20 June 2009

Fun with Lasers

Posted by Ryan Anderson

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.