MESSENGER Spacecraft Research Articles

Precise Orbit Determination of the MESSENGER Spacecraft

The NASA MESSENGER mission explored Mercury for more than four years to investigate the properties of the planet. To safely operate in the harsh conditions around Mercury, the spacecraft was in a highly eccentric orbit with a low periapsis altitude. The radiation environment had a strong impact on the spacecraft orbit evolution because of the proximity of Mercury to the Sun. A detailed modeling of the nonconservative forces is then a key factor to enhance the precise orbit determination of the spacecraft. We present here refined models of the nonconservative forces, including thermal reradiation effects, that enabled significant improvements in the trajectory reconstruction. A crossover analysis based on the Mercury Laser Altimeter (MLA) data was carried out to cross-check the accuracy of the orbit determination results. The trajectories retrieved by using the refined spacecraft dynamical model provide reduced height misfit at crossover points, indicating a high-quality reconstruction. Our new solutions of the spacecraft orbits are then archived to be used as auxiliary information for the data analysis of other MESSENGER instruments.

Mercury’s Bow Shock and Magnetopause Variations According to MESSENGER Data

Using data from the MESSENGER spacecraft magnetometer that describes the magnetopause and the bow shock crossing points of the Mercury’s magnetosphere, we have calculated the parameters of the paraboloids of revolution approximating the obtained points. For each spacecraft orbit, the subsolar magnetopause and bow shock standoff distances were obtained, based on the paraboloid parameters for each crossing point. The dependences of the magnetopause and bow shock subsolar standoff distances on the Mercury’s position relative to the Sun have been obtained. These profiles agree with decreases of the solar wind plasma dynamic pressure and the interplanetary magnetic field strength with heliocentric distance. The variations of the interplanetary and magnetosheath magnetic field were investigated. The average subsolar magnetosheath thickness and the value of the magnetic field jump at the bow shock during the transition from the upstream interplanetary magnetic field region to the magnetosheath were obtained.

North–South Plasma Asymmetry Across Mercury's Near‐Tail Current Sheet

AbstractAmong nearly 300 near‐Mercury tail current sheet crossings performed by the MESSENGER spacecraft, we identified 37 traversals of an asymmetric current sheet, wherein the lobe densities on opposite sides differ by a factor of three or more. These asymmetric current sheet crossings primarily occur on the dawnside. A global magnetohydrodynamic (MHD) simulation was found to be in excellent agreement with the observations. The results suggest that the north–south density asymmetry is caused by solar wind entering via an upstream‐connected window in one hemisphere. Furthermore, the Parker spiral interplanetary magnetic field (IMF) controls the near‐tail density asymmetries, whereas Mercury's offset dipole magnetic field controls those in mid‐ or distant‐tail regions. We propose that hemispheric asymmetries in Mercury's magnetospheric convection occur under strong IMF conditions.

The Taurid Resonant Swarm at Mercury

ABSTRACT It has previously been suggested that ejection and vaporization of Hermean surface material by meteoroids from comet 2P/Encke causes a seasonal enhancement in Mercury’s Ca exosphere observed by the NASA MESSENGER spacecraft in 2011-2015. The ESA/JAXA BepiColoen mission, now routeute to Mercury, will likely provide the next set of observational tests of this hypothesis after it enters orbit in late 2025. Here we study the Taurid Swarm Complex (IAU Code: STS), a population of cm-sized or larger meteoroids from Encke’s comet that encounters the Earth every 3–7 yr. Through analysis of previous observations of the STS and many-particle numerical simulations, we study the circumstances of encounters between the STS and Mercury and find that, unlike the Earth where STS encounters is observed in some years but not others, each time the STS is at perihelion it encounters Mercury on three consecutive planetary orbits. We further predict that the STS will encounter this planet during the early stages of BepiColombo’s orbital mission. The temporal flux profile during each encounter will be broad and possibly double-peaked with total number fluence 0.4×–1.7× that of the sporadic fluence for >1 kg meteoroids on the sub-radiant hemisphere of the planet. The meteoroid arrival direction and sub-radiant point strongly depend on True Anomaly Angle, switching from mainly nightside to mainly dayside impacts as Mercury travels from orbital perihelion to aphelion. Our predictions may be used to create detailed models of exosphere generation by Encke stream meteoroids.

Anomalous Response of Mercury’s Magnetosphere to Solar Wind Compression: Comparison to Earth

Magnetic field intensity increases when solar wind compresses a planet’s magnetosphere. The compression can be measured using the ratio of compressed magnetic fields to purely dipolar magnetic fields just inside the magnetopause. For Earth, the ratio is proportional to the subsolar standoff distance of the magnetopause. Data from in-orbit observations by the MESSENGER spacecraft indicate an opposite ratio for Mercury; the compression ratio is inversely proportional to the subsolar standoff distance. The additional magnetic fields induced by currents at the top of Mercury’s core enhance the total magnetic field strength. We also evaluated differences in the subsolar standoff of Mercury’s magnetopause according to the north–south polarity of the interplanetary magnetic field (IMF). Previous studies have not identified meaningful differences in subsolar standoff distance between those in northward versus southward IMF polarities for Mercury; however, we found that the difference is statistically significant at a large IMF B Z (15–20 nT). The magnetic reconnection that occurs behind the cusp for a large northward IMF transfers the magnetic flux to the dayside and increases the subsolar standoff distance. The eroded magnetic flux for a large southward IMF is compensated by the induced magnetic fields.

MESSENGER Observations of Multiple Magnetic Energy Releases during Mercury’s Substorm

Substorms are energy storage and dissipation processes that occur in planetary magnetospheres accompanied by numerous phenomena, such as magnetic dipolarizations, plasma sheet thinning and thickening, magnetic flux ropes, and plasmoid ejections. Recent MESSENGER observations have revealed substorm activity in Mercury’s near magnetotail, with average durations of 2–3 minutes for both the growth and expansion phases. Herein, we report MESSENGER spacecraft observations of sporadic and short-term energy dissipation processes during the substorm expansion phase in Mercury’s magnetotail. Three magnetic dipolarizations lasting for ∼4 minutes were detected during the expansion phase and manifested as step-like enhancements in the northward component of the magnetic field. The magnetic field strength decreased during each dipolarization, indicating an intermittent release of magnetic energy. They arise owing to multiple magnetic pileup effects caused by planetward bursty bulk flows ejected from distant magnetotail reconnections. Additionally, in close proximity to each dipolarization, we observed the occurrence of pulse-like fluctuations in the B y component. These new MESSENGER observations suggest that during the expansion phase of Mercury’s substorm, the energy dissipation can be caused by multiple local transient processes, although the duration is much shorter than that of Earth.

Morphostructural mapping of Borealis Planitia, Mercury

ABSTRACT Orbital data from the MESSENGER spacecraft show that a significant portion of Mercury’s northern hemisphere is covered by smooth plains, which are interpreted to be flood volcanic material and/or impact melt. The smooth plains show pervasive tectonic structures and encompass a broad raised bulge of uncertain geophysical interpretation. In this work, we focus on the mapping of all the morphostructures within the northern smooth plains, aiming at providing a useful dataset for further studies about the mapped area. The structural map is obtained through a twofold process: first with an automatic mapping, using an algorithm to identify all the lineaments from a DEM; and second with a visual inspection and classification of the results of the algorithm in a GIS environment. The final maps are drafted at two different scales, 1:300,000 and 1:600,000. With this approach, we mapped and characterized more than fifty thousand lines marking scarps on the surface, creating a database with several morphometric attributes for each of the identified scarps (e.g. length, azimuth, and height), which can be used for geostatistical study of smooth plains tectonics. Our structural map reveals that: (i) the area is broadly dominated by wrinkle ridges, ghost crater assemblages of lineaments, and scarps related to impact crater processes (e.g. radial faults, secondary crater chains, ejecta emplacement) and that (ii) the amount of strain was not evenly accommodated throughout the northern smooth plains.

Principal Component Analysis applied on MASCS/MESSENGER data for the spectral investigation of Mercury's surface

The surface of Mercury hosts several spectral units with variations in reflectance and spectral slope, mainly identified after the application of Principal Component Analysis (PCA) on MDIS data, the camera onboard the MESSENGER spacecraft. The major terrains identified on Mercury include the smooth plains such as High-reflectance Red Plains (HRP), Intermediate Plains (IP) and Low-reflectance Blue Plains (LBP), the spectrally Intermediate Terrains (IT) and Low-Reflectance Material (LRM). The minor terrains involve hollows, fresh ejecta, faculae and red material connected to impact craters.This work aims to examine the surface of Mercury by applying the PCA on photometrically corrected and quality-filtered spectra acquired by the MASCS-MESSENGER spectrometer.By combining PC1, PC2 and PC3 components in the red-green-blue image, major terrains (HRP, IP/IT, LBP, LRM) can be distinguished from each other; similarly, among the minor terrains, fresh ejecta and hollows are spectrally distinct from the faculae (connected to volcanism) and red material (related to impact craters). Furthermore, the composite image allows discrimination of the faculae with a spectrally distinct deposit from those with a faint spectral contrast. Examination of PC1 and PC2 separates the surface of Mercury into three sections, characterized by a progressive reddening of the Visible (VIS) spectral slope. Section A includes “spectrally blue” units (LRM, LBP, hollows and fresh ejecta) with lower VIS slope values, whereas Section B involves the “spectrally red” units (RM, HRP, faculae and IT) with higher VIS slope values and with an origin connected to volcanism. The variation in VIS slope could be ascribed to differences in composition or space weathering alterations. Section C, with intermediate PC2 and VIS Slope values, includes terrains with variations in reflectance and spectral slopes. Furthermore, Section C brings to the attention areas distributed around the cold poles longitudes and high latitudes in the southern hemisphere with the maximum surface temperature spanning from 450 K to 650 K. Finally, localized regions corresponding to the walls of craters and explosive vents are also revealed from the Section C, suggesting possible exposure of material slightly altered by solar ion irradiation and/or thermal effects.

Automatic Detection of Bow Shock and Magnetopause Positions at Mercury’s Magnetosphere Using MESSENGER Magnetometer Data

The MESSENGER spacecraft completed more than 4000 orbits around Mercury during 4 years of operation on orbit around the planet. Using the onboard magnetometer data, we determine the bow shock and magnetopause crossings in Mercury’s magnetosphere by automatic methods and compare the accuracy of these methods with the results of manual data processing. The developed algorithm makes it possible to significantly increase the magnetometer data processing rate and can be used in the future when analyzing data from the BepiColombo spacecraft, which is going to enter Mercury’s orbit in 2025.

MESSENGER Observations of Reconnection in Mercury's Magnetotail Under Strong IMF Forcing

AbstractThe MESSENGER spacecraft typically crossed Mercury's magnetotail current sheet relatively close to the planet, that is, less than 2.5 RM (planet radius; 2,440 km). Magnetometer measurements are used to detect active reconnection events by identifying the quadrupole Hall magnetic field signatures that form about X‐lines. Statistical analyses of the 51 active reconnection events detected in this manner indicate that they occur most frequently on the duskside and typically at a mean altitude greater than 1.5 RM. In contrast, the dawnside events occur at altitudes of ∼1 RM. In addition, a higher recurrence rate of flux ropes formed in the Hall region was observed on the dawnside. Applying the Kan‐Lee solar wind‐magnetosphere coupling function confirmed that these near‐tail reconnection events at Mercury are observed under strong forcing by the interplanetary magnetic field. We further propose that the reconnection‐driven nightside magnetosphere‐planet interaction is characterized by a pronounced dawn‐dusk asymmetry and may significantly influence the near‐Mercury space environment.

Spectral identification of pyroclastic deposits on Mercury with MASCS/MESSENGER data

The pyroclastic deposits on Mercury are proof of explosive volcanism that occurred on the innermost planet of the Solar System; pyroclastic deposits are usually characterized by a central pit (vent) surrounded by a spectrally bright and red deposit (facula).The pyroclastic deposits on Mercury were mainly identified by analysing images acquired from MDIS onboard the MESSENGER mission, which revealed about 200 deposits.In this work, we focus on the identification of pyroclastic deposits on Mercury by applying spectral conditions on radiometrically and photometrically corrected data acquired by the MASCS spectrometer onboard the MESSENGER spacecraft. In addition, to further optimize the quality of the used spectra, the spectral dataset was limited by applying additional filters on observing conditions and on detector temperature.We selected 8 pyroclastic deposits, out of the 25 deposits recognized by the IAU, as a training field, and we spectrally characterized the area of the deposits by using four spectral parameters, i.e. reflectance and spectral slopes. We defined the range of variability of the parameters able to distinguish the pyroclastic deposits from the Average Mercury Terrain, and we filtered the MASCS data set by applying the spectral conditions. As a result, 52 previously recognized pyroclastic deposits and 16 new potential candidates were revealed across the portion of Mercury's surface observed by MASCS. The method also exposed extended areas such as the High-Reflectance Red Plains (HRP) in Asparangi Planitia, Borealis Planitia, and the floor of Rembrandt crater, in addition to Intermediate Terrains (IT) and Intermediate Plains (IP).Nathair Facula, Agwo Facula, Abeeso Facula and an unnamed facula that we termed “Facula 5” show a redder UV slope than most of the recognized pyroclastic deposits, possibly associated with fine regolith and/or a minor amount of Fe or C-phases in the minerals composing the facula.Among the 16 new bright and red areas, 6 potential candidates strongly suggest a nature connected to explosive volcanism and could be considered as pyroclastic deposits.

The Influence of Dynamic Solar Oblateness on Tracking Data Analysis from Past and Future Mercury Missions

When the BepiColombo spacecraft arrives at Mercury in late 2025, it will be able to measure the orbit of the planet with unprecedented accuracy, allowing for more accurate measurements of the perihelion advance of the planet, as predicted by the Theory of General Relativity (GR). A similar effect is produced by the gravitational oblateness of the Sun through the zonal coefficient J2⊙. The gravitational field of the Sun has been hard to determine despite centuries of observations, causing great uncertainties in experiments on GR. Recent publications in heliophysics suggest that J2⊙ is not a constant, but a dynamic value that varies with solar magnetic activity. The aim of this paper is to analyse what the effect is of suggested higher-order effects of the solar gravitational field on experiments of the perihelion advance of Mercury as predicted by GR. The orbit of Mercury and observations of the MESSENGER and BepiColombo spacecraft are simulated, and parameters corresponding to gravitational theory, as well as the oblateness J2⊙ including a time-variable component are estimated using a least-squares approach. The result of the estimation is that the amplitude of a periodic component can be found with an uncertainty of 3.7×10−11, equal to 0.017% the value of J2⊙. From analysis of published experiments that used MESSENGER tracking data, it can already be deduced that the amplitude of the periodic variation cannot be higher than 5% of the value of J2⊙. It is also found that if a periodic component exists with an amplitude greater than 0.04% the value of J2⊙ and it is not considered, it can lead to errors in the experiments of GR using BepiColombo data to the point that results falsely confirm or contradict the Theory of General Relativity.

Geology of the Derain quadrangle (H10), Mercury

ABSTRACT We present the results of geological mapping of Mercury’s Derain (H10) quadrangle (0°-72°E and 22.5°N-22.5°S) using data from the MESSENGER spacecraft. The map is presented on a scale of 1:3,000,000, for which linework was drawn at 1:300,000. We distinguish three major morphological plains units: Smooth, Intermediate, and Intercrater Plains. We produced two versions of the map, with craters classified according to a 3- and 5-class degradation system. This allows compatibility with other MESSENGER-era maps and Mariner 10-era maps. This map will help provide science context for the ESA-JAXA BepiColombo mission to Mercury.

Electron-scale Magnetic Peaks Upstream of Mercury’s Bow Shock: MESSENGER Observations

Electron-scale magnetic peaks (ESMPs) with spatial sizes less than one local ion gyroradius have been recently revealed to exist in the terrestrial magnetosheath and solar wind at 1 au. Whether they widely exist in the astrophysical plasma is unclear. Here, we investigate the magnetic peaks with a period of 0.1–100 s upstream of Mercury’s bow shock by using the magnetic field data from the MESSENGER spacecraft. Based on the distribution of their durations, these magnetic peaks can be divided into two groups: one with durations less than 0.6 s and the other with durations larger than 0.6 s. The durations in each group obey a log-normal distribution. The magnetic peaks with durations less than 0.6 s are inferred to be electron scale, suggesting that ESMPs exist in the solar wind at Mercury’s orbit. The median duration of these ESMPs is ∼0.3 s. The ESMPs have a larger occurrence rate near the bow shock and prefer to occur when the ambient interplanetary magnetic field (IMF) can be connected to the bow shock, which suggests that the foreshock could be one source region of these ESMPs. Their occurrence rate also tends to be larger when the IMF strength is weaker. Our observations also suggest that some ESMPs originate from the upstream solar wind. The properties of the ESMPs found here could help to shed light on their generation mechanisms and their roles in the astrophysical plasma.

Arecibo S-band Radar Characterization of Local-scale Heterogeneities within Mercury’s North Polar Deposits

Abstract Ground-based planetary radar observations first revealed deposits of potentially nearly pure water ice in some permanently shadowed regions (PSRs) on Mercury’s poles. Later, the MESSENGER spacecraft confirmed the icy nature of the deposits, as well as their location within PSRs. Considering the geologic context provided by MESSENGER, we further characterized the north polar deposits by pairing spacecraft data with new Arecibo S-band radar observations. Here we show that some ice deposits within PSRs have a gradational pattern in their radar properties that is likely associated with differences in ice purity. Radar-bright features with a circular polarization ratio μ c > 1 can be characterized by water ice with ≳3% impurities by volume while those with μ c < 1 by ≳20% impurities. Furthermore, areas in PSRs with μ c < 1 typically surround locations of stronger radar backscatter with μ c > 1. Therefore, deposits of nearly pure water ice are likely surrounded by lower-purity material, such as water-ice-rich regolith, which could be the result of impact gardening or the crater’s thermal environment. However, such deposits are not always colocated within large polar craters where ice should be the most stable, even at the surface. In fact, we found that there is no significant difference between the radar backscattering properties of deposits thought to have surficial ice and those with buried ice. Our results also help improve the identification of icy reservoirs elsewhere, such as the Moon. Indeed, we found that μ c is not an adequate diagnostic, but rather the radar backscatter in each circular polarization independently provides information to identify water-ice deposits.

Identification of the planetary magnetosphere boundaries with the wavelet multi-resolution analysis

The study of the properties and variations of planetary boundaries such as magnetosheaths and bow shocks is an important subject for magnetospheric dynamics and interaction with solar wind. The identification of these boundaries is important for those studies. Thus, the Haar wavelet decomposition technique is used to detect the planetary magnetosphere boundaries and discontinuities. We use the magnetometer data from the CASSINI and MESSENGER spacecraft to identify the abrupt changes in the magnetic field when the spacecraft crossed the magnetospheric bow shocks and magnetopauses of Saturn and Mercury, respectively. The methodology based on variance obtained by scale and edge identifications was shown to be a simple tool to perform this task. The results confirm that the Haar transform can efficiently identify the planetary magnetosphere boundaries characterized by the abrupt magnetic field changes. Due to this wavelet function to be a discrete function it promotes the abrupt and sharp identification of the boundaries. It is suggested that this technique can be applied to detect the planetary boundaries as well as the discontinuities such as the shock waves in the interplanetary space.

Bifurcated Current Sheets in Mercury's Magnetotail: Observations and Implications

AbstractCross‐tail current sheets play an important role in magnetospheric dynamics and plasma transport. We present a comprehensive study of the structure and behavior of Mercury's cross‐tail current sheet, as well as constraints on the plasma environment imposed by this structure, using magnetic field measurements from the MESSENGER spacecraft. We find that the current sheet dominantly exhibits a bifurcated, or double‐peaked, current density structure throughout the tail region, but that a single‐peaked sheet is sometimes observed. The time‐averaged current distribution in the nightside magnetosphere is as follows: close to the planet on the midnight plane at an altitude of 0.15 RM (where RM is Mercury radius = 2,440 km), current density peaks are 0.2 RM north and south of the magnetic equator. The peak‐to‐peak separation decreases with down‐tail distance, until the current density profile resembles that of a single‐peaked sheet. We find no correlation between the sheet geometry and long‐period variations in solar wind and magnetospheric conditions. We attribute the observed bifurcated structure in the current sheet to non‐adiabatic behavior (Speiser orbits) of the major plasma constituents in Mercury's magnetosphere (H+, Na+ and, to a lesser extent e−). Finally, we infer the spatial distribution of the current carriers by comparing a simple model of their resultant current sheets to observations. We find that, close to the planet, a combination of planetary sodium ions and solar wind protons with energies of a few keV are the main current carriers, but in the far‐tail region, the current is dominantly carried by solar wind protons.

Detection of Large Exospheric Enhancements at Mercury due to Meteoroid Impacts

Abstract The Ultraviolet and Visible Spectrometer (UVVS) on the MESSENGER spacecraft observed three large transient events in Mercury’s nightside “tail” in which the exospheric brightness increased by an order of magnitude. Meteoroid impacts are the best explanation given that the events are brief, can be simulated with instantaneous injections of vapor, and were not associated with unusual solar wind conditions. Data–model comparisons suggest that the impactors are 10–20 cm in diameter and produce vapor temperatures of ∼104 K, much warmer than usually assumed for impact vapor. We estimate the impact frequency to be on the order of once per Earth day for meteoroids 10 cm diameter and larger, consistent with a pre-MESSENGER prediction. UVVS observed three atomic species during one event: sodium, magnesium, and calcium. Na and Mg brightened simultaneously, and their modeled ejection ratio roughly matches Mercury’s surface abundance. Ca showed no sign of an enhancement, consistent with earlier predictions that Ca in impact vapor is bound in a molecule that is undetectable to UVVS. This event provides an unprecedented opportunity to see three species respond (or not) to a single source and has implications for our understanding of Mercury’s exosphere.

Middle Corona Magnetic Field Strength Determined by Spacecraft Radio Faraday Rotation

Abstract Faraday rotation (FR) of MESSENGER spacecraft X-band transcoronal radio transmissions were studied to evaluate the mid-coronal magnetic field strength, with the line-of-sight closest solar approach at heliocentric altitudes ranging 1.647–1.817 R ⊙. A Community Coordinated Modeling Center (CCMC) CORHEL-MAS 3D coronal field map revealed this region to contain closed magnetic fields. By FR analysis we found a field strength of 118,000 nT at r = 1.647 R ⊙, with fall-off to 40,000 nT by r = 1.817 R ⊙. These values straddle estimates provided by the CCMC model. The mean value of 79,000 nT at r = 1.732 R ⊙ is comparable to the value provided by an established empirical model for average coronal magnetic field strength. FR can be used to evaluate middle coronal magnetic fields, but improved methods to constrain concurrent electron column density will be needed to produce the most accurate results.

Updated values of solar gravitational moments J2n using HMI helioseismic inference of internal rotation

ABSTRACT The solar gravitational moments J2n are important astronomical quantities whose precise determination is relevant for solar physics, gravitational theory and high precision astrometry, and celestial mechanics. Accordingly, we propose in the present work to calculate new values of J2n (for n = 1, 2, 3, 4, and 5) using recent two-dimensional rotation rates inferred from the high-resolution Solar Dynamics Observatory / Helioseismic and Magnetic Imager helioseismic data spanning the whole solar activity cycle 24. To this aim, a general integral equation relating J2n to the solar internal density and rotation is derived from the structure equations governing the equilibrium of slowly rotating stars. For comparison purpose, the calculations are also performed using rotation rates obtained from a recently improved analysis of Solar and Heliospheric Observatory / Michelson Doppler Imager heliseismic data for solar cycle 23. In agreement with earlier findings, the results confirmed the sensitivity of high-order moments (n > 1) to the radial and latitudinal distribution of rotation in the convective zone. The computed value of the quadrupole moment J2 (n = 1) is in accordance with recent measurements of the precession of Mercury’s perihelion deduced from high precision ranging data of the MESSENGER spacecraft. The theoretical estimate of the related solar oblateness Δ⊙ is consistent with the most accurate space-based determinations, particularly the one from Reuven Ramathy High-Energy Solar Spectroscopic Imager/Solar Aspect Sensor.