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

Characterizing Interplanetary Coronal Mass Ejection-related Forbush Decreases at Mercury Using MESSENGER Observations: Identification of a One- or Two-step Structure

The large-scale magnetic structure of interplanetary coronal mass ejections (ICMEs) has been shown to cause decreases in the galactic cosmic ray (GCR) flux measured in situ by spacecraft, known as Forbush decreases (Fds). We use measurements of the GCR count rate obtained by NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during its orbital phase around Mercury to identify such Fds related to the passage of ICMEs and characterize their structure. Of the 42 ICMEs with corresponding high-quality GCR data, 79% are associated with a Fd. Thus, a total of 33 ICME-related Fds were identified, 24 of which (73%) have a two-step structure. We use a superposed epoch analysis to build an average Fd profile at MESSENGER and find that despite the variability of individual events, a two-step structure is produced and is directly linked with the magnetic boundaries of the ICME. By using results from previous studies at Earth and Mars, we also address whether two-step Fds are more commonly observed closer to the Sun; we found that, although likely, this is not conclusive when comparing to the wide range of results of previous studies conducted at Earth. Finally, we find that the percentage decrease in GCR flux of the Fd is greater at MESSENGER on average than at Earth and Mars, decreasing with increasing heliocentric distance. The relationship between the percentage decrease and maximum hourly decrease is also in agreement with previous studies, and follows trends relating to the expansion of ICMEs as they propagate through the heliosphere.

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.

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.

MESSENGER Observations and Global Simulations of Highly Compressed Magnetosphere Events at Mercury

AbstractWe identify and examine all MErcury Surface Space ENvironment, GEochemistry, and Ranging (MESSENGER) crossings of Mercury's dayside magnetopause with magnetospheric field intensities ≥300 nT. The eight such events, which occurred under highly compressed magnetosphere conditions, are analyzed in the identical manner utilized by Slavin et al. (2014, https://doi.org/10.1002/2014JA020319). The results suggest that the eight highly compressed magnetosphere events represent the highest solar wind dynamic pressures for which the MESSENGER's orbit still passed below the magnetopause and provided measurements of the dayside magnetosphere. Using the magnetohydrodynamic model by Jia et al. (2015, https://doi.org/10.1002/2015JA021143) that electromagnetically couples Mercury's interior with its magnetosphere, a series of global simulations are conducted to quantitatively characterize the response of Mercury's magnetosphere to solar wind forcing. Combining the MESSENGER observations with the simulations, we have obtained a consistent picture of how Mercury's dayside magnetospheric configuration is controlled, separately and in combination, by induction‐driven shielding and reconnection‐driven erosion. For solar wind pressures of ∼40–90 nPa, compared with the average ∼10–15 nPa at Mercury's orbit, the shielding effects of induction in Mercury's core in standing‐off the solar wind typically exceed the erosion of the dayside magnetosphere due to reconnection for these events, most of which occurred under low magnetic shear conditions. For high magnetic shear across the magnetopause our simulation predicts that reconnection would dominate. Mercury's effective magnetic moment as inferred from magnetopause standoff distance ranges from 170 to 250 for these events. These findings are of crucial importance for understanding the space weathering at Mercury and its contribution to the generation of Mercury's exosphere.

Global Structure and Sodium Ion Dynamics in Mercury's Magnetosphere With the Offset Dipole

AbstractWe conducted global magnetohydrodynamics (MHD) simulation of Mercury's magnetosphere with the dipole offset, which was revealed by MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) observations, in order to investigate its global structure under northward interplanetary magnetic field conditions. Sodium ion dynamics originating from the Mercury's exosphere is also investigated based on statistical trajectory tracing in the electric and magnetic fields obtained from the MHD simulations. The results reveal a north‐south asymmetry characterized by open field lines around the southern polar region and northward deflection of the plasma sheet in the far tail. The asymmetry of magnetic field structure near the planet drastically affects trajectories of sodium ion and thus their pressure distributions and precipitation pattern onto the planet. Weaker magnetic field strength in the southern hemisphere than in the north increases ion loss by precipitation onto the planetary surface in the southern hemisphere. The “sodium ring,” which is formed by high‐energy sodium ions drifting around the planet, is also found in the vicinity of the planet. The sodium ring is almost circular under nominal solar wind conditions. The ring becomes partial under high solar wind density, because dayside magnetosphere is so compressed that there is no space for the sodium ions to drift around. In both cases, the sodium ring is formed by sodium ions that are picked up, accelerated in the magnetosheath just outside the magnetopause, and reentered into the magnetosphere due to combined effects of finite Larmor radius and convection electric field in the dawnside magnetosphere.

MESSENGER observations of the energization and heating of protons in the near‐Mercury magnetotail

AbstractThe energization and heating processes for protons in the near‐Mercury tail are examined with MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) observations. In a case study, suprathermal proton particle flux (STPF) and proton temperature are observed to be clearly enhanced during near‐Mercury substorm dipolarizations, indicating the proton energization and heating processes. STPF and proton temperature distributions in near‐Mercury central plasma sheets display dawn‐dusk asymmetries, with higher values in the dawnside plasma sheet, i.e., postmidnight, than in the duskside, i.e., premidnight. Further investigations reveal that these asymmetries are more prominent during active periods in Mercury's magnetosphere, as compared to quiet periods. Magnetic field variations in the ZMSM component display a similar feature, with variations being more prominent on the dawnside than the duskside during active periods. We propose that the dawn‐dusk asymmetry in the distributions of protons could be due to the fact that more substorm dipolarizations were initiated on the dawnside of Mercury's magnetotail.

MESSENGER observations of cusp plasma filaments at Mercury

AbstractThe MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft while in orbit about Mercury observed highly localized, ~3‐s‐long reductions in the dayside magnetospheric magnetic field, with amplitudes up to 90% of the ambient intensity. These magnetic field depressions are termed cusp filaments because they were observed from just poleward of the magnetospheric cusp to midlatitudes, i.e., ~55° to 85°N. We analyzed 345 high‐ and low‐altitude cusp filaments identified from MESSENGER magnetic field data to determine their physical properties. Minimum variance analysis indicates that most filaments resemble cylindrical flux tubes within which the magnetic field intensity decreases toward its central axis. If the filaments move over the spacecraft at an estimated magnetospheric convection speed of ~35 km/s, then they have a typical diameter of ~105 km or ~7 gyroradii for 1 keV H+ ions in a 300 nT magnetic field. During these events, MESSENGER's Fast Imaging Plasma Spectrometer observed H+ ions with magnetosheath‐like energies. MESSENGER observations during the spacecraft's final low‐altitude campaign revealed that these cusp filaments likely extend down to Mercury's surface. We calculated an occurrence‐rate‐normalized integrated particle precipitation rate onto the surface from all filaments of (2.70 ± 0.09) × 1025 s−1. This precipitation rate is comparable to published estimates of the total precipitation rate in the larger‐scale cusp. Overall, the MESSENGER observations analyzed here suggest that cusp filaments are the magnetospheric extensions of the flux transfer events that form at the magnetopause as a result of localized magnetic reconnection.

MESSENGER observations of induced magnetic fields in Mercury's core

AbstractOrbital data from the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft allow investigation of magnetic fields induced at the top of Mercury's core by time‐varying magnetospheric fields. We used 15 Mercury years of observations of the magnetopause position as well as the magnetic field inside the magnetosphere to establish the presence and magnitude of an annual induction signal. Our results indicate an annual change in the internal axial dipole term, g10, of 7.5 to 9.5 nT. For negligible mantle conductivity, the average annual induction signal provides an estimate of Mercury's core radius to within ±90 km, independent of geodetic results. Larger induction signals during extreme events are expected but are challenging to identify because of reconnection‐driven erosion. Our results indicate that the magnetopause reaches the dayside planetary surface 1.5–4% of the time.

MESSENGER observations of solar energetic electrons within Mercury's magnetosphere

AbstractDuring solar energetic particle (SEP) events, the inner heliosphere is bathed in MeV electrons. Through magnetic reconnection, these relativistic electrons can enter the magnetosphere of Mercury, nearly instantaneously filling the regions of open field lines with precipitating particles. With energies sufficient to penetrate solid aluminum shielding more than 1 mm thick, these electrons were observable by a number of sensors on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. Because of its thin shielding, frequent sampling, and continuous temporal coverage, the Fast Imaging Plasma Spectrometer provided by far the most sensitive measurements of MeV electrons of all MESSENGER sensors. Sharp changes in energetic electron flux coincided with topological boundaries in the magnetosphere, including the magnetopause, polar cap, and central plasma sheet. Precipitating electrons with fluxes equal to ~40% of their corresponding upstream levels were measured over the entire polar cap, demonstrating that electron space weathering of Mercury's surface is not limited to the cusp region. We use these distinct precipitation signatures acquired over 33 orbits during 11 SEP events to map the full extent of Mercury's northern polar cap. We confirm a highly asymmetric polar cap, for which the dayside and nightside boundary latitudes range over ~50–70°N and ~30–60°N, respectively. These latitudinal ranges are consistent with average models of Mercury's magnetic field but exhibit a large variability indicative of active dayside and nightside magnetic reconnection processes. Finally, we observed enhanced electron fluxes within the central plasma sheet. Although these particles cannot form a stable ring current around the planet, their motion results in an apparent trapped electron population at low latitudes in the magnetotail.

MESSENGER observations of the dayside low‐latitude boundary layer in Mercury's magnetosphere

AbstractObservations from MErcury Surface Space ENvironment GEochemistry, and Ranging (MESSENGER)'s Magnetometer and Fast Imaging Plasma Spectrometer instruments during the first orbital year have resulted in the identification of 25 magnetopause crossings in Mercury's magnetosphere with significant low‐latitude boundary layers (LLBLs). Of these crossings 72% are observed dawnside and 65% for northward interplanetary magnetic field. The estimated LLBL thickness is 450 ± 56 km and increases with distance to noon. The Na+ group ion is sporadically present in 14 of the boundary layers, with an observed average number density of 22 ± 11% of the proton density. Furthermore, the average Na+ group gyroradii in the layers is 220 ± 34 km, the same order of magnitude as the LLBL thickness. Magnetic shear, plasma β and reconnection rates have been estimated for the LLBL crossings and compared to those of a control group (non‐LLBL) of 61 distinct magnetopause crossings which show signs of nearly no plasma inside the magnetopause. The results indicate that reconnection is significantly slower, or even suppressed, for the LLBL crossings compared to the non‐LLBL cases. Possible processes that form or impact the LLBL are discussed. Protons injected through the cusp or flank may be important for the formation of the LLBL. Furthermore, the opposite asymmetry in the Kelvin‐Helmholtz instability (KHI) as compared to the LLBL rules out the KHI as a dominant formation mechanism. However, the KHI and LLBL could be related to each other, either by the impact of sodium ions gyrating across the magnetopause or by the LLBL preventing the growth of KH waves on the dawnside.

MESSENGER observations of large dayside flux transfer events: Do they drive Mercury's substorm cycle?

AbstractThe large‐scale dynamic behavior of Mercury's highly compressed magnetosphere is predominantly powered by magnetic reconnection, which transfers energy and momentum from the solar wind to the magnetosphere. The contribution of flux transfer events (FTEs) at the dayside magnetopause to the redistribution of magnetic flux in Mercury's magnetosphere is assessed with magnetic field data acquired in orbit about Mercury by the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. FTEs with core fields greater than the planetary field just inside the magnetopause are prevalent at Mercury. Fifty‐eight such large‐amplitude FTEs were identified during February and May 2012, when MESSENGER sampled the subsolar magnetosheath. The orientation of each FTE was determined by minimum variance analysis, and the magnetic flux content of each was estimated using a force‐free flux rope model. The average flux content of the FTEs was 0.06 MWb, and their durations imply a transient increase in the cross‐polar cap potential of ~25 kV. For a substorm timescale of 2–3 min, as indicated by magnetotail flux loading and unloading, the FTE repetition rate (10 s) and average flux content (assumed to be 0.03 MWb) imply that FTEs contribute at least ~30% of the flux transport required to drive the Mercury substorm cycle. At Earth, in contrast, FTEs are estimated to contribute less than 2% of the substorm flux transport. This result implies that whereas at Earth, at which steady‐state dayside reconnection is prevalent, multiple X‐line dayside reconnection and associated FTEs at Mercury are a dominant forcing for magnetospheric dynamics.

Thermal evolution of Mercury as constrained by MESSENGER observations

AbstractOrbital observations of Mercury by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft provide new constraints on that planet's thermal and interior evolution. Specifically, MESSENGER observations have constrained the rate of radiogenic heat production via measurement of uranium, thorium, and potassium at the surface, and identified a range of surface compositions consistent with high‐temperature, high‐degree partial melts of the mantle. Additionally, MESSENGER data have placed new limits on the spatial and temporal variation in volcanic and tectonic activity and enabled determination that the planet's core is larger than previously estimated. Because Mercury's mantle layer is also thinner than previously thought, this result gives greater likelihood to the possibility that mantle convection is marginally supercritical or even that the mantle is not convecting. We simulate mantle convection and magma generation within Mercury's mantle under two‐dimensional axisymmetry and a broad range of conditions to understand the implications of MESSENGER observations for the thermal evolution of the planet. These models demonstrate that mantle convection can persist in such a thin mantle for a substantial portion of Mercury's history, and often to the present, as long as the mantle is thicker than ~300 km. We also find that magma generation in Mercury's convecting mantle is capable of producing widespread magmas by large‐degree partial melting, consistent with MESSENGER observations of the planet's surface chemistry and geology.

Distribution and compositional variations of plasma ions in Mercury's space environment: The first three Mercury years of MESSENGER observations

AbstractWe have analyzed measurements of planetary ions near Mercury made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Fast Imaging Plasma Spectrometer (FIPS) over the first three Mercury years of orbital observations (25 March 2011 through 31 December 2011). We determined the composition and spatial distributions of the most abundant species in the regions sampled by the MESSENGER spacecraft during that period. In particular, we here focus on altitude dependence and relative abundances of species in a variety of spatial domains. We used observed density as a proxy for ambient plasma density, because of limitations to the FIPS field of view. We find that the average observed density is 3.9 × 10–2 cm–3 for He2+, 3.4 × 10–4 cm–3 for He+, 8.0 × 10–4 cm–3 for O+‐group ions, and 5.1 × 10–3 cm–3 for Na+‐group ions. Na+‐group ions are particularly enhanced over other planetary ions (He+ and O+ group) in the northern magnetospheric cusp (by a factor of ~2.0) and in the premidnight sector on the nightside (by a factor of ~1.6). Within 30° of the equator, the average densities of all planetary ions are depressed at the subsolar point relative to the dawn and dusk terminators. The effect is largest for Na+‐group ions, which are 49% lower in density at the subsolar point than at the terminators. This depression could be an effect of the FIPS energy threshold. The three planetary ion species considered show distinct dependences on altitude and local time. The Na+ group has the smallest e‐folding height at all dayside local times, whereas He+ has the largest. At the subsolar point, the e‐folding height for Na+‐group ions is 590 km, and that for the O+ group and He+ is 1100 km. On the nightside and within 750 km of the geographic equator, Na+‐group ions are enhanced in the premidnight sector. This enhancement is consistent with nonadiabatic motion and may be observational evidence that nonadiabatic effects are important in Mercury's magnetosphere.

MESSENGER observations of magnetopause structure and dynamics at Mercury

On 18 March 2011, MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) became the first spacecraft to orbit Mercury, providing a new opportunity to study the outer boundary of the planet's magnetosphere—the magnetopause. Here we characterize Mercury's magnetopause using measurements collected by MESSENGER's Magnetometer and Fast Imaging Plasma Spectrometer. Analysis of measurements from two of MESSENGER's “hot seasons,” when the orbital periapsis is on Mercury's dayside and the magnetopause crossing takes place in the subsolar region, resulted in 43 events with well‐determined boundary normals. The typical duration of a magnetopause traversal was ~5 s. The average normal magnetic field component was ~20 nT, and the dimensionless reconnection rate, i.e., the ratio of the normal magnetic field component to the total field magnitude just inside the magnetopause, was 0.15 ± 0.02. This rate is a factor of ~3 larger than values found during the most extensive surveys at Earth. The ratio of the reconnection rate at Mercury to that of the Earth is comparable to the ratio of the solar wind Alfvén speeds at their respective orbits. We also find that the magnetopause reconnection rate at Mercury is independent of magnetic field shear angle, but it varies inversely with plasma β, the ratio of total thermal pressure to magnetic pressure, in the magnetosheath. These results suggest that reconnection at Mercury is not only more intense than at Earth but also that it occurs for nearly all orientations of the interplanetary magnetic field due to the low‐β nature of the solar wind in the inner heliosphere.

An MHD simulation of the inner heliosphere during Carrington rotations 2060 and 2068: Comparison with MESSENGER and ACE spacecraft observations

We present results from a new magnetohydrodynamic (MHD) model of the inner heliosphere. The model is adapted from the well-established Lyon–Fedder–Mobarry (LFM) MHD simulation code, which until recently mostly applied to studies of the terrestrial magnetosphere. We perform quasi steady-state simulations of two Carrington rotations: 2060 and 2068. During both of these periods, the heliosphere remained quiet and undisturbed by transient phenomena, making them well-suited for simulation studies of Corotating Interaction Regions (CIRs). The MHD model of the solar wind is driven at the inner boundary by the Wang–Sheeley–Arge (WSA) model of the corona augmented with empirical relations to infer the solar wind velocity, density, and temperature. Here we report on a validation exercise whereby LFM-helio simulation results are compared with in situ data from the Advanced Composition Explorer (ACE) and MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft. We find that the model successfully reproduces the large-scale configuration of the inner heliosphere, namely timing and duration of high-speed streams and heliospheric current sheet crossings, as reflected in ACE and MESSENGER observations. Discrepancies between in situ measurements and simulations, such as 1–2 day errors in the time of arrival of a CIR or the strength of the simulated magnetic field at the spacecraft, are attributed to the uncertainty in the specification of the coronal conditions, rather than a poor performance of the solar wind model. More comparisons between different inner heliosphere models driven with identical coronal conditions are suggested as a way to explore their comparative strengths and weaknesses.

The dayside magnetospheric boundary layer at Mercury

Magnetic field and plasma data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft on the outbound portions of the first (M1) and second (M2) flybys of Mercury reveal a region of depressed magnetic field magnitude and enhanced proton fluxes adjacent to but within the magnetopause, which we denote as a dayside boundary layer. The layer was present during both encounters despite the contrasting dayside magnetic reconnection, which was minimal during M1 and strong during M2. The overall width of the layer is estimated to be between 1000 and 1400 km, spanning most of the distance from the dayside planetary surface to the magnetopause in the mid-morning. During both flybys the magnetic pressure decrease was ∼1.6 nPa, and the width of the inner edge was comparable to proton gyro-kinetic scales. The maximum variance in the magnetic field across the inner edge was aligned with the magnetic field vector, and the magnetic field direction did not change markedly, indicating that the change in field intensity was consistent with an outward plasma-pressure gradient perpendicular to the magnetic field. Proton pressures in the layer inferred from reduced distribution observations were 0.4 nPa during M1 and 1.0 nPa during M2, indicating either that the proton pressure estimates are low or that heavy ions contribute substantially to the boundary-layer plasma pressure. If the layer is formed by protons drifting westward from the cusp, there should be a strong morning–afternoon asymmetry that is independent of the interplanetary magnetic field (IMF) direction. Conversely, if heavy ions play a major role, the layer should be strong in the morning (afternoon) for northward (southward) IMF. Future MESSENGER observations from orbit about Mercury should distinguish between these two possibilities.

The MESSENGER mission: Results from the first two Mercury flybys

NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is well into its voyage to initiate a new era in our understanding of the innermost planet. During 2008 MESSENGER flew by Mercury twice and completed the initial spacecraft reconnaissance of the planet begun 34 years earlier by Mariner 10. MESSENGER observations have illuminated how the 1500-km-diameter Caloris basin was a focus for magmatic and deformational activity. High-resolution images show evidence for volcanic vents, pyroclastic deposits, and volcanic flooding of its interior. Lobate scarps, discovered by Mariner 10, are the dominant tectonic landform across the planet and collectively record greater contraction of the surface than inferred from Mariner 10 images. On portions of the surface not seen by Mariner 10 are young craters with prominent ray systems that span most of a hemisphere. Variations in visible and infrared spectral reflectance correlate with geological units. The lack of an infrared absorption band near 1 μm indicates a low ferrous oxide content in surface silicates, but absorption of thermal neutrons by surface material suggests the presence of surface iron in concentrations similar to low- to intermediate-Fe lunar soils. The most unanticipated result to date is the dynamic and complex nature of Mercury’s exosphere–magnetosphere system. Magnetometer measurements point to a dynamo origin for Mercury’s main magnetic field, but that field is substantially modified by the planet’s small, dynamic magnetosphere. Emission-line measurements have revealed the distribution of neutral sodium, calcium, and magnesium in the exosphere and tail; plasma spectrometer observations indicate that these and other species become ionized to populate the planet’s magnetosphere. MESSENGER’s third Mercury flyby on 29 September 2009, the last prior to orbit insertion in 2011, largely duplicated the geometry of the second but added new observations of the planet’s surface and a new perspective on the in situ environment as solar activity continues to change. A Participating Scientist Program has brought the Science Team to its current strength of 48 scientists and their associates. The spacecraft continues to perform as designed in its challenging thermal environment. Use of solar radiation pressure for maintaining momentum balance and supplementing maneuver capability is helping to maintain propellant reserves that could enable an extended mission. The MESSENGER team is continuing its informal interaction with members of the BepiColombo project to maximize the overall scientific return from both the missions.

Whole-disk spectrophotometric properties of Mercury: Synthesis of MESSENGER and ground-based observations

Disk-integrated and disk-resolved measurements of Mercury’s surface obtained by both the Mercury Dual Imaging System (MDIS) and the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) onboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft were analyzed and compared with previous ground-based observations of Mercury at 11 wavelengths. The spectra show no definitive absorption features and display a red spectral slope (increasing reflectance with increasing wavelength) typical of space-weathered rocky surfaces. The MDIS spectra show evidence of phase reddening, which is not observed in the MASCS spectra. The MDIS spectra are commensurate with ground-based observations to within 10%, whereas the MASCS spectra display greater discrepancies with ground-based observations at near-infrared wavelengths. The derived photometric calibrations provide corrections within 10% for observations taken at phase angles less than ∼100°. The derived photometric properties are indicative of a more compact regolith than that of the lunar surface or of average S-type asteroids. The photometric roughness of the surface is also much smoother than the Moon’s. The calculated geometric albedo (reflectance at zero phase) is higher than lunar values. The lower reflectance of immature units on Mercury compared with immature units on the Moon, in conjunction with the higher geometric albedo, is indicative of more complicated grain structures within Mercury’s regolith.

An international program for Mercury exploration: synergy of MESSENGER and BepiColombo

Mariner 10 has been the only spacecraft to visit the innermost planet Mercury. Its three flybys, more than 25 years ago, yielded the first view of this little-understood world. With advances in spacecraft technology and a growing realization of how important Mercury is to our understanding of the solar system and its formation, two missions are now in development for more intensive Mercury exploration. The first is the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission, competitively selected under the NASA Discovery Program, that will send a spacecraft to fly by Mercury in 2007 and 2008 and to orbit Mercury for one Earth year beginning in April 2009. The second is the more comprehensive BepiColombo mission, consisting of three elements: the Mercury Planetary Orbiter (MPO), the Mercury Magnetospheric Orbiter (MMO), and the Mercury Surface Element (MSE). Still in final definition stage, BepiColombo is a partnership between the European Space Agency (ESA) and the Japanese Institute of Space and Astronautical Science (ISAS). With one or two launches (depending upon the final mission architecture) BepiColombo will use solar electric propulsion to send two orbiters (MPO from ESA and MMO from ISAS) and a lander (MSE) to Mercury as early as 2011. The BepiColombo orbiters, in orbits complementary to that of MESSENGER, will extend geochemical, spectral, and photometric mapping of the planet. With its factor-of-ten larger downlink, BepiColombo will complete the intensive study of Mercury begun with the exploration by MESSENGER. Synergistic strategies of exploration will enable efficient use of BepiColombo resources in a more detailed study of the planet than can be accomplished by MESSENGER alone. For example, the earlier MESSENGER mission can help identify appropriate landing sites for the MSE, while BepiColombo can provide complementary orbital measurements of surface features from different phase angles and exploit MESSENGER observations to target high-resolution measurements. Possible joint activities include cooperative use of ground stations and extension of the temporal baseline for geophysical measurements by combining data from all spacecraft. Were MESSENGER to be approved for an extended orbital phase, there is also the possibility of simultaneous dual-spacecraft measurements in the magnetosphere.