Surface modifications of comet-exposed aerogel from the Stardust cometary collector


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Abstract– Keystones removed from the Stardust cometary collector show varying degrees of visible fluorescence when exposed to UV light, with the brightest fluorescence associated with the space-exposed surface. We investigated the spatial characteristics of this phenomenon further by using fluorescence microscopy, confocal Raman microscopy, and synchrotron Fourier transform infrared (FTIR) spectromicroscopy. Twenty-four keystones, extracted from the Stardust cometary collector, were analyzed. 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 about 30–50 μm below the surface. Confocal Raman measurements did not reveal any changes associated with the surface; however, only features associated with aliphatic hydrocarbons were strong enough to be observed. FTIR measurements, on the other hand, show two distinct distributions at the space-exposed surface: (1) a narrow, surface-confined distribution originating from −O3SiH groups and (2) a broader, sub-surface distribution originating from −O2SiH2 groups. These functional groups were not observed in keystones extracted from the cometary flight spare or from the Stardust interstellar collector, indicating that they may result at least partially from cometary exposure. The presence of O3SiH and O2SiH2 groups at the comet-exposed surface suggests that the enhanced surface fluorescence is caused by defects in the O-Si-O network and not by organic contamination.