Paul Stefan Szabo

Charged‐particle sputtering is a fundamental process that contributes to the generation of the Moon’s neutral exosphere. Neutral sputtering rates are dependent on the speed, flux, and composition of the incident charged‐particle populations, all of which are variable at the Moon’s orbit. Dramatic increases in the neutral sputtering rate are likely driven by the impact of interplanetary coronal mass ejections (ICMEs). ICMEs have enhanced densities and speeds, as well as changes in their underlying composition with respect to typical solar wind values. Here, we study the role that ion composition has on the overall contributions to neutral sputtering at the Moon by analyzing in situ observations by NASA’s Advanced Composition Explorer (ACE) mission. A pre‐existing catalog of ICMEs at 1 au is used to extract relevant time periods and convolve compositionally resolved ion fluxes with laboratory sputtering yields. Median sputtered fluxes increase by factors of 2.4× and 1.5× the average solar wind value during ICME sheath and ejecta periods, respectively. Protons typically dominate sputtered fluxes from the Moon for all data, including during ICME sheath and ejecta periods; however, the contributions from alphas increases during ICME ejecta due to enhanced alpha‐to‐proton ratios. Heavy ions, including C, O, Ne, Mg, Si, and Fe typically contribute less than 10% of the total sputtered flux. Furthermore, the relative contribution of heavy ions decreases during ICME ejecta driven by strong increases in the sputtered contribution from alphas. Overall, solar wind heavy ions play an important, although not dominant, role in sputtering of the Moon’s surface.

Mercury is exposed to various space weathering processes, which alter the surface and cause emission of atoms and molecules into the exosphere. The exosphere thus allows remote insights into the surface composition and erosion processes, with the emission of refractory elements such as Ca and Mg typically being dominated by micrometeoroid‐impact vaporization. Here we present simulations of the enhanced sputtering by ion impacts during interplanetary coronal mass ejections (ICMEs), predicting an exosphere regime change for strong space weather events. Due to higher ion fluxes and erosion of Mercury's dayside magnetosphere, the sputter source increases by up to a factor of 380. As a result, sputtering becomes the dominant release process for Ca and Mg, temporarily causing a reconfiguration from a dawn‐centered, MIV‐dominated exosphere to a dayside‐centered, sputter‐dominated exosphere. Our study thus highlights and quantifies the variability in Mercury's space environment and the system's strong coupling to space weather effects.

Electron radiation belts, reservoirs of stably trapped energetic electrons hazardous to space technology, are a prevalent feature of most magnetized planetary environments. However, their existence in the compact magnetosphere of Mercury has been a subject of debate for decades. The presence of energetic electrons was first identified by Mariner 10’s observations in the 1970s but was subsequently questioned by MESSENGER’s limited energetic particle measurements 40 years later. Here we integrate MESSENGER’s more sensitive indirect energetic electron measurements with new analysis techniques, particle simulations and theory to discover a structured radiation belt with a variable morphology subject to solar wind activity. This radiation belt can persist for several Earth days and, thus, seems to be quasi-permanent under weak solar wind forcing. However, it becomes unstable under strong driving as electron drift orbits bifurcate about the magnetic equator within the dayside magnetosphere and particles are rapidly lost. Our results establish that even compact magnetospheres can host radiation belts and, furthermore, that Mercury is a natural laboratory for radiation-belt physics usually inaccessible at Earth: one of an extreme, rapidly driven state.

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).

Solar wind ions that reach the surface of Mercury cause surface erosion via sputtering. Among several other processes, sputtering can then contribute to the planet’s exosphere [1]. For the refractory elements Ca and Mg, NASA’s MESSENGER spacecraft provided extensive observations of their occurrence [2, 3]. However, the dawn-centered characteristics and yearly periodicity of the exospheric emission have been attributed to micrometeoroid impact vaporization (MIV) with no significant contribution from sputtering under regular SW conditions [4-6].Here we present a simulation study that predicts a significant change in dominant sources of Ca and Mg emission during strong interplanetary coronal mass ejections (ICMEs). We use the AMITIS hybrid plasma code to calculate surface precipitation of solar wind H+ and He++ ions during such strong solar storms [7]. We specifically focus on disappearing dayside magnetosphere (DDM) events [8], where the planet’s magnetic field cannot stand off the solar wind pressure and exposure of Mercury’s surface increases significantly. Based on the ion impact results from Amitis, we calculate the sputter emission using regolith sputter yields simulated with SDTrimSP-3D and model the formation of the sputtered Ca and Mg exosphere in the Harrah’s direct simulation Monte Carlo code (DSMC) [9,10].Considering the ranges of ICME intensities that have been observed by MESSENGER at Mercury [11], our simulations clearly show that strong ICMEs allow full access of the solar wind to Mercury’s surface and increase both the surface precipitation and the sputtering by more than two orders of magnitude. Above a dynamic pressure of around 100 nPa, our calculated sputter rates exceed the surface emission that was reported from MESSENGER and attributed to MIV.Our DSMC simulations further show that the increased surface erosion rates are also expected to significantly affect the distribution of refractories in the exosphere. Contrary to the dawn-centered MIV emission, the sputtering adds an additional noon-centered emission feature that overwhelms the otherwise preferential dawn emission. The Ca and Mg exospheres show differences in their detailed response to the ICME impact due to the highly efficient photoionization of Ca, which limits the transport of the atoms across the exosphere.ICME-induced exosphere enhancements as modeled here are expected to be temporary and to last a few hours, due to the highest solar wind pressures typically occurring in the short shock phases of an ICME and the exospheric lifetime of Ca and Mg of similar duration. Such features have not been reported so far, but should be observable similar to exospheric enhancements following meteoroid impacts reported by MESSENGER [12]. Future measurements of such events with BepiColombo and further analysis of the MESSENGER exosphere observations will help understand the response of Mercury’s system to ICME impacts and the role of solar wind sputtering in the exosphere formation. References[1] P. Wurz, et al., Space Science Reviews 218.3 (2022), 10.[2] M.H. Burger, et al. Icarus 238 (2014), 51.[3] A.W. Merkel, et al., Icarus 281 (2017), 46.[4] P. Pokorný et al., The Astrophysical Journal 863.1 (2018), 31.[5] J.-Y. Chaufray, et al., Icarus 384 (2022), 115081.[6] P.S. Szabo, et al., Journal of Geophysical Research: Planets 130 (2025), e2025JE009058.[7] S. Fatemi, et al., Journal of Physics: Conference Series 837.1 (2017).[8] J.A. Slavin, et al., Journal of Geophysical Research: Space Physics 124 (2019), 6613.[9] U. Von Toussaint, et al., Physica Scripta, 2017.T170 (2017), 014056.[10] S.R. Carberry Mogan, et al., Journal of Geophysical Research: Planets 127 (2022), e2022JE007294.[11] R. Winslow, et al., The Astrophysical Journal 889 (2020), 184.[12] T.A. Cassidy, et al., The Planetary Science Journal 2 (2021), 175.

Even though it is the closest planet to the Sun, Mercury’s polar craters harbor large volatile deposits [1]. Observations of radar brightness [2], neutron absorption [3], and optical reflection [4] have led to the conclusion that the deposits are largely made up of water ice. Ultimately, a total water ice mass of 1013 – 1015 kg has been estimated [3].Due to the low number of craters on the icy surfaces, it has been suggested that the ice has been deposited in the past few 100 million years [4], for example by a single cometary impact [5]. However, recent analysis of the ages of ice-filled craters indicates a significant correlation of crater age and water ice amount [6]. Such a behavior would rather point to a contribution from a steadily ongoing process. Previous work has discussed dust impacts and solar wind delivery as potential sources [7, 8]. Jones et al. [8] developed a model to quantify the accumulation of water in Mercury’s permanently shadowed regions (PSRs) following solar-wind H+ implantation. They estimate a small contribution on the order of 1013 kg to today’s deposits over the past billion years, assuming identical delivery rates as today.However, observations of other stars have allowed us to constrain the behavior of the younger Sun and it has been established that it was much more active in the past. In particular, the mass loss rate via solar wind emission was orders of magnitude higher [9], which potentially provides a much-increased source of solar-wind implanted water. At the same time, the Sun’s Lyman alpha emission was much brighter [10], which increases the H2O photodissociation rate and limits the efficiency of migration into Mercury’s PSRs. Several studies have also suggested that Mercury’s dynamo history has evolved significantly in the past, likely requiring a much stronger early dynamo to explain today’s remanent surface magnetization [11].In this study, we aim to quantify the potential solar wind contribution to Mercury’s H2O deposits by accounting for these different aspects. We perform hybrid simulations using the Amitis code [12] for a more intense young solar wind, considering a range of different dynamo strengths. Based on the results of solar wind H+ implantation, we calculate rates of water delivery into Mercury’s PSRs by accounting for the increased Lyman alpha strength in the past (up to a factor of 3).When calculating the estimated total water amount accumulated over the past 3.7 billion years, we find that the result significantly depends on the assumptions regarding Mercury’s dynamo history. A gradual decrease of dynamo strength to today’s value typically leads to similar estimates as the original calculation from Jones et al. (2020) of a minor contribution of 1013 kg. However, if there were extensive time periods with a dynamo comparable to today’s strength or no dynamo at all, the calculated water deposits increase up to several 1014 kg, which would represent an important contribution to today’s ice deposits.Our study thus further supports that solar wind implantation is a possible source of water ice in Mercury’s PSRs. The planet’s volatile deposits might also be closely connected to the history of the planet’s dynamo. Further characterization of Mercury’s PSRs with BepiColombo, as well as more detailed modeling and additional characterization of Mercury’s interior and its dynamo history will allow us to better understand the accumulation of water ice through solar-wind implantation. References[1] A.N. Deutsch, et al., Icarus 280 (2016), 158.[2] J.K. Harmon, et al., Icarus 211 (2011), 37.[3] D.J. Lawrence, et al., Science 339 (2013), 292.[4] D.A. Paige, et al., Science 339 (2013), 300.[5] C.M. Ernst, et al., Journal of Geophysical Research: Planets 123 (2018), 2628.[6] S. Bertoli, et al., Planetary and Space Science 264 (2025), 106150.[7] K. Frantseva, et al., Icarus 383 (2022), 114980.[8] B.M. Jones, et al., The Astrophysical Journal Letters 891 (2020), L43.[9] V.S. Airapetian, et al., The Astrophysical Journal 916 (2021), 96.[10] I. Ribas, Proceedings of the International Astronomical Union 5 (2009), 3.[11] I.S. Narrett, et al., EPSC-DPS2025-1118.[12] S. Fatemi, et al., Journal of Physics: Conference Series 837.1 (2017).

As a body without a thick atmosphere or global magnetic field, the Moon is directly exposed to incident ion fluxes from the solar wind and the terrestrial magnetosphere. As ions strike the lunar surface, they sputter individual regolith atoms, thereby contributing one component of the lunar exosphere. Previous work has studied the lunar sputtered exosphere during both nominal solar wind conditions and extreme space-weather activity such as coronal mass ejections (CMEs). These studies have suggested greater-than-tenfold increases in the lunar exospheric density during CME events due to elevated sputtering rates. Here, we analyze the effects that CMEs may have on the production and equilibrium of the sputtered neutral exosphere at the Moon via the use of in-situ solar wind measurements during CMEs. In particular, we investigate the role that heavy, highly charged minor ions in the solar wind may play during CME impacts at the Moon.For this purpose, we use measurements of the plasma moments and heavy ion composition during CMEs observed by the ACE/SWICS instrument at Sun-Earth L1 over the period of 1998–2011. We extract the solar wind flux and heavy ion composition during the event intervals listed in the publicly available “Richardson and Cane CME list” and convolve the heavy ion fluxes with appropriate sputtering yields for the lunar regolith. Generally, we find that solar wind heavy ions nominally contribute ~5% of the total sputtering yield while extreme events can reach contributions of ~20%. In no cases, however, do solar wind minor ions dominate the sputtering rates at the Moon. Finally, we discuss the implications of this work for our understanding of the full variability of the Moon’s exosphere.

We perform Low-energy Ion Scattering with He ions ranging in energy from 1 keV to 3 keV on CaSiO 3 using a commercial electrostatic detector system and determine the charge fraction of scattered ions from comparison with binary collision approximation simulations. The simulations take composition changes due to surface cleaning Xe sputtering into account. Scattered He particles are separated in the simulation into single, dual, and multiple scattering trajectories. We find that the charge fraction of single and dual scattered He is about ten times higher than the one for multiple collisions. Our results show that the charge fraction of scattered He can be reliably extracted for different scattering regimes and that for a given He incidence energy, the entire scattered He spectrum can be modeled with only few constant parameters.

The interactions between micrometeoroid impacts and solar wind on the surface of the Moon remain a major area of investigation in the space weathering field. In this study, we first irradiated Fe-bearing enstatite [(Mg,Fe) ₂ Si ₂ O ₆ ] (1.50 at% Fe) with a nanosecond-pulse UV excimer laser ( λ = 248 nm) to simulate micrometeoroid impacts, using fluences above and below the ablation threshold to produce four different impact-processed regions. Next, we irradiated the surfaces with 4 keV He ⁺ to explore how prior impact processing changes the way solar wind ions interact with mineral surfaces. We performed comprehensive micro-scale to nano-scale characterization of the experimental products using scanning electron microscopy, scanning transmission electron microscopy, electron energy loss spectroscopy, and energy dispersive X-ray spectroscopy. In the samples irradiated above the ablation threshold, we observed the coexistence of nanophase Fe ( np Fe ⁰ ) and Fe ³⁺ -bearing glass, indicating that np Fe ⁰ formation was driven by charge disproportionation. In the sample irradiated at the highest laser fluence, we produced helium-bearing vesicular np Fe ⁰ , an enigmatic space weathering product known to occur widely on the Moon. Finally, we show that the temperature of impact melting and the phases produced by impact processing have a direct influence on the target’s sputtering behavior and overall helium capture capacity. Our findings demonstrate that micrometeoroid impacts play a critical role in modifying the crystal structure and chemistry of regolith grain surfaces and that variation in impact processing can fundamentally shift the subsequent interactions that solar wind ions have with affected surfaces.