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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.05.018 Byline: Miriam A. Riner (a), Francis M. McCubbin (b), Paul G. Lucey (a), G. Jeffrey Talyor (a), Jeffrey J. Gillis-Davis (a) Keywords: Mercury; Mercury, Surface; Geological processes Abstract: A significant opaque component in Mercury's crust is inferred based on albedo and spectral observations. Previous workers have favored iron-titanium bearing oxide minerals as the spectrally neutral opaque. A consequence of this hypothesis is that Mercury's surface would have a high FeO content. An array of remote sensing techniques have not provided definitive constraints on the FeO content of Mercury's surface. However, spectral observations have not detected a diagnostic 1[mu]m absorption band and have thus limited the FeO in coexisting silicates to Author Affiliation: (a) Hawaii Institute of Geophysics and Planetology, University of Hawaii, Manoa, Honolulu, HI 97822, United States (b) Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd., N.W. Washington, DC 20015, United States Article History: Received 16 December 2009; Revised 25 May 2010; Accepted 25 May 2010

Theoretical calculations and experimental simulations indicate that the surprisingly low sulfur abundance on the surfaces of spacecraft visited S‐type asteroids 433 Eros and 25143 Itokawa may be due to space weathering. Two current missions, Osiris‐Rex and Hayabusa 2, are studing asteroids 101955 Bennu (B‐type) and 162173 Ryugu (C‐type), respectively. Understanding space weathering effects related to sulfur containing species is only in the beginning stages and has not been studied from the point of view of C‐type asteroids. This laboratory study details the formation of hydrogen sulfide (H2S) and hydrogen disulfide (H2S2) from conversion of nonvolatile‐sulfuretted species in the Murchison meteorite by exposure to energetic electrons and laser processing, which mimics the synergic effect of secondary electrons generated by galactic cosmic rays and high‐energy solar wind particles plus micrometeorite impact on airless bodies. The results indicate that space weathering processes likely induce depletion of sulfur on the surface of C‐type and undifferentiated S‐type asteroids. For keeping scientific integrity of these fragile species, cold/cryogenic curation of future collected samples from asteroids might be required. Plain Language Summary: The Near Earth Asteroid Rendezvous (NEAR) Shoemaker and Hayabusa spacecraft have detected surprisingly low sulfur abundance on the surfaces of asteroids 433 Eros and 25143 Itokawa. The physical and chemical properties of the surfaces of asteroids are affected by space weathering processes including irradiation by cosmic rays and bombardment of micrometeorites. However, understanding space weathering effects related to sulfur containing species is only in the beginning stages. Here we show that hydrogen sulfide (H2S) and hydrogen disulfide (H2S2) can be generated from conversion of nonvolatile sulfuretted species in the Murchison meteorite by simulated weathering processes. The Murchison meteorite is suggested to originate from asteroids. These volatile sulfur‐containing species sublimed at temperatures lower than the maximum surface temperatures (with the Sun overhead) on asteroids. These results indicate that space weathering likely induces depletion of sulfur on the surfaces of asteroids. Key Points: Murchison meteorite samples were irradiated with electrons and laser to model effects of space weatheringHydrogen sulfide (H2S) and hydrogen disulfide (H2S2) were generated from sulfur‐containing minerals at low temperatureConversion of sulfur‐containing mineral to volatile H2S may induce the surficial sulfur depletion on asteroids [ABSTRACT FROM AUTHOR]

The source of water (H2O) and hydroxyl radicals (OH), identified on the lunar surface, represents a fundamental, unsolved puzzle. The interaction of solar-wind protons with silicates and oxides has been proposed as a key mechanism, but laboratory experiments yield conflicting results that suggest that proton implantation alone is insufficient to generate and liberate water. Here, we demonstrate in laboratory simulation experiments combined with imaging studies that water can be efficiently generated and released through rapid energetic heating like micrometeorite impacts into anhydrous silicates implanted with solar-wind protons. These synergistic effects of solar-wind protons and micrometeorites liberate water at mineral temperatures from 10 to 300 K via vesicles, thus providing evidence of a key mechanism to synthesize water in silicates and advancing our understanding on the origin of water as detected on the Moon and other airless bodies in our solar system such as Mercury and asteroids. [ABSTRACT FROM AUTHOR]

Abstract: Reflectance spectroscopy is important for placing lunar samples into a regional and global geologic context. To this end, the ultraviolet–visible (UV–vis) color ratio, used to estimate the TiO2 composition of mature mare basalts, has been one of the most widely used spectral parameters in lunar exploration. We examine the correlation between UV–vis color and TiO2 content using a combination of Clementine, Lunar Prospector, and sample data to document the extent to which color is dependent upon TiO2. Examination of the remotely sensed data reveals that the correlation between UV–vis ratio and TiO2 composition is best represented by a sigmoidal trend rather than the canonical linear or curvilinear correlation. With this information, we are then able to evaluate between two models that propose different explanations for the relationship between UV–vis color and TiO2. The first model attributes the correlation between TiO2 and UV–vis color to spectrally neutral opaques (i.e., ilmenite), while the other emphasizes the effect of Fe–Ti charge-transfer in lunar glasses and dual scattering mechanisms between high- and low-Ti basalts. We do not find evidence in the spectral data to support the occurrence of Fe–Ti charge-transfer in lunar glass as the principal cause for color in high-Ti basalts. The data also do not substantiate the existence of different scattering mechanisms (e.g., volume v. surface scattering) between high- and low-TiO2 basalts. Instead, our analyses substantiate that the spectral effects of ilmenite exhibit a major influence over the UV–vis ratio. By including sample data we find that in addition to ilemenite/TiO2 content, factors such as FeO content, ilmenite grain size, modal abundance of plagioclase, and the olivine-to-pyroxene ratio in a mare soils can influence the UV–vis continuum. These findings point to promising avenues of research that future UV–vis spectral techniques can exploit in order to yield more accurate TiO2 estimates and potentially additional petrologic information. [Copyright &y& Elsevier]