Fun with Lasers

Ladies and gentlemen: I just spent a week vaporizing rocks with a laser!

Now, after your first thought of “Whoa, awesome” wears off, you may be wondering why I would do such a thing. Because it’s fun, obviously. But also because the Mars Science Laboratory rover “Curiosity” will be doing the same thing on Mars.

The ChemCam instrument uses an infrared laser to shoot pulses of light at rocks. The light is so intense and deposits so much energy into a tiny spot on the target, that the molecules break up into their constituent atoms and the electrons on those atoms are ripped off. This creates an expanding cloud of super-hot plasma (many thousands of degrees). Of course, atoms really would prefer to be quietly sitting in molecules with all of their electrons peacefully orbiting their nucleus: that’s the lowest energy configuration, and nature always tries to minimize the energy in a system.

So as the plasma cools, the electrons join back up with the atoms, and in the process they give up some of their energy in the form of light. Since electrons can only orbit atoms at certain energy levels, the light given off can only be certain wavelengths as well. Even better, every element on the periodic table has different energy levels, so they each give off distinctive colors of light. If one were able to collect that light and measure its wavelength, one could tell what atoms were in the target.

That’s why we shoot rocks with lasers. By zapping the rocks on Mars we will be able to calculate their chemistry and determine what kind of rocks they are and how they got that way. This technique, which goes by the official name of “Laser-Induced Brakdown Spectoscopy” or LIBS also has the advantage that it is fast. The laser pulses ten times per second, and each pulse returns a spectrum full of information. For most analyses, we use the average of a few spectra, but still, it only takes seconds to collect the data.

Analyzing it is the tricky part, and that’s what I will be doing over the next few years: figuring out the best way to look at LIBS data to extract the information quickly and accurately.

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6 Comments on “Fun with Lasers”

  1. That’s cool.

    Will the laser be able to burn through the layer of caked-on dust to the rock underneath, or will Curiosity usually scrape that off first with a RAT type of tool? The picture implies LIBS will be used on out-of reach rocks but is that the plan?

    • Ryan Says:

      ChemCam will be able to blast away loose dust, and with repeated shots in the same place can penetrate very thin weathering layers. The amount that it vaporizes depends a lot on the material: some rocks that we shot were essentially unaffected, while others drilled in quite quickly.

      On of the most important things ChemCam will do is identify rocks that are interesting for the other instruments to analyze. So given the choice between half a dozen targets, a quick ChemCam analysis could tell the team which rock is worth the much lengthier process of using the in-situ instruments.

  2. Infamousginger Says:

    Very good work.

  3. parxier Says:

    It looks like R2-D2’s laser from Star Wars :-)

    • Ryan Says:

      The actual laser is invisible, but for illustration purposes they show it as green, which is the color it would be if you doubled the frequency.

  4. vinny Says:

    I am reporting you to PETMR – People for the Ethical Treatment of Mars Rocks.

    Expect to hear from them very soon.

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