How to Examine Rocks 163 Million Miles Away

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How do you tell what something is made of without picking it up, touching it, or even coming within 163 million miles of it? The Curiosity rover, which landed on Mars two weeks ago, has a clever method to do just this, which was tested on Sunday.

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Mockup of Curiosity testing the composition of a Mars rock (LANL)

NASA's latest and greatest Mars rover has yet to move an inch. Right now all of the onboard systems are being checked to see if they survived the journey and landing. One of these systems is the rock analyzer. The method it employs is called Laser-Induced Breakdown Spectroscopy (LIBS), and the system for performing it is named "ChemCam."

Have you noticed the spindly mast on top of Curiosity in its publicity shots? It looks like a small head mounted on a long giraffe neck. The one giant left eye on this head is a laser. It is an infrared beam, with a wavelength of 1067 nanometers (nm). (This is invisible to the human eye which can see wavelengths of roughly 400 nm to 800 nm.) The beam is produced from an exotic material: a neodymium doped potassium-gadolinium titanate crystal. (Break out your periodic tables to locate these uncommonly mentioned elements.)

How does it work? The basic principle relies on the fact that the electrons surrounding atoms can only lie in certain very precise positions, called orbitals. Due to the complexities of quantum mechanics, every single element (and every isotope of each element) has a slightly different set of electron orbitals. When an electron switches from one orbital to a lower one, light is emitted from the atom with a very certain color. Each color corresponds to the difference in the electron's energy in its final orbital relative to its original orbital.

Neon signs and sodium street lights are characteristically red and orange, respectively, because those are the colors of light emitted when their electrons change orbitals inside the bulb. We colloquially refer to all glowing signs as "neon" signs, but really, blue "neon" signs are made from mercury. The atoms emitting light in these tubes are in a gaseous state. To get the gas to emit light all you have to do is run an electric current through it. By doing this, you can identify everything inside only by the colors that shine out of it, without ever touching the gas in any way.

But, what if you want to test a rock? This is what Curiosity's laser is for. It pulses several times to blast the dust and outermost surface off of a rock from up to 7 meters (23 feet) away. With the bare inside exposed, the laser fires several more pulses. These pulses blast small pieces of the rock into a plasma, which cools rapidly into a gas. In the chaos of the explosions, electrons switch wildly between states, emitting the particular colors of the atoms that they are orbiting. Cameras on the bottom of Curiosity's "head" then capture this light and carefully analyze how much of each color there is. (Think of how a prism separates light into all of its colors.) Scientists back on earth can then match up the pattern of colors to known elements to identify every element in the rock individually. They can even tell the relative amounts of each.

The LIBS ChemCam test was just one of a series of trials which Curiosity is currently undergoing. On Monday the rover's 7-foot long mechanical arm was flexed. On Tuesday the rover budged a wheel, to test its driving machinery and software. It is truly amazing how well everything seems to be working so far. Soon, Curiosity should be off and rolling. A round of applause for everyone involved in the project!
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