Mercury Science Goals, Objectives and Investigations - Version 1.0

Mercury Science Goals, Objectives, and Investigations Mercury Exploration Assessment Group (MExAG) Version 1.0 Version released on September 3rd, 2025 The MExAG Science Goals, Objectives, and Investigations document is the first of three MExAG goals documents: Science, Technology, and Community. This document is recommended to be cited as: “The Mercury Exploration Assessment Group Science Goals, Objectives, and Investigations, MExAG (2025). Version 1.0, XX pp., <insert LPI Link>”. The purpose of this document is to provide an overview of the science goals of the MExAG community for understanding and exploring Mercury. The intended audiences for the document are: (1)NASA, as a means for communicating the community-driven goals, and (2) the planetary science community, as a mechanism of sharing knowledge about important themes for understanding and exploring Mercury. This document will be updated every three years, to maintain relevance and utility. Community members wishing to provide input can do so at any time by emailing me[email protected]. The science goals were devised by the MExAG Steering Committee after community discussions during the February 2021 MExAG Annual Meeting. This document was conceptualized and compiled between 2022 and 2025 by the MExAG Goals Working Group, including contributions from many members of the MExAG and Mercury science community. The text was edited by the MExAG Science Goals Working group and MExAG steering committee released for comment by the community in January 2025. Community comments were incorporated and the final document (Version 1.0) was released on August 1, 2025. Current and Previous MExAG Goals Working Group Members: Ryan Dewey, Gina DiBraccio, Carolyn Ernst, Mallory Kinczyk, Christian Klimczak, Stephen Parman, Alain Plattner, Jim Raines, Ron Vervack, Jack Wright Contributors: Sae Aizawa, Brendan Anzures, David Baker, Jeff Balcerski, Océane Barraud, Sébastien Besse, Paul Byrne, Nancy Chabot, Clark Chapman, Matt Clement, Kelsey Crane, Gabriele Cremonese, Brett Denevi, Ariel Deutsch, Ryan Dewey, Mathieu Dumberry, Nick Dygert, Denton Ebel, Carolyn Ernst, Caleb Fassett, Sander Goossens, Steven Hauck, Robert Herrick, Daniel Heyner, Lon Hood, Dana Hurley, Suzanne Imber, Peter James, Xianzhe Jia, Catherine Johnson, Rosemary Killen, Mallory Kinczyk, Misha Kreslavsky, Laura Lark, David Lawrence, Francois Leblanc, Valeria Mangano, Simone Marchi, Elena Martellato, Erwan Mazarico, Tim McCoy, Jeff Morgenthaler, Liam Morrissey, Megan Mouser, Larry Nittler, Stephen Parman, Michael Phillips, Alain Plattner, Gangkai Poh, Petr Pokorný, Anne Pommier, Parvathy Prem, Jim Raines, Edgard Rivera-Valentin, Dave Rothery, Daniel Savin, Carl Schmidt, Jim Slavin, Gregor Steinbrügge, Weijie Sun, Nicola Tosi, Ron Vervack, Thomas Watters, Shoshana Weider, Jack Wright 2 MExAG Science Goals Overview The document is structured into goals and objectives, with objectives broken into several more-specific sub-objectives. The goals are the high-level outcomes to be achieved and should remain fairly stable over time. The objectives identify strategies to achieve the goals. The objectives are intended to be broad, so that the ways in which they can be conducted are not overly constrained. Once the Technology Goals Document has been published, the intention is to revise the Science Goals document to include specific investigations. These investigations will describe specific missions or studies that could be undertaken to achieve an objective. Goals and objectives are listed in the outline below. Goal 1 is focused on how Mercury can be used to address scientific questions about our solar system and beyond. It demonstrates how Mercury is a keystone for understanding the origin and evolution of our solar system, the planets within it, and the processes that affect them. Goals 2–4 are more directly focused on understanding the evolution of Mercury itself. These goals include all aspects of Mercury’s formation and evolution, and are intended to be a comprehensive resource for proposers. 3 MExAG Science Goals Outline Document Goals Objectives Science 1) Establish Mercury’s context in our solar system and beyond 1.1) Use Mercury’s uniqueness as a compositional and physical end-member to constrain the formation of the solar system 1.2) Apply our knowledge of Mercury to understand other planetary bodies and vice versa 1.3) Inform our understanding of rocky exoplanets 2) Understand the origins of Mercury, from accretion to solidification 2.1) Investigate compositional building blocks 2.2) Explore formation scenarios 2.3) Understand global differentiation/evolution 3) Characterize Mercury’s evolution since solidification 3.1) Study past geological processes 3.2) Understand processes behind the chemical and mineralogical diversity of the crust 3.3) Investigate the origin and evolution of the polar deposits 3.4) Explore the history of the intrinsic magnetic field 3.5) Infer long-term effects of variations in exospheric and magnetospheric source and loss mechanisms 4) Investigate processes currently ongoing at Mercury 4.1) Characterize present-day geologic activity 4.2) Investigate magnetospheric dynamics 4.3) Understand exospheric and magnetospheric source and loss processes 4 MExAG Science Goals Table of contents (click on topic to go to text) Outline 4 Executive Summary 8 Abbreviations 9 1. Establish Mercury’s context in our solar system and beyond 10 1.1 Use Mercury’s uniqueness as a compositional and physical end-member to constrain the formation of the solar system 10 a) Cosmochemistry: Is Mercury the missing piece? 10 b) Dynamics of the protoplanetary disk: Why is Mercury so small and so dense? 10 1.2 Apply our knowledge of Mercury to understand other planetary bodies and vice versa 10 a) Observed surface geology 10 b) Impact cratering record and flux 11 c) Polar deposits 12 d) Exospheric and magnetospheric processes 12 e) Core structure and evolution 13 1.3 Inform our understanding of rocky exoplanets 14 a) Mercury as an exoplanet 14 b) Exoplanet exospheres and magnetospheres 14 2. Understand the origins of Mercury, from accretion to solidification 15 2.1 Investigate compositional building blocks 15 a) Understanding meteoritic contributions 15 b) Origin and retention of volatiles during formation 16 c) Constraints on bulk composition from surface chemistry 16 2.2 Explore formation scenarios 17 a) Dynamics of early accretion of materials (dust to planetesimals) 17 b) Origin scenarios (planetesimal collisions, orbit migration and large core/mantle ratio) 17 c) Volatile record (before and after gas loss from the inner disk) 18 2.3 Understand global differentiation/evolution 18 a) Initial differentiation (core and mantle) 18 b) Solidification of the magma ocean and production of a primary crust 19 c) Core structure and dynamics during magma ocean solidification 20 3. Characterize Mercury’s evolution since solidification 20 3.1 Study past geological processes 20 5 MExAG Science Goals a) Cratering record 20 b) Timing and styles of volcanism 21 c) Timing and styles of tectonics 22 d) Relationships between volcanism, tectonism, volatiles 23 e) Long-wavelength topography 23 f) Structure of the crust and lithosphere 24 g) Heat flow 24 h) Regolith formation and mixing 25 3.2 Understand processes behind the chemical and mineralogical diversity of the crust 25 a) Geochemical variations 25 b) Variations in mineralogy 26 c) Interior volatiles 27 d) Low-reflectance material 27 e) Low crustal iron 27 3.3 Investigate the origin and evolution of the polar deposits 28 a) Distribution and volume 28 b) Composition 28 c) Volatile transport 29 3.4 Explore the history of the intrinsic magnetic field 30 a) Remanent magnetism 30 b) Dipole field strength 31 3.5 Infer long-term effects of variations in exospheric source and loss mechanisms 31 a) Origin of the exosphere 31 b) Space weathering processes and rates 32 4. Investigate processes currently ongoing at Mercury 33 4.1 Characterize present-day geologic activity 33 a) Present-day cratering rate 33 b) Recent/current volcanic activity 33 c) Recent/current tectonic activity 34 d) Hollows 35 e) Polar deposits 35 4.2 Investigate magnetospheric dynamics 36 a) Generation of the current magnetic field 36 b) Magnetic reconnection 37 c) Induction effects 38 6 MExAG Science Goals d) Dawn-dusk asymmetries 38 e) Magnetospheric currents 39 f) Flux rope formation 40 g) Effect of solar events on the system 40 4.3 Understand exospheric and magnetospheric source and loss processes 41 a) Magnetospheric ions 41 b) Distributions of neutral and ionized species and their temporal / spatial variations 42 5. References 44 7 MExAG Science Goals Executive Summary The ongoing advances in cosmochemistry and planet formation dynamics places Mercury firmly at the center of understanding how our solar system formed. Its composition is likely a key geochemical end-member in the solar system, but it is as yet unsampled. Measuring its composition would not only revolutionize our understanding of Mercury’s formation, but also shed light on the makeup of the Earth and other planets, the origin of life-essential volatiles in the inner solar system, and how planets accreted in the early protoplanetary disk. As the closest planet to the Sun, Mercury provides a unique vantage point to study a range of key planetary processes, including: accretion in the protoplanetary disk, past and current cratering rates, space weathering, and volatile delivery to the inner planets. Its proximity to the Sun also provides a unique window into many exoplanets, a substantial fraction of which orbit as close, or closer, to their host stars than Mercury. Due to its proximity to the Sun, Mercury formed in an oxygen-poor, carbon-rich environment, distinct from all other terrestrial planets. Many exoplanets also formed in such low-oxygen conditions, making Mercury uniquely suited to understanding their formation and evolution. Mercury is truly an ‘exoplanet in our backyard’. The large (relative) size of Mercury’s core is also unique within the solar system. The core produces a global magnetic field that leads to complex interactions between the Sun and the planet’s surface. This interaction provides insights into the dynamics of planetary magnetospheres, exospheres, and space weathering. Surprisingly, there is thought to be more water ice on Mercury than on the Moon. As with the Moon, it has been found in permanently shadowed regions of craters at Mercury’s poles, but in Mercury’s case, there are actually large areas of exposed ice. How and when this ice formed is unknown, but it is clearly a unique record of volatile delivery and deposition in the inner solar system. By advancing the understanding of Mercury, key contributions to many of the Priority Science Questions identified in the National Academies Origins, Worlds, and Life Decadal Survey* will be made, including: Q1) Evolution of the protoplanetary disk, Q3) Origin of the Earth and inner solar system bodies, Q4) Impacts and dynamics, Q5) Solid body interiors and surfaces, Q6) Solid body atmosphere, exospheres, magnetospheres and climate evolution, and Q12) Exoplanets. * National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. https://doi.org/10.17226/26522. 8 MExAG Science Goals Abbreviations CSFD - crater size frequency distribution CMB - core mantle boundary ESD - electron stimulated desorption EUV - extreme ultraviolet FAC - field-aligned current fO2 - oxygen fugacity FR - flux rope fS - sulfur fugacity FTE - flux transfer event GRAIL - Gravity Recovery And Interior Laboratory (mission) HMR - high magnesium region ICME - interplanetary coronal mass ejection IMF - interplanetary magnetic field IW - Iron-wüstite oxygen buffer IR - infrared JWST - James Webb Space Telescope KPLO - Korea Pathfinder Lunar Orbiter (mission) LCROSS - Lunar CRater Observation and Sensing Satellite (mission) LRM - low-reflectance material MASCS - Mercury Atmospheric and Surface Composition Spectrometer (instrument) MDIS - Mercury Dual Imaging System (instrument) MDM - Mercury Dust Monitor (instrument) MERTIS - MErcury Radiometer and Thermal Infrared Spectrometer (instrument) Mio - Mercury Magnetospheric Orbiter (mission) MORE - Mercury Orbiter Radio-science Experiment (instrument) MPO - Mercury Planetary Orbiter (spacecraft) MESSENGER - MErcury Surface, Space ENvironment, GEochemistry and Ranging (mission) NEO - near-Earth orbit PSD - photon stimulated desorption PSR - permanently shadowed region SEP - solar energetic particle SIMBIO-SYS - Spectrometer and Imaging for MPO BepiColombo Integrated Observatory SYStem (instrument) UVVS - Ultraviolet and Visible Spectrometer (instrument) VISTIR - visible-to-thermal infrared VNIR - visible-to-near-Infrared XRF - X-ray fluorescence XRS - X-Ray Spectrometer (instrument) 9 MExAG Science Goals b) Origin and retention of volatiles during formation To explain many of Mercury’s unusual features, including its high bulk-Fe content, many models invoke a high-temperature formation for the planet, and therefore predict a volatile-depleted composition. MESSENGER results demonstrated, however, that Mercury appears not to be volatile depleted. For example, its K/Th ratio (volatile element/non-volatile element) is the same as that of Earth and Mars. In addition, Mercury’s surface is rich in S, Na, and Cl, and pyroclastic deposits (driven by volatiles) abound. How Mercury retained its volatiles while forming so close to the Sun is not understood. It may be due to changing geochemical behavior under reducing conditions where nominally lithophile elements become more chalcophile and siderophile (McCubbin et al. 2012; Wohlers and Wood 2015; Boujibar et al. 2019; Pirotte et al. 2023; McCubbin and Anzures 2025). Reduced enstatite chondrites are also enriched in moderately volatile lithophiles such as Na and K (Fogel et al. 1989; Keil 2010; Weisberg and Kimura 2012; Wilbur et al. 2022; Hammouda et al. 2022). Better analysis of moderately volatile elements on Mercury’s surface, as well as more detailed studies of volatiles in reduced meteorites, will provide insight into how the innermost planet retained its volatile-rich surface. The speciation of volatile elements during Mercury’s formation would have depended on conditions in the early hot inner disk. Modeling and experiments on volatile partitioning and speciation at relevant conditions would help to determine their chalcophile/lithophile character and how refractory their host solids are as a function of fO2. Preliminary calculations indicate that at least some elements (S, Cl, K) become more refractory in solar-system vapor enriched in carbon (low fO2) (Ebel and Sack, 2013). A landed mission with the ability to analyze volatile element concentrations in-situ would provide invaluable insight into the building blocks of Mercury. Additionally, a sample return mission would allow detailed isotopic analysis of volatile elements as well as analysis of volatiles in polycrystalline material. c) Constraints on bulk composition from surface chemistry As with all of the terrestrial planets, the bulk composition of Mercury must be inferred from a combination of geophysical measurements (some of which can sample to the center of the planet), geochemical measurements (which are restricted to the surface), and laboratory experiments at relevant pressure and temperature (P, T) values and compositions (Nittler et al., 2018; Cartier and Wood, 2019). Geologic processes, including cratering and faulting, can exhume material from depth, allowing direct analysis of interior materials. However, much of our understanding of the interior composition of planets, including their overall bulk composition, comes from the analysis of erupted lavas combined with understanding of melting processes through experiments and thermodynamic modeling (Charlier et al., 2013; Namur et al., 2013, 2016a; Boujibar et al., 2014, 2025). Thus, improved analysis of the composition of Mercury’s surface, in both spatial resolution and accuracy of geochemical and mineralogical measurements, will directly lead to improved constraints on the compositional structure of the planet’s interior. Landed missions with the capability to analyze major and trace element composition, mineralogy, and isotopic composition, would provide a huge leap forward in our understanding of Mercury’s surface 16 MExAG Science Goals chemistry. Key targets might be areas where material from depth is exhumed by impact craters and areas where lava flows with a range of compositions could be accessed with a rover. Mercury’s high-magnesium region (HMR) could also be a key target for a landed mission as it may represent part of the mantle exhumed by an early, large, impact or an area covered by highly-magnesian (and sulfur-rich) lavas (Weider et al., 2015). Landed missions would also provide critical ‘ground truth’ for remotely sensed compositional and mineralogical datasets. High-pressure experiments will also be key to interpreting the surface compositional and mineralogical variations, but to date, relatively few experiments have been conducted under Mercury-relevant conditions. Experiments exploring a wider range of compositions, fO2, and volatile contents are thus required. As the magmas undoubtedly underwent cooling and fractionation as they migrated from the mantle to the surface, models of how mercurian magmas would differentiate are also needed. A range of mantle compositions, magma ocean solidification scenarios, and melting models should be explored through a combination of experiments, thermodynamic modeling, and physical modeling of melting processes. 2.2 Explore formation scenarios a) Dynamics of early accretion of materials (dust to planetesimals) Understanding the dynamics of accretion—from dust, to planetesimals, to planets—is critical to constraining the building blocks of Mercury. Most dynamical collision models predict Mercury should have a substantially larger mass and a less inclined/eccentric orbit (Nesvorny et al., 2021; Woo et al., 2022; Clement et al., 2023). The mass deficit and orbit are explained by post-formation stripping of mass by one or more large collisions or by the inward migration of Mercury from farther out in the disk. Each of these models has implications for the composition of Mercury, the timing of its formation, and the size of the other terrestrial planets. Improved dynamical models are thus needed that explore longer periods of early solar system evolution, more realistic collision physics, and the effects of mass-depleted zones within the disk. b) Origin scenarios (planetesimal collisions, orbit migration and large core/mantle ratio) Mercury’s large core, diminutive mass, and dynamically isolated orbit are rarely reproduced in terrestrial accretion models (Chambers, 2001; Clement et al., 2023; Lykawka and Ito, 2023) and have long been interpreted to imply a unique formation history (Morgan and Anders, 1980; Chapman, 1988). Mercury’s mysterious origin is even more enigmatic when viewed within the context of numerous exoplanet systems; as many as 50% of Sun-like stars possess >Earth-mass planets with orbital periods much less than ~100 days (Howard et al., 2012; Petigura et al., 2018). In light of MESSENGER’s findings, two broad classes of models aiming to explain Mercury’s large core mass fraction remain viable. In the first, the mantle of a primordially larger Mercury is stripped in one or more violent collisions with another proto-planet (a “chaotic” origin: Benz et al., 2007; Asphaug and Reufer, 2014). However, in such an event, Mercury would likely re-accrete substantial amounts of the lost material (Gladman and Coffey, 2009). Moreover, such an event would require improbably large impact velocities (Jackson et al., 2018). 17 MExAG Science Goals In the alternative class of models, Mercury forms within a reservoir of objects with Fe/Si ratios that are boosted as a result of a process that differentially affected the infant Sun’s protoplanetary disk (an “orderly” origin: Ebel and Alexander, 2011; Wurm et al., 2013; Kruss and Wurm, 2018; Johansen and Dorn, 2022). However, it remains difficult to envision how this process only affected Mercury because of the high degree of mixing known to occur during the epoch of terrestrial planet formation (Raymond et al., 2006; Woo et al., 2022). An “orderly” origin for Mercury axiomatically implies that it formed much closer to the Sun, within a different reservoir of material than Earth and Venus. In contrast, the most likely impact scenario would be one where Mercury originated in the vicinity of Earth and Venus, and was scattered out of the region after its core mass fraction was altered (Clement et al., 2019; Franco et al., 2022; Izidoro et al., 2022). Thus, improved constraints on the age of Mercury’s surface and its bulk and isotopic compositions from high-resolution surface images, in-situ lander measurements, and/or age-dating of returned samples of the oldest crust could break degeneracies between these models. It is also possible that an as-yet unidentified mercurian meteorite currently exists in our collection. Indeed, there are undifferentiated meteorites with similarly low fO2 whose origins remain murky (Ebel and Alexander, 2011; Weisberg and Kimura, 2012), and in-situ elemental and isotopic measurements could be used to establish such a link. c) Volatile record (before and after gas loss from the inner disk) Mercury is surprisingly volatile-rich both in its interior (Peplowski et al., 2012). The interior composition indicates that Mercury retained volatiles throughout its formation and solidification, despite its proximity to the Sun. Experiments on volatile solubility in mantle and core phases are needed, as well as physical models of magma ocean differentiation, to constrain how Mercury could have retained its volatiles. Analysis of samples on the surface would constrain how much of Mercury’s volatile inventory was lost, as well as their distribution in the interior, both in the silicate and metallic portions of the planet. Improved models of the early evolution of Mercury’s magnetic field are also needed to constrain the extent to which it could have shielded Mercury from volatile stripping by the Sun (Johnson et al., 2015). 2.3 Understand global differentiation/evolution a) Initial differentiation (core and mantle) Models for the formation of Mercury’s core are strongly linked to the formation of Mercury itself. If Mercury formed directly from metal-rich building blocks, then there was little mantle for the core-forming metal to react with, possibly promoting a mantle and core that are not in chemical equilibrium with each other. Whereas if Mercury were larger, then core-formation processes may have been similar to those hypothesized for Earth, where the traverse of metal through a sizable mantle induced equilibrium, at least at the start (Trønnes et al., 2019). Traces of Mercury’s initial differentiation should be recorded in the composition of Mercury’s mantle (and lavas), in their trace elements and isotopic compositions, including for the 18 MExAG Science Goals siderophile elements, as well as W, Si, and Fe isotopes (Righter, 2003; Helfrich et al., 2019). Measurements either in situ or on returned samples would thus provide valuable insights into core formation on Mercury. Experimental studies to constrain partitioning of heat-producing elements between mantle and core analogs at pressure and temperature conditions relevant to Mercury are needed to interpret the geochemical observations and provide valuable insights into core formation (Boujibar et al., 2019; Pirotte et al., 2023). Geophysical constraints on the size of the core and putative inner core, as well as the core density also inform formation models, because they help constrain the concentrations of the light elements in the core (e.g. S, Si, C) and heat loss. Improved geodynamic models are needed to understand the potential effects of Mercury’s thin mantle on core segregation. These should include the effects of low fO2 on the physical properties of silicate melts (Anzures et al., 2020; Mouser et al., 2021; Pommier et al., 2023) and metallic melts (e.g., Pommier et al., 2019; Berrada et al., 2022). b) Solidification of the magma ocean and production of a primary crust The solidification of Mercury’s magma ocean established its initial internal structure and set the stage for its subsequent evolution. Due to the high-S and low-Fe solubility in the magma ocean, and the presence of C, the solidification and differentiation of Mercury’s magma ocean is very different from that of the Moon (Boukaré et al., 2019). Constraining what phases formed from Mercury’s magma ocean, as well as their chemical and physical properties (Anzures et al., 2020; Mouser et al., 2021; Pommier et al., 2023), is key to addressing several questions about Mercury’s early evolution, including: Did graphite and/or Ca-Mg sulfide float to form a primary crust (Vander Kaaden and McCubbin, 2015)? Are sulfides abundant in the mantle (Lark et al., 2022)? Was there enough density increase during fractionation to drive subsequent overturn, as suggested for the Moon (Mouser and Dygert, 2023)? Did the initial layering produced by magma ocean solidification survive to produce the variety of magmas seen on Mercury’s surface, or was it homogenized by convection (Charlier et al., 2013)? Experiments under Mercury-relevant conditions are needed to constrain how Mercury’s magma ocean differentiated. Critical measurements include phase compositions as well as their physical properties (e.g., density, electrical and thermal conductivity, viscosity, surface energy), where it is critical to understand the behavior of silicate magmas under highly reducing conditions. Models of crystal/melt separation in the magma ocean are needed to define layering and melt retention in the mantle. Geophysical measurements (e.g., gravity, topography, shape, and heat flow) can constrain current mantle density and structure, which may help to inform magma ocean models. New measurements of lava composition of lavas (major/trace elements and isotopic) and diversity would greatly increase our understanding of how the magma ocean formed and crystallized. c) Core structure and dynamics during magma ocean solidification Once Mercury’s core formed, subsequent cooling was controlled by heat loss through the core–mantle boundary (CMB) and internal heat production. As the magma ocean cooled and solidified, the core would have responded to the changing boundary conditions, which would have affected the thermal gradient and the possible formation of a sulfide-rich layer at the CMB (Smith et al., 2012). Such a layer 19 MExAG Science Goals might have incorporated some amount of radioactive heat-producing elements (U, Th, K) in the sulfides (Boukaré et al., 2019; Pirotte et al., 2023). However, cooling models of the planet showed that this FeS layer should have negligible effects on core dynamics and the magnetic field (Davies et al., 2024). Future observations from BepiColombo will help to improve our existing models of core convection, and further our understanding of the very early stages of Mercury’s core. Better characterization of the timing and extent of planet contraction may constrain the earliest core history. The discovery of a crustal magnetic field spatially associated with Caloris Planitia may indicate magnetic remanence acquired during the Caloris impact (3.8 to 3.9 Ga) and that the core field was active early in Mercury’s history (Johnson et al., 2015). Combined geochronologic and paleomagnetic analysis of minerals (e.g., zircon) or rock fragments in regolith, either in situ or in a returned sample, could constrain the early evolution of the core. Studies of remnant magnetism in the crust and determination of its geological context could further elucidate the history of Mercury’s dynamo. 3. Characterize Mercury’s evolution since solidification 3.1 Study past geological processes a) Cratering record A true understanding of Mercury’s impact flux through time, which forms the basis for Mercury’s chronology (Spudis and Guest, 1988), requires measuring the absolute age from a known geological terrain on Mercury via in situ age dating or sample return. In the absence of such measurements, our understanding is extrapolated from the Moon, compensating for Mercury’s distinct gravity and impact velocity. Observed primary crater size frequency distributions (CSFDs) are translated into a production function and absolute chronology through assumed models of impact flux and crater scaling relationships (e.g., Spudis and Guest, 1998; Marchi et al., 2009; Le Feuvre and Wieczorek, 2011). The timing of geological events, including stratigraphic markers like Tolstoj and Caloris, can be significantly affected by what models are used (e.g., Denevi et al., 2018). Further, secondary craters may contaminate the CSFDs for diameters <10 km (Bierhaus et al., 2018). Thus, crater-based dating of the youngest or smallest units and understanding of the primary impact flux that relies on these small crater sizes requires an understanding of secondary crater production and distribution. To improve knowledge of Mercury’s impact flux through time, one aim is to study the characteristics and dynamics of small-body populations in the solar system, to understand the sources of the impactors. Further research on lunar and martian chronology, along with theoretical studies of the evolution of small-body populations in the solar system, could narrow the uncertainties concerning Mercury’s geological evolution. Detailed searches, especially by spacecraft orbiting near Mercury, for small body populations interior to Mercury’s orbit would determine the potential role of such impactors in affecting the changing rate of crater formation on the planet. Geophysical and geochemical 20 MExAG Science Goals studies could shed light on the effect that basin-forming impacts have on modifying the geological and geochemical characteristics of the surface. Another aim is to better characterize primary crater-size frequency distributions and the contribution of secondary craters to the cratering record. By investigating the morphology of small impact craters and the variations in post-formation modification processes, we may better understand the differences in the secondary crater distribution on Mercury compared with other bodies such as the Moon. Additionally, the spatial variation of crater modification will elucidate the processes that have shaped different parts of the surface. High-resolution orbital images and digital terrain models over a larger percentage of the surface, such as will be acquired from SIMBIO-SYS on the BepiColombo mission (Da Deppo et al., 2017; Cremonese et al., 2020), will allow detailed characterization of crater morphology and improve the ability to identify secondary craters. To improve Mercury-specific crater scaling models, an aim is to better understand the compositional and physical attributes of the surface (strength, bulk density, porosity, layering) and how they change laterally across the surface and as a function of depth (e.g., Genova et al., 2021). Orbital spectroscopic observations and in situ sample analysis would improve our understanding of the composition of the surface and its inferred mechanical properties. Experimental and numerical impact studies would also improve models for how target properties, higher projectile velocities, and higher target temperatures might affect crater size. b) Timing and styles of volcanism An aim is to better understand the composition, volatile content, and speciation that drove the vesiculation, eruption, and timing of volcanic deposits across Mercury. Volcanic deposits on Mercury are generally characterized as either smooth plains or explosive/pyroclastic deposits. Smooth plains occupy about a quarter of the planet's surface. The vast majority by volume of these units were emplaced as effusive volcanic products by ~3.5 billion years ago. Subsequently, global contraction appears to have suppressed large effusive volcanic episodes (Byrne et al., 2016). Some deposits have morphological and spectral characteristics of lavas, whereas others may be ponded impact melt (Denevi et al., 2013). Many smooth plains units are collocated with impact features, suggestive of a causal relationship (Byrne et al., 2018b) that needs to be further investigated. Other aims are to identify if the spectral characteristics of smooth plains have systematic temporal or spatial variations, and eruption rates and compositions. Pyroclastic deposits have been found to be long-lived, with some of them less than 1 billion years old (Thomas et al., 2014b), but there is an intriguing paucity of constructional volcanic features such as shield or cone-like volcanoes (Wright et al., 2018). Determining the timing, rate, and volatile speciation of explosive volcanic eruptions and how they continued after the cessation of major effusive activity and the onset of global contraction is an important aim. Carbon and sulfur in such deposits are depleted relative to Mercury’s surface (Weider et al., 2016), which has been interpreted as indicating that C and S were the driving volatile species, but the key species that drove the explosive volcanism still remain to be confirmed (Iacovino et al., 2023). 21 MExAG Science Goals To achieve these aims, high-resolution images and spectral data, together with detailed mapping of unit superposition and onlap relations and improved crater production models for Mercury, are required—some of which will be provided by BepiColombo (Cremonese et al., 2020; Bunce et al., 2020; Hiesinger et al., 2020). Moreover, more experiments are needed to constrain the solubility of volatiles in magmas at relevant compositions and fO2 and what gas species magmas would emit as they erupt. New thermodynamic models of low-fO2 and high-sulfur-fugacity (fS) conditions will need to be developed. Ultimately in situ geochemical and geochronological analysis of smooth plains, or returned samples from these regions and/or sites of explosive volcanism would answer open questions related to these aims. c) Timing and styles of tectonics Mercury has undergone global contraction since at least the Calorian era, evinced by the global distribution of crustal shortening features (Byrne et al., 2018a) and superposition relationships with craters of various degradation stages (Crane and Klimczak, 2017). Tectonic activity prior to that phase of global shortening, however, is not well understood. Mercury was likely much more active—volcanically, tectonically, and otherwise—early in its geologic history. It is therefore an aim to identify, describe, and model the oldest preserved (thrust) faults on the body to understand their structural style and subsurface geometry, timing, and locations. It is also an aim to understand the topographic expressions of thrusts, their tectonic patterns, and their relationship to other long-wavelength variations in the lithosphere. This is important for constraining the total tectonic history with individual contributions from contraction, changes in rotation, vertical loads on the lithosphere, and the spatial pattern of variations in thickness and strength of the crust and lithosphere. To achieve this aim, high-resolution topography measurements with global coverage are needed, as well as detailed estimates of the timing of faulting. Furthermore, model-derived geologically reasonable interpretations of subsurface geometries of thrust faults are needed to constrain estimates of Mercury’s radial contraction. These geologically reasonable subsurface geometries might also illuminate the physical properties of Mercury’s rocks and its crustal structure. It will be important to validate the model solutions of subsurface geometries at appropriate Earth analogue sites. Extensional deformation on Mercury is much more spatially restricted than shortening and is mostly found in volcanically flooded impact basins (Freed et al., 2012) associated with slumps on crater walls, or along the fold axes of major shortening structures (Man et al., 2023). However, Mercury may have undergone an early phase of planetary expansion leading to rifting and giant dike formation, similar to what is found on the Moon (Andrews-Hanna et al., 2013). To address this topic, high-resolution gravity field measurements, comparable to that returned by the GRAIL mission, are required to detect linear gravity anomalies associated with ancient rifts or dikes. Further, only limited instances of strike-slip deformation have been reported on Mercury (e.g., Galluzzi et al., 2015). High-resolution images and topographic datasets are required to determine if this kind of deformation is more widespread and to elucidate the nature of ancient tectonic activity global. 22 MExAG Science Goals d) Relationships between volcanism, tectonism, volatiles Connecting volcanism and tectonism on Mercury is key to understanding the formation and spatial distribution of volcanic vents, the emplacement of the smooth plains materials, and the connection between intrusive and extrusive igneous processes. Since fractures in fumarole systems serve as conduits for gases (Munaretto et al., 2023), it is important to understand the interplay of fracture systems and fumarolic activities for Mercury-relevant conditions. To achieve that, it is essential to conduct field work at analog sites on Earth at permafrost terrains or sulfur-rich volcanic crater formations like solfataras and their connection to fractures (Phillips et al., 2021). To connect intrusive and extrusive volcanism, analysis of upcoming data from the Mercury Orbiter Radio Science Experiment (MORE) (Iess et al., 2021) and geologic mapping using SIMBIO-SYS (Cremonese et al., 2020) on BepiColombo will be needed. Gravity anomaly mapping can be used to: detect intrusive bodies; determine the location, orientation, and characteristics of volcanic conduits that produced the smooth plains; more tightly constrain models of crustal thickness; and address why smooth plains are concentrated in the northern hemisphere. Improved spectral and spatial observations from orbiters, together with in-situ sampling of Mercury’s crust, will ultimately constrain the compositional variations between the explosive and effusive volcanic units, determine the volatiles driving volcanism, and further elucidate the relationship between tectonics and volcanism. e) Long-wavelength topography MESSENGER-derived topography of Mercury’s northern hemisphere reveals broad regions of undulating relief, which appear to be genetically independent of other, major topographic modifications such as faulting, cratering, and volcanism. These variations occur at a range of scales from the degree 2 shape of the planet (Phillips et al., 2018) to the undulatory patterns in the Caloris basin or broad, isolated rises, the most prominent of which is seen in Borealis Planitia (the Northern Rise) (Zuber et al., 2012; Klimczak et al., 2013). The superposition relationships of geologic features indicate that long-wavelength topographic changes occurred subsequent to the craters and volcanically resurfaced areas that they affect. There is minimal correlation between gravity and topography, suggesting that the observed topography is supported by multiple processes. Modeling indicates that folding is unlikely to have been produced by global contraction (Kay and Dombard, 2019). Such processes may include top loading of an elastic lithosphere, bottom loading, crustal thickening, and/or dynamic topography. It is an aim to elucidate how, where, and when each of these processes operated. Because of Mercury’s thin mantle, processes at the core-mantle boundary (CMB) may have an observable effect on long-wavelength topography. Thus, it is an aim to characterize how plausible different CMB processes are and how they affect the long-wavelength topography. To achieve these aims, some improvements in observations on topography and gravity from BepiColombo, especially for the southern hemisphere, and ultimately major such improvements by a mission similar in design to GRAIL, will provide further insight into the distribution of the long wavelength undulations. Their causes must be further studied by modeling plausible CMB processes and their expressions in gravity, topography and the magnetic field (Plattner and Johnson, 2021). 23 MExAG Science Goals f) Structure of the crust and lithosphere Characterizing the structure of the crust and lithosphere, and their relationship to geologic features and surface compositions, is vital to understanding how the planet’s crust formed and the endogenic and exogenic processes that have shaped the surface. MESSENGER provided a first look at the topography, crustal thickness, and crustal density over Mercury’s northern hemisphere (e.g., James et al., 2015; Padovan et al., 2015; Sori, 2018; Beuthe et al., 2020.). Improved characterization of enigmatic features of the lithosphere, such as long-wavelength topographic variations, will provide new insights into mantle structure and thermal state. Constraining lithospheric thickness through time is important to put observed surface features in context and to estimate how interior heat has declined with time. Like the Moon, Mercury’s shape and gravity figure are not in hydrostatic equilibrium. The reason for this is yet unexplained but is important for formation and early evolution scenarios. Global high-resolution gravity and topography data are important for revealing the support mechanisms for major features such as the Northern Rise and the Caloris basin, the history of heat flow, and the origin of the global, non-hydrostatic shape of the planet. g) Heat flow The loss of heat through the surface of a planet is a crucial window into the current and historical operation of the planet’s interior. Understanding how Mercury has lost heat is directly related to the composition of the planet, its tectonic and volcanic activity, and how the magnetic field is generated. The primary long-term source of heat in Mercury is the decay of radioactive elements in the interior (e.g., Hauck et al., 2004). Constraining the distribution of heat-producing elements across and below the surface is therefore essential for understanding how the interior has evolved. This could be accomplished through improved measurement of heat producing elements at a variety of scales across the surface, determination of surface rock and mineral compositions that host heat-producing elements, determination of partitioning relationships for heat-producing elements in Mercury analog materials, and identification and compositional analysis of any Mercury meteorites. Determining variations in lithospheric thickness at a variety of scales (e.g., through studies of lithospheric deformation at faults, characterization of high-resolution gravity and topography data, determination of the depth of seismogenic activity) can also place constraints on past heat flow from the mantle (e.g., Grott et al., 2011; Byrne et al., 2014). In addition, estimating the thermal structure of the shallow subsurface (e.g., via direct measurement with heat flow probes on a landed mission, high resolution thermal mapping, estimation of surficial porosity via high-resolution gravity measurements, and modeling based on surface properties, etc.) is critical for constraining modern heat flow, tectonic activity, and the sequestration and liberation of volatiles. h) Regolith formation and mixing Understanding of Mercury’s regolith can benefit from our knowledge of the Moon’s regolith, but there are significant differences in meteoroid, micrometeoroid, and proton fluxes, surface composition, and daytime temperature between the two bodies. By characterizing the differences in regolith production at Mercury in comparison to the Moon, we may better understand the 24 MExAG Science Goals environmental and compositional factors that influence regolith development on airless bodies in general. In situ analyses of Mercury regolith from a landed investigation (akin to information gleaned from returned Apollo drill cores) would inform understanding of the rate of impact gardening and the depth to which processes affect the regolith. Regolith formation and mixing models can be improved by understanding the mass, velocity, and flux of the meteoroid population near Mercury. Regolith models can also be improved through observations of degrading features (e.g., depth-diameter ratio and the shape of ejecta blankets around fresh craters on Mercury) and models of topographic diffusion such as those conducted for the Moon (Fassett and Thomson, 2014; Fassett et al., 2017). Such studies are most robust when remote sensing techniques covering several depth scales are used together (e.g., radar observations on the decameter scale and visible light on the surface). It is also necessary to understand the mechanical properties of Mercury’s regolith and how it may differ from other airless bodies. Remote thermal infrared radiometry (e.g., from MERTIS on BepiColombo; Hiesinger et al., 2010; 2020) will allow analysis of thermophysical properties. Future landed images of regolith grains could be used to investigate particle size, strength, cohesion, and porosity. Space weathering at Mercury is thought to be dominated by the formation of agglutinitic deposits through micrometeorite bombardment and to be lacking in solar wind ion sputtering (Hapke, 2001). Additionally, Mercury’s extreme temperature range has implications for the diffusion in glass and crystal growth processes in the regolith (Noble and Pieters, 2003). These properties have important effects on the spectral properties of the soil. Photometric modeling has found that Mercury’s regolith is likely smoother at micrometer scales and has a smaller mean particle size distribution compared with lunar regolith, which is consistent with a larger flux and higher velocity of impactors than at the Moon (Domingue et al., 2016). The intense space weathering environment may have resulted in a higher abundance of agglutinates, leading to smoother grains thereby reducing surface roughness. While Mercury’s deficient 1-micron band could be due to a lack of iron, it is plausible that the lack of a 1-micron band is the result of an amorphous surface rich in agglutinates (McClintock et al., 2008; Robinson et al., 2008). Color imaging and analysis before and after a physical interaction with the regolith (e.g., with a lander arm) would help to elucidate the effects of space weathering on Mercury’s regolith as well as regolith depth. Characterizing regolith optical maturation rates on Mercury (and on airless bodies in general) is essential to interpreting multispectral reflectance measurements (Braden et al., 2013). 3.2 Understand processes behind the chemical and mineralogical diversity of the crust a) Geochemical variations Data from MESSENGER’s geochemical instruments revealed the presence of geochemical terranes in Mercury’s northern hemisphere—regions with geographically distinct compositions (e.g., McCoy et al., 2018; Peplowski and Stockstill-Cahill, 2019) that are thought to be primarily igneous in origin. Several petrologic studies have been conducted to ascertain the conditions that produced the geochemical terranes (Charlier et al., 2013; Cartier et al., 2014; Namur et al., 2016a, Vander Kaaden et al., 2016; Boujibar et al., 2025), but further progress in understanding Mercury’s geochemical diversity requires improvements in the experimental (melting/crystallization) data available for diverse, 25 MExAG Science Goals not quantitatively explain the observed spatial and temporal variations observed in these elements. Pathways of material loss from the surface to create each of these exospheric elements are not fully understood. Improved observations of Mercury’s exosphere and improved theoretical and numerical modeling are needed. The link between meteoroid impacts and different species in Mercury’s exosphere has been established thanks to MESSENGER data. BepiColombo is expected to provide a variety of new measurements and constraints that would provide a tighter connection of the Ca, CaO, and Mg exosphere structure to the imprint of the meteoroid influx at Mercury from both the sporadic meteoroid background (Pokorný et al. 2018) and the individual parent bodies such as comet 2P/Encke (Christou et al 2015, 2024). b) Space weathering processes and rates Particles, photons, and micrometeorites can desorb material from the surfaces of regolith grains and cause changes in their chemical composition, optical properties, and morphologic structure, cumulatively known as space weathering. Micrometeoroid impacts vaporize regolith grains, produce impact gardening of the regolith, contaminate the surface with exogenic materials, and cause electrostatic effects on surface processes. Photon and electron stimulated desorption (PSD and ESD, respectively) can drive release of Na, K, and H2O, and potentially other moderately volatile species including Ca and S, through electronic excitations. Solar wind ion impacts lose energy through binary collisions with the target material and release atoms, molecules, and ions from the top-most layers of the solid surface in a process known as sputtering. Open questions relating to these space weathering effects include: What is the relative importance of each of the above-described space weathering processes for the various surface species found on Mercury? What are the binding energies of volatile materials adsorbed onto the surfaces of regolith grains, and how do these change when the species are incorporated into the crystalline structure of the grains? How are binding energies affected by the presence of voids, cracks, and other surface defects? How do binding energies change the desorption energies of volatile species (and thus their exospheric densities)? What are the delivery rates of volatiles to the poles? How does the temperature of the surface influence the relative importance of the various desorption processes? These questions can be addressed with laboratory space weathering experiments, combined with more detailed observations of spatial and temporal variations in Mercury’s exosphere. 4. Investigate processes currently ongoing at Mercury 4.1 Characterize present-day geologic activity a) Present-day cratering rate Estimates for the current impact flux at Mercury have been derived using numerous techniques including dynamical calculations (Marchi et al., 2005; Pokorný et al., 2018), extrapolating measurements from the 32 MExAG Science Goals Moon (Marchi et al., 2009), and by searching for surface changes in spacecraft images (Speyerer et al., 2022). The latter observational analysis identified 19 surface changes that were attributed to the formation of craters >100 m. If this estimate is correct, the implied crater formation rate is a factor of 1000 times higher than existing production models, necessitating further work to understand the discrepancy between observations and models. Understanding the role dust impacts play in shaping Mercury’s surface and exosphere is an important ongoing field of work. To date, studies have been conducted to understand the dust population (<10 micron particles) using measurements in the inner solar system from Parker Solar Probe and Solar Orbiter (e.g., Mann et al., 2019; Krüger et al., 2024) and more specifically at Mercury (Krüger et al., 2024). For particle sizes up to 10 micrometers, the Mercury Dust Monitor (MDM) on BepiColombo will provide a direct measurement. In the future, an orbiter hosting a dedicated dust detector with a mass spectrometer (e.g., Sternovsky et al., 2011) could be used to derive particle impact velocity vector, mass, and composition. For particle sizes from 10 micrometers to 1 centimeter, work could be done to indirectly estimate the flux by better understanding the excitation of the Mercury’s exosphere via micrometeoroid impacts (Killen and Hahn, 2015; Pokorný et al., 2018). Further work is needed to improve our knowledge of collisional processes between particles, the influence of dust/meteoroid shapes on magnitudes of radiative effects, and hypervelocity chemistry (melting, vaporization, ionization of different species). For particle sizes from 1 centimeter and above, more insights with change detection (e.g., inter MESSENGER–BepiColombo high-resolution image comparisons) will aid in further characterizing the present-day meteoroid impact flux. Telescopic or orbital observations of impact flashes on Mercury’s surface (c.f. lunar observations Suggs et al., 2014) or detection with seismic instruments or infrasound techniques (Silber and Brown, 2018) on a lander could help to estimate the present-day impactor flux, mass, and energy. Additionally, characterizing the current impact rate on Mercury will shed light on the dynamical evolution of the near-Earth orbit (NEO) population into Mercury crossers. b) Recent/current volcanic activity Identifying recent effusive volcanism is challenging as it can be difficult to distinguish smooth lava flows from impact melts. Finding volcanic vents would be diagnostic, however locating vents of effusive lavas is often difficult because they tend to self-seal. It is therefore an aim to identify and obtain stereo image pairs of candidate vent areas at the sub-10 m/pixel-scale and identify and characterize the most recent effusive volcanism by defining spectrally homogeneous areas as small as about 400 km2. Crater counts down to crater diameters of 1 km should be done on these areas to test whether spectral units have different CSFDs and to quantify these as ages using a crater production function. To achieve this aim, high-resolution spectral imaging of smooth plains units is needed, as well as absolute age calibrations that can be achieved via sample return or in situ radiometric dating. In addition, geologic context can help distinguish between small young effusive volcanism and impact melt, but compositional information from X-ray fluorescence (XRF) and visible-to-thermal infrared (VISTIR) spectroscopy will ultimately only provide the important disambiguation. 33 MExAG Science Goals Mercury has more than 100 candidate sites of explosive eruption. Two of the youngest explosive vents have been dated as Kuiperian (<280 Ma) on the basis of a superposition relationship with crisp craters (Thomas et al., 2014b; Jozwiak et al., 2018). The eruptive lifetime of each explosive vent site is poorly defined, but many vents consist of multiple closely spaced loci of eruption, suggestive of multiple eruptions spaced over perhaps long intervals. Most explosive deposits (faculae) could be a cumulative product of many separate eruptions (Pegg et al. 2021). The composition and rate at which faculae lose their spectral distinctiveness with age remains unknown. It is thus an aim to better define the timing and duration of the explosive eruptions. To achieve this aim, high-resolution imaging can provide fine-scale superpositional relationships and textural differences within compound vents, whereas compositional information via high-resolution XRF and VISTIR spectroscopy and space weathering experiments can provide information on composition and longevity of spectral distinctiveness of faculae. c) Recent/current tectonic activity Large thrust fault-related scarps, 100s of km long, crosscut impact craters as old as pre-Tolstojan (> ~4 Ga) but as young as Kuiperian (< 280 Ma). Despite such evidence for a long duration of contraction, it is not known how active tectonics on Mercury are at present (Crane and Klimczak, 2017). The crosscutting of scarps with small craters and very small-scale thrust fault scarps suggests that tectonic activity occurred geologically recently and is perhaps ongoing (Watters et al., 2016). Very small grabens on large, old lobate scarps also suggest long-lived and/or recent tectonism on Mercury (Man et al., 2023). To understand how geologically recent the landforms are, an aim is to understand their spatial and kinematic relations to larger shortening structures, and what process(es) drive their formation. Another aim is to identify the current rate of tectonic deformation to specify how tectonically active Mercury is at present. The mechanisms driving the tectonic activity on Mercury are not fully quantified. It is crucial to understand the respective roles of continuing global contraction, topographic relaxation, thermally and tidally induced deformation, seismic surface modification leading to mass wasting, and even local-scale deformation from magma ascent or impact bombardment. Understanding the relative contribution of these processes will provide insights into the evolution of Mercury’s interior. Characterization of relationships between tectonic features and impact craters, cataloging of indicators of recent surface change such as landslides and boulder trails from higher-resolution-than-currently-available images, and determination of potential surface changes between data from MESSENGER, BepiColombo, and future orbiters are approaches that can indicate tectonic sources of geologic activity. Furthermore, using tectonic observations to inform modeling of the potential for seismicity from Mercury’s solid body (solar) tides—suggested to be as much as 17 times larger than those on the Moon (Van Holst and Jacobs, 2003)—as well as modeling coseismic slip on thrust faults will inform current deformation rates. A Mercury lander containing one (or more) seismometer(s) would ultimately provide information on deformation rates by measuring magnitudes, frequencies, and sources of quakes. Finally, the investigation of Mercury's tectonic and volcanic activity requires understanding of its interior, especially the composition and structure of the silicate portion of the planet. New insight from 34 MExAG Science Goals BepiColombo's instruments as well as new laboratory measurements on crust and mantle analogs and numerical modeling will help to constrain the mineralogy and thermal state of the silicate portion. For instance, induction data can be used to obtain an electrical conductivity-depth profile of the mantle (Genova et al., 2021), which can be used to place constraints on the mineralogy and thermal state (e.g., Pommier et al., 2025). Elucidating the interior of the planet is necessary to model the different deformation processes and related volcanism that shaped the surface. d) Hollows Although much progress has been made in understanding hollows, how they form remains an open question. It is generally agreed that the hollows are young (Blewett et al., 2011, 2013), that sublimation or space weathering processes are involved in hollow formation (Blewett et al., 2011, 2013; Vaughan et al., 2012; Phillips et al., 2021), and that hollows are associated with Mercury’s Low Reflectance Material (LRM) (Blewett et al., 2013; Thomas et al., 2014a). Most hollows occur within impact craters or material excavated by impacts (Thomas et al., 2014a; Blewett et al., 2018); however, whether impacts simply excavate material or play some additional role, such as providing necessary heat or melt products, is unknown. It has also been proposed that ejecta blocks from Caloris that have been modified by long-term and ongoing mass wasting had substantial volatile contents and may be related to hollows (Wright et al., 2020). A first aim is to determine the identity of the volatile-bearing phase that is lost in the hollow-formation process. High-resolution compositional and reflectance orbital measurements of hollows and LRM could help to unveil their composition and relationship to one another. Landed elemental (e.g., sulfur and carbon) and mineralogical (e.g., sulfide, carbide) measurements will be critical for understanding the volatile-bearing phase. A second aim is to constrain the mechanism by which hollows form. High-resolution images of hollow morphology could shed light on hollow formation. High-resolution image coverage of Mercury’s southern hemisphere would allow a mapping of the global distribution of hollows and their associations with other landforms. A third aim is to constrain the ages, formation timescales, and growth-rates of hollows. Higher resolution imaging of the surface would determine the ages of host units and could reveal “fossil” hollows (not bright compared to their surroundings). Change-detection imaging (visible or radar) of hollows could illuminate whether hollows are actively growing and at what rate. e) Polar deposits The polar ice deposits in Mercury PSRs have few craters and so appear to be recent and/or active features. In addition, their covering of dark, possibly organic, material appears to correlate closely with the extent of the permanently shadowed areas (Chabot et al., 2014a). Potential current processes acting on the deposits include sublimation/de-sublimation, micrometeoroid impact, space weathering, ion implantation and interaction with the magnetic field. To improve our understanding of the processes currently affecting the polar deposits, along with their current accumulation and loss rates, a number of key measurements are needed. These include detailed measurements of polar thermal environments, measurements of solar wind flux at multiple locations (including at latitudes where water ice is exposed at the surface and at latitudes where water ice is 35 MExAG Science Goals insulated by a layer of low-albedo materials), and measurements of the ongoing regolith gardening rates. High-resolution measurements of the gravity field would provide key insights into the distribution of ice, and better measurements of magnetic fields at the poles would improve constraints on ice formation and loss processes. A lander (even one not at the polar regions) would provide key constraints on ongoing activity. High-resolution images, reflectance measurements, and topographic measurements of the polar deposits are also important, particularly if they can be made repeatedly over long timespans, to directly measure changes and rates of changes. Additionally, measurements of polar deposit purity as a function of depth (e.g., by radar sounding) would provide insight into ice evolutionary history. A variety of experiments and model improvements would also improve our understanding of ongoing processes at the polar deposits, including experiments and models of space weathering rates at Mercury’s polar conditions, experiments of cometary-like volatiles to test what low-reflectance volatiles may form after exposure to Mercury-like environmental conditions, and experiments to determine how thermal lag deposits may develop and evolve in Mercury-like conditions. 4.2 Investigate magnetospheric dynamics a) Generation of the current magnetic field The global magnetic field of Mercury is likely generated by a convection-driven dynamo in the core (e.g., Christensen, 2006) that is thought to have operated early in the planet's history (~ 3.7–3.9 billion years; Johnson et al., 2015). Using magnetic and geodetic measurements, it has been suggested that the metallic core is at least partially liquid at present (e.g., Steinbrügge et al., 2021). Core cooling models have shown that iron snow is unlikely to occur in Mercury's core, and the growth of a solid inner core is the only expected crystallization mechanism (Knibbe and van Westrenen, 2018; Davies et al., 2024). Models that best reproduce the existing observational constraints are obtained for a layered core made of a molten outer portion, composed of a thick thermally stable layer and a convecting region where the dynamo is generated, and a solid inner core. The presence of a hypothetical molten FeS layer atop the core would have minimal effect on Mercury's long-term thermal and magnetic evolution (Davies et al., 2024). The size of the expected solid inner core is debated (up to 1500 km; e.g., Genova et al., 2019), partly due to the lack of constraints on core chemistry, and new data from BepiColombo's Mercury Orbiter Radio Science Experiment (MORE) (Iess et al., 2021) are expected to shed light on the structure and evolution of the core. Characterizing the compositional and physical properties of Mercury’s core is key to modeling core dynamics and the origin of the magnetic field. Information about the chemistry of the metallic core has been obtained by the combination of surface composition data and laboratory experiments. The low-Fe and high-S contents in mercurian lavas suggest that the core formed under highly reducing conditions (e.g., McCubbin et al., 2012). At these fO2 conditions, high-pressure and high-temperature experiments showed that only small amounts (<~2.0 wt.%) of sulfur are expected in the core, whereas Si could alloy with Fe in significant proportions (>10 wt.%; e.g., Knibbe and van Westrenen, 2018; Goossens et al., 2022). A non-negligible amount of Ni (several wt.%) is expected, based on iron meteorite cosmochemistry, and substantial amounts of C might also be present in the core (e.g., Vander 36 MExAG Science Goals Kaaden et al., 2020). In addition, partitioning experiments of Si between liquid and solid Fe alloys indicated that large amounts of silicon are expected to partition to the solid inner core (e.g., Tao and Fei, 2021). Current understanding of the physical properties of Mercury’s core rests largely on MESSENGER-based determinations of the rotational state (i.e., amplitude of the physical libration and the orientation of the planet), the second-degree harmonics of the gravity field (e.g., Margot et al 2018, Bertone et al., 2021) and laboratory measurements of the physical properties (e.g., Pommier et al., 2019). Better measurements of Mercury’s gravity field, along with higher precision topography would greatly advance our understanding of the origin of Mercury’s magnetic field, as would experimental studies focused on the relevant core compositions and conditions. b) Magnetic reconnection Mercury and Earth both have reconnection-driven magnetospheres, with energy and momentum from the solar wind driving the circulation of magnetic flux through the system. The relative reconnection rates on the dayside and in the magnetotail govern the change in open magnetic flux content of the system, and thus are important parameters for describing the global magnetospheric dynamics (Imber and Slavin, 2017; Slavin et al., 2021), the distribution and acceleration of magnetospheric plasma (e.g. Dewey et al., 2018), and the location of particle precipitation to the surface (e.g. Poh et al., 2016; Lindsay et al., 2016; Raines et al., 2022). Although MESSENGER rarely observed reconnection sites directly (e.g., Zhong et al., 2018), it recorded ample evidence of the frequency and importance of reconnection to Mercury’s magnetosphere. Due to the single-point nature of MESSENGER’s in situ plasma and magnetic field observations, many outstanding features of magnetic reconnection in Mercury’s magnetosphere remain, including: the number and locations of simultaneous reconnection sites at the magnetopause and/or in the magnetotail; the influence of planetary ions on reconnection occurrence and strength; the relative importance of low-latitude Kelvin-Helmholtz waves for plasma entry and flux circulation compared to magnetic reconnection; the relation between reconnection and dawn-dusk asymmetries; particle energization (both ion and electron) directly from reconnection and from reconnection products. c) Induction effects The small dimensions of Mercury’s magnetosphere and the large volume occupied by the planet’s conducting core result in electromagnetic coupling between the planet’s interior and its magnetosphere. Since Mercury lacks an ionosphere, changes in the external magnetic field directly affect the planet as, in response, inductive currents form on the surface of Mercury’s highly conducting core to oppose these changes. Induction currents introduce additional magnetic flux to the magnetosphere that, like the intrinsic magnetic field, can undergo dynamic processes like magnetic reconnection. The effect of induction and its role in the coupling between Mercury’s magnetosphere and interior are not well understood. Initial studies of induction using MESSENGER data have confirmed annual and episodic changes in the planet’s axial dipole magnetic moment due to changes in solar wind dynamic 37 MExAG Science Goals pressure, with a few-percent change due to annual dynamic pressure variation (Johnson et al., 2016) and effects as strong as ~50–100% due to solar wind transients (e.g., Slavin et al., 2014). However, other causes for induction effects, such as those related to magnetotail current systems and dynamics, remain unconfirmed. Further, the consequences of the additional magnetic flux in the magnetosphere generated by induction currents remain poorly constrained. During high-pressure events, the flux added to the dayside helps to prevent the collapse of the dayside magnetosphere to the surface of the planet. However, strong magnetopause reconnection can transport this flux to the magnetotail, eroding the dayside magnetosphere anyway. The interplay between induction and reconnection, and their effects on the magnetosphere’s structure and content, require further investigation. While induction is present at many solar system objects, such as the Galilean moons, only Mercury and Ganymede are known to possess both global magnetic fields and at least weakly conducting interiors. As Ganymede is deeply embedded in Jupiter’s magnetosphere, Mercury is a unique place to study induction effects as its magnetosphere is subject to the more variable solar wind. Expected data from BepiColombo's MPO-MAG instrument have the potential to answer several questions related to Mercury's magnetosphere (Heyner et al., 2021). d) Dawn-dusk asymmetries Mercury’s magnetosphere exhibits differences between its duskside and dawnside in the characteristics and dynamics of its plasma and magnetic field. These dawn-dusk asymmetries are present throughout the system, but are pronounced in the magnetotail, where, for example, the dawnside means current sheet thickness is greater (e.g., Poh et al., 2017; Rong et al., 2018), the plasma density is higher (e.g., Korth et al., 2014; Zhao et al., 2020), and signatures related to magnetic reconnection are more frequent than on the duskside magnetotail (e.g., Sun et al., 2016; Smith et al., 2017; Dewey et al., 2017; Dewey et al., 2020). Some dawn-dusk asymmetries are similar to Earth, however, others are opposite in their dawn-dusk preference (see, e.g., a review by Sun et al., 2022). Characterization of dawn-dusk asymmetries to date has relied on statistical accumulation of MESSENGER observations within the magnetosphere. While these accumulations describe average local properties, they integrate over the time history of the system, lack context of solar wind driving, and compare temporally and spatially distant measurements. As a result, the origins and consequences of these asymmetries remain unresolved. One aim is therefore to understand the physical origins of the magnetosphere’s dawn-dusk asymmetries, while another aim is to determine the influence of these asymmetries on the magnetosphere’s internal dynamics. Characterization of relationships between dawn-dusk asymmetries and external solar wind driving conditions; comparison of dayside and nightside magnetosphere mass, energy, and momentum budgets and the transfer of these quantities throughout the system and with the solar wind; resolution of meso-scale dynamics (~0.1 of Mercury’s reference radius) and constraining their contribution to global magnetosphere configuration; and tracing of particles and field lines through the magnetosphere as a function of magnetospheric state can each help to resolve the origins and effects of dawn-dusk asymmetries. Such activities rely on simultaneous multi-point in situ measurements within or across 38 MExAG Science Goals magnetospheric regions, knowledge of upstream solar wind conditions, and comparison between global simulations and localized spacecraft measurements. e) Magnetospheric currents Mercury’s major magnetospheric currents generally resemble those of Earth, but there are significant differences in current closure at Mercury due to its lack of a conducting ionosphere. The major current systems at Mercury include magnetopause current, cross-magnetotail current sheet, a ring-like current about the planet, and field-aligned currents. At the dayside magnetopause, a Chapman-Ferraro current separates the closed dayside magnetosphere field from the magnetosheath. Mercury’s magnetopause is more conducive to reconnection than Earth’s, including instances of symmetric reconnection, and the resulting differences in magnetopause current strength and coherence compared to Earth’s are currently unknown. In the magnetotail, the dawn-to-dusk cross-tail current sheet is a consequence of the elongation of nightside field lines. There is some evidence for bifurcation of this current sheet (Al Asad et al., 2021), but the origins and consequences of this bifurcation are not well understood. Close to the planet, trapped particles injected from the magnetotail into the dipole-dominated region of the magnetosphere gradient-curvature drift about the planet. Although the existence and frequency of drift paths remain under debate, recent work has suggested a sufficient amount of plasma drifts to form the equivalent of an Earth-like ring current (Shi et al., 2022; Zhao et al., 2022). However, the direction and primary current carriers cannot currently be reconciled. At the dayside, this current may close about equatorial dayside, but during moderate magnetosphere forcing the current is expected to split into the northern and southern hemispheres in a Shabansky-like configuration. The current systems at Mercury that are least likely to resemble Earth’s are its field-aligned current (FAC) systems. Since Mercury lacks a conducting atmosphere, the interior of the planet is expected to provide necessary closure for these types of current systems. While the exact mechanics and location of current closure in the planet require further constraints, conceptual models suggest the currents flow radially through the resistive crust and mantle, and close at the highly conductive core-mantle boundary. Current closure over the core and the resistive crust/mantle imposes constraints on temporally-variable FACs. MESSENGER observations in the magnetotail suggest a substorm current wedge may form that connects the near-tail region to the planetary interior (e.g., Dewey et al., 2020) but the small spatiotemporal scales of the magnetosphere and the resistive regolith hamper the formation of this system compared to Earth. The frequency and similarity of a substorm current wedge at Mercury remain open. In addition to transient FACs, Mercury’s magnetosphere possesses a more permanent Region-1 style current that was consistently observed by MESSENGER over Mercury’s northern pole (e.g., Anderson et al., 2014, 2018). These large-scale polar FACs are quasi-stationary and omnipresent. The exact origins of this system at Mercury are unknown, but flow shear interior to the magnetopause near the terminator is a possible mechanism. A Region-2 style FAC system has not been observed in the MESSENGER data to date. However, it is expected to exist to complete the Dungey cycle by transporting closed magnetotail field lines to the dayside. New observations of the magnetic field and its evolution with time are required. 39 MExAG Science Goals f) Flux rope formation Flux ropes (FRs) are helical bundles of magnetic flux that form between magnetic reconnection sites and are convected away from their location of formation by a combination of magnetic tension forces and dominant plasma flows in the system. Flux ropes at Mercury have been observed by MESSENGER to form in the magnetotail current sheet and along the dayside magnetopause. Dayside magnetopause flux ropes are also known as flux transfer events (FTEs) and are observed moving anti-sunward along the magnetopause where they may transport significant quantities of magnetic flux from the dayside magnetopause into the magnetotail (e.g. Imber et al., 2014). ‘Flux transfer event showers’ appear to be a phenomenon unique to Mercury (Slavin et al., 2012) and consist of hundreds of FTEs observed over very short time periods. While individual FTEs within the showers were resolved by MESSENGER’s magnetometer, their small spatial scale (and corresponding short timescale as they passed over the spacecraft) has limited analysis from plasma observations. These magnetic field observations suggest that FTEs may play a significant or even dominant role in flux transport from Mercury’s dayside magnetosphere to the magnetotail. However, without corresponding detailed plasma observations, questions remain as to the role of FTEs in flux and particle transport, the location of particle precipitation to the planetary surface and subsequent contribution to the generation of the exosphere, and the storage and dissipation of energy within Mercury’s magnetosphere. Observations of the magnetic field over longer time periods with improved coverage are needed. g) Effect of solar events on the system Interplanetary Coronal Mass Ejections (ICMEs) have the strongest effect on Mercury’s magnetosphere of any solar events. They typically bring significant increases in plasma flux and magnetic field strength. They mainly drive the magnetosphere via two mechanisms: increased dayside magnetopause reconnection and increased solar wind dynamic pressure. When dayside reconnection dominates, significant magnetic flux is stripped from the dayside magnetosphere and loaded into the magnetotail (Imber and Slavin, 2017), stimulating magnetic reconnection across the central current sheet and increasing magnetospheric circulation. In extreme cases, this erosion of the dayside magnetic field could expose substantial portions of Mercury’s dayside surface to direct solar wind impact (Slavin et al., 2019). When pressure dominates, the whole magnetosphere is squeezed, leading to an increase in dayside magnetosphere field strength from magnetic induction on the core (Slavin et al., 2014). The net result can be similar to the reconnection-dominated events: in extreme cases portions of the dayside surface can be exposed to direct solar wind impact. Increased solar wind sputtering likely increases the density of the neutral exosphere and heavy ion content of the magnetosphere (Sun et al., 2022), as well as the space weathering of the surface (Domingue et al., 2015). The combination of the two BepiColombo orbiters will provide two-point measurements that will greatly improve understanding of these events compared with the single-spacecraft measurements of MESSENGER. MPO will continuously monitor near-planet conditions with its lower, nearly circular orbit while Mio will either be upstream monitoring the solar wind or deep in the magnetotail monitoring loading and tail reconnection. 40 MExAG Science Goals Two other classes of solar events have an indirect and likely much smaller effect on Mercury’s magnetosphere: Solar Energetic Particle (SEP) events and solar extreme ultraviolet (EUV) flares. SEP events have been shown to rain energetic electrons down onto Mercury’s polar cap (Gershman et al., 2015a). Solar flares cause increases in photon-stimulated desorption of material from the surface as well as likely produce more planetary ions in the magnetosphere, which may precipitate back on to the surface at higher energies. Both of these effects likely lead to more sputtering of neutrals and ions from the surface, but evidence of this effect has not been reported. Future observations using multiple spacecraft are needed to improve our understanding of the Sun’s influence on the magnetosphere. 4.3 Understand exospheric and magnetospheric source and loss processes a) Magnetospheric ions Because Mercury’s exosphere is collisionless, magnetospheric ion dynamics are completely decoupled from exospheric neutrals. The primary sources of magnetospheric ions are entry of the solar wind through dayside magnetopause reconnection and photoionization of exospheric neutrals (Zurbuchen et al., 2008; 2011; Raines et al., 2013). Much smaller quantities likely come from ionization of material from surface micrometeoroid impacts in the hot vaporization cloud (Pokorný et al., 2017; 2018) as well as charge exchange of exospheric neutrals with solar wind protons in the magnetosheath or cusps, though there are currently no measurements to conclusively support either of these cases. Magnetospheric ions are lost primarily through magnetospheric circulation and kinetic processes. Following magnetic reconnection in the central plasma sheet, as part of Dungey Cycle circulation, ions are carried by helical magnetic structures called plasmoids down the magnetotail and back into the solar wind. Plasma is also carried toward the nightside of the planet by this process, with some being lost to precipitation on the nightside surface through the plasma sheet horns or possibly even across the entire nightside during very active times (Glass et al., 2022). While plasmoids have been observed (Slavin et al., 2008; DiBraccio et al., 2015), estimates of the plasma lost via this process have not been made. The retained ions will tend to drift around Mercury’s night side toward the dusk terminator. Mercury’s weak magnetic field results in a very small magnetosphere (Slavin et al., 2007), where, on the dayside, the distance between the magnetopause and the planetary surface is often the same scale as ion gyroradii. A large portion of ions drifting through this space will be lost by gyrating either into the planet’s surface (precipitation) or into the magnetopause (magnetopause shadowing). The significance of this kinetic process increases with increasing ion energy and mass, so that planetary ions, such as Na+, which have gyroradii more than 20 times that of protons at the same energy, can pass through this space only at the lowest energies, 100s of eV (Raines et al., 2014). 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