Paper AS-ThP11
Space Weathering Effects on Ceres: Novel Application of Surface Analytical Techniques to Questions in Planetary Science

Thursday, November 2, 2017, 6:30 pm, Room Central Hall

Introduction: The solar wind plasma continuously streams from the Sun, interacting with the surfaces of airless bodies throughout the solar system. Sulfates, suggested by the thermal emission [1], and carbonates, identified by the 3.4 and 4.0 μm absorption features [2] on the surface of Ceres by NASA’s Dawn spacecraft, will be exposed to solar wind H⁺ and He⁺ at ~1 keV/amu irradiation. We investigate the stability of these salts under 4 keV He⁺ irradiation as solar-wind proxy at the low pressure/temperature conditions found in the Main Asteroid Belt.

Experiment: Anhydrous MgSO₄ and Na₂CO₃ powders are pressed into pellets and compositions are confirmed by X-ray diffraction. We measure diffuse optical reflectance prior and subsequent to irradiation through 0.2-2.5 µm (Lambda 1050) and 0.6-16 µm (Thermo Nicolet 670). Pellet samples are then introduced to ultra-high vacuum (10^-9 Torr) and maintained at 110 K and effects of in situ 4 keV He⁺ irradiation are monitored by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy. Variations in surface composition and chemistry are identified and quantified as a function of fluence.

Results: Solar wind type ion irradiation of MgSO₄ damages the crystal structure, preferentially removing oxygen along with sulfur. XPS measurements imply the formation of MgO after 5 x10¹⁷ He⁺ cm^-2 (~15,000 years at 2.7 AU); a small shoulder on the sulfur peak suggests the presence of trapped SO₂, also confirmed by the IR feature observed at ~7.8 µm with irradiation. McCord et al. (2001) provides a potential decomposition pathway for MgSO₄ to SO₂ consistent with our observations. We observe secondary ion ejection (Mg⁺, MgO⁺, O^-, OH^-, H^-, S^-, and SO^- ) from MgSO₄ with He-impact, and neutral spectra show loss of SO₂. Spectral darkening and reddening in the UV-Vis region is observed by ex situ optical spectroscopy after irradiation.

Bright Na₂CO₃ deposits darken as a function of solar wind exposure in the visible spectrum on a timescale of 1-10 thousand years. Visible darkening, caused by enhancement in surface Na as C and O are preferentially removed, can be completely reversed by exposure to H₂O vapor. For Ceres’ bright regions, this suggests that brightest areas are likely to be the most recent material deposits or the most recently exposed to water.

Acknowledgements: We thank the NASA SSW and NSF-Astronomy programs. I thank Dr. Petra Reinke for her support and encouragement.

References: [1] Bu et al (2017) GRL (submitted) [2] Palumbo et al (2016)

LPSC 47, #2166 [3] Hapke et al (1981) Icarus 47, 361-367 [4] Hodyss et al LPSC 44,

#2328 [5] Lane (2007) Am. Mineral 92, 1-18 [6] McCord et al (2001) JGR 106, 3311-

3319.