Preparing for NEO Sample Return: Simulating the Effects of Laser Space Weathering on Macromolecular Carbon

Abstract:

The NASA OSIRIS-REx mission plans to visit a B-type asteroid and return pristine regolith samples to Earth. These regolith samples, like those returned by the JAXA Hayabusa mission from Itokawa, will likely exhibit some modification by space weathering (SW). Further, these samples may contain up to ~5% organic carbon, mainly in the form of macromolecular carbon (MMC). MMC in meteorites can be studied with Raman spectroscopy; changes in its Raman spectral parameters have been shown to correlate with the petrographic grade of the meteorite. But these petrographic studies are calibrated with internal pieces of meteorite samples, so the MMC seen in meteorites has not experienced SW. Hence, it is important to determine the effects of SW may have on the MMC and its Raman spectrum. Laser pulse heating experiments that simulate the micrometeorite impact component of SW have been carried out in samples of pure graphite, and carbonaceous chondrites Allende (CV3) and Murchison (CM2). Pulse heating was done in vacuum (1×10^-6 torr) with a 20 Hz 1064 nm Nd:YAG laser, a 6 ns pulse duration (30 mJ/pulse), and a 200 μm spot size. Raman spectra were collected on the each sample using a WITec alpha300 R confocal Raman microscope, with a 1 mW 532 nm continuous laser and a ~10 μm laser spot size. UVVIS-NIR (0.4-2.5 µm) reflectance was measured using an Analytical Spectral Devices Inc. FieldSpec 4 spectrometer. Based on its Raman spectra, the original pure graphite is modified to nanocrystalline graphite by 10 minutes (12,000 laser pulses), and further modified to glassy carbon (amorphous 3-coordinate carbon) within 20 minutes (24,000 laser pulses). Vapor deposited on the side of the sample holder has a Raman spectrum consistent with amorphous carbon glass (3- and 4-coordinate carbon). Laser SW carried out on a slab of Murchison resulted in the production of glassy carbon inside siliceous melt blobs in the laser craters. Surprisingly, the Raman spectrum for MMC in Allende powder does not change, despite the production of melt glass and changes to its reflectance spectra. To confirm our Raman results, compositional and structural analyses of the carbon bearing SW products in Allende and Murchison will be done using transmission electron microscopy. Future work will expand our samples to other aqueously altered meteorites such as CI1 and CM1 chondrites