This is no “d-swinging” stunt- the instrument (“ChemCam”) will return valuable data on rock compositions. This in turn will let the mission team decide where to extend the arm and take rock samples with the other instruments. These instruments constitute the most decked-out chem lab ever sent to Mars, and off the top of my head, to any other planet or small body.
The laser delivers over a megawatt. Awesome… kinda. It fires megawatt pulses, but each pulse is a few billionths of a second, in a train of a few dozen pulses. Thus, the total energy is comparable to the old incandescent bulb you may still have hanging in the attic.
What that pulse train does is called LIBS (Laser Induced Breakdown Spectroscopy). The energy of the laser is focused on an interesting rock. The target spot on the rock then vaporizes and glows. By examining the glow, the rover sends back information that can determine the composition of the rock.
See, besides the laser transmitter is a receiver telescope. The telescope is focused on the laser spot, and gathers the glow emitted from the rock target. Attached to the telescope, then, are spectrometers- instruments which measure the amount of different colors in a given light. Certain chemicals will glow in certain colors, and not in others. The pattern of light and dark colors (a spectrum) can be a fingerprint for a given chemical, if your spectrometer is sensitive enough. Needless to say, you don’t risk ‘Seven Minutes of Terror’ for an insensitive instrument. The spectrometers go from ultraviolet light, through visible and a bit into the infrared, and with a pretty fine degree of color readout too (“spectral resolution”).
In use, the rover will aim at rocks up to 7 meters away (~23 feet). Should any rocks appear interesting, the rover’s mast can blast deeper, to peel back any crust that may have formed (“weathering rind”) and get at the bulk material. The Sojourner rover from 1996 could not do anything about rock rinds, and thus all the nearby rocks looked alike. The Spirit and Opportunity rovers were then given little grinders. MSL now has a frickin’ laser.
Mission planners anticipate peeling back even better paydirt. It is pretty obvious that there was liquid water on Mars at some point. Even if there wasn’t, rocks still form layers, with the oldest usually on the bottom, and getting younger with height (“rock strata”). Anyone who’s been to the Grand Canyon or similar geologic features has seen the obvious strata, but they don’t have to be obvious to be informative. The site on Mars, Gale Crater, is expected to display lots of strata on its rock faces and walls. Zapping up and down these layers should tell us the history of the Gale site for millions, possibly billions of years. And, again, the rover also has more tools in the box if a particularly juicy strata is found. Microscopes, drills, samplers… a whole bag of Shhh.
Need to drill deeper? See The ChemCam Instrument Suite on the Mars Science Laboratory (MSL) Rover: Science Objectives and Mast Unit Description, a paper submitted by the instrument managers. Still want more? Try The ChemCam Instrument Suite on the Mars Science Laboratory Rover Curiosity: Remote Sensing by Laser-Induced Plasmas.