Identification of Stardust Analogs in Aerogel Using Raman Spectroscopy

Introduction: In 2006, NASA’s Stardust mission returned a capsule to Earth with particles collected from the coma of comet 81P/Wild 2. Cometary particles were collected by impact into low-density silica aerogel at a relative velocity of 6.1 km/s [1]. Upon impacting the aerogel, particles left behind tracks of melted and compressed aerogel and void space, shedding material along the way. In order to maximize the scientific return from these samples, it is imperative to record their initial character using a variety of nondestructive techniques while they remain embedded in the capture medium. At the Microscopy and Imaging Facility at the American Museum of Natural History (AMNH), a Zeiss LSM710 Laser Scanning Confocal Microscope (LSCM) is used to perform 3-dimensonal mapping of whole Stardust tracks at high resolution (<80 nm/pixel.) Typically, these results are complemented with chemical data acquired using the GSECARS Xray microprobe on beamline 13-IDE at the Advanced Photon Source (APS) of Argonne National Laboratory. However, time at APS is limited and highly competitive, which has motivated us to develop a system with which we can characterize Stardust grain mineralogy relying solely on instruments at our disposal at AMNH. Experimental Methods: Pioneering work [2] has demonstrated that Raman spectroscopy is a practical, effective method for mineralogical identification of grains trapped in aerogel. To employ this technique at AMNH, a Princeton Instruments IsoPlane SCT 320 imaging spectrograph paired with a Princeton Instruments PIXIS 100BR CCD camera was coupled to the Zeiss LSM710 (Fig.1). This LSCM is equipped with several lasers of differing wavelengths. This arrangement facilitates the high-resolution location of grains in situ, followed by the capability of switching to Raman spectroscopy immediately, without losing focus or disturbing the sample. This unique LSCM-Raman setup presents the ability to perform mineralogical identification experiments with superior spatial resolution. There are many benefits of using Raman spectroscopy; for example, [3] reports the possibility of quantifying the relative abundance of Mg and Fe in olivine based on the position of major peaks in the Raman spectrum. In addition, Raman permits the identification of silicates, unlike synchrotron X-ray fluorescence mapping, which is incapable of distinguishing between silicon in cometary particles versus that of the aerogel Figure 1: Schematic of LSCM-Raman system at AMNH