On the Curiosity rover, a tool called CheMin (Chemistry and Mineralogy) is helping scientists determine what minerals make up the Martian landscape, and whether single-celled or more complex organisms could once have thrived there. CheMin sends an X-ray beam through tiny samples of Martian soil or rock, recording how the beam scatters as it bounces against atomic planes of the different minerals contained within. The technique, called X-ray powder diffraction (XRD), has been around for a long time, but the tools commonly used were not practical for a robotic mission millions of miles from the nearest human hands.
“To do powder diffraction and get good data, you need to have roughly a million grains of the same type within the volume you're analyzing,” said Philippe Sarrazin, who was the lead developer of CheMin at Ames Research Center. Technicians would grind a fine powder with grains just 10 to 50 microns in diameter, press it into a flat cake between two pieces of plastic, and put it in a massive XRD machine. On Mars, however, everything relies on very precise motions and very heavy equipment.
The biggest problem was that to make the instrument smaller, the sample chamber also had to be smaller. But since traditional XRD still requires a million grains to get enough data points, that meant the grains needed to be even smaller as well. NASA engineers first considered making grains smaller than one micron in diameter. The task proved basically impossible: by the time you ground the material that finely, even if you could do so reliably on Mars, you'd ruin the crystalline structure you were attempting to analyze.
Then they stumbled onto a game-changing discovery. When you vibrate a bed of grains, it flows in a predictable, cyclical pattern. It's called granular convection, and it had been seen before, but no one had ever thought to apply it to XRD. The CheMin team realized this phenomenon could solve the problems they were encountering. Because they were able to see grains from so many different orientations, they could use fewer, bigger grains — about the texture of sand. When the drill operates on Mars, it creates samples that are about 150 microns and less. All the sample-handling instrument had to do was pour the dirt into a sieve and shake that into the CheMin sample chamber.
Sarrazin filed for a patent covering the vibration technique; shortly afterward, he left NASA to form his own company, inXitu, but his work on the instrument continued. Ames granted inXitu two Small Business Innovation Research (SBIR) contracts to pursue the project, which was now destined to travel to Mars on Curiosity. Although the vibration concept worked, there were still a number of practicalities to figure out, including how to vibrate the sample at high intensity without vibrating everything else at the same time. The design for CheMin was completed at Jet Propulsion Laboratory, and the instrument is successfully analyzing samples gathered during Curiosity's trek across Mars. Sarrazin was able to use the work he did for CheMin to create a product for his new company. inXitu's XRD mineral analyzer was small, easy to use, and extremely rugged, and the company could sell it at a fraction of the cost of other instruments.
His new design, which can be used in the field, requires the user to take a hammer and knock out a rock fragment; the hammer is then used to crush it. Then it's poured into the sample cell; the XRD analysis is a one-button operation. The unit can be used by anyone in the field, eliminating the need for a highly trained technician to perform the XRD analysis, or the long wait for a sample to be sent to a lab.
Olympus Scientific Solutions America (Waltham, MA) bought inXitu and now sells the XRD device in two models: TERRA and BTX II Bench top. One of their largest markets is oil and gas exploration. By analyzing the minerals the drill is encountering, you can determine when you've hit the “pay zone” of oil or gas.
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