Adam K. Woodson

Complex organic molecules are widely detected in star-forming regions and are important precursors to prebiotic species. Because these molecules are thought to form primarily within icy grain mantles, their desorption is assumed to be regulated by the sublimation of the surrounding H ₂ O ice matrix. However, observations increasingly reveal gas-phase abundances of methyl formate (MF) at temperatures well below the sublimation temperature of water ice ( T ∼ 100 K), challenging this view. To investigate how ice composition and phase influence MF desorption, we present laboratory temperature-programmed desorption measurements of MF from four astrophysically relevant substrates: amorphous and crystalline H ₂ O and CH ₃ OH. Desorption kinetics were analyzed using pre-exponential factors derived from transition-state theory with leading-edge analysis and non-negative least-squares inversion to recover binding-energy distributions. We find that MF binds more weakly to CH ₃ OH ice than to H ₂ O-rich substrates, with representative binding energies of 6247 ± 43 K for amorphous H ₂ O, 6213 ± 38 K for crystalline H ₂ O and 5469 ± 40 K for amorphous CH ₃ OH. MF deposited on crystalline CH ₃ OH exhibits coverage-independent leading edges consistent with multilayer or island desorption, yielding an effective binding energy of 5333 ± 17 K. Enhanced surface mobility and reduced trapping efficiency on methanol ice, combined with weaker MF binding, provide a natural explanation for the early appearance of gas-phase MF observed in star-forming environments, with characteristic desorption temperatures of ∼85 K on CH ₃ OH and ∼95–100 K on H ₂ O. Together, these effects support a two-step thermal desorption pathway in which MF associated with CH ₃ OH-rich ice can be released prior to bulk H ₂ O sublimation.

Without continuous diffusion from the Hermean subsurface to the planetary epiregolith, Mercury’s Na exosphere cannot be sustained [Sprague 1990; Killen and Morgan 1993; Leblanc & Johnson 2010; Gamborino et al. 2019; Verkercke et al. 2024]. Meteoritic impacts and solar wind sputtering excavate Na from regolith minerals into the exosphere where a fraction of the ejected atoms will resettle on the planetary surface. These excavated, adsorbed Na atoms may be subsequently re-ejected into the exosphere either in the same manner or by lower energy (< 10 eV) release processes such as thermal desorption and photodesorption. However, this “surface reservoir” appears unsustainable, with theoretical sublimation fluxes via thermal desorption surpassing the expected available atomic concentrations on Mercury’s sunlit hemisphere [Gamborino et al. 2019; Killen et al. 2007; Leblanc and Johnson 2003]. If not for Na diffusion from the subsurface, exospheric Na would be depleted, inconsistent with current observation [e.g., Millano et al. 2021; Schmidt et al. 2020]. Therefore, sustained diffusion of Na from the subsurface. We have measured the rate of Na and K diffusion through a porous regolith analog using X-ray photoelectron spectroscopy (XPS). For each measurement, alkali vapor was deposited onto the underside of a puck-shaped quartz glass frit using an alkali metal evaporator, and the atomic concentration was measured. After this, the frit was flipped to monitor the Na/K concentration on the top surface while the bottom of the frit was held at a constant temperature between 300 and 700 K. The high-purity quartz frits used in these experiments are commercial porous filters composed of sintered quartz-glass beads, with puck thickness ranging from 2 to 3 mm and pore sizes ranging from 16 to 40 microns (porosity ~ 53%). (Fig. 1). For initial Na and K surface concentrations of 3.1 at-% and 19 at-%, respectively, on the underside of the frit, we found the rate of diffusion through the frit at 600 K to follow an exponential curve: y = 1.16 – 1.06*exp [-t/163] at-% for Na and y = 0.4909 - 0.5277*exp [-t/30.30] at-% for K. It is notable that diffusion appears to start immediately with heat exposure – before the initial measurement can be acquired (~ 25 min) for Na – leading to a positive y-intercept for both species. Additionally, the time constant – a measure of the rate to reach an equilibrium surface concentration via diffusion – is 5.4x larger for Na than K in this preliminary data set; this implies that the Na concentration at the surface will continue increasing to its equilibrium value for a longer period than K, suggesting a slower diffusion rate. This rather surprising result will be investigated at additional temperatures to confirm. Acknowledgements: This research was supported by the International Space Science Institute (ISSI) in Bern, through ISSI International Team project 'Multi-scale Understanding of Surface-Exosphere Connections (MUSEC)'. References: Gamborino et al. 2019, Ann. Geophys., 37, 455–470. Killen and Morgan 1993, J. Geophys. Res., 98, E12, 23589–23601. Killen et al. 2007, Space Sci Rev 132, 433–509. Leblanc and Johnson 2003, Icarus, 164, 261-281. Leblanc & Johnson 2010, Icarus, 209, 2, 280-300. Millano et al. 2021, Icarus, 355, 114179. Schmidt et al. 2020, Planet. Sci. Journal, 1, 14. Sprague 1990, Icarus, 84, 1, 93-105. Verkercke et al. 2024, Geophys. Res. Lett., 51, e2024GL109393. Fig 1. Front face of a quartz disc-shaped frit, where sintered glass particles form irregular channels of 16-40 microns. The image horizontal field width is 519 microns. Alkali atoms are deposited at room temperature on the bottom face of the frit (radius: 5 mm; depth: 2.5 mm), diffusing isothermally through the channels to reach the front face. We monitor the frit front face Na concentration as a function of time, at constant temperature.Fig. 2. [Left] Elemental concentrations are shown for K vs. time at 600K on the top surface of a porous quartz frit (16-40 um pore dia.) after vapor deposition onto the opposite (bottom) surface. K appears to grow asymptotically on the top surface of the frit over time. [Right] Na increases monotonically at 600K through a similarly specified quartz frit. Both curves are fit with a function of the form y = y0 + A exp ( -t/τ ) utilizing parameters provided in each figure and discussed in the text.

Regolith roughness is expected to modulate the sputter flux of surface atoms into the exospheres of airless bodies such as Mercury, and studies have shown that roughness from the nanometer scale upward promotes redeposition and diminishes total sputtering yields. Experiments involving irradiation of olivine, plagioclase, and single-element powders demonstrate yield reductions from 15% to 67% as compared to smooth targets, but the grain size dependence of this effect has not been adequately parameterized. Understanding this mechanism is therefore of critical importance for quantifying desorption and sputter ejection and for predicting the relative contributions of release processes from regolith surfaces.We measured the sputter depletion of adsorbed Na from polished natural olivine and from synthetic granular forsterite targets with narrow grain size distributions from 45 μm up to 520 μm. Na vapor was deposited onto each target in an ultrahigh vacuum system and then irradiated at either 15° or 60° incidence (from global surface normal) to prescribed fluence steps using 4 keV He+ ions. All experimental steps were carried out at room temperature (~300 K). After each fluence step an X-ray photoelectron spectrum was acquired and used to quantify the remaining Na surface concentration. Depletion rates and cross sections were then extracted from the concentration vs. fluence data for each target.For the polished targets, Na concentrations exhibited single-exponential decay with increasing fluence, and depletion at 60° incidence outpaced that at 15° due to increased energy deposition near the surface as expected from theory. Conversely, for all granular samples the Na concentrations exhibited at least double-exponential decay, suggesting a bimodal removal process, and depletion from the granular targets occurred more quickly at 15° than at °60, implying enhanced access to grain facets at near normal incidence. We propose that this reflects faster removal of Na that is directly exposed to the incident ion flux, convolved with slower removal of shadowed Na by reflected incident ions. We found that the shadowed sodium was sputtered away 10–100 times more slowly than the exposed Na, with a transition from single-exponential to double-exponential decay at some threshold between nanoscale roughness (polished targets) and microscale roughness (

Angular distributions of sputtered ejecta are critical inputs for simulating regolith sputtering on airless bodies, where surface topography and porosity determine if particles escape or redeposit. We present new analysis and simulations demonstrating that, although current binary collision approximation (BCA) frameworks can reliably predict either total sputter yields or angular distributions for minerals, they cannot reproduce both simultaneously. This is a limitation with direct implications for exosphere and surface-evolution models that require realistic, three-dimensional ejecta plumes. Using SDTrimSP[1] for flat targets, SPRAY[2] for AFM-resolved rough surfaces, and SDTrimSP-3D[3] for porous regolith analogs, we quantify how morphology and binding energies control plume tilt and shape via a dedicated three-dimensional lobe-fitting approach designed for full-plume implementation in modeling tools. This framework addresses the need to move beyond simplified cosine laws and to incorporate azimuthal structure when propagating sputtered particles in regolith and exosphere simulations.Across surface morphologies, rough and porous targets deflect ejecta toward the surface normal, reducing the strongly forward-facing plumes typical of flat surfaces and introducing geometry-mediated effects such as shadowing and redeposition that reshape both yield and energy distributions. In this regime, we find a robust tradeoff between reproducing sputter yield and plume tilt: the “compound/hybrid” binding-energy model[4] capable of matching experimental mineral yields[5] fails to reproduce the experimentally observed forward tilt at oblique incidence on flat and, more significantly, on rough surfaces (Fig. 1). Conversely, adopting increased surface-binding energies (SBEs) informed by recent molecular dynamics work[6] strengthens the forward tilt and better matches rough-surface angular data but underestimates absolute mass yields by factors of 3–4.Figure 1: Model–data comparison of sputter yields and angular distributions for MgSiO3 under 4 keV He irradiation. Left: SDTrimSP model yields that result from the Jäggi et al. (2023) hybrid binding energy model (HB-C, blue dashed) with the increased surface-binding case (SB-C) based on Morrissey et al. (2024) surface binding energies, compared to the Biber et al. (2022) laboratory measurements (black, with 2 SD errors). Center: Normalized mass yield polar distributions from a flat surface at an incidence angle αin = 60° for said simulations. Right: Analogous to the center figure, but for a rough pellet surface. Methodologically, we map flat-surface SDTrimSP ejecta to measured roughness via ray tracing in SPRAY and construct porous regolith stacks in SDTrimSP-3D, then fit the resulting plumes with a single- or tri-lobe model that captures central and side “winglet” components associated with single-collision recoils (Fig. 2). This procedure allows for consistent comparison to laboratory angular data and direct use of fitted plumes in forward models. For rough surfaces, the simulations confirm a forward tilt relative to the sample surface normal but with magnitudes that depend sensitively on the binding-energy scheme. Only the higher-SBE case reproduces the observed forward-tilted distributions. For porous targets, preliminary comparisons and prior 3D simulations indicate stronger backscattering, further emphasizing the need for morphology-aware angular inputs in regolith sputtering models.Figure 2: Comparison of O sputter ejecta distributions sputtered from a rough surface (SPRAY) by 4 keV He at an incidence angle of αin = 75° from the surface normal (black arrow) resulting from (left) the Jäggi et al. (2023) hybrid binding energy (HB) model and from (right) a surface binding energy (SB) model with increased binding energies from Morrissey et al. (2024), fitted by a central plume (blue) and two forward-facing “winglet” plumes (red). These findings have two key implications. First, exosphere and regolith models should incorporate full 3D angular distributions rather than rely on simple cosine laws, especially for rough and porous substrates where plume tilt and azimuthal structure are non-negligible. Second, the inability to match yield and angle concurrently suggests missing physics in BCA implementations—most notably, yield-enhancing processes such as molecule and cluster sputtering that are not represented in single-atom BCA ejecta—and cautions against parameter tuning. In practice, our results recommend using binding-energy models that best reproduce angular distributions when plume directionality controls transport and redeposition, while acknowledging that total yields must then be corrected for non-BCA processes if absolute source strengths are required.In summary, angular distribution data are indispensable for realistic regolith sputtering simulations, yet existing BCA parameterizations cannot simultaneously reproduce experimental mass yields and angular distributions across flat, rough, and porous mineral surfaces. This demonstrates a fundamental limitation of current BCA-based models for planetary surfaces and motivates hybrid or extended frameworks that incorporate missing yield mechanisms while retaining accurate, morphology-dependent plume geometry.References[1] Mutzke, A., Toussaint, U. v, Eckstein, W., Dohmen, R. & Schmid, K. (2024). SDTrimSP Version 7.00. Technical Report[2] Cupak, C. et al. Appl. Surf. Sci. 570, 151204 (2021).[3] von Toussaint, U., Mutzke, A. & Manhard, A. Phys. Scr. T170, 014056 (2017).[4] Jäggi, N. et al. Planet. Sci. J. 4, 86 (2023).[5] Biber, H. et al. Planet. Sci. J. 3, 271 (2022).[6] Morrissey, L.S. et al. Planet. Sci. J. 5, 272 (2024).

Introduction: KEVION, a KiloElectron Volt ION irradiation facility for space science – one of NASA’s new Planetary Science Enabling Facilities – will begin full operation this Fall (2026) as part of the Laboratory for Astrophysics and Surface Physics at the University of Virginia (LASP-UVa). This resource facility is comprised of four major components: (1) a 25-300 keV Pelletron ion accelerator to provide positive atomic/molecular ions over a broad range of species, charges, and energies; (2) a novel, multi-technique analytical chamber (GRAINS) with low-energy ion gun built around an X-ray photoelectron spectrometer and in situ UV-Vis-NIR optical spectroscopy; (3) a specialized cryogenic chamber (ICE) for radiolytic astrochemistry studies of solid-state ices; and (4) a minimally-equipped chamber (TEST) for ion implantation, instrument performance testing, and user-customized experiments.With the deployment of Juno, Juice and Europa Clipper to Solar System bodies with radiation-exposed surfaces, along with the anticipated Dragonfly and CNSA’s Tianwen-4 missions, and new galactic and interstellar science from the James Webb Telescope, the KEVION facility addresses an identifiable gap in available community tools for high-impact Planetary Science research. The KEVION high-energy linear accelerator, a Peabody Scientific light-ion accelerator, and low-energy (< 5 keV) ion guns will deliver positive ions appropriate for simulating magnetospheric, cosmic-ray, and solar wind interaction with surfaces to provide transformative research in studies of space weathering, radiolysis, radiosynthesis, sputtering, and surface charge. Meanwhile, the TEST chamber can be utilized to facilitate science instrument development and radiation damage.Details: Investigators proposing across all NASA, ESA, JAXA, and other programs are invited to utilize the KEVION facility over the course of their research. A full-time facility instrument scientist (Dr. Adam Woodson) is available to assist with experiment planning, accelerator operation, instrument instruction, and analytical procedure. Specific details of the Pelletron accelerator and available analytical techniques associated with each end-chamber are outlined below and summarized on the KEVION website and at NASA’s science link, https://science.nasa.gov/wp-content/uploads/2024/01/kevion-2024.pdf.Ion Accelerator: The National Electrostatics Corp. Pelletron ion implanter will generate isotopically pure ion beams with energies between 25 and 300 keV for simulated space weathering, radiolysis, materials characterization, measurement of fundamental parameters, and instrument prototyping applications. This range is sufficient to simulate a substantial cross section of energetic particles originating from the Sun, which vary in energy from ~300 eV/amu in the slow Solar Wind, to ~50 keV in transient Interplanetary Coronal Mass Ejection events, to > 1 MeV in the suprathermal tail. Similarly, magnetospheric ion energies generally range from a few keV to a few tens of keV as measured in Mercury’s and Titan's interaction regions, to MeV particle fluxes at the Moons of Jupiter [1-3]. The NEC instrument is designed to provide beam currents of 300 μA into the beamline Faraday Cup. Lower energy (0.2 – 4 keV) ions, appropriate for solar wind simulation, are available from a standard or m/q select-ed ion gun mounted on the GRAINS chamber.GRAINS Chamber: The GRAINS chamber will enable comprehensive analytical measurement and monitoring of surface (1–3 monolayers, ML), near-surface (to ~10 nm depth) and bulk (to ~10 μm depth) material characteristics. In situ sample characterization can be done by: quadrupole secondary-ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), Rutherford scattering (forward and RBS), and UV-Vis-NIR optical reflectance (or trans-mission). A low-energy ion gun for 3D compositional analysis by sputter depth profiling, charge neutralization, and irradiation can provide fluence dependent details over decades of energies/depths/time; and an electron flood gun for positive surface charge neutralization is available. Multiple, integrated analytical techniques on the same chamber eliminates the need to transfer samples between systems, thereby reducing the logistic complexity of such experiments as well as the risk of contamination due to atmospheric exposure.ICE Chamber: Investigation into solid-phase radiochemistry is of high-importance, as icy bodies and granular surfaces act as substrates for simple and com-plex molecular species formation by gas-grain chemistry and photolytic / radiolytic reactions, and also serve as reservoirs for important reactive species in interstellar cold clouds. The ICE cryogenic end-chamber is equipped with an in-situ gas-dosing system, FT-IR spectrometer for Vis-IR reflectance / transmission, an in-situ quartz crystal microbalance system (QCM), a UV-Vis interferometer for coupled mass and density/porosity/thickness measurement, and a +/- ion-neutral mass spectrometer.TEST Chamber: The TEST chamber is a large-diameter, mu-metal-lined UHV vacuum vessel, intended for instrument response testing over varied ion species/energies. In some instances, the TEST chamber may be outfitted with additional research tools from an investigator’s home institution for experiments beyond the capabilities of GRAINS or ICE.Usage Information: The KEVION accelerator and analytical chambers are available at no cost for NASA awarded projects (~6 months/year), and we also welcome other ESA, JAXA, academic, governmental, and industrial clients as users at a nominal, tier-ed cost. Facility resources are available to clients “in person” with training, or analyses can be carried out “remotely” by facility personnel in consultation with users.Contact Information: For more information on how to utilize the KEVION or to solicit information for...

Saturn's magnetosphere is continuously supplied with neutrals from the Enceladus plume and the icy rings, which undergo ionization and charge‐exchange to form a complex water‐group plasma environment. While the Cassini Plasma Spectrometer (CAPS) instrument has provided extensive compositional information, detailed separation of individual water‐group ion species in time‐of‐flight (TOF) data has not previously been achieved. In this study, we perform forward modeling of CAPS‐IMS energy‐per‐charge (E/Q) and TOF spectra obtained between 2004 and 2012 to resolve O ⁺ , OH ⁺ , H ₂ O ⁺ , and H ₃ O ⁺ and to characterize their plasma properties, including number density, temperature, and thermodynamic κ. Our results demonstrate that O ⁺ is the dominant thermal ion species throughout Saturn's magnetosphere, comprising up to ∼70% of the total ion population beyond ∼5 Saturn radii (R _S ). In contrast, molecular ions such as OH ⁺ , H ₂ O ⁺ , and H ₃ O ⁺ dominate closer to Enceladus but rapidly dissociate into atomic ions between ∼5 and 10 R _S . This radial region is also characterized by the steepest increase in plasma flow speed, which rises from ∼40% to ∼80% of rigid corotation. Simultaneously, ion velocity distributions approach Maxwell–Boltzmann equilibrium, as indicated by high kappa values. These findings provide new constraints on the ion–neutral chemistry that regulates the balance between molecular and atomic ions in Saturn's magnetosphere. They also emphasize the critical role of the 5–10 R _S region as a transition zone for both plasma composition and dynamics. Our results refine previous CAPS‐based studies and underscore the need to incorporate seasonal variability and ionospheric coupling into future global models of Saturn's plasma environment.

Without continuous diffusion from the Hermean subsurface to the planetary epiregolith, Mercury’s Na exosphere cannot be sustained [1-4]. Meteoritic impacts and solar wind sputtering excavate Na from regolith minerals and replenish the supply, redepositing it across the planetary surface. These excavated, adsorbed Na atoms may be subsequently re-ejected into the exosphere by lower energy (< 10 eV) release processes such as thermal desorption and photodesorption. However, this “surface reservoir” appears unsustainable, with theoretical sublimation fluxes via thermal desorption surpassing the expected available atomic concentrations on Mercury’s sunlit hemisphere [3, 5-6]. With no additional source of Na diffusing from the subsurface, exospheric Na would be depleted, inconsistent with current observation [e.g., 7-8]. Therefore, sustained diffusion of Na from the subsurface is necessary to maintain the observed exospheric abundances, and the rate at which this subsurface Na migrates to the surface is critically important.We have measured the rate of Na diffusion through a porous regolith analog using X-ray photoelectron spectroscopy (XPS). For each measurement, Na vapor was deposited onto the underside of a puck-shaped quartz glass frit, after which the Na concentration on the top side was monitored over time while the bottom of the frit was held at a constant temperature between 300 and 700 K. The high-purity quartz frits used in these experiments are commercial porous filters composed of sintered quartz-glass beads, with puck thickness ranging from 2 to 3 mm and pore sizes ranging from 16 to 40 microns. (Fig. 1). For an initial Na surface concentration of 3.1 at-% on the underside of the frit, we found the rate of diffusion through the frit at 700 K to be 6.4 x 10-7 at-% s-1, or roughly 1.3 x 109 Na cm-2 s-1. Fig 1. Front face of disc-shaped frit, where sintered glass particles form irregular channels of 16-40 microns. Image field width is 519 microns; irregular particles create channels. Na is deposited at RT on the bottom face of the frit, diffusing through the channels to reach the front face. The frit front Na concentration is monitored as a function of time, at constant temperature. Fig. 2. (Left) Elemental concentrations vs. time on the top surface of the porous 16-40 um pore dia. quartz frit at 700 K after Na vapor deposition onto the bottom surface. The profiles show adventitious carbon and minor N, in addition to SiO2. (Right) Zoomed region of the left-hand plot showing a steady increase in Na on the frit top over time. During ~1900 thru ~2400 minutes, the temperature was maintained but XPS spectra were not acquired. References:[1] Sprague 1990, Icarus, 84, 1, 93-105.[2] Killen and Morgan 1993, J. Geophys. Res., 98, E12, 23589–23601.[3] Gamborino et al. 2019, Ann. Geophys., 37, 455–470.[4] Verkercke et al. 2024, Geophys. Res. Lett., 51, e2024GL109393.[5] Killen et al. 2007, Space Sci Rev 132, 433–509.[6] Leblanc & Johnson 2003, Icarus, 164, 261-281.[7] Schmidt et al. 2020, Planet. Sci. Journal, 1, 14.[8] Millano et al. 2021, Icarus, 355, 114179.

The KiloElectron Volt Ion (KEVIon) irradiation facility for space science — a new NASA Planetary Science Enabling Facility — is under development within the Laboratory for Astrophysics and Surface Physics at the University of Virginia (LASP-UVa). This user-focused facility is comprised of four integrated components: (1) multiple ion sources including a low energy ion gun (< 5 keV), a medium energy light ion accelerator (< 50 keV), and a Pelletron ion accelerator (< 300 keV) to provide positive atomic or molecular ions over a wide range of species, charges, and energies; (2) a novel ultrahigh vacuum (UHV) chamber called "GRAINS" that integrates X-ray photoelectron spectrometry, mass spectrometry, medium-energy ion scattering, hyperspectral imaging, surface charge measurement, and more for holistic studies of mineralogical samples and other materials; (3) an established cryogenic UHV chamber, named "ICE", for studies involving irradiation, temperature-programmed desorption, and mass spectrometry of condensed gas targets; and (4) a user-configurable UHV chamber called "TEST" for instrument testing, calibration, and prototyping. KEVIon is expected to facilitate transformative research in space weathering, radiolysis, radiosynthesis, sputtering, radiation damage, surface charging, and instrument development/response testing. A variety of experimental results obtained using KEVIon instrumentation will be presented and discussed to showcase the capabilities of the new facility.KEVIon is accessible to researchers either in-person—contingent upon required safety and instrument training—or remotely as a pay-for-services facility. A full-time instrument scientist is available to assist with experiment planning, instrument operation, instrument training, data acquisition, and data analysis. Nominal hourly rates for academic, industrial, and government-affiliated users are provided on the KEVIon website at https://engineering.virginia.edu/kevion, along with details about facility instrumentation and analytical techniques. For more information or to schedule a consultation, please email Cathy Dukes (PI) at cdukes@virginia.edu, or Adam Woodson (Instrument Scientist) at akw8r@virginia.edu.Acknowledgments: The authors would like to thank the NASA PSEF program for making this facility possible through award 80NSSC23K0200.