Ion sputtering from loose powders remains poorly understood despite its relevance to planetary science and industry. We developed a multiscale Monte Carlo model to simulate sputtering from powders, using a higher-fidelity approach for the target geometry compared to voxel-based methods. Simulating Kr+ ions impacting Cu powders and flat slabs, we show that sputtering from loose powders differs markedly from that of flat slabs or rough surfaces. The main differences are: (1) for incident angles alpha > 0 degrees relative to the bulk normal, the escaping sputtering yield is dominated by backward-directed ejecta for all ion energies; (2) for alpha <= 60 degrees, the yield peaks toward the ion-beam origin, similar to the opposition effect seen in optical observations of airless bodies; (3) the angular distribution peak is half or less than that of a flat slab; (4) as ion energy increases, no evolution occurs from primary to secondary knock-on sputtering in the ejecta angular distribution. We attribute these behaviors to the powder's interconnected voids. Ions penetrate these voids and sputter underlying grains; the ejecta then preferentially escape toward the ion-beam origin, where shadowing is minimal. We derive two fitting functions: (1) relating the escaping sputtering yield of a powder to that of a flat surface, depending only on porosity, incident angle, mean local incidence angle, and the corresponding flat slab yield; (2) providing the double-differential angular distribution of the escaping ejecta for porosities >= 0.49. These provide a potentially universal fitting function of the absolute doubly differential escaping sputtering yield from loose powders.

We have measured the absolute doubly differential angular sputtering yield for 20 keV Kr+ impacting a loose Cu powder at an incidence angle of θi = 45°. We find that ion sputtering from a loose Cu powder differs dramatically from a flat Cu slab under the same irradiation conditions, being most pronounced for the angular distribution. The ejecta lobe for the powder is directed primarily backward, relative to the ion velocity vector. This contrasts with the slab, where the lobe is peaked nearly normal to the surface. We attribute the backward-directed ejecta lobe to two effects. First, ions can penetrate through the interconnected voids of the powder and cause sputtering from underlying grains. Atoms sputtered from these underlying grains are most likely to escape the powder if they are ejected roughly toward the origin of the incident ions, while ejecta in other directions are more likely to be shadowed by neighboring grains. Second, for θi = 45°, the grain surfaces exposed to the ions are primarily oriented in the backward direction, leading to shadowing of forward-directed ejecta. We find qualitatively good agreement between our experimental measurements and the results of our multiscale Monte Carlo simulation of sputtering from granular targets. We attribute the remaining discrepancies to ion-induced surface modifications not included in the simulations. Our findings indicate that ion sputtering from loose powders is distinctly different from that of flat surfaces, rough surfaces, and pressed powders due to the lack of interconnected voids in these latter three targets.

We have explored a combination of two methods to control the internal excitation of H 3 + produced in a duoplasmatron ion source. The H 3 + was formed starting from a gas of H2. The first control method varied the H2 pressure in the ion source to collisionally relax any internally excited ( H 3 + ) ∗ . The second method added Ar to the source to chemically destroy ( H 3 + ) ∗ with internal excitation energies E int ⩾ 0.57 eV, using the endoergic reaction H 3 + + Ar → ArH + + H 2 . To infer the H 3 + internal temperature, T int , representative of internal excitation under the hypothesis of thermodynamic equilibrium, we used merged-beams rate coefficient measurements of the endoergic deuterating reaction H 3 + + D → H 2 D + + H 2 and a semi-empirical theoretical model. We found that using collisional relaxation alone, we could vary T int over the range ≈ 1300 − 2200 K. Combining collisional relaxation and chemical destruction, we reduced the minimum to T int ≈ 1130 K. Over this temperature range, the fraction of H 3 + where all vibrational modes are in their v = 0 level varies from ≈ 0.88 at the lowest temperature to ≈ 0.44 at the highest temperature. This combination of cooling methods offers a potentially powerful means for studying reactive scattering processes as a function of the internal excitation of the H 3 + .

We have explored a combination of two methods to control the internal excitation of H-3(+) produced in a duoplasmatron ion source. The H-3(+) was formed starting from a gas of H-2. The first control method varied the H-2 pressure in the ion source to collisionally relax any internally excited (H-3(+))& lowast;. The second method added Ar to the source to chemically destroy (H-3(+))& lowast; with internal excitation energies E-int >= 0.57 eV, using the endoergic reaction H-3(+)+Ar -> ArH+ +H-2. To infer the H-3(+) internal temperature, T-int, representative of internal excitation under the hypothesis of thermodynamic equilibrium, we used merged-beams rate coefficient measurements of the endoergic deuterating reaction H-3(+)+D -> H2D++H-2 and a semi-empirical theoretical model. We found that using collisional relaxation alone, we could vary T-int over the range approximate to 1300-2200 K. Combining collisional relaxation and chemical destruction, we reduced the minimum to T-int approximate to 1130 K. Over this temperature range, the fraction of H-3(+) where all vibrational modes are in their v = 0 level varies from approximate to 0.88 at the lowest temperature to approximate to 0.44 at the highest temperature. This combination of cooling methods offers a potentially powerful means for studying reactive scattering processes as a function of the internal excitation of the H-3(+).

Our ability to understand the formation of the exospheres of airless bodies such as the Moon and Mercury has been hindered by uncertainties in how surface processes influence exospheric sources. Ejection processes important for exosphere formation rely on the notion that an emitted atom must first overcome an attractive energy with the surface to be ejected into the exosphere (the surface binding energy, SBE). Recent studies have shown that atoms from minerals are more tightly bound than commonly assumed, making it difficult to reconcile how such high volatile concentrations are being observed in the exospheres of airless bodies. Here, we used molecular dynamics modeling to explain the physics underlying the interaction of low-energy returning atoms, initially ejected below the escape energy of the body, with mineral surfaces. Global exosphere models make ill-informed assumptions for these interactions due to a lack of SBEs for adsorbed atoms. Results provide first-of-their-kind SBE distributions for adsorbed atoms and can be used by global models to better understand exosphere formation on airless bodies. We highlight the importance of adsorbate coverage and the atomic arrangement of a surface on the SBE. At low absorbate coverage sodium forms ionic bonds with oxygen, leading to tightly bound adsorbates (SBE ∼6 eV). At 1 ML of coverage the free O is terminated and Na is unable to form strong ionic bonds, leading to loosely bound adsorbates (SBE 1–3 eV). Emission processes from covered surfaces will be far more efficient than those without adsorbates. These improvements will allow for better interpretation of mission data such as from MESSENGER, BepiColombo, LADEE, Europa Clipper, and Artemis.

Our understanding of the ion-sputtering contribution to the formation of exospheres on airless bodies has been hindered by the lack of accurate surface binding energies (SBEs) of the elements in the various mineral and amorphous compounds expected to be on the surfaces of these bodies. The SBE for a given element controls the predicted sputtering yield and energy distribution of the ejecta. Here, we use molecular dynamics computations to provide SBE data for the range of elements sputtered from plagioclase feldspar crystalline end members, albite and anorthite, which are expected to be important mineral components on the surfaces of the Moon and Mercury. Results show that the SBE is dependent on the crystal orientation and the element’s coordination, meaning multiple SBEs are possible for a given element. Variation in the SBEs among the different surface positions has a significant effect on the predicted yield and energy distribution of the ejecta. We then consider sputtering by H, He, and a solar wind mixture of 96% H and 4% He. For each of these cases, we derive best-fit elemental SBE values to predict the ejecta energy distribution from each of the (001), (010), and (011) cleavage planes. We demonstrate that the He contribution to the sputtering yield cannot be accounted for by multiplying the 100% H results by some factor. Lastly, we average our results over all three possible lattice orientations and provide best-fit elemental SBE values that can be easily incorporated into sputtering yield models.

We have measured the absolute doubly differential angular sputtering yield for 20 keV Kr+ impacting a polycrystalline Cu slab at an incidence angle of θi = 45° relative to the surface normal. Sputtered Cu atoms were captured using collectors mounted on a half dome above the sample, and the sputtering distribution was measured as a function of the sputtering polar, θs, and azimuthal, ϕs, angles. Absolute results of the sputtering yield were determined from the mass gain of each collector, the ion dose, and the solid angle subtended, after irradiation to a total fluence of ∼1 × 1018 ions/cm2. Our approach overcomes shortcomings of commonly used methods that only provide relative yields as a function of θs in the incidence plane (defined by the ion velocity and the surface normal). Our experimental results display an azimuthal variation that increases with increasing θs and is clearly discrepant with simulations using binary collision theory. We attribute the observed azimuthal anisotropy to ion-induced formation of micro- and nano-scale surface features that suppress the sputtering yield through shadowing and redeposition effects, neither of which are accounted for in the simulations. Our experimental results demonstrate the importance of doubly differential angular sputtering studies to probe ion sputtering processes at a fundamental level and to explore the effect of ion-beam-generated surface roughness.

Space weathering is a key process in the interpretation of airless planetary surfaces. As we engage new missions to planetary objects with potentially novel surfaces such as 16 Psyche, there is renewed interest in expanding our knowledge of space weathering effects to a wider variety of analog materials, including the physical/chemical effects of solar-wind ions on planetary regoliths. We have experimentally simulated the effects of solar ions on two polished thick sections of meteoritic troilite (FeS) via irradiation with 1 keV hydrogen (H+) and 4 keV helium (He+), to investigate effects resulting from different ion species. We detected depletion of sulfur over the course of each irradiation using in situ X-ray photoelectron spectroscopy. Sulfur depletion rates were surprisingly similar for H+ and He+, interpreted as a function of subsurface ion-activated diffusion. By comparing XPS-derived elemental abundances with SDTrimSP computer simulations, we further quantified sulfur diffusion, sputtering yield, and altered-layer composition with respect to incident-ion fluence, and accounted for the influence of surface oxidation due to atmospheric sample storage. Using scanning electron microscopy, we detected an increase in nanoscale surface roughness resulting from the irradiation, which we quantified using atomic force microscopy. Based on these results, we estimate that an exposure time of order 103 Earth-years is required for troilite on Psyche to reach equilibrium sulfur depletion within the first atomic layer.

Modeling the Effect of Solar Ion Irradiation of Troilite with SDTrimSP – Implications for (16) Psyche. G.M. Minesinger, J.M. Christoph, C.A. Dukes, C. Bu, and L.T. Elkins-Tanton, Laboratory for Astrophysics and Surface Physics, University of Virginia, Charlottesville, VA 22904 (gmm9uf or cdukes@virginia.edu), School of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287, Astrophysics Laboratory, Columbia University, New York, NY 10027.

Introduction: Space weathering by solar wind ion irradiation has long been acknowledged as one of the main processes altering the surfaces of airless planetary objects such as the Moon and stony asteroids [1,2] through the chemical alteration of surfaces e.g. production of nanophase iron [3,4]. With missions now exploring worlds with novel compositions such as 16 Psyche [5], it is critical to determine the behavior of additional materials in solar wind. Troilite (FeS) is the most abundant sulfide in carbonaceous chondrite and iron meteorites [6], as well as an anticipated major mineral component of 16 Psyche [5]. Investigations of troilite solar wind ion space weathering at 433 Eros [7,8] and 25143 Itokawa [9,10] have identified the production of a metallic iron and iron-rich sulfide layer and sub-micron surface structures. Here we report results of new ion irradiation experiments to identify differences between effects of H and He bombardment, and to investigate the possibility that irradiation-induced surface roughening may affect the detectability of weathering via reflectance spectrum alteration Experiment: We carried out four ion irradiation experiments on three troilite samples provided by the ASU Center for Meteorite Studies: thick section slabs (x3) taken from troilite nodule inclusions in the Canyon Diablo and Toluca iron meteorites and a pressed troilite powder pellet. We used electron-bombardment-type ion guns on a PHI Versaprobe III and a highly-customized PHI-560 X-ray Photoelectron Spectrometer (XPS) at the University of Virginia. All four irradiations occurred at room temperature (~295K) and were designed to reach a total ion fluence >10 ions/cm, determined to achieve equilibrium S-depletion and simulating ~10 years of solar wind bombardment at 16 Psyche [8]. Solar wind consists of ~96% 1 keV H, ~4% 4 keV He, and <0.1% heavier ions, with a total ion flux at 1 AU of ~2×10 ions/cm/s [11]. For two of the irradiations we used a combination of 1 keV H and 4 keV He ions approximating this solar wind ratio, while the other two used exclusively 1 keV H or 4 keV He. Ion fluxes were ~10 ions/cm for both H and He. Before and after each irradiation we obtained visible to near-infrared (VNIR) optical reflectance spectra using apparatus at ASU, survey and high-resolution XPS spectra, and Scanning Electron Microscope (SEM) imaging using the FEI XL-30 at ASU’s Center for High Table 1: summary parameters of the four irradiations. Sample Surface Instrument H cm He cm Canyon Diablo Rough PHI560 1.40 ×10 0

Introduction: Numerous bright deposits of carbonates across the dark background of dwarf-planet Ceres have been identified by Dawn’s Visible NearInfrared Mapping Spectrometer (VIR). Carbonate compositions vary from natrite with minor amounts of ammonium bicarbonate at the Cerealia and Vinalia Faculae to magnesite, calcite, and dolomite in other high-albedo regions [1,2]. These salt deposits are dominantly anhydrous, but small amounts of hydrated Nacarbonates have also be observed at Oxo, Kahukura, and Kupalo craters [2]. Such deposits are expected to derive from the aqueous alteration of volatilecontaining silicates below Ceres’ solid crust and deposited on the surface by extrusion through vents or coejected by jets of sub-surface water ice. Fresh material is expected to dehydrate once exposed to the low pressure environment on Ceres’ surface even at temperatures < 240 K [3], forming anhydrous carbonates. Without an atmosphere, Ceres’ surface lies unprotected from solar radiation and cosmic rays, undergoing chemical and physical changes (space weathering) which can be remotely identified by optical reflectance. Darkening by space weathering has been proposed to affect Ceres’ optical spectra in the UV-visible [4] and Dawn photometric observations of darkened crater rim/wall faculae [5]; however, no systematic laboratory investigations are available to confirm these interpretations. X-rays from the solar corona are emitted continuously, dominated by low energies (< 5 keV). The effects of X-rays can be significantly different than for solar ions, with a greater penetration depth (>1 μm) and deeper energy deposition, providing a better match to vis-IR spectral information depths. Here, we investigate the stability of carbonates under soft-X-ray irradiation and investigate whether the measured irradiation-induced optical changes can be used as a potential geologic chronometer for Ceres’ bright deposits. Experiment: X-ray irradiation with in-situ diffuse reflectance measurements under Ceres’ low-pressure and low-temperature conditions were performed on a customized Physical Electronics (PHI 560) X-ray photoelectron spectrometer (XPS) system, maintained at ultra-high vacuum (UHV, ~10 Torr). Reagent-grade carbonate (Na2CO3, CaCO3, and dolomite) powders (grain size < 45μm) were pressed into pellets in dry air, then introduced into the UHV chamber. Samples were at 90–296 K, simulating Ceres’ surface temperature range, and irradiated by Al-Kα X-rays (1486.7 eV). The X-ray flux was ~1.0 × 10 γ cm s, except when investigating potential flux effects where a higher flux of 1.5 × 10 γ cm s was used. A FilmTek 2000 ultraviolet-visible spectrometer was coupled to the UHV chamber, performing in-situ, in-vacuo reflectance measurements at λ = 250–850 nm. Spectra were measured at varied X-ray fluence, equivalent to solar irradiation of ~8,000 – 2,000,000 years on Ceres. Complementary XPS measurements in a PHI Versaprobe III were performed to investigate X-ray-induced composition and molecular chemistry changes. Results: Optical Measurements. Fig. 1 shows the relative reflectance spectra for natrite (Na2CO3) with X-ray irradiations, at sample temperatures of 296 K, 200 K, and 90 K. The visible reflectance decreased (darkening) as X-ray fluence increased, with the rate of darkening decreasing at lower temperature. Two absorption features developed with irradiation at~400 nm and ~500 nm. The position of the first feature was independent of X-ray fluence but shifted from ~415 nm at 296 K to ~405 nm at 90 K; the position of the second feature was independent of temperature but depended on X-ray fluence – starting at ~508 nm at low fluence and shifting to ~480 nm at ~5 × 10 γ cm (1.6 × 10 Ceres Years).

LABORATORY-IRRADIATED NORTHWEST AFRICA 12008. M. L. Shusterman1, T. G. Sharp1, M. S. Robinson1, Z. Rahman2, L. P. Keller2, C. A. Dukes3, C. Bu3 and M. A. Roldan4, 1School of Earth and Space Exploration, Arizona State University (E-mail: Morgan.Shusterman@asu.edu). 2ARES, XI3, NASA/JSC. 3Laboratory for Astrophysics and Surface Physics, University of Virginia, 4Eyring Materials Center, Arizona State University.

Micrometeoroid impacts, solar wind plasma interactions, and regolith gardening drive the complicated and nuanced mechanism of space weathering (or optical maturation); a process by which a material’s optical properties are changed as a result of chemical and physical alterations at the surface of grains on airless bodies. Reddened slopes, attenuated absorption bands, and an overall reduction in albedo in the visible and near-IR wavelength ranges are primarily the result of native iron nanoparticle (npFe0) production within glassy rims that form from sputtering and vaporization. The sizes and abundance of these particles provide information about the relative surface exposure age of a particular grain. In addition, many studies have indicated that composition greatly affects the rate at which optical maturation occurs. Despite our understanding of how npFe0 affects optical signatures, the relative roles of micrometeoroid bombardment and solar wind interactions remains undetermined. To simulate the early effects of weathering by the solar wind and to determine thresholds for optical change with respect to a given mineral phase, we irradiated a fine-grained lunar basalt with 1 keV H+ to a fluence of 6.4 x 1016 H+ per sq.cm. Surface alterations within four phases have been evaluated using transmission electron microscopy (TEM). We found that for a given fluence of H+, the extent of damage acquired by each grain was dependent on its composition. No npFe(0) was produced in any of the phases evaluated in this study. These results are consistent with many previous studies conducted using ions of similar energy, but they also provide valuable information about the onset of space weathering and the role of the solar wind during the early stages of optical maturation.

Deposits of carbonates have been observed and definitively identified by Dawn's Visible Near-Infrared Mapping Spectrometer (VIR), particularly in the faculae that lie within the central portion of Occator, Oxo, and Haulani craters, implying geologically recent cryo-volcanism or extrusion with sub-surface CO(2 )and H2O. Carbonate composition varies from primarily sodium at the Cerealia and Vinalia Faculae and at Oxo crater, where carbonate deposits are most abundant, to magnesium and calcium for most other bright regions. The formation of hydrated salts is expected from the aqueous alteration of silicates; however, VIR measurements of the faculae show no water signature, potentially the result of dehydration after exposure to Ceres' surface conditions. We investigate the stability and decomposition pathway for hydrated sodium-carbonate, natron (Na2CO3 center dot 10H(2)O), grains in the laboratory under Ceres' cryogenic, low-pressure environment by UV-vis-NIR reflectance spectroscopy and X-ray powder diffraction. H2O-loss begins simultaneously with vacuum-exposure, altering natron's spectral signature by attenuation of the water bands, enhancement of the carbonate features, and concurrent reduction of the NIR blue spectral slope. We find that the water absorption features in natron reduce below VIR's detection limit (<2%) within a time scale of < 6 days at temperatures >= 200K, eliminating hydrous sodium carbonate from Ceres' surface mineralogy in the equatorial region and the mid-latitudes without continuous rehydration. A temperature-dependent systematic shift of the 1.9-um band center to lower wavelengths is observed with vacumm-exposure time at 200 and 240 K. In Ceres' polar-regions (similar to 120 K), natron retains water longer, depleting the 1.9-mu m water band to <2% within a few hundred years (similar to 300 years). No significant changes in the visible relative reflectance or spectral slope result from vacuum-exposure of hydrous or anhydrous sodium carbonate, which does not match the observed red-slope in the Framing Camera (FC) measurements for Occator Crater's faculae. In the UV, extended exposure to vacuum in all sodium carbonates examined here causes significant reddening due to an increase in short-range crystallographic defects and reduction of the conduction band energy; in addition, the development of new UV electronic transition features at ( )similar to 275 and 235 nm is observed in hydrous and anhydrous sodium-carbonates with vacuum-processing. Similar transitions in carbonates and organics may contribute to the unidentified 280 nm absorption feature on Ceres. (C) 2017 Elsevier Inc. All rights reserved.

CONDITIONS RELEVANT TO CERES. C. Bu , G. Rodriguez Lopez, C.A. Dukes, L.A. McFadden, J-Y. Li, and O. Ruesch University of Virginia (Laboratory for Astrophysics and Surface Physics, Materials Science and Engineering, Charlottesville, VA 22904; cb8nw@virginia.edu; gr3dw@virginia.edu; car8r@eservices.virginia.edu); NASA Goddard (NASA/GSFC, Mail Code: 693, Greenbelt, MD 20771; lucyann.a.mcfadden@nasa.gov; ottaviano.ruesch@esa.int); Planetary Science Institute (1700 E. Ft. Lowell Rd., Tucson, AZ 85719; jyli@psi.edu).

The formation of hydrated salts is an expected consequence of aqueous alteration of Main Belt objects, particularly for large, volatile-rich protoplanets like Ceres. Sulfates, present on water-bearing planetary bodies (e.g., Earth, Mars, and carbonaceous chondrite parent bodies) across the inner solar system, may contribute to Ceres' UV and IR spectral signature along with phyllosilicates and carbonates. We investigate the presence and stability of hydrated sulfates under Ceres' cryogenic, low-pressure environment and the consequent spectral effects, using UV-Vis-IR reflectance spectroscopy. H2O loss begins instantaneously with vacuum exposure, measured by the attenuation of spectral water absorption bands, and a phase transition from crystalline to amorphous is observed for MgSO(4)6H(2)O by X-ray powder diffraction. Long-term (>40h), continuous exposure of MgSO(4)nH(2)O (n=0, 6, 7) to low pressure (10(-3)-10(-6)Torr) causes material decomposition and strong UV absorption below 0.5m. Our measurements suggest that MgSO(4)6H(2)O grains (45-83m) dehydrate to 2% of the original 1.9m water band area over similar to 0.3Ma at 200K on Ceres and after similar to 42Ma for 147K. These rates, inferred from an Avrami dehydration model, preclude MgSO(4)6H(2)O as a component of Ceres' surface, although anhydrous and minimally hydrated sulfates may be present. A comparison between Ceres emissivity spectra and laboratory reflectance measurements over the infrared range (5-17m) suggests sulfates cannot be excluded from Ceres' mineralogy.