Search

Over 200 molecules have been detected in multiple extraterrestrial environments, including glycolaldehyde (C2(H2O)2, GLA), a two-carbon sugar precursor that has been detected in regions of the interstellar medium. Its recent in situ detection on the nucleus of comet 67P/Churyumov-Gerasimenko and via remote observations in the comae of others, provides tantalizing evidence that it is common on most (if not all) comets. Impact experiments conducted at the Experimental Impact Laboratory at NASA's Johnson Space Center have shown that samples of GLA and GLA mixed with montmorillonite clays can survive impact delivery in the pressure range of 4.5 GPa to 25 GPa. Extrapolated to amounts of GLA observed on individual comets and assuming a monotonic impact rate in the first billion years of solar system history, these experimental results show that up to 10^23 kg of cometary GLA could have survived impact delivery, with substantial amounts of threose, erythrose, glycolic acid, and ethylene glycol also produced or delivered. Importantly, independent of the profile of the impact flux in the early solar system, comet delivery of GLA would have provided (and may continue to provide) a reservoir of starting material for the formose reaction (to form ribose) and the Strecker reaction (to form amino acids). Thus, comets may have been important delivery vehicles for starting molecules necessary for life as we know it., Accepted manuscript, to appear in the journal Astrobiology

The U.S. Army Research Laboratory (ARL) and Lawrence Livermore National Laboratory have developed a high speed computed tomography system that is capable of tracking unbounded material flux through a reconstruction volume at ballistic rates. It achieves this feat by creating three individual time-registered volume reconstructions throughout the duration of a single dynamic event. Each individual volume reconstruction is computed from five attenuation projections, which are collected using five flash X-ray sources and five digital imaging plate detectors. This work summarizes the numerous complexities in combining the 15 sources and 15 detectors into a single diagnostic, the complexities of analyzing multiple reconstruction volumes from a limited number of views, and complications associated with designing an experiment to fit within the bounding conditions of the diagnostic. It continues to demonstrate an example of how free-flowing material propagates through a reconstruction volume during a bullet/target interaction.

Geochronology, or determination of absolute ages for geologic events, underpins many inquiries into the formation and evolution of planets and our Solar System. Absolute ages of ancient and recent magmatic products provide strong constraints on the dynamics of magma oceans and crustal formation, as well as the longevity and evolution of interior heat engines and distinct mantle/crustal source regions. Absolute dating also relates habitability markers to the timescale of evolution of life on Earth. However, the number of geochronologically-significant terrains across the inner Solar System far exceeds our ability to conduct sample return from all of them. In preparation for the upcoming Decadal Survey, our team formulated a set of medium-class (New Frontiers) mission concepts to three different locations (the Moon, Mars, and Vesta) where sites that record Solar System bombardment, magmatism, and/or habitability are uniquely preserved and accessible. We developed a notional payload to directly date planetary surfaces, consisting of two instruments capable of measuring radiometric ages in situ, an imaging spectrometer, optical cameras to provide site geologic context and sample characterization, a trace element analyzer to augment sample contextualization, and a sample acquisition and handling system. Landers carrying this payload to the Moon, Mars, and Vesta would likely fit into the New Frontiers cost cap in our study (~$1B). A mission of this type would provide crucial constraints on planetary history while also enabling a broad suite of investigations such as basic geologic characterization, geomorphologic analysis, ground truth for remote sensing analyses, analyses of major, minor, trace, and volatile elements, atmospheric and other long-lived monitoring, organic molecule analyses, and soil and geotechnical properties., Comment: Submitted to the Planetary Science Journal, October 2020

This work investigates how one can postprocess a series of flash-generated X-ray radiographs with known point spread functions and collection geometries to produce radiographs with enhanced image quality and/or to approximate a multiframe X-ray movie by spatially correlating temporally sequenced images. To produce images with enhanced quality, one collects multiple individual projections, simultaneously, and then uses the known point spread function and acquisition geometry to correlate and integrate the numerous projections as if they were acquired from approximately the same perspective. If using sources of similar spectral characteristics, the outcome correlates with that which would be produced from a multiflash integration, where one benefits from an increase in flux of X-ray photons. If using sources of different spectral characteristics, a composite radiograph can be produced where the unique source spectrum can be matched to known material absorption cross-sections to accentuate radiographic features within the target material. To produce multiframe movies, similar mathematics are applied to images collected throughout a timespan over which the object changes. The aforementioned processes’ mathematics and examples are demonstrated for the case in which the individual projections are completely separable on the detector with no overlap. Finally, a potential method is discussed for separation of images in the case where multiple radiograph projections are overlapped and entangled on the detector.

A series of bare silicon carbide ceramic armour disks were manufactured with high-strength steel containment to induce varying magnitudes of radial pre-stress. The induced radial stresses, ranging from 0 to 900 MPa, were measured via neutron diffraction and verified by comparison with numerical calculations. Four targets of each configuration, referred to as slip-fit, moderate pre-stress, and high pre-stress, were subject to ballistic testing with hemispherical-nose, tungsten heavy alloy long rod projectiles to determine the interface defeat transition velocity. In-situ diagnostics were unsuccessful in aiding the identification of interface defeat, necessitating a reliance on post-mortem assessment. A transition velocity of approx. 1000 m/s was identified for the unstressed target, increasing to approx. 1200 m/s for the pre-stressed configurations. No performance effect was discernible between the moderate (372 MPa) and high (899 MPa) pre-stress configurations, suggesting that an optimal performance may be achieved for lower pre-stress levels (i.e.

Impact cratering is the dominant process for transporting material on the Moon's surface. An impact transports both proximal material (continuous ejecta) locally and distal ejecta (crater rays) to much larger distances. Quantifying the relative importance of locally derived material versus distal material requires understandings of lunar regolith evolution and the mixing of materials across the lunar surface. The Moon has distinctive albedo units of darker mare basalt and brighter highland materials, and the contacts between these units are ideal settings to examine this question. Information on the amount of material transported across these contacts comes from both the sample collection and remote sensing data, though earlier interpretations of these observations are contradictory. The relatively narrow (~4–5 km wide) mixing zone at mare/highland contacts had been interpreted as consistent with most material having been locally derived from underneath mare plains. However, even far from these contacts where the mare is thick, highland material is abundant in some soil samples (>20%), requiring transport of highland material over great distances. Any model of impact transport on the Moon needs to be consistent with both the observed width of mare/highland contacts and the commonality of nonmare material in mare soil samples far from any contact. In this study, using a three-dimensional regolith transport model, we match these constraints and demonstrate that both local and distal material transports are important at the lunar surface. Furthermore, the nature of the distal material transport mechanism in discrete crater rays can result in substantial heterogeneity of surface materials.

The U.S. Combat Capabilities Development Command Army Research Laboratory and Lawrence Livermore National Laboratory are currently developing a Multi-Energy Flash Computed Tomography (MEFCT) diagnostic for multi-frame, in situ, three-dimensional radiographic assessment of ballistic impact phenomena. To accomplish this, we combine the capabilities of medical X-ray computed tomography and high-speed computed tomography, to produce a system that captures three independent, time-sequenced volume reconstructions throughout the timespan of a typical dynamic ballistic event. Because this system has the capability to image an event across three spatial dimensions and time, it is the first of its kind to track mass/material-flux of an un-bounded system through a volume at ballistic timescales. To demonstrate the diagnostic’s capabilities, an assessment of a bullet penetrating an aluminum plate is performed. A compilation of the three volume reconstructions were computed to describe the event. The results were compared to a state-of-the-art simulation of the event using EPIC, a Lagrangian hydrocode with penetration applications. This comparison shows how using a four-dimensional computed tomography system can benefit the validation of physical failure and mass/material-flow models.

Proton radiography’s utilization of positively charged probing particles makes it susceptible to potential influences from electromagnetic forces generated at the target object. In this work, we investigate how the 800 MeV proton radiography system of Los Alamos National Laboratory would respond to such influences with intentions to develop proton radiography/deflectometry diagnostics capable of probing electromagnetic fields inside dynamically changing objects. To understand how the proton radiography system responds to object-generated electromagnetic fields, a series of simulations were performed, which map proton trajectories through the system. This leads to an understanding of how the radiographic images, normally dominated by multiple Coulomb scattering, would be modified. The simulations also highlight novel ways in which additional data could be collected to glean more information about electromagnetic influences within and around the target object.

The U.S. Army Research Laboratory, in conjunction with Lawrence Livermore National Laboratory, is developing a Multi-Energy Flash Computed Tomography (MEFCT) diagnostic that will be used to capture tomographic image(s) of dynamic impact and detonation events. To accomplish dynamic tomography, the diagnostic uses numerous source–detector pairs to accumulate up to fifteen two-dimensional images, which are subsequently used to compute up to three three-dimensional tomographic reconstructions. The diagnostic is designed to provide either: a single-frame, three-dimensional tomographic reconstruction that delineates material specificity throughout the field, or a three-frame tomographic reconstruction movie spaced in time, while lacking the information pertaining to the material specificity. This work assesses aspects of the diagnostic development including structural design, dynamic capability, instrument resolution and computational reconstruction. Examples of real-time measurements are provided from static phantom fiducials, as well as a dynamic experiment depicting a non-symmetric ballistic penetration to demonstrate the usefulness of the capability.

An application of nuclear physics, a facility for using protons for flash radiography, has been developed at the Los Alamos Neutron Science Center (LANSCE). Protons have proven far superior to high energy x-rays for flash radiography because of their long mean free path, good position resolution, and low scatter background. Although this facility is primarily used for studying very fast phenomena such as high explosive driven experiments, it is finding increasing application to other fields, such as tomography of static objects, phase changes in materials and the dynamics of chemical reactions. The advantages of protons are discussed, data from some recent experiments will be reviewed and concepts for new techniques are introduced.

This work describes the use of proton radiography and continuum simulations to investigate the mechanics of a copper jet penetrating unconfined ALON ® transparent ceramic. Use of proton radiography enabled characterization of the jet and ceramic material at 21 time steps, in situ , throughout the penetration process. These radiographs provide time-evolution data pertaining to the material densities, cavity growth, and material failure. The data were compared to legacy analytical penetration models and to a simulation of the event computed using a continuum multi-physics code. These comparisons revealed additional insights into the penetration mechanics as well as strengths and weaknesses of the computational algorithms and material models used in the simulations.

We present a methodology for concurrent Photon Doppler Velocimetry (PDV) and in-situ flash X-ray imaging during ballistic impact of metallic penetrators into monolithic armor grade ceramic targets. Tungsten heavy alloy long rod penetrators were launched from a smooth bore cannon into silicon carbide targets at velocities of 0.753 and 1.403mm/μs. Penetrator lengths and penetration depths were measured as a function of time using an array of four 450kV flash X-ray sources and digital film. A four channel PDV system coupled to a 16GHz oscilloscope was used to measure the velocity history of the penetrator and at three positions on the back surface of the target. This experimental technique shows great promise for measurements of the dwell time, penetration velocity, and the time of final break-out from the back of the target.

It has been shown recently that MCs are required for differential regulation of the immune response by granulocytic versus monocytic MDSCs. Granulocytic MDSCs promoted parasite clearance, whereas monocytic MDSCs enhanced tumor progression; both activities were abrogated in MC-deficient mice. Herein, we demonstrate that the lack of MCs also influences MDSC trafficking. Preferential trafficking to the liver was not seen in MC-deficient mice. In addition, evidence that the MC mediator histamine was important in MDSC trafficking and activation is also shown. MDSCs express HR1–3. Blockade of these receptors by HR1 or HR2 antagonists reversed the histamine enhancement of MDSC survival and proliferation observed in cell culture. In addition, histamine differentially influenced Arg1 and iNOS gene expression in MDSCs and greatly enhanced IL-4 and IL-13 message, especially in granulocytic MDSCs. Evidence that histamine influenced activity seen in vitro translated to in vivo when HR1 and HR2 antagonists blocked the effect of MDSCs on parasite expulsion and tumor metastasis. All of these data support the MDSC-mediated promotion of Th2 immunity, leading to the suggestion that allergic-prone individuals would have elevated MDSC levels. This was directly demonstrated by looking at the relative MDSC levels in allergic versus control patients. Monocytic MDSCs trended higher, whereas granulocytic MDSCs were increased significantly in allergic patients. Taken together, our studies indicate that MCs and MC-released histamine are critical for MDSC-mediated immune regulation, and this interaction should be taken into consideration for therapeutic interventions that target MDSCs.

An introductory Astronomy survey course is often taken to satisfy a college graduation requirement for non-science majors at colleges around the United States. In this course, material that can be broadly categorized into topics related to "the sky", "the Solar System", "the Galaxy", and "cosmology" is discussed. Even with the wide variety of topics in these categories, though, students may not be 100% interested in the course content, and it is almost certain that a specific topic about which a student wishes to learn is not covered. To at least partly address these issues, to appeal to all of the students in this class, and to allow students to explore topics of their choice, a video project has been assigned to students at Albion College as a class activity. In this assignment, students are asked to create a video of a famous (or not) astronomer, astronomical object or discovery, or telescope observatory to present to the class. Students work in pairs to create a video that is original and imaginative and includes accurate scientific content. For this project, then, students use a familiar technology and exercise their creativity while learning a little (or a lot of) science along the way. Herein data on types and topics of videos, examples of videos, assignment requirements and grading rubrics, lessons learned, and student comments will be discussed and shared., Comment: 18 pages, activity instructions & evaluation criteria included

Compression and fracture during high rate impacts were quantified in a series of model transparent polymer networks using high speed videography with sub-microsecond temporal resolution. Sample transparency enabled direct quantification of the compression front velocities, as well as time-resolved measurements of crack initiation and growth. Compression front propagation reveals multiple fronts and reflections, indicating a complex loading profile under these conditions. The front velocity is found to correlate linearly with the longitudinal sound velocity. Radial crack growth was measured both in the radial direction as well inward growth towards the sample impact surface. Measurements of the crack length reveal a deceleration period at short times, transitioning to a steady-state velocity at longer times, and crack arrest at still longer times. Crack lengths were found to correlate with quasi-static fracture toughness, and crack velocities were compared to longitudinal sound velocities.

Table 1 had incorrect densities for the RH and Poly samples. The correct density for RH samples is 1.1892 ± 0.0003 g/cm3. The correct density for Poly samples is 1.1842 ± 0.0004 g/cm3. The authors thank Christopher Neel from the Air Force Research Laboratory for bringing this error to our attention.

B cell A disintegrin and metalloproteinase 10 (ADAM10) is required for the development and maintenance of proper secondary lymphoid tissue architecture; however, the underlying mechanism remains unclear. In this study, we show disturbances in naive lymph node architecture from B cell–specific ADAM10-deficient mice (ADAM10B−/−) including loss of B lymphocyte/T lymphocyte compartmentalization, attenuation of follicular dendritic cell reticula, excessive collagen deposition, and increased high endothelial venule formation. Because TNF-α signaling is critical for secondary lymphoid tissue architecture, we examined compensatory changes in ADAM17 and TNF-α in ADAM10B−/− B cells. Surprisingly, defective follicular development in these mice was associated with increased rather than decreased TNF-α expression. In this article, we describe an increase in TNF-α message, mRNA stability, soluble protein release, and membrane expression in ADAM10B−/− B cells compared with wild type (WT), which coincides with increased ADAM17 message and protein. To assess the mechanistic contribution of excessive TNF-α to abnormal lymphoid architecture in ADAM10B−/− mice, we performed a bone marrow reconstitution study. Rectification of WT architecture was noted only in irradiated WT mice reconstituted with ADAM10B−/− + TNF knockout bone marrow because of normalization of TNF-α levels not seen in ADAM10B−/− alone. We conclude that ADAM17 overcompensation causes excessive TNF-α shedding and further upregulation of TNF-α expression, creating an aberrant signaling environment within B cell cortical regions of ADAM10B−/− lymph nodes, highlighting a key interplay between B cell ADAM10 and ADAM17 with respect to TNF-α homeostasis.

A method to permit wave separation with a Kolsky bar is described. A photon Doppler velocimeter (PDV) is used to measure particle velocity at the location of each strain-gage. These measurements are used with the measured strain to separate wave-trains that in general exist in each bar (for example, an incident and reflected pulse) even when they are superimposed at the gage location. This can extend the duration of the experiment and permit more freedom in the types of loadings that can be applied to a specimen. It was found that the PDV measurement of particle velocity often contains a significant component due to bending waves. A method to account for bending is described but requires multiple PDV measurements at each gage position.

Photon Doppler Velocimetry was used to characterize the early time formation of the jet from a 65-mm diameter shaped charge with a copper liner. This tool provided a high fidelity, high temporal resolution technique to characterize the axial velocity evolution of the shaped charge jet's first motion to exit from the base of the conical liner. The measurements were then compared with continuum mechanics shock physics simulations, using both CTH and ALEGRA, to understand the jet formation process and evolution.

We have developed and implemented a low-cost peer-mentoring program that blends personal and academic support to help students achieve academic success in the introductory courses required for the Physics Major or the Dual-Degree Program in Engineering. As a result of this program, retention has increased among entering science, technology, engineering and mathematics students at Albion College as they move through the introductory classes, as shown by a 20 per cent increase in retention from first-semester to third-semester physics courses compared to years when this program was not in place., accepted (European Journal of Physics), 17 pages, 1 figure, 1 table

The decomposition pathways of methanol and water on Pt-modified W(110) bimetallic surfaces have been investigated using density functional theory (DFT), temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). The reaction of methanol on submonolayer and monolayer Pt-modified W(110) surfaces is compared to that on clean Pt(111) and W(110). Similar to clean W(110), the Pt/W(110) bimetallic surfaces remain active toward the dissociation of methanol, although the reaction pathway leading to the production of CH 4 is reduced on the bimetallic surfaces. The Pt/W(110) surfaces are also active toward the decomposition of water. These results are compared with previous studies of the reactions of H 2 and ethylene on Pt/W(110) bimetallic surfaces to reveal the different Pt-modification effects for the dissociation of oxygen-containing molecules.

Geochemical and 40Ar/39Ar data on nine impact glasses from the Apollo 14, 16, and 17 landing sites indicate at least seven distinct impact events with ages ∼800 Ma. Rock fragments analyzed by Barra et al. [Barra F., Swindle T. D., Korotev R. L., Jolliff B. L., Zeigler R. A., and Olsen E. (2006) 40Ar–39Ar dating of Apollo 12 regolith: implications for the age of Copernicus and the source of nonmare materials, Geochim. Cosmochim. Acta, 70, 6016–6031] from the Apollo 12 landing site and some Apollo 12 spherules reported by Levine et al. [Levine J., Becker T. A., Muller R. A., Renne P. R. (2005) 40Ar/39Ar dating of Apollo 12 impact spherules, Geophys. Res. Let., 32, L15201, doi: 10.1029/2005GL022874.] show ∼800 Ma ages, close to the accepted age of the Copernicus event, 800 ± 15 Ma [Bogard D. D., Garrison D. H., Shih C. Y., and Nyquist L. E. (1994) 39Ar–40Ar dating of two lunar granites: The age of Copernicus, Geochim. Cosmochim. Acta, 58, 3093–3100]. These Apollo 12 samples are thought to have been affected by material from the Copernicus event since there is a Copernicus ray going through the Apollo 12 landing site. When all of these data are viewed collectively, including an Apollo 16 glass bomb [Borchardt R., Stoffler D., Spettel B., Palme H. and Wanke H. (1986) Composition, structure, and age of the Apollo 16 subregolith basement as deduced from the chemistry of post-Imbrium melt bombs. In Proceedings, 17th Lunar and Planetary Science Conference, pp. E43–E54], and in the context of diverse compositional range and sample location, there is a suggestion that there may have been a transient increase in the global lunar impact flux at ∼800 Ma. Therefore, the Copernicus impact event could have been one of many. If correct, there should be evidence for this increased impact flux around 800 Ma ago in the age statistics of terrestrial impact samples.

The current review focuses on the general trends in the electronic and chemical properties of monolayer bimetallic surfaces (MBS). The MBS is defined in this review as a monolayer of admetal deposited on a single crystal metal substrate. Depending on the deposition temperature and reaction environment, the admetal in MBS can be in one of the three structures: occupying the topmost surface sites to produce the surface monolayer, diffusing into the subsurface region to form the subsurface monolayer, and alloying with surface to produce intermixed bimetallic surfaces. The review will summarize recent experimental and theoretical studies of the surface monolayer, subsurface monolayer, and inter-mixed bimetallic structures to demonstrate their unique electronic and chemical properties. In the current review we will use one the most thoroughly studied bimetallic systems, surface and subsurface Ni monolayer on Pt(111), as a model system to introduce the synthesis, characterization, and DFT modeling of MBS structures. We will then utilize the adsorption of atomic hydrogen, atomic oxygen, and CO to demonstrate the correlation between the surface d-band center and the binding energies of both atomic and molecular adsorbates for various bimetallic surfaces. We will also address the issue of stability of the subsurface and surface MBS structures in vacuum, and in the presence of adsorbed hydrogen and oxygen. We will then use the hydrogenation of alkenes on subsurface MBS structures, and the reforming of oxygenates on surface MBS structures, to illustrate the trend between binding energies of adsorbates and chemical activities.

An effective anode electrocatalyst in direct methanol fuel cell (DMFC) should have high activity for the oxidation of methanol and the decomposition of water, while remaining stable under the relatively harsh anode environment. Although the Pt/Ru bimetallic alloy is currently the most effective anode electrocatalyst, both Pt and Ru are expensive due to limited supplies and both are susceptible to CO poisoning. Consequently, the discovery of less expensive and more CO tolerant alternatives to the Pt/Ru catalysts would help facilitate the commercialization of DMFC. In this paper we will discuss the possibility of using tungsten carbides (WC) and Pt-modified WC as potential anode electrocatalysts in DMFC. We will provide an overview of our recent work, using a combined approach of fundamental surface science studies and in-situ electrochemical evaluation of the activity and stability of tungsten carbides. We will demonstrate the feasibility to bridge fundamental surface science studies on single crystals with the electrochemical evaluation on polycrystalline WC films. We will also discuss the synergistic effect by supporting low coverages of Pt on the WC substrate to further enhance the electrochemical performance of WC.

Polycrystalline tungsten monocarbide (WC) surfaces are synthesized and evaluated as a potential replacement of platinum (Pt) electrocatalysts. A combined approach utilizing ultrahigh vacuum (UHV) and electrochemical techniques demonstrates that WC possesses the following prerequisites for alternative electrocatalysts: high activity toward methanol decomposition, improved resistance to poisoning by CO, and promising electrochemical stability.

The major- and minor-element abundances were determined by electron microprobe in 1039 glasses from regoliths and regolith breccias to define the compositional topology of lunar glasses at the Apollo 16 landing site in the central highlands of the Moon. While impact glasses with chemical compositions similar to local materials (i.e., Apollo 16 rocks and regoliths) are abundant, glasses with exotic compositions (i.e., transported from other areas of the Moon) account for up to ~30% of the population. A higher proportion of compositionally exotic, angular glass fragments exists when compared to compositionally exotic glass spherules. Ratios of non-volatile lithophile elements (i.e., Al, Ti, Mg) have been used to constrain the original source materials of the impact glasses. This approach is immune to the effects of open-system losses of volatile elements (e.g., Si, Na, K). Four impact glasses from one compositionally exotic group (low-Mg high-K Fra Mauro; lmHKFM) were selected for 40Ar/39Ar dating. The individual fragments of lmHKFM glass all yielded ages of ~3750 ± 50 Ma for the time of the impact event. Based on the petrography of these individual glasses, we conclude that the likely age of the impact event that formed these 4 glasses, as well as the possible time of their ballistic arrival at the Apollo 16 site from a large and distant cratering event (perhaps in the Procellarum KREEP terrain) (Zeigler et al. 2004), is 3730 ± 40 Ma, close to the accepted age for Imbrium.

Sugars of extraterrestrial origin have been observed in the interstellar medium (ISM), in at least one comet spectrum, and in several carbonaceous chondritic meteorites that have been recovered from the surface of the Earth. The origins of these sugars within the meteorites have been debated. To explore the possibility that sugars could be generated during shock events, this paper reports on the results of the first laboratory impact experiments wherein glycolaldehyde, found in the ISM, as well as glycolaldehyde mixed with montmorillonite clay, have been subjected to reverberated shocks from ~5 to >25 GPa. New biologically relevant molecules, including threose, erythrose and ethylene glycol, were identified in the resulting samples. These results show that sugar molecules can not only survive but also become more complex during impact delivery to planetary bodies., Please contact the authors for supplemental data and text

Lunar impact glasses, quenched melts produced during cratering events on the Moon, have the potential to provide not only compositional information about both the local and regional geology of the Moon but also information about the impact flux over time. We present in this paper the results of 73 new 40Ar/39Ar analyses of well-characterized, inclusion-free lunar impact glasses and demonstrate that size, shape, chemical composition, fraction of radiogenic 40Ar retained, and cosmic ray exposure (CRE) ages are important for 40Ar/39Ar investigations of these samples. Specifically, analyses of lunar impact glasses from the Apollo 14, 16, and 17 landing sites indicate that retention of radiogenic 40Ar is a strong function of post-formation thermal history in the lunar regolith, size, and chemical composition. Based on the relationships presented in this paper, lunar impact glasses with compositions and sizes sufficient to have retained 90% of their radiogenic Ar during 750 Ma of cosmic ray exposure at time-integrated temperatures of up to 290 K have been identified and are likely to have yielded reliable 40Ar/39Ar ages of formation. Additionally, ~50% of the identified impact glass spheres have formation ages of, Please contact Zellner (nzellner@albion.edu) for data tables and other supplemental information

The reaction pathways of methanol on V(1 1 0), carbide-modified V(1 1 0), Ti(0 0 0 1), and carbide-modified Ti(0 0 0 1) have been studied using high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and temperature-programmed desorption (TPD). On V(1 1 0) and C/V(1 1 0), methanol undergoes complete dissociative adsorption producing a methoxy intermediate for exposures less than 2 L at 100 K. On Ti(0 0 0 1) and C/Ti(0 0 0 1), methanol dissociates to produce methoxy at exposures of 3 L at 100 K. The combination of TPD and AES reveals that the number of methoxy per surface metal atom is 0.13 on C/V(1 1 0) and 0.41 on C/Ti(0 0 0 1) at 100 K. All methoxy species undergo further decomposition on C/V(1 1 0) at higher temperatures. However, a significant fraction (∼39%) of methoxy species undergo recombinatory reaction to produce gas-phase methanol on C/Ti(0 0 0 1). The reaction pathways on the C/V(1 1 0) and C/Ti(0 0 0 1) surfaces are compared with previous studies of methanol on other carbide-modified surfaces, as well as on single crystal VC and TiC surfaces.

The decomposition, dehydrogenation, self-hydrogenation, and hydrogenation of cyclohexene were used as probe reactions to determine the effects of supporting monolayer Pt on W(110) and C/W(110) surfaces. The reaction pathways were studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). On the clean and carbon-modified W(110) surfaces, the dominant reaction pathway of cyclohexene is the complete decomposition to produce hydrogen and atomic carbon. Monolayer Pt-modification of the W(110) and C/W(110) surfaces reduces the complete decomposition and enhances the selectivity toward the dehydrogenation of cyclohexene to produce gas phase benzene. More importantly, unlike previous studies of supporting monolayer Pt on Ni and Co surfaces, neither self-hydrogenation of cyclohexene nor hydrogenation of cyclohexene with coadsorbed hydrogen is detected on the monolayer Pt-modified W(110) and C/W(110) surfaces. The comparison is consistent with our previous density functional theory (DFT) modeling of monolayer Pt on different substrates, confirming that the chemical properties of supported Pt vary significantly on different substrates.