MErcury Surface, Space ENvironment, GEochemistry, And Ranging Research Articles

Magma eruption ages and fluxes in the Rembrandt and Caloris interior plains on Mercury: Implications for the north-south smooth plains asymmetry

The MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission by the National Aeronautics and Space Administration revealed that ~27% of the surface of Mercury is occupied by smooth plains potentially formed by magma eruptions. Most of these smooth plain units are located on the floors of the impact basins; these plains occupy a surface area that is approximately seven times larger in the northern hemisphere than in the southern hemisphere. This suggests a difference in thermal conditions in the mantle and/or the ease of magma eruption between hemispheres. This study re-estimated magma eruption ages and volumes within the Rembrandt and Caloris basins; these are representative impact basins in the southern and northern hemispheres, respectively. The estimations of eruption ages and volumes were carried out by measuring crater size-frequency distributions and the diameters of partially buried craters. The formation and eruption ages of the Rembrandt and the Caloris basins were estimated by adopting the porous-target chronology model; the formation ages were estimated as 3.93 ± 0.06 and 3.94 ± 0.04 Gy, while their eruption ages were from 3.87 ± 0.04 to 3.76 ± 0.01 Gy and 3.88 ± 0.03 to 3.74 ± 0.01 Gy, respectively. The observed crater size-frequency distributions in both basins might show multiple resurfacing from episodic eruptions, as suggested by previous numerical simulations of Mercurian thermal evolution. The estimated magma eruption fluxes in the Caloris basin (9.3–15.1 km/Gy) were at most, three times larger than that in the Rembrandt basin (5.2–12.3 km/Gy). Given its larger diameter, the crust beneath the Caloris basin is likely to be ~1.8 times thinner than the crust beneath the Rembrandt basin. This will make it easier for magma to ascend to the surface beneath Caloris than beneath Rembrandt; as such, quantities of magma production in the mantle beneath the two basins should not differ by a factor of <3. These results suggest that there are no large spatial variations in the abundance of heat-producing elements in the mantle and lower crust of Mercury unlike the concentration of heat-production elements on the nearside of the Moon.

Science Goals and Mission Concept for a Landed Investigation of Mercury

Abstract Mercury holds valuable clues to the distribution of elements at the birth of the solar system and how planets form and evolve in close proximity to their host stars. This Mercury Lander mission concept returns in situ measurements that address fundamental science questions raised by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission’s pioneering exploration of Mercury. Such measurements are needed to understand Mercury's unique mineralogy and geochemistry, characterize the proportionally massive core's structure, measure the planet's active and ancient magnetic fields at the surface, investigate the processes that alter the surface and produce the exosphere, and provide ground truth for remote data sets. The mission concept achieves one full Mercury year (∼88 Earth days) of surface operations with an 11-instrument, high-heritage payload delivered to a landing site within Mercury's widely distributed low-reflectance material, and it addresses science goals encompassing geochemistry, geophysics, the Mercury space environment, and geology. The spacecraft launches in 2035, and the four-stage flight system uses a solar electric propulsion cruise stage to reach Mercury in 2045. Landing is at dusk to meet thermal requirements, permitting ∼30 hr of sunlight for initial observations. The radioisotope-powered lander continues operations through the Mercury night. Direct-to-Earth communication is possible for the initial 3 weeks of landed operations, drops out for 6 weeks, and resumes for the final month. Thermal conditions exceed lander operating temperatures shortly after sunrise, ending operations. Approximately 11 GB of data are returned to Earth. The cost estimate demonstrates that a Mercury Lander mission is feasible and compelling as a New Frontiers–class mission.

Observational evidence of ring current in the magnetosphere of Mercury

The magnetic gradient and curvature drift of energetic ions can form a longitudinal electric current around a planet known as the ring current, that has been observed in the intrinsic magnetospheres of Earth, Jupiter, and Saturn. However, there is still a lack of observational evidence of ring current in Mercury’s magnetosphere, which has a significantly weaker dipole magnetic field. Under such conditions, charged particles are thought to be efficiently lost through magnetopause shadowing and/or directly impact the planetary surface. Here, we present the observational evidence of Mercury’s ring current by analysing particle measurements from MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft. The ring current is bifurcated because of the dayside off-equatorial magnetic minima. Test-particle simulation with Mercury’s dynamic magnetospheric magnetic field model (KT17 model) validates this morphology. The ring current energy exceeds 5times {10}^{10} J during active times, indicating that magnetic storms may also occur on Mercury.

Relating the Solar Wind Turbulence Spectral Break at the Dissipation Range with an Upstream Spectral Bump at Planetary Bow Shocks

Abstract At scales much larger than the ion inertial scale and the gyroradius of thermal protons, the magnetohydrodynamic (MHD) theory is well equipped to describe the nature of solar wind turbulence. The turbulent spectrum itself is defined by a power law manifesting the energy cascading process. A break in the turbulence spectrum develops near-ion scales, signaling the onset of energy dissipation. The exact mechanism for the spectral break is still a matter of debate. In this work, we use the 20 Hz Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) magnetic field data during four planetary flybys at different heliocentric distances to examine the nature of the spectral break in the solar wind. We relate the spectral break frequencies of the solar wind MHD turbulence, found in the range of 0.3–0.7 Hz, with the well-known characteristic spectral bump at frequencies ∼1 Hz upstream of planetary bow shocks. Spectral breaks and spectral bumps during three planetary flybys are identified from the MESSENGER observations, with heliocentric distances in the range of 0.3–0.7 au. The MESSENGER observations are complemented by one Magnetospheric Multiscale observation made at 1 au. We find that the ratio of the spectral bump frequency to the spectral break frequency appears to be r- and B-independent. From this, we postulate that the wavenumber of the spectral break and the frequency of the spectral bump have the same dependence on the magnetic field strength ∣B∣. The implication of our work on the nature of the break scale is discussed.

Reconstructing Mercury's magnetic field in magnetosphere using radial basis functions

Based on the Magnetometer observations of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit around Mercury, we construct a global magnetic field model of Mercury's magnetosphere using radial basis functions (RBFs). This model is parameterized by magnetic disturbance level and the fields are a sum of internal and external sources in the magnetosphere, including magnetopause and magnetospheric currents. The RBF-based model does not impose preconceived assumptions on the fields and constructs the system of tail currents and field-aligned currents (FACs) simultaneously. Compared with the previous models, the residuals between the observed and modeled magnetic field are significantly reduced at different magnetic activity levels and this indicates that our model is reliable and robust. Some features in Mercury's magnetosphere are also constructed based on the RBF model. The magnetic field is compressed in the boundary layer inside the dayside magnetosphere and stretched at the nightside due to the strong dawn-dusk cross-tail currents beyond 1.5 RM(RM is Mercury's radius). The X-line location at the nightside is reduced from 4.6 RM in the magnetic quiet period to 2.9 RM in the magnetic active period and fits the magnetic disturbance indices in linear form. The FACs are downward (upward) in the dawn (dusk) sector and are increased with magnetic disturbance levels. In summary, the RBF-based model could reveal all principal features in the magnetosphere and their changes associated with magnetic disturbance levels.

A 3D MHD‐Particle Tracing Model of Na+ Energization on Mercury's Dayside

AbstractData collected by the Fast Imaging Plasma Spectrometer (FIPS) aboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft showed singly charged Na+‐group ions at energies of between 1 and 13 keV in Mercury's northern planetary cusp. Most of these ions are likely formed by either photoionization or charge exchange of exospheric Na atoms, with initial energies of approximately 1 eV or less. FIPS observations did not establish which acceleration mechanism most reasonably accounts for this energy gain. Using the Adaptive Mesh Particle Simulator (AMPS) model, we undertake kinetic simulations of 1 eV Na+ test particles through the electric and magnetic fields output from the Block Adaptive Tree Solar wind Roe‐type Upwind Scheme (BATSRUS) global magnetohydrodynamic (MHD) model of Mercury's magnetosphere, in search of plausible explanations for the source of this energization. We find that Na+ with initial energy of 1 eV are readily picked up by the Dungey cycle return flow in the dayside magnetosphere. In some cases, this flow provides the energy for the ions to escape into the magnetosheath, and in other cases it energizes the ions to hundreds of eV before they pass immediately into the cusp. Those that escape can be rapidly picked up into the magnetosheath flow, where they are accelerated by pickup again up to tens of keV. These one‐ and two‐stage pickup processes on Mercury's dayside can account for the energies of many of the Na+ ions observed in Mercury's northern magnetospheric cusp by MESSENGER.

Variability of the Interplanetary Magnetic Field as a Driver of Electromagnetic Induction in Mercury's Interior

AbstractMercury's magnetosphere is a unique and dynamic system, primarily due to the proximity of the planet to the Sun and its small size. Interactions between solar wind and embedded interplanetary magnetic field (IMF) and the dayside Hermean magnetosphere drive an electric current on the system's magnetopause boundary. So far, electromagnetic induction due to magnetopause motion in response to changing external pressure has been used to constrain Mercury's iron core size. Here we assess the impact a changing IMF direction has on the Hermean magnetopause currents, and the resulting inducing magnetic field. Observations made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during dayside magnetopause boundary crossings in the first “hot season,” are used to demonstrate the importance of the IMF direction to Mercury's magnetopause currents. Our 16 boundary crossings show that introduction of external IMFs change the magnetopause current direction by 10° to 100°, compared to the case where only the internal planetary field is considered. Analytical modeling was used to fill in the bigger picture and suggests for an east‐west reversal of the IMF, typical of the heliospheric current sheet sweeping over Mercury's magnetosphere, the inducing field at Mercury's surface caused by the resulting magnetopause current dynamics is on the order of 30% of the global planetary field. These results suggest that IMF variability alone has an appreciable effect on Mercury's magnetopause current and generates a significant inducing magnetic field around the planet. The arrival of the BepiColombo mission will allow this response to be further explored as a method of probing Mercury's interior.

Multiscale Features of the Near-Hermean Environment as Derived Through the Hilbert-Huang Transform

The interaction between the interplanetary medium and planetary environments gives rise to different phenomena on several temporal and spatial scales. Here, we propose for the first time, the application of the Hilbert-Huang Transform (HHT) to characterize both the local and global properties of Mercury's environment as seen during two Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) flybys. In particular, we compute the energy-time-frequency distribution of the observed magnetic field components and the reconstruction of these signals at large, magnetohydrodynamics (MHD) and kinetic scales through the empirical mode decomposition. We show that the HHT analysis allows to capture and reproduce some interesting features of the Hermean environment such as flux transfer events (FTEs), Kelvin-Helmholtz vortices, and ultralow frequency (ULF) wave activity. Moreover, our findings support the ion kinetic nature of the Hermean plasma structures, the characterization of the magnetosheath by anisotropic ion-kinetic intermittent fluctuations, superimposed to both MHD fluctuations and large-scale field structure. Our approach has proven to be very promising for characterizing the structure and dynamics of planetary magnetic field at different scales, for identifying the boundaries, and for discriminating the different scale-dependent features of global and local source processes that can be used for modeling purposes.

Near‐Ultraviolet to Near‐Infrared Spectral Properties of Hollows on Mercury: Implications for Origin and Formation Process

AbstractAmong the geological features revealed by the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) mission on the surface of Mercury, hollows are the most surprising and least understood. Possibly related to volatile components, hollows are small depressions, surrounded by bright halos and are not observed on any other surfaces in our solar system. Previous analysis of multispectral data obtained by Mercury Dual Imaging System (MDIS) has shown that some hollows have weak spectral absorption features centered at around 600 nm. In this work, we analyzed four hollows with observations acquired by the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on board MESSENGER with more than 230 spectral channels from the near‐ultraviolet to the near‐infrared. Unlike previous MDIS multispectral data, the MASCS reflectance spectra exhibit no absorption features in the MDIS wavelength range. However, we found that hollows have unique spectral properties in the near‐ultraviolet, with a spectral curvature between 300 and 600 nm that is distinctly different from other geological units. Moreover, we used MASCS observations with the best available spatial resolution (&lt;0.5 km/pixel) to analyze both parts of a hollow: the flat floor and the surrounding halo. Our results support the hypothesis that hollows form by a sublimation process and scarp retreat.

MESSENGER Observations of Planetary Ion Characteristics in the Vicinity of Kelvin‐Helmholtz Vortices at Mercury

AbstractThe MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft regularly observed the magnetospheric flanks of Mercury during its orbital phase. Data from the Magnetometer (MAG) and the Fast Imaging Plasma Spectrometer (FIPS) allow us to investigate the statistical properties of planetary ions (Na+) in the presence of Kelvin‐Helmholtz (KH) waves at the duskside magnetopause. We collect the data from orbits with clear signatures of KH waves under northward interplanetary magnetic field, as well as from adjacent orbits that do not have KH signatures, and we compare the energy characteristics between the KH and non‐KH events. Although low planetary counts in FIPS data make the comparison of these characteristics difficult, we find that in the presence of KH waves: (1) large counts of planetary ions are observed and (2) differences in Na+ energy spectra are only seen inside the magnetosphere, where they show a deceleration signature for ions with energies above 2.0 keV/e. These results suggest that planetary ions are not escaping from the magnetosphere and that electric field structures related to KH waves can decelerate planetary ions originating from the magnetotail region. The understanding of the energy distribution of planetary ions in the magnetospheric flanks of Mercury is important for a better understanding of plasma convection in the magnetosphere.

A Global Gravity Reconstruction Method for Mercury Employing Deep Convolutional Neural Network

Mercury, the enigmatic innermost planet in the solar system, is one of the most important targets of space exploration. High-quality gravity field data are significant to refine the physical characterization of Mercury in planetary exploration missions. However, Mercury’s gravity model is limited by relatively low spatial resolution and stripe noises, preventing fine-scale analysis and applications. By analyzing Mercury’s gravity data and topography data in the 2D spatial field, we find they have fairly high spatial structure similarity. Based on this, in this paper, a novel convolution neural network (CNN) approach is proposed to improve the quality of Mercury’s gravity field data. CNN can extract the spatial structure features of gravity data and construct a nonlinear mapping between low- and high-degree data directly. From a low-degree gravity input, the corresponding initial high-degree result can be obtained. Meanwhile, the structure characteristics of the high-resolution digital elevation model (DEM) are extracted and fused to the initial data, to get the final stripe-free result with improved resolution. Given the paucity of Mercury’s data, high-quality lunar datasets are employed as pretraining data after verifying the spatial similarity between gravity and terrain data of the Moon. The HgM007 gravity field obtained by the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission at Mercury is selected for experiments to test the ability of the proposed algorithm to remove the stripes caused by quality differences of the highly eccentric orbit data. Experimental results show that our network can directly obtain stripe-free and higher-degree data via inputting low-degree data and implicitly assuming a lunar-like relation between crustal density and porosity. Albeit the CNN-based method cannot be sensitive to subsurface features not present in the initial dataset, it still provides a new perspective for the gravity field refinement.

PHEBUS on Bepi-Colombo: Post-launch Update and Instrument Performance

The Bepi-Colombo mission was launched in October 2018, headed for Mercury. This mission is a collaboration between Europe and Japan. It is dedicated to the study of Mercury and its environment. It will be inserted into Mercury orbit in December 2025 after a 7-year long cruise. Probing of Hermean Exosphere By Ultraviolet Spectroscopy (PHEBUS) is an ultraviolet Spectrograph and is one of the 11 instruments on-board the Mercury Planetary Orbiter (MPO). It is dedicated to the study of the exosphere of Mercury, its composition, dynamics and variability and its interface with the surface of the planet and the solar wind. The PHEBUS instrument contains four distinct detectors covering the spectral range from 55 nm up to 315 nm and two additional narrow windows at 404 nm and 422 nm. It also has a one-degree of freedom mechanism that allows observations along a cone with an half angle of $80^{\circ }$ . This paper follows a detailed presentation of the PHEBUS instrument design that was presented by Chassefiere et al. (Planet. Space Sci. 58:201–223, 2010). Here we present an update of the science objectives and measurement requirements following the results published by the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission. We also present results of the ground calibration campaigns of the flight unit that is currently on-board MPO. In the last part, we present some details of the observations that will be performed during the cruise to Mercury, such as stellar observation campaigns, interplanetary background observations and planetary flybys.

Upstream Ultra‐Low Frequency Waves Observed by MESSENGER's Magnetometer: Implications for Particle Acceleration at Mercury's Bow Shock

AbstractWe perform the first statistical analysis of the main properties of waves observed in the 0.05–0.41 Hz frequency range in the Hermean foreshock by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Magnetometer. Although we find similar polarization properties to the “30 s” waves observed at the Earth's foreshock, the normalized wave amplitude (δB/|B0|∼0.2) and occurrence rate (∼0.5%) are much smaller. This could be associated with relatively lower backstreaming proton fluxes, the smaller foreshock size and/or less stable solar wind (SW) conditions around Mercury. Furthermore, we estimate that the speed of resonant backstreaming protons in the SW reference frame (likely source for these waves) ranges between 0.95 and 2.6 times the SW speed. The closeness between this range and what is observed at other planetary foreshocks suggests that similar acceleration processes are responsible for this energetic population and might be present in the shocks of exoplanets.

The Shape of Mercury's Magnetopause: The Picture From MESSENGER Magnetometer Observations and Future Prospects for BepiColombo

AbstractThe MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft orbited Mercury between March 2011 and April 2015, typically passing through the magnetosphere, magnetosheath, and interplanetary magnetic field, on each orbit. Using data from the Magnetometer, we identify magnetopause crossings for the complete orbital mission and model the average boundary shape. We find that Mercury's average magnetopause is well modeled by both an axisymmetric shape and a three‐dimensional shape containing indentations in the cusp regions and a magnetotail that is wider in the north‐south versus east‐west direction. Examination of MESSENGER data and the use of simulated crossings show that coverage is very limited in the cusp region. Parameters describing the indentation are poorly constrained, and model fits may be biased due to the combination of orbital geometry and magnetospheric dynamics. The distribution of MESSENGER data inside the magnetopause, together with minimum variance analysis of a representative subset of magnetopause crossings, hints at the presence of a cusp indentation, though likely shallower than that estimated from the observed crossing positions. The BepiColombo spacecraft will arrive at Mercury in 2025. We simulate expected data coverage and magnetopause crossings using both axisymmetric and three‐dimensional magnetopause models. BepiColombo will improve our understanding of the magnetopause shape by providing data over the southern hemisphere and regions of the northern cusp indentation not covered by MESSENGER. There is also the potential for simultaneous measurements of the interplanetary magnetic field and magnetopause position.

The Chaotic Terrains of Mercury Reveal a History of Planetary Volatile Retention and Loss in the Innermost Solar System

Mercury’s images obtained by the 1974 Mariner 10 flybys show extensive cratered landscapes degraded into vast knob fields, known as chaotic terrain (AKA hilly and lineated terrain). For nearly half a century, it was considered that these terrains formed due to catastrophic quakes and ejecta fallout produced by the antipodal Caloris basin impact. Here, we present the terrains’ first geologic examination based on higher spatial resolution MESSENGER (MErcury Surface Space ENvironment GEochemistry and Ranging) imagery and laser altimeter topography. Our surface age determinations indicate that their development persisted until ~1.8 Ga, or ~2 Gyrs after the Caloris basin formed. Furthermore, we identified multiple chaotic terrains with no antipodal impact basins; hence a new geological explanation is needed. Our examination of the Caloris basin’s antipodal chaotic terrain reveals multi-kilometer surface elevation losses and widespread landform retention, indicating an origin due to major, gradual collapse of a volatile-rich layer. Crater interior plains, possibly lavas, share the chaotic terrains’ age, suggesting a development associated with a geothermal disturbance above intrusive magma bodies, which best explains their regionality and the enormity of the apparent volume losses involved in their development. Furthermore, evidence of localized, surficial collapse, might reflect a complementary, and perhaps longer lasting, devolatilization history by solar heating.

A New In Situ Quasi-continuous Solar-wind Source of Molecular Water on Mercury

Abstract Radar observations of Mercury and the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft data indicate the probable existence of water ice in the permanently shadowed polar regions. Generally, water is accepted to be of exogenous origin through delivery via comets and meteoritic impact. However, a continuous water formation process that involves thermal transformation of chemically stable mineral-bound hydroxyl groups produced by implanted solar-wind protons is readily available on the surface of Mercury. At typical temperatures prevailing on Mercury’s dayside surface, H2O can be produced from reactions involving OH groups on or within the H-saturated regolith grain interfaces. Similar reactions will also occur due to micrometeorite impact events on both the dayside and nightside. Once produced, H2O is released into the exosphere and then transported and processed via Jeans escape, photodissociation, dissociative adsorption, or condensation. Water reaching cold traps will be bound over geological periods. This simple water cycle will produce a highly chemically reduced surface and can deliver significant amounts of H2O to the permanently shadowed regions of Mercury over geological time periods. The overall process is an important but hitherto unnoticed source term that will contribute to the accumulation of water in the cold traps and polar regions of Mercury.

The Flapping Motion of Mercury's Magnetotail Current Sheet: MESSENGER Observations

AbstractThe flapping motion of Mercury's magnetotail current sheet is investigated based on the observations of MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER). The typical flapping period in Mercury's magnetotail is much shorter (~13 s), comparing with ~10–20 min at Earth's magnetotail. The magnetic field oscillation amplitude induced by flapping motion is larger near both flanks than that around tail center. Similar to Earth's magnetotail, there are two flapping types existent in Mercury's magnetotail, one is the kink‐like flapping that can propagate as traveling waves, and the other one is the steady flapping that does not propagate. Both flapping types distribute widely in magnetotail. The kink‐like flapping waves propagating either dawnward or duskward can be observed across the entire magnetotail current sheet, which suggests that the energy sources for triggering the kink‐like flapping waves should locate around both tail flanks instead of tail center as is for the Earth's magnetotail.

Observations of Extreme ICME Ram Pressure Compressing Mercury’s Dayside Magnetosphere to the Surface

Abstract Mercury’s magnetosphere is known to be affected by the enhanced ram pressure and magnetic fields inside interplanetary coronal mass ejections (ICMEs). Here we report detailed observations of an ICME compressing Mercury’s dayside magnetosphere to the surface. A fast CME launched from the Sun on 2013 November 29 impacted first the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, which was orbiting Mercury, on November 30 and later STEREO-A near 1 au on December 1. Following the ICME impact, MESSENGER remained in the solar wind as the spacecraft traveled inwards and northwards toward Mercury’s surface until it reached and passed its closest approach to the planet (at 371 km altitude) without crossing into the magnetosphere. The magnetospheric crossing finally occurred 1 minute before reaching the planet’s nightside at 400 km altitude and 84°N latitude, indicating the lack of dayside magnetosphere on this orbit. In addition, the peak magnetic field measured by MESSENGER at this time was 40% above the values measured in the orbits just prior to and after the ICME, a consequence of the magnetospheric compression. Using both a proxy method at Mercury and measurements at STEREO-A, we show that the extremely high ram pressure associated with this ICME was more than high enough to collapse Mercury’s weak magnetosphere. As a consequence, the ICME plasma likely interacted with Mercury’s surface, evidenced by enhanced sodium ions in the exosphere. The collapse of Mercury’s dayside magnetosphere has important implications for the habitability of close-in exoplanets around M dwarf stars, as such events may significantly contribute to planetary atmospheric loss in these systems.

A morphological evaluation of crater degradation on Mercury: Revisiting crater classification with MESSENGER data

Observations of impact crater morphology can be used to gain insight into the geological history and evolution of a planet's surface. Image data from the Mariner 10 mission revealed the diversity of impact crater morphologies and degradational states on Mercury, leading to early studies that sought to establish a stratigraphic column for the planet, despite only acquiring image data for ~45% of the surface. In 2011, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft entered orbit around Mercury, returning a high-resolution global image dataset that enables a robust analysis of crater morphology and degradation to be completed for the entirety of Mercury's surface. In this study, we conducted a visual classification of crater degradation according to initial crater morphology, and assigned a degradation state to all craters on Mercury ≥40 km in diameter. In our scheme, Class 1 craters are those that are heavily degraded, and Class 5 craters are very fresh with bright ray systems. We discuss the processes involved in crater degradation and erasure, and the challenges associated with applying crater degradation to derive the timing of geological events. We found that, based on the global spatial density of craters in each class, there appears to be a dearth of Class 1 craters within the intercrater plains, likely due to several ancient basin-sized impacts effectively obliterating a considerable portion of craters ≥40 km in diameter in this region. The crater degradation database we present here will serve as a useful tool for future analyses of Mercury's geological evolution.

Constraining the Early History of Mercury and Its Core Dynamo by Studying the Crustal Magnetic Field

AbstractLow‐altitude magnetic field data acquired by MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) over a small portion of Mercury's surface revealed weak crustal magnetic field signatures. Here we study the crustal magnetic anomalies associated with impact craters on Mercury. We assume that the sources of these anomalies consist of impact melt, enriched in impactor iron. We assume that the subsurfaces of Mercury's impact craters have cooled in the presence of a constant global magnetic field, thus becoming thermoremanently magnetized. We invert for the crustal magnetization direction within five craters using a unidirectional magnetization model which assumes that the melt impact rocks recorded the constant core magnetic field present when the crater was formed and that the crater's magnetization has not been altered since its formation. From the best fitting magnetization direction we then obtain the corresponding north magnetic paleopole position assuming a centered core dipolar field. Results show that all five magnetic paleopoles lie in the southern hemisphere but are not required to be located near the present‐day magnetic pole, which lies near the south geographic pole. Accounting for the uncertainties, we show that our results all agree in a common small region that excludes the current magnetic pole. This strongly suggests that the dynamo has evolved with time. Our results represent valuable information for understanding the evolution of Mercury and emphasize the importance of including more anomaly analyses to complete and refine our conclusions.