^1,2C. Royer,³S. Bernard,³O. Beyssac,³E. Balan,⁴O. Forni,³M. Gauthier,³M. Morand,³Y. Garino,³P. Rosier
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2023.115894]
¹LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, Meudon, France
²Purdue University Earth, Atmospheric and Planetary Sciences department, West Lafayette, IN, USA
³Institut de Minéralogie, Physique des Matériaux et Cosmochimie, CNRS UMR 7590, Muséum National d’Histoire Naturelle, Sorbonne Université, Paris, France
⁴IRAP, CNRS, Université de Toulouse, UPS-OMP, Toulouse, France
Copyright Elsevier

Perseverance is on Mars, collecting samples which will inform about Martian paleoenvironmental conditions. However, the surface of Mars is continuously bombarded by ionizing radiation, including UVs, which may significantly alter hydrated mineral phases such as sulfates, phosphates and carbonates. To explore and constrain this effect, we experimentally exposed pellets of more or less hydrated minerals to UV radiation within a Martian chamber at a temperature relevant for the rocks at the surface of Mars. Results show that exposure to UV leads to a strong alteration of the Raman and IR signals of sulfates, phosphates and carbonates. The strong increase of the luminescence signals coupled to the decrease of the Raman signals relatively to the background and the clear attenuation of the IR signals are interpreted as caused by an increasing concentration of electronic defects. The present results have strong implications for the ongoing exploration of Mars: one should not expect to detect pristine materials, except over freshly excavated surfaces. Still, as a precaution, all the targets measured or collected on Mars should be considered as having been exposed to UV radiation to some extent.

¹D.L. Laczniak,¹M.S. Thompson,²R. Christoffersen,³C.A. Dukes,⁴R.V. Morris,⁴L.P. Keller
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2023.115883]
¹Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907, United States of America
²Jacobs, NASA Johnson Space Center, Mail Code X13, Houston, TX 77058, United States of America
³Laboratory for Astrophysics and Surface Physics, University of Virginia, 395 McCormick Road, Charlottesville, VA 22904, United States of America
⁴ARES, Mail Code X13, NASA Johnson Space Center, Houston, TX 77058, United States of America
Copyright Elsevier

We present results from a set of low and high flux 1 keV/amu H+ and He+ irradiation experiments performed on slabs of the Murchison CM2 carbonaceous chondrite. The low flux conditions for H+ and He+ irradiation were ~ 1–1.5 orders of magnitude lower than the high flux conditions, and each experiment was irradiated to a total fluence between ~3 × 1016 to ~6 × 1016 ions/cm2. Irradiation-induced changes in the surface chemistry and optical properties of the Murchison samples were evaluated using in situ X-ray photoelectron spectroscopy (XPS) and visible and near-infrared spectroscopy (VNIR). We characterized the microstructure and composition of ion damaged rims in focused ion beam (FIB) cross-sections extracted from olivine and matrix material in each irradiated Murchison slab using transmission electron microscopy (TEM). XPS results suggest that both low flux and high flux H+ and He+ irradiation cause minor sputtering of surface carbon as well as a reduction in the valence state of iron, from Fe3+ to Fe2+. Slope bluing is observed in VNIR spectra of the irradiated samples which may reflect carbonization and dehydrogenation of organic species and contrasts with reddening trends associated with npFe0 formation. Although we do not observe a strong flux-dependence on the crystallinity of ion-damaged olivine, TEM analyses reveal a variety of microstructures in all olivine FIB-sections, suggesting that crystallographic orientation affects amorphization efficiency. Analyses of matrix FIB-sections indicate that phyllosilicate alteration is mainly driven by He+ irradiation, where the higher incident flux leads to greater amorphization and the formation of more distinct ion-damaged layers, similar to smooth layers in returned Ryugu particles. TEM results also provide some evidence that higher ion flux leads to greater vesiculation, with He+ irradiation being more efficient at vesiculation than H+ irradiation, and that higher ion flux may promote the segregation of Mg and Si into laterally extensive lenses and layers in olivine samples. We discuss the implications of these findings for constraining the role that ion flux plays in the development of space weathering characteristics in silicate phases present in carbonaceous asteroidal regoliths. These results will be important for understanding the complexity of this process and how it operates on carbon-rich airless bodies like asteroids Bennu and Ryugu.