May 2008 Archives

In May 2007, NASA's Mars rover Spirit found that Martian soil has a high concentration of silica. This is considered as very strong evidence that water could have existed on ancient Mars, because certain hydrothermal reactions are most likely to produce silica. The discovery was announced in brief at the time (see http://www.nasa.gov/mission_pages/mer/mer-20070521.html), but scientists led by Professor S. Squyres (Cornell University, United States) have now had time to fully analyze the mineral deposits. In addition to the Miniature Thermal Emission Spectrometer (Mini-TES), the Alpha Particle X-ray Spectrometer (APXS) contributed significantly to the analysis. Analysis of the elemental composition of the deposits revealed that Si is strongly enriched relative to typical soil, and there are weaker enrichments in Ti, Cr, and Zn. Other major elements appear to be depleted. For more information, see the paper, "Detection of Silica-Rich Deposits on Mars", S. W. Squyres et al., Science, 320, 1063 (2008).

Analysis of X-ray and neutron reflectivity is usually done by modeling the scattering length density profile (such as multilayers) of the sample and performing a least square fit to the measured, phaseless reflectivity data. Professor T. Salditt (Institute for X-ray Physics, Universitat Gottingen) and his colleague recently attempted to extend the inversion technique. The research group discussed conditions for uniqueness, which are applicable in the kinematic limit (Born approximation), and for the most relevant case of box model profiles with Gaussian roughness. They also demonstrated that an iterative method to reconstruct the profile based on regularization works well. For more information, see the paper, "Iterative reconstruction of a refractive-index profile from x-ray or neutron reflectivity measurements", T. Hohage et al., Phys. Rev. E77, 051604 (2008).

There appears to be increasing demand for learning analytical techniques for surfaces and interfaces. In Japan, the 2^nd tutorial course on the analysis of thin films and multilayers by X-ray reflectivity was held on March 26. Although a similar school was run only 4 months earlier, an additional 50 young participants came to Tsukuba for the course. In France, the 3^rd school was held at Giens on May 4-8. The organizers were Professors A. Gibaud (Université du. Maine), R. Lazzari (Institut des NanoSciences de Paris) and J. Daillant (Institut Rayonnement Matière de Saclay). Of particular note is that SAXS, GI-SAXS and In-plane XRD have been newly included in the program, in addition to ordinary X-ray reflectivity. Further information is available at http://www.nims.go.jp/xray/ref/ (in Japanese only) and http://www.univ-lemans.fr/~gibaud/ecoledegiens/ (in French only), respectively