when the rock set hard. The exposure age tells how long it has been near the surface.

Common mistake

"An old rock has been at the surface all its life." Cumberland is about four billion years old, but has been near the surface for only about the last 2% of that time.

Where it leads