Understanding the Nature of Metal Segregation in Asteroid Regolith

Metal/Silicate Fractionation: The process of re-golith formation on an asteroid normally results in the sorting of the asteroid's regolith, a process that would involve segregation of the denser, less brittle metallic from silicate particles. It has been proposed that, as a result of mobilization of volatiles, the position of this particle layer in the regolith would depend on initial size and abundance of metal particles, as well as availability of volatiles for fluidization [1]. According to this model, less abundant, smaller diameter metal particles , present in L and LL Chondrites, would tend to rise to the surface, whereas more abundant, larger diameter particles, present in H Chondrites, would tend to sink. We discuss here how segregating a metallic component in this way would modify the apparent element and mineral abundance ratios relative to the parent material. We also discuss the impact such a process would have on X-ray and other remote spectral measurements, as well as on the availability of Fe or volatiles such as S for 'space weathering' processes. Impact on XRS measurements: Heavier elements, the most prevalent of which is iron, have a higher absorption cross-section for solar X-rays. The presence of metallic iron particles will induce 'matrix effects' on X-ray fluorescent lines. Matrix effects, which create non-linearity in the relationship between fluorescent line intensity and elemental abundance, are caused by macroscopic and microscopic physical het-erogeneities in the medium being measured which result in variation in the degree to which the medium absorbs X-rays. In a light, fine-grained matrix with metallic particles, less effective absorption by the lighter matrix causes relatively lower intensity for light element lines, and relatively greater intensity for Fe lines than linear relationships with abundance would predict, an effect which becomes more pronounced as the metallic particle size decreases [2,3]. For chon-dritic compositions, Si, Al and Ca are anti-correlated with Fe, and their anti-correlation, particularly in the case of the low abundance components Al and Ca, would tend to be exaggerated downward in areas of greater metallic Fe abundance. The downward effect on Mg, which is generally correlated with metallic Fe, would be less pronounced. Because " a fluorescent element has a sensitivity which varies with the nature of the matrix it is in " [4], the relationship between intensity and abundance depends critically on particle size, composition, and distribution for each element. In an asteroid regolith, the depth of penetration for X-rays …