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Mercury Exploration Assessment Group (MExAG) Meeting

February 6–8


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Day 3 Agenda

Thursday, February 8, 2024, 10:00 AM

10:00 AM
Welcome, Presentation of Preliminary Findings
Mercury's Exosphere
10:15 AM
Morrissey L. S. Huang Z. Tucker O. J. Killen R. M. Leblanc F. et al.
New Insights into the Surface Binding Energies of Elements in Plagioclase Feldspars and the Effect on Ion Sputtering [#6013]
The exospheres of Mercury and the Moon are formed, in part, by solar wind (SW) ion sputtering from the mineral surfaces of these bodies. The sputtering yield for the different elements in these minerals depends on the surface binding energy (SBE). The importance of the SBE in understanding SW-induced sputtering has led to several researchers calling for further modelling of SBEs relevant to planetary science. We have used molecular dynamics to predict the elemental SBEs from different albite and anorthite crystalline surfaces, the endmembers of plagioclase feldspars. Our results are the first to show that the SBE is dependent on the lattice position and surface orientation, meaning multiple SBEs are possible for a given element within a mineral. We then used SDTrimSP to quantify the effect of these SBEs on the sputtering yield, energy distribution, and escape rate. Future exosphere models should consider multiple SBEs for each element type instead of single values.
10:23 AM
Killen R. M. McLain J. L. Burger M. H. Vervack R. J. Tucker O. J. et al.
The Effect of Electron-Stimulated Desorption on Mercury's Exosphere [#6008]
We have simulated the effects of an electron-stimulated desorption (ESD) source for sodium in our models for Mercury's exosphere in addition to those previously used: photon-stimulated desorption (PSD), ion sputter and micrometeoroid vaporization. We use the ESD cross-section measurements of McLain et al. (2011) for Na-bearing silicate glass desorbing Na+. Additional data obtained by one of us (McLain) showing the ESD yield as a function of electron energy to 600 eV has been used to quantify the yield at Mercury. We use the estimates of electron flux and energy in the cusp region from Lindsay et al. (2011), who found fluxes on the order of 10^9 electrons/ cm^2/ s with energies of hundreds of eV, based on X-ray fluxes seen by the X-Ray Spectrometer (XRS) onboard the MESSENGER spacecraft. We incorporated this source into the Monte Carlo code "nexoclom" (Burger, 2023) to simulate the exosphere, using the electron footprint and flux from Lindsay et al. (2011) and Lavorenti et al. (2023).
10:31 AM
Moroni M. Milillo A. Mura A. Plainaki C. Mangano V. et al.
Seasonal Variation of Ca and Ca-Bearing Molecules in Mercury's Exosphere as a Product of Micro-Meteoroids and Comet Stream Particles Impact [#6029]
We use the exospheric Monte Carlo model by Mura et al. (2007) to simulate the spatial distribution of the Ca-bearing molecule and atomic Ca Mercury’s exospheres generated by the Micro-Meteoroids Impact Vaporization process. Since exact temperature, photolysis lifetimes of the produced molecules, and excess energy during photolysis processes are still not well constrained by observations, we investigate different model assumptions. The theoretical calculations agree better with observations at shorter photolysis lifetimes and higher excess energy of Ca atoms. In that case, we show the presence of two Ca components: energetic Ca component, more intense at high altitudes, and the contribution of a low energy component in the post-dawn low altitude. We investigate the possible contribution due to the comet 2P/Encke for explaining the excess Ca emission at specific orbit positions, but the simulation results show an underestimation compared to the observations.
10:39 AM
Mangano V. Villard E. Zender J. Bunn H. Spezzano S. et al.
New Mercury Earth-Based Observations with ALMA [#6036]
The Earth-based studies of the exosphere of Mercury up to now were performed mainly through solar telescopes and in the sodium D line, providing information on the morphology and dynamics of the exosphere and its interactions with the solar wind, IMF, meteorites etc., and datasets over more than 2 decades. MESSENGER has largely improved our knowledge of both the exosphere and the surface composition that is the main source of the exospheric refilling. New species have been identified for the first time. New modeling efforts reinforced the idea of the existence of exospheric molecules, precursors of the atomic species already observed. More will come from BepiColombo in 2026, but there is still much that can be done now from the ground. Here, we report our idea of new observations in the sub-mm range to study the exosphere of Mercury in its molecular components. A feasibility study with APEX and the Atacama Large Millimeter/submillimeter Array (ALMA) of 12 m antennas is ongoing.
10:47 AM
Mercury's Surface 2
10:53 AM
Loveless S. R. Klimczak C.
A Comparison of Thrust Fault Systems Underlying Wrinkle Ridges and Lobate Scarps on Mercury [#6010]
Mercury’s shortening landforms are formed by thrust fault displacement-gradient folds. Most of them have been grouped into two categories: lobate scarps (LS) and wrinkle ridges (WR). Using the 2D Move on Fault Fault-Bend-Fold algorithm in the Petroleum Experts MOVE software, we produced balanced cross-sections of fault and fold geometries for 25 LS and 25 WR to identify structural differences between the two landform types. We find that individual faults under LS have steeper dips, penetrate to greater depths, and accommodate more slip than WR, a result consistent with previous studies. But the morphology of many WR requires more than one fault, either as pop-up structures or imbricate fans, making the shortening strain for WR and LS nearly at -0.8%. This implies that these landforms may have originated from one and the same process, i.e., global contraction, as opposed to previously suggested separate origins from global contraction and subsidence.
11:01 AM
Clark J. D. Bernhardt H. Preusker F. Klimczak C. Banks M. E. et al.
Constraining the Ages of Shortening Landforms in Mercury’s Western Hemisphere [#6043]
The temporal trends of the geographic, morphometric, and structural parameters of shortening structures on Mercury are key to better understanding the planet’s history of contraction, tidal despinning, and lithologic/rheologic variation in its current crust. Employing the best resolution image mosaics (166 m/px) and a novel, global 222 m/px stereo-DTM, we are producing a new global map and parameter catalog of shortening structures on Mercury. To assess the temporal trends of scarp parameters and to better understand global/regional stress, we will constrain the maximum ages of shortening structures by deriving absolute model ages for their host units wherever possible (currently expecting ~100 dateable host units). Our database will enable us to discern spatial and temporal trends within the measured parameters. Although this ongoing work will cover the entire planet, we present here the preliminary results of a selection of shortening structures in the western hemisphere of Mercury.
11:09 AM
11:13 AM
Caminiti E. Lantz C. Besse S. Brunetto R. Carli C. et al.
Effects of Ions Irradiation on Mercury Terrestrial Analogues in the Visible to Mid-Infrared [#6018]
The surface of Mercury is subject to space weathering that complicates remote sensing data analysis. We present an experimental study to provide a better constraint on spectral changes induced by solar wind. We simulated for the first time solar wind ion irradiation on terrestrial Mercury’s analogs (boninite, basaltic komatiite, and komatiite) in the VMIR using strong energy and fluences (20 kev He+ with fluences up to 5.1017 ions/cm2). Several spectral modifications induced by irradiation are observed. The VNIR samples show an exponential darkening, a reddening, and a flattening of spectra. Above a certain irradiation dose, the darkening reaches a steady state, while the reddening and flattening do not show any clear trend. In the MIR, we observe a red shift in Restrahlen bands. The Christiansen feature is red or blue shifted according to the irradiation dose. This study provides ground-truth data for the future BepiColombo observations, especially the SIMBIO-SYS and MERTIS instruments.
11:21 AM
Deutsch A. N. Neumann G. A. Kreslavsky M. A. Pokorny P. Martinez Camacho J. M. et al.
Temperature-Dependent Space Weathering on Mercury [#6004]
It has been predicted that high equatorial temperatures on Mercury could promote thermal annealing by Ostwald ripening, where nanophase metal particles (a product of space weathering) coalesce and grow into larger microphase particles, resulting in lower albedo. We tested this prediction by studying the correlation between albedo and temperature in 1° spatial bins using newly recalibrated 1064-nm reflectance data acquired by the Mercury Laser Altimeter (MLA), low-incidence angle data from the Mercury Dual Imaging System (MDIS), and newly modeled maximum surface temperatures (MSTs). After accounting for local geology and latitude, we found longitudinal reflectance variations correlate with temperature variations rather than variations in micrometeoroid or solar wind fluxes. We will place these new findings in the context of the BepiColombo mission with measurement and targeting opportunities for future exploration of thermal-regime-dependent space weathering on Mercury.
11:29 AM
Frizzell K. R. Nittler L. R.
Extracting Potassium Abundances on Mercury from MESSENGER X-ray Spectrometer Data [#6024]
Potassium is a moderately volatile element that can be used to trace refractory/volatile fractionation processes in the early solar system as well as geochemical and thermal fractionation processes in planetary crusts. The isotope 40K can be measured in orbit from the decay of gamma rays in the near-subsurface. MESSENGER’s gamma-ray instrument (GRS) unfortunately failed after just one year, but the X-Ray Spectrometer (XRS) continued collecting data for the full 4 years of the mission duration. The x-ray fluorescence line for K is not resolved from that of Ca in the XRS spectra, but simulations show that its presence affects the shape of the Ca peak and that with careful fitting, K abundances likely can be inferred from spectra that were acquired during large solar flares, where the signal to noise ratio is highest. We aim to produce an accurate map of K abundance across Mercury’s surface with greatly enhanced spatial coverage and resolution (compared to GRS) using XRS data.
11:37 AM
11:43 AM
Mercury's Interior
11:53 AM
Del Vecchio E. Gargiulo A. M. Petricca F. Genova A. Andolfo S. et al.
First Analysis of BepiColombo Radio Science and Accelerometer Data Acquired During Venus and Mercury Flybys [#6040]
The ESA/JAXA BepiColombo mission is en route to Mercury to conduct multidisciplinary investigations of the innermost planet in the Solar System. By processing radio measurements in combination with onboard accelerometer readings, a precise spacecraft navigation will be achieved to study Mercury’s gravity and to support the operations of other instruments. We analyzed radio science and accelerometer data acquired during the second Venus flyby and Mercury flybys to assess the performance of both instruments. The analysis of the combined dataset is carried out through a precise orbit determination process (POD) and by comparing modeled and measured accelerations. To enhance the trajectory reconstruction and gravity modeling, we developed a least-squares filter that enables to jointly invert BepiColombo's radio measurements with the whole MESSENGER dataset. This method leads to an updated solution of Mercury's gravity field, enabling a precise local definition of the gravitational anomalies.
12:01 PM
Davies C. Pommier A. Greenwood S. Wilson A.
Investigation of the Thermal and Magnetic Evolution of Mercury: Effect of an FeS Layer Atop the Core [#6034]
An FeS shell around Mercury's core has been suggested by several studies but is still debated. If present, this layer may affect the global magnetic field and growth of a solid inner core. We developed a parameterized model of coupled core-mantle thermal and magnetic evolution to test the effect of an immiscible liquid FeS layer atop the core. We first ran a large suite of cases with no layer and required successful models to reproduce constraints on Mercury’s dynamo, the total global contraction, and the present-day crustal thickness. Successful models have a Si concentration in the core > 13 wt.%, a present-day inner core radius of 1000-1200 km, and a 500-800 km thick stable thermal layer atop the core. Including a 5-100 km thick FeS layer in these models has little effect on thermal evolution, energetics, and present-day core structure. The main effect of a thick FeS layer is to lower the entropy production of the dynamo following inner core formation by a factor of about 1.5.
12:09 PM
Dunnigan A. H. Liu D. Steinbrügge G. Rivoldini A. Dumberry M. et al.
Constraining Mercury’s Interior with Fe-S-Si Core Models and Snow Zones [#6039]
We employ interior structure models with Fe-S-Si core compositions to better understand Mercury’s interior. In order to further constrain the models after exploring their parameter spaces, we consider iron snow as a potential driver of Mercury’s weak dynamo. Towards this end, we determine which models have no iron snow zones, which have snow zones that are not compatible with magnetic field generation (i.e., snow only occurs directly above the inner core), and which could have compositionally-driven dynamo action (i.e., snow occurs in a mid-core layer). We further identify which models are consistent with Mercury’s thermal evolution. A complementary Monte Carlo simulation of how iron snow-containing models vary as a function of moment of inertia emphasizes the need for convergence on geodetic constraints. The European Space Agency’s BepiColombo mission is expected to improve these measurements and reduce their error for a more accurate understanding of Mercury’s interior structure.
12:17 PM
12:23 PM
Fischer E. L. Parman S. W.
Geochemical and Geodetic Constraints on the Initial Size of Mercury [#6017]
Mercury’s mantle is depleted in Si by ~25% compared to an average enstatite chondrite. Assuming all of the missing Si was lost to the core through equilibrium partitioning, and with Mercury’s current mantle and core sizes, mass balance yields 2-3 wt% Si in an Fe-Si core. This implies a mantle fO2 of ~IW-4. However, geodetic estimates of Si in the core are substantially higher (up to ~15 wt%). This larger amount of Si could be explained by a larger initial mantle volume. For example, a core Si content of 15 wt% implies an initial Mercury (Mi) ~2 times more massive than it is today, an fO2 of ~IW-4.8, and a metal/silicate mass fraction comparable to other terrestrial planets. This fits within Mi values predicted by dynamical simulations of 1.5-4 times the current mass. The Mi estimates depend strongly on fO2. Fugacities below IW-5.4 imply an Mi greater than 6 times current mass (outside the dynamical estimates) and metal/silicate ratios below the other terrestrial planets.
12:31 PM
Anzures B. A. McCubbin F. M. Vander Kaaden K. E. Iacovino K. Prissel K. et al.
Understanding Mercury’s Magmatic History: Geochemical Affinity, Compatibility, and Volatility Changes Due to Reduction [#6023]
Mercury’s depletion in O and abundance of S substituting in as S2- has fundamental effects on the physicochemical properties of minerals and melts because O2- is the dominant rock-forming anion. These changes exert a first-order effect on planetary body evolution, including the initial distribution of elements, crystallization paths, stability of phases, and mineral chemistry. Experimental and empirical data have revealed that most nominally lithophile elements become more chalcophile and/or siderophile at reducing conditions, which can be visualized through a re-interpreted Goldschmidt classification table prepared for McCubbin & Anzures’s Treatises of Geochemistry 3rd edition chapter under review. Additionally, the S bonding environment influences Mercurian phase equilibria by decreasing the stability of forsterite and anorthite while increasing the stability of enstatite, silica, and albite through the re-projection of silicate ternaries.
12:39 PM
12:45 PM
Meeting Wrap-up Discussion, Including Preliminary Findings

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