Investigating Space Weathering Effects Using Coordinated Analysis of a H+- and He+-Irradiated Carbonaceous Chondrite

Dara Laczniak, Michelle Thompson, Catherine Dukes, Richard Morris, Simon Clemett, Lindsay Keller, Roy Christoffersen
2020 Microscopy and Microanalysis  
Space weathering processes such as micrometeorite bombardment and solar wind irradiation alter the microstructural, compositional, and optical properties of airless body regoliths. As a result, space weathering complicates the interpretation of remote sensing data from spacecraft missions [1] . Unlike the Moon and asteroids similar in composition to unaltered ordinary chondrites, very little is known about space weathering of hydrated, organic-rich (carbonaceous or C-type) planetary bodies. In
more » ... he next three years, the Hayabusa2 and OSIRIS-REx missions will return samples from C-type asteroids Ryugu and Bennu, respectively [2, 3] . To maximize the scientific return of these missions, we simulated solar wind irradiation in the laboratory on CM2 Murchison-a suitable analog for C-type asteroids [4] . We present results from a coordinated analysis investigating how 1 keV/amu H + and He + ion irradiation modifies the physical, chemical, and spectral properties of the organic and inorganic components of carbonaceous chondrites. H + and He + irradiations were performed separately under ultra-high vacuum on two discrete 6x6 mm regions of a dry-cut Murchison slice. Using a flux of 1.1x10 13 ions/cm 2 /s, the first region was irradiated with 4 keV He + to a total fluence of 1.1x10 18 ions/cm 2 (~6000 years of exposure at Bennu) while the second region was irradiated with 1 keV H + to a total fluence of 8.1x10 17 ions/cm 2 (~200 years of exposure at Bennu). To comprehensively analyze the samples, we employed four analytical techniques: (1) X-ray photoelectron spectroscopy (XPS), (2) visible to near infrared (VNIR; 0.35-2.50 µm) spectroscopy, (3) two-step laser-desorption mass spectrometry (μL 2 MS), and transmission electron microscopy (TEM). We observed changes in surface chemistry via in situ XPS using a PHI Versaprobe with a monochromatic, scanning X-ray source (AlKα: 1486.7 eV) and hemispherical electron-energy analyzer. We collected VNIR spectra using a fiber-optic ASD FieldSpec 3 Spectrometer (Malvern Panalytical) under ambient laboratory conditions to understand trends in reflectance. Changes in organic functional group chemistry were evaluated using a 118 nm photoionization spectral map acquired with a µL 2 MS instrument. Using an FEI Quanta 3D focused ion beam scanning electron microscope (FIB-SEM), we prepared eight electron transparent thin sections consisting of matrix material, pyroxene, Mgrich olivine, and Fe-rich olivine from both the H + -and He + -irradiated regions. TEM analysis of these FIB-sections revealed microstructural and chemical changes driven by ion irradiation. Specifically, we acquired bright field (BF), high-angle annular dark field (HAADF), and high-resolution TEM (HRTEM) images, as well as elemental distribution maps, using a JEOL 2500SE 200 kV field-emission scanning
doi:10.1017/s143192762002214x fatcat:7fn7a7hjv5hfjfuts6lziytt2y