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Mars Science Laboratory

Chemistry & Mineralogy X-Ray Diffraction (CheMin)

Chemistry & Mineralogy X-Ray Diffraction Instrument
Designed to be about the size of a laptop computer inside a carrying case, the Chemistry and Mineralogy Instrument identifies and measures the abundances of minerals on Mars. A rotating wheel in the center of the rectangular housing carries individual rock and soil samples for chemical analysis. Credit: NASA/JPL-Caltech

The Chemistry and Mineralogy instrument, or CheMin for short, identifies and measures the abundances of various minerals on Mars. Examples of minerals found on Mars so far are olivine, pyroxenes, hematite, goethite, and magnetite.

Minerals are indicative of environmental conditions that existed when they formed. For example, olivine and pyroxene, two primary minerals in basalt, form when lava solidifies. Jarosite, found in sedimentary rocks by NASA's Opportunity rover on Mars, precipitates out of water.

Using CheMin, scientists are able to study further the role that water, an essential ingredient for life as we know it, played in forming minerals on Mars. For example, gypsum is a mineral that contains calcium, sulfur, and water. Anhydrite is a calcium and sulfur mineral with no water in its crystal structure. CheMin is able to distinguish the two. Different minerals are linked to certain kinds of environments. Scientists use CheMin to search for mineral clues indicative of a past Martian environment that might have supported life.

To prepare rock samples for analysis, the rover is able to drill into rocks, collect the resulting fine powder, sieve it, and deliver it to a sample holder. It uses a scoop for collecting soil.

CheMin then directs a beam of X-rays as fine as a human hair through the powdered material. X-rays, like visible light, are a form of electromagnetic radiation. They have a much shorter wavelength that cannot be seen with the naked eye. When the X-ray beam interacts with the rock or soil sample, some of the X-rays will be absorbed by atoms in the sample and re-emitted or fluoresced at energies that are characteristic of the particular atoms present.

In X-ray diffraction, some X-rays bounce away at the same angle from the internal crystal structure in the sample. When this happens, they mutually reinforce each other and produce a distinctive signal. Scientists can measure the angle at which X-rays are diffracted toward the detector and use that to identify minerals. For example, if the mineral halite (common table salt, or NaCl), were placed in CheMin, the instrument would produce a specific diffraction pattern that would identify the structure of halite.

Because all minerals diffract X-rays in a chacteristic pattern and all elements emit X-rays with a unique set of energy levels, scientists use the information from X-ray diffraction to identify the crystalline structure of materials the rover encounters on Mars. A Charge-Coupled Device (CCD) collects both diffraction and fluorescence information.

Chemistry and Mineralogy Instrument Installed in Mars Rover
Members of NASA's Mars Science Laboratory team carefully steer the hoisted Chemistry and Mineralogy (CheMin) instrument during its June 15, 2010, installation into the mission's Mars rover, Curiosity. Credit: NASA/JPL-Caltech

Say 'Ahh' on Mars
This image from NASA's Curiosity rover shows the open inlet where powdered rock and soil samples are funneled down for analysis. The Mars Hand Lens Imager (MAHLI) was about 8 inches (20 centimeters) away from the mouth of the Chemistry and Mineralogy (CheMin) instrument when it took the picture.