CheMin: Martian Mineralogy
- Published: Aug 15, 2012
- Author: David Bradley
- Channels: X-ray Spectrometry
Minerals on Mars
Rocks are important. The ground beneath our feet, and all that…but on another horizon it is the chemistry of Martian rocks that are the focus of the CheMin system aboard NASA's recently landed Curiosity vehicle. The principal investigator NASA Ames Research Center heading the studies is David Blake.
Blake's primary objective with the CheMin system, which incorporates both a powder X-ray Diffraction (XRD) instrument and an X-ray Fluorescence (XRF) spectrometer is to determine whether Mars may have had or even still has habitable environments by characterising the sediments and rocks encountered by Curiosity as it roves the surface of the Red Planet. Critically, CheMin will carry out the definitive mineralogy on materials assimilated by the rover's Sample Acquisition, Sample Processing and Handling (SA/SPaH) system. The money shot, as it were, will be seen should CheMin reveal evidence of the effects of water on the formation, deposition, or alteration of those samples. CheMin has a secondary role too, in that it is also capable of looking for putative bio-signatures in the samples, signs of energy sources involved in life processes.
The CheMin system sits snug within the onboard Analytical Laboratory of the MSL rover located within the vehicle's main body. It is tasked with analysing up to about 74 samples collected by the SA/SPaH system during the initial mission. However, careful design means that the sample cells are reusable and so many more samples might be analysed subsequently. Each analysis can take up to 10 Earth hours of analysis time, which would be spread over two or more Martian nights. The hub of the CheMin system uses a microfocus cobalt X-ray source, a transmission sample cell, and an energy-discriminating X-ray sensitive charge-coupled device (CCD) to generate and record simultaneous two-dimensional X-ray diffraction patterns and energy-dispersive histograms from the collected powder samples.
The CCD detector is operated in single-photon counting mode (it is read at a frequency that ensures most pixels contain charge from either zero or one photon) and repeatedly erased (1000 or more exposures) so that the charge generated by each photon (and so its energy) can be measured. Thus, diffracted X-rays strike the detector and are identified by their energy, producing a two-dimensional image that constitutes the diffraction pattern. The detected X-rays are summed into a histogram of photon count versus photon energy to give an energy-dispersive perspective. The raw CCD frames are processed initially on Mars to strip out extraneous information and then the much-compressed packets are transmitted to Earth for further processing and analysis.
The Martian rock plots
The plots are comparable to conventional diffractometer data and quantitative mineralogical results can be obtained from the XRD data by Rietveld refinement, FULLPAT and other full-pattern fitting techniques. The team explains that both crystalline and amorphous materials can be analyzed in this fashion. Ultimately, CheMin can unequivocally identify and quantify minerals above its detection limits in complex natural samples such as basalts, multicomponent evaporite systems, and soils.
Article by David Bradley
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.