ASSESSMENT OF THE PRESERVATION OF IMPACT RESIDUES IN STARDUST ANALOGUE CRATERS USING ADVANCED EDS IMAGERY WITH AN ANNULAR SDD

摘要

Introduction: Studies of experimental Stardust analogue craters from projectiles impacted onto aluminium foil have shown dependence of crater ellipticity and rim morphology on projectile size, shape, velocity and trajectory [1]. When particles impact perpendicular to the target, residue can be found throughout the bowl shape [2], and as much as -55 % of the impacting particle can be preserved [3]. Here we show how novel energy-dispersive X-ray spectroscopy (EDS) reveals more information as to preservation, modification and distribution of impact residue. Method: Light gas gun shots at the University of Kent fired 22 μm soda lime glass projectiles onto 100 μm thick Al 1100 foil targets at ~6 km/s. EDS images of craters with different impact angles were acquired using an annular Bruker silicon drift detector (SDD) on an FEI Quanta field emission scanning electron microscope. To enhance spatial resolution for element analysis, revealing features < 100 nm in size, low acceleration voltage (9 kV and 2.6 kV) were applied. Results: The oblique impacts show an uneven crater floor with multiple steps in the downrange direction. The maps show a link between distribution of impact residues and impact angle (Fig. 1). As the impact angle departs from perpendicular, the residue distributions initially appear to shift so that they are concentrated in the downrange direction of the crater. The crater surface shows that both the silicate projectile residue and aluminium target have characteristic flow textures, the silicate showing filaments which resembles a Pele's hair type morphology (Fig. 2). At crater steps, aluminium also grades into elongated filaments in the downrange direction. Both components have clearly undergone melting. Previous work using focussed ion beam sectioning and transmission electron microscopy [4] has demonstrated intimate mixing and compositional blending between impactor and aluminium deeper within melt layers during impact. The presence of micrometre-scale aluminium blebs on top of silicate melt suggests that in the latest stages of melt movement, the two melts were essentially immiscible, and aluminium with a flow like morphology was deposited on top of already solidifying silicate melt. Conclusions: An annular SDD can show relationships between even tiny impact residues throughout complex crater shapes, without the necessity of applying a conductive coating or working in low vacuum - both of which might contaminate the surface. The technique should be used as a preliminary reconnaissance method on all Stardust cometary dust craters to give vital information as to the location of specific dust residue components (e.g., distinctive refractory phases) before selection and preparation for other analysis techniques.