Gasda, Patrick - Laser Space Weathering of Carbon: What can we expect from NEO sample return?

Abstract: 
Two upcoming missions, OSIRIS-REx and Hayabusa-2, plan to visit B- and C-type asteroids in order to return pristine samples of these asteroids. If these asteroids are indeed carbonaceous, then they 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 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 may not have experienced space weathering. Carbonaceous meteorites that contain space-exposed asteroid regoliths are uncommon in meteorite collections and it is uncertain if asteroid regolith samples we have are representative of asteroid regolith. Hence, it is important to determine the effects space weathering may have on the MMC and on its Raman spectrum.

Laser pulse heating experiments that simulate the micrometeorite impact component of space weathering have been carried out in 30 s increments on uncompressed powders of pure graphite and a sample of Allende, which is a CV petrologic type 3 carbonaceous chondrite. Powders were contained within a glass sample cup. Pulse heating was done in vacuum (1×10-6 torr) with a 1064 nm Nd:YAG laser running at 20 Hz, a 6 ns pulse duration (30 mJ/pulse), and a 200 mm spot size. Raman spectra were collected on the each sample using a WITec alpha300 R confocal Raman microscope, with a 45.5 μW 532 nm continuous laser and a ~10 mm laser spot size.

Preliminary results show that Raman spectra of the original graphite powder exhibit dramatic changes. The original pure graphite is modified to disordered graphite by 10 minutes (60,000 laser pulses), and further modified to glassy carbon (nanocrystalline 3-coordinate carbon) within 20 minutes (120,000 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). Changes to the Raman spectrum for Allende are much more subtle than the graphite experiments and no amorphous carbon is detected as a vapor deposit. There is a much lower concentration of carbon in Allende compared to pure graphite, so the weathered MMC Raman signals might be below the detection limit. The majority of Allende is silicate minerals. It is possible that melt and vapor deposited silica rims may contain a few percent carbon, but TEM measurements are needed to confirm this hypothesis. TEM studies of the space-weathered graphite are needed to confirm the phase change from graphite to glassy carbon, to confirm the presence of amorphous carbon, and determine the carbon coordination number of these phases. TEM studies of Allende might show small amount of weathering of the MMC, which may exhibit a different carbon coordination number compared to the space-weathered graphite. Additional meteorite classes will be studied, such as type 2 CM (e.g. Murchison) and type 3.0 ordinary chondrites (e.g. Bishanpur).