David Blake, is the principal investigator for CheMin, NASA Ames Research Center, Moffett Field, California
The Chemistry and Mineralogy experiment, or CheMin, is one of two investigations that will analyze powdered rock and soil samples delivered by Curiosity’s robotic arm.
CheMin is the size of a laptop computer inside a carrying case. A rotating wheel in the center of the rectangular housing will carry individual rock and soil samples for chemical analysis. Image credit: NASA/JPL-Caltech
CheMin will identify the minerals in the samples and measure how much of each mineral is present. We are studying the minerals because they give us a durable record of past environmental conditions. They also give us information about possible ingredients and energy sources for life.
CheMin uses X-ray diffraction, a first for a mission to Mars. This is a more definitive method for identifying minerals than was possible with any instrument on previous missions. This method of X-ray diffraction is added to the diffraction measurements with the CheMin X-ray fluorescence (XRF), a method it shares with the XRF on the APXS instrument. X-ray diffraction gives us further details of the composition of our sample by telling us the amount of each specific element.
On previous missions to Mars instruments used for studying Martian were not capable of providing us with a definitive identification of all types of minerals.
CheMin will be able to do so for minerals present in samples above minimal detection limits of about 3 percent of the sample composition. The instrument will also indicate the approximate concentrations of different minerals in the sample. The X-ray fluorescence instrument can add information about the ratio of elements in types of minerals with variable elemental composition, such as the proportion of iron to magnesium in iron magnesium sulfate (olivine).
It can also aid in identifying non-crystalline ingredients in a sample, such as volcanic glass.
Each type of mineral forms under a certain set of environmental conditions: the chemistry present (including water), the temperature and the pressure.
CheMin’s identification of minerals will provide information about the environment at the time and place where the minerals in the rocks and soils formed or were altered. Some minerals the instrument might detect, such as phosphates, carbonates, sulfates and silica, can help preserve biosignatures. Whether or not the mission determines that the landing area has offered a favourable habitat for life, the inventory of minerals identified by CheMin will provide information about processes in the evolution of the planet’s environment.
X-ray diffraction works by directing an X-ray beam at a sample and recording how X-rays are scattered by the sample at the atomic level. All minerals are crystalline, and in crystalline materials, atoms are arranged in an orderly, periodic structure, causing the X-rays to be scattered at predictable angles. From those angles, researchers can deduce the spacing between planes of atoms in the crystal. Each different mineral yields a known, characteristic series of spacings and intensities, its own fingerprint.
On Curiosity’s deck, near the front of the rover, one funnel with a removable cover leads through the deck top to the CheMin instrument inside the rover. The instrument is a cube about 10 inches (25 centimeters) on each side, weighing about 22 pounds (10 kilograms).
The rover acquires rock samples with a percussive drill and soil samples with a scoop. A sample processing tool on the robotic arm puts the powdered rock or soil through a sieve designed to remove any particles larger than 150 microns (0.006 inch) before delivering the material into the CheMin inlet funnel. Vibration helps move the sample material — now a gritty powder — down the funnel. Each sample analysis will use about as much material as in a baby aspirin.
The funnel delivers the sample into a disc-shaped cell, about the diameter of a shirt button and thickness of a business card. The walls of the sample cell are transparent plastic. Thirty-two of these cells are mounted around the perimeter of a sample wheel. Rotating the wheel can position any cell into the instrument’s X-ray beam. Five cells hold reference samples from Earth to help calibrate the instrument. The other 27 are reusable holders for Martian samples.
Each pair of cells is mounted on a metal holder that resembles a tuning fork. A tiny piezoelectric buzzer excites the fork to keep the particles in the sample moving inside the cell during analysis of the sample. This puts the particles in a random mix of orientations to the X-ray beam, improving detection of how the mineral crystals in the sample scatter the X-rays.
The piezoelectric vibration, at about 200 cycles per second (middle C on a piano is 261 cycles per second) also helps keep the powder flowing during filling and dumping of the cell.
With the invention of this tuning-fork powder vibration system NASA Ames Research Center won the 2010 Commercial Invention of the Year Award from NASA for the. Blake and Philippe Sarazin of inXitu Inc., Campbell, Calif., a co-investigator on the CheMin team, developed the technology while Sarazin was working as a post-doctoral fellow at Ames.
CheMin generates X-rays by aiming high-energy electrons at a target of cobalt. The X-rays emitted by the cobalt are then directed into a narrow beam. During analysis, the sample sits between the incoming beam on one side and the instrument’s detector on the other.
The detector is a charge-coupled device like the ones in electronic cameras, but sensitive to X-ray wavelengths and cooled to minus 76 degrees Fahrenheit (minus 60 degrees Celsius).
Each CheMin analysis of a sample requires up to 10 hours of accumulating data while the X-rays are hitting the sample. The time may be split into two or more Martian nights of operation.
The X-ray diffraction data show the angles at which the primary X-rays from the beam are deflected and the intensity at each angle. The detector also reads secondary X-rays emitted by the sample itself when it is excited by the primary X-rays. This is the X-ray fluorescence information. Different elements emit secondary X-rays at different frequencies. CheMin’s X-ray fluorescence capability can detect elements with an atomic number greater than 11 (sodium) in the periodic table.
David Blake the principal investigator for CheMin is an expert in cosmochemistry and exobiology at NASA Ames. He began work in 1989 on a compact X-ray diffraction instrument for use in planetary missions. His work with colleagues has resulted in commercial portable instruments for use in geological field work on Earth, as well as the CheMin investigation. The spinoff instruments have found applications in screening for counterfeit pharmaceuticals in developing nations and in analyzing archaeological finds.
Decades of work preparing a miniaturized laboratory for identifying minerals on Mars have also yielded spinoff versions with diverse applications on Earth and, possibly, the moon.
This image shows one of the spinoffs, in the orange case, in use during a November 2008 expedition to the Mauna Kea volcano in Hawaii.
CheMin Principal Investigator David Blake, of the NASA Ames Research Center, Moffett Field, Calif., is seen here collecting data from a CheMin cousin called Terra. The scene is from a NASA field test of technology for producing water and oxygen from soil, using the Hawaiian site as an analog for the moon. In such an application, Terra could analyze the starting soils as well as products from the extraction process.