Surface Modifications of Space-exposed Aerogel from the Stardust Cometary Collector

Keystones removed from the Stardust Cometary Collector show varying degrees of visible luminescence when exposed to UV light, with the brightest luminescence associated with the space-exposed surface. The spatial characteristics of this phenomenon were investigated further by using fluorescence microscopy and synchrotron FTIR spectromicroscopy to analyze over 24 keystones extracted from the Stardust Cometary Collector. Fluorescence measurements show two distributions with different excitation characteristics, indicating the presence of at least two distinct fluorophores. The first distribution is confined to within about ~10 μm of the space-exposed surface, whereas the second distribution is much broader with a maximum that is typically ~30-50 μm below the surface. Although the aerogel keystones contain varying amount of organic contaminants, FTIR measurements indicate that the fluorescence is not likely to come from a carbon-based source. However, they do show two distributions of different widths and depths near the space-exposed surface: a narrow, surface-confined distribution that can be assigned to O₃SiH, and a broader, subsurface distribution that can be assigned to O₂SiH₂. These functional groups were not observed in keystones extracted from the cometary flight spare or from the Stardust Interstellar Tray, suggesting that the surface modifications may result from cometary exposure. The correlations between the fluorescence and the presence of O₃SiH and O₂SiH₂ groups supports the hypothesis that the surface fluorescence is caused by defects in the O-Si-O network, as suggested by Sandford et al. [1]. However, because all keystones exhibit some fluorescence, the presence of O₃SiH and O₂SiH₂ are not the only indicators of this fluorescence. Reference: [1] Sandford S.A. et al. Meteorit. Planet. Sci. 45, 406 (2010).