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Reading rocks with lasers

Waves, sound and light · level 8, lesson 5 of 6

A question to start

Perseverance's SuperCam can tell what a rock is made of without touching it. In one of its first tests, it zapped a rock 30 times from about 3 m away. How does a laser read a rock?

The idea

SuperCam uses light in three ways.

1. Laser-induced breakdown spectroscopy (LIBS). A short, intense laser pulse heats a pinhead-sized spot on the rock so much that it turns into a plasma, a glowing gas of atoms and ions. As the plasma cools, each element gives out its own emission lines. A spectrometer spreads the light out, and the lines show which elements are there: silicon, iron, calcium, magnesium, sodium, hydrogen and more. The brighter a line, the more of that element.

2. Raman spectroscopy. A green laser excites the chemical bonds in the rock. The scattered light comes back slightly shifted in wavelength, by amounts that depend on which atoms are bonded together. That shows the minerals, not just the elements.

3. Visible and infrared reflectance (VISIR). SuperCam also looks at sunlight reflected from rocks. Minerals absorb particular infrared bands, which show what they are made of.

The sound of each zap, picked up by SuperCam's microphone, adds one more clue: how hard the rock is.

laser pulseplasmaelement lines

Worked examples

1. A hydrogen line. A bright line at 656.3 nm in the plasma means hydrogen, often from water locked in minerals.

2. The energy of the light. A plasma line at 589 nm, from sodium, has photons of \(hc/\lambda = 2.1\) eV.

3. Resolving the lines. Sodium's two lines, 0.6 nm apart, need a resolving power of about 1,000: easy for a good grating spectrometer.

Try it

  1. What does the laser do to the rock in LIBS?
  2. Why does each element show up as its own set of lines?
  3. What extra information does Raman spectroscopy give that LIBS does not?
  4. How does the sound of the zaps help?
Answers
  1. It heats a tiny spot into a glowing plasma of atoms and ions.
  2. Each element has its own energy levels, so its own photon energies and wavelengths.
  3. Which atoms are bonded together, and so which minerals the rock contains.
  4. Its loudness gives a hint of how hard the rock is.

Common mistake

"The laser melts away the rock." Each pulse removes only a pinhead of material, from the surface. That also blows off dust, so later pulses read the rock beneath.

Where it leads