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

Low sulfide concentration in Mercury’s smooth plains inhibits hollows

MESSENGER (Mercury, Surface, Space Environment, Geochemistry, and Ranging) mission to Mercury led to the discovery of hollows. These geological landforms have no close counterpart on other airless silicate bodies. Multispectral images and geochemical measurements by MESSENGER suggest that hollows are formed by the loss of volatile-bearing minerals. We investigated the mineralogical composition of the hollows using near-ultraviolet to near-infrared spectra obtained by MESSENGER. We compared reflectance spectra of hollows with laboratory spectra of Mercury's analogs: sulfides, chlorides, silicates, and graphite. The best candidates to reproduce the curvature of the hollow spectra are calcium sulfide, magnesium sulfide, and sodium sulfide. In addition, we performed spectral modeling with spectra obtained at the highest spectral and spatial resolution within the hollows. Our results show that the enrichment of sulfides in hollow material is up to two times higher than the sulfide concentration derived from chemical measurements of Mercury's high-reflectance smooth plains. This result explains the small percentage of hollows found within these plains.

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 (<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.

Spectral behavior of sulfides in simulated daytime surface conditions of Mercury: Supporting past (MESSENGER) and future missions (BepiColombo)

To detect the mineral diversity of a planet's surface, it is essential to study the spectral variations over a broad wavelength range at relevant simulated laboratory conditions. The MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) mission to Mercury discovered that irrespective of its formation closest to the Sun, Mercury is richer in volatiles than previously expected. This is especially true for sulfur (S), with an average abundance of 4 wt%. It has been proposed that sulfur in the interior of Mercury can be brought to the surface through volcanic activity in the form of sulfides as slag deposits in Mercury hollows and pyroclastic deposits. However, comprehensive spectral library of sulfide minerals measured under vacuum conditions in a wide spectral range (0.2–100 μm) was lacking. This affects the detectability and understanding of the distribution, abundance, and type of sulfides on Mercury using remote-sensing spectral observations. In the case of Mercury, the effect of thermal weathering affecting the spectral behavior of these sulfides must be studied carefully for their effective detection. In this study, we present a spectral library of synthetic sulfides including MgS, FeS, CaS, CrS, TiS, NaS, and MnS. For each sample, we performed emissivity measurements in the thermal infrared range (TIR: ∼7–14 μm) for sample temperatures from 100°C–500°C, covering the daytime temperature cycle on Mercury's surface. In addition, for each sample we measured the spectral reflectance of fresh and thermally processed sulfides over a wide spectral range (0.2–100 μm) and at four different phase angles, 26°, 40°, 60°, 80°. This spectral library facilitates the detection of sulfides by past and future missions to Mercury by any optical spectrometer of any spectral range. Specifically, the emissivity measurements in this study will support the Mercury Radiometer and Thermal Imaging Spectrometer (MERTIS) instrument on the ESA/JAXA BepiColombo mission, which will study the surface mineralogy over a wavelength range of 7–14 μm at a spatial resolution of 500 m/pixel. The measured reflectance of these sulfides in 0.2–100 μm at various phase angles will support the interpretation of measurements from past (MDIS, MASCS on MESSENGER) and future missions (SIMBIO-SYS on BepiColombo).

Interplanetary magnetic field properties and variability near Mercury's orbit

AbstractThe first extensive study of interplanetary magnetic field (IMF) characteristics and stability at Mercury is undertaken using MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) magnetometer data. Variations in IMF and solar wind conditions have a direct and rapid effect upon Mercury's highly dynamic magnetosphere; hence, understanding of the time scales over which these variations occur is crucial because they determine the duration of magnetospheric states. We characterize typical distributions of IMF field strength, clock angle, and cone angle throughout the duration of MESSENGER's mission. Clock and cone angle distributions collected during the first Earth year of the mission indicate that there was a significant north‐south asymmetry in the location of the heliospheric current sheet during this period. The stability of IMF magnitude, clock angle, cone angle, and IMF Bz polarity is quantified for the entire mission. Changes in IMF Bz polarity and magnitude are found to be less likely for higher initial field magnitudes. Stability in IMF conditions is also found to be higher at aphelion (heliocentric distance r ∼ 0.31 AU) than at perihelion (r ∼ 0.47 AU).

Radiative transfer modeling of MESSENGER VIRS spectra: Detection and mapping of submicroscopic iron and carbon

We model the spectral effects of submicroscopic Fe and submicroscopic C particles within a transparent mineral host in order to investigate whether such materials could reproduce the major spectral characteristics of Mercury's reflectance spectra (i.e., low reflectance relative to the Moon, a lack of Fe absorption features in the near infrared, and an increasing continuum slope between visible and near-infrared wavelengths). By using the radiative transfer technique to model the VIRS (Visible and Infrared Spectrograph) spectral dataset obtained from the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission, we found that our spectral models based on nanophase and microphase Fe and C consistently fit the VIRS data (except of regions of Mercury's low-reflectance material). Our models show that the mean global submicroscopic Fe abundance is 2.5wt%, which exceeds the total Fe abundances obtained from the MESSENGER Gamma-Ray Spectrometer (GRS) and the X-Ray Spectrometer (XRS) and the global average submicroscopic C abundance is 1.9wt%, which is within the three-standard-deviation level of the MESSENGER GRS C measurements for Mercury's northern hemisphere. We also produced nanophase and microphase Fe and C abundance maps that show: (1) submicroscopic C is present at percent levels across the surface, (2) the spatial variations of submicroscopic particle abundances are correlated to the maximum surface temperature, (3) lower concentrations of nanophase Fe in fresh craters and their ejecta, (4) the submicroscopic Fe abundance is lower in the northern volcanic plains and Caloris basin than the global average, (5) the submicroscopic C and Fe abundances are very low around NE Rachmaninoff, a pyroclastic deposit, and (6) the submicroscopic particle abundances vary between low-reflectance material (LRM) deposits. In correlating these maps to geology and surface temperatures, we concluded that Ostwald ripening is responsible for the longitudinal and latitudinal variations and that the C is endogenic in origin.

Recent tectonic activity on Mercury revealed by small thrust fault scarps

The planet Mercury has contracted over its history. The identification of small thrust fault scarps suggests the occurrence of tectonic activity on Mercury within the past 50 million years and thus a slow-cooling planetary interior. Large tectonic landforms on the surface of Mercury, consistent with significant contraction of the planet, were revealed by the flybys of Mariner 10 in the mid-1970s1. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission confirmed that the planet’s past 4 billion years of tectonic history have been dominated by contraction expressed by lobate fault scarps that are hundreds of kilometres long2,3,4,5. Here we report the discovery of small thrust fault scarps in images from the low-altitude campaign at the end of the MESSENGER mission that are orders of magnitude smaller than the large-scale lobate scarps. These small scarps have tens of metres of relief, are only kilometres in length and are comparable in scale to small young scarps on the Moon6,7,8. Their small-scale, pristine appearance, crosscutting of impact craters and association with small graben all indicate an age of less than 50 Myr. We propose that these scarps are the smallest members of a continuum in scale of thrust fault scarps on Mercury. The young age of the small scarps, along with evidence for recent activity on large-scale scarps, suggests that Mercury is tectonically active today and implies a prolonged slow cooling of the planet’s interior.

Plasma distribution in Mercury's magnetosphere derived from MESSENGER Magnetometer and Fast Imaging Plasma Spectrometer observations

AbstractWe assess the statistical spatial distribution of plasma in Mercury's magnetosphere from observations of magnetic pressure deficits and plasma characteristics by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. The statistical distributions of proton flux and pressure were derived from 10 months of Fast Imaging Plasma Spectrometer (FIPS) observations obtained during the orbital phase of the MESSENGER mission. The Magnetometer‐derived pressure distributions compare favorably with those deduced from the FIPS observations at locations where depressions in the magnetic field associated with the presence of enhanced plasma pressures are discernible in the Magnetometer data. The magnitudes of the magnetic pressure deficit and the plasma pressure agree on average, although the two measures of plasma pressure may deviate for individual events by as much as a factor of ~3. The FIPS distributions provide better statistics in regions where the plasma is more tenuous and reveal an enhanced plasma population near the magnetopause flanks resulting from direct entry of magnetosheath plasma into the low‐latitude boundary layer of the magnetosphere. The plasma observations also exhibit a pronounced north‐south asymmetry on the nightside, with markedly lower fluxes at low altitudes in the northern hemisphere than at higher altitudes in the south on the same field line. This asymmetry is consistent with particle loss to the southern hemisphere surface during bounce motion in Mercury's offset dipole magnetic field.

MESSENGER at Mercury: Early orbital operations

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, launched in August 2004 under NASA's Discovery Program, was inserted into orbit about the planet Mercury in March 2011. MESSENGER's three flybys of Mercury in 2008–2009 marked the first spacecraft visits to the innermost planet since the Mariner 10 flybys in 1974–1975. The unprecedented orbital operations are yielding new insights into the nature and evolution of Mercury. The scientific questions that frame the MESSENGER mission led to the mission measurement objectives to be achieved by the seven payload instruments and the radio science experiment. Interweaving the full set of required orbital observations in a manner that maximizes the opportunity to satisfy all mission objectives and yet meet stringent spacecraft pointing and thermal constraints was a complex optimization problem that was solved with a software tool that simulates science observations and tracks progress toward meeting each objective. The final orbital observation plan, the outcome of that optimization process, meets all mission objectives. MESSENGER's Mercury Dual Imaging System is acquiring a global monochromatic image mosaic at better than 90% coverage and at least 250m average resolution, a global color image mosaic at better than 90% coverage and at least 1km average resolution, and global stereo imaging at better than 80% coverage and at least 250m average resolution. Higher-resolution images are also being acquired of targeted areas. The elemental remote sensing instruments, including the Gamma-Ray and Neutron Spectrometer and the X-Ray Spectrometer, are being operated nearly continuously and will establish the average surface abundances of most major elements. The Visible and Infrared Spectrograph channel of MESSENGER's Mercury Atmospheric and Surface Composition Spectrometer is acquiring a global map of spectral reflectance from 300 to 1450nm wavelength at a range of incidence and emission angles. Targeted areas have been selected for spectral coverage into the ultraviolet with the Ultraviolet and Visible Spectrometer (UVVS). MESSENGER's Mercury Laser Altimeter is acquiring topographic profiles when the slant range to Mercury's surface is less than 1800km, encompassing latitudes from 20°S to the north pole. Topography over the remainder of the southern hemisphere will be derived from stereo imaging, radio occultations, and limb profiles. MESSENGER's radio science experiment is determining Mercury's gravity field from Doppler signals acquired during frequent downlinks. MESSENGER's Magnetometer is measuring the vector magnetic field both within Mercury's magnetosphere and in Mercury's solar wind environment at an instrument sampling rate of up to 20 samples/s. The UVVS is determining the three-dimensional, time-dependent distribution of Mercury's exospheric neutral and ionic species via their emission lines. During each spacecraft orbit, the Energetic Particle Spectrometer measures energetic electrons and ions, and the Fast Imaging Plasma Spectrometer measures the energies and mass per charge of thermal plasma components, both within Mercury's magnetosphere and in Mercury's solar-wind environment. The primary mission observation sequence will continue for one Earth year, until March 2012. An extended mission, currently under discussion with NASA, would add a second year of orbital observations targeting a set of focused follow-on questions that build on observations to date and take advantage of the more active Sun expected during 2012–2013. MESSENGER's total primary mission cost, projected at $446M in real-year dollars, is comparable to that of Mariner 10 after adjustment for inflation.

Constraints on Mercury's surface composition from MESSENGER neutron spectrometer data

The composition of Mercury's surface is poorly known, but the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission has provided a wealth of new data from three flybys. In particular, MESSENGER Neutron Spectrometer (NS) observations reveal a surface enriched in neutron absorbing elements, consistent with interpretations of color and albedo observations suggesting a surface composition enriched in Fe–Mg–Ti oxides. In this study, we have computed the neutron absorption cross sections for all of the available proposed surface compositions of Mercury and evaluated the plausibility of each surface composition based on the neutron absorption cross section observed by MESSENGER. For identified plausible compositions, the implications for the thermal and magmatic evolution of Mercury are discussed. The measured macroscopic neutron absorption cross section of Mercury is inconsistent with a crust formed from partial melting of plausible bulk mantle compositions, flotation in a magma ocean or adiabatic melting of upwelling cumulates during magma ocean overturn. However, the observed neutron absorption is consistent with model compositions of late-stage magma-ocean cumulates and some proposed compositions from spectral modeling and equilibrium modeling. This suggests that the enrichment of neutron absorbing elements may be indicative of the processes that acted to form Mercury's crust. The enrichment in neutron absorbing elements, in combination with spectral observations that constrain FeO in silicates (< 2 wt.%), offers strong evidence of a magma ocean on Mercury since global scale melting appears to be required to concentrate the major neutron absorbing elements while minimizing Fe in silicate minerals. We also find that iron plays a secondary role in the neutron absorption of plausible surface compositions and its variations within different Fe–Mg–Ti oxide solid solution series does not cause any overlap among the various oxide series in neutron absorption cross section. High-Fe oxides are not required and more Mg-rich oxides may even be favored as the Ti-contents can sufficiently account for the observed neutron absorption.

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.

MESSENGER Mission Overview

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, launched on August 3, 2004, is nearing the halfway point on its voyage to become the first probe to orbit the planet Mercury. The mission, spacecraft, and payload are designed to answer six fundamental questions regarding the innermost planet: (1) What planetary formational processes led to Mercury’s high ratio of metal to silicate? (2) What is the geological history of Mercury? (3) What are the nature and origin of Mercury’s magnetic field? (4) What are the structure and state of Mercury’s core? (5) What are the radar-reflective materials at Mercury’s poles? (6) What are the important volatile species and their sources and sinks near Mercury? The mission has focused to date on commissioning the spacecraft and science payload as well as planning for flyby and orbital operations. The second Venus flyby (June 2007) will complete final rehearsals for the Mercury flyby operations in January and October 2008 and September 2009. Those flybys will provide opportunities to image the hemisphere of the planet not seen by Mariner 10, obtain high-resolution spectral observations with which to map surface mineralogy and assay the exosphere, and carry out an exploration of the magnetic field and energetic particle distribution in the near-Mercury environment. The orbital phase, beginning on March 18, 2011, is a one-year-long, near-polar-orbital observational campaign that will address all mission goals. The orbital phase will complete global imaging, yield detailed surface compositional and topographic data over the northern hemisphere, determine the geometry of Mercury’s internal magnetic field and magnetosphere, ascertain the radius and physical state of Mercury’s outer core, assess the nature of Mercury’s polar deposits, and inventory exospheric neutrals and magnetospheric charged particle species over a range of dynamic conditions. Answering the questions that have guided the MESSENGER mission will expand our understanding of the formation and evolution of the terrestrial planets as a family.

The MESSENGER mission to Mercury: Development history and early mission status

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, launched 3 August 2004, will be the first spacecraft to orbit the planet Mercury. Mercury holds answers to several critical questions regarding the formation and evolution of the terrestrial planets. The MESSENGER mission will fly by Mercury in 2008 and 2009 and will orbit Mercury for one Earth year beginning in March 2011. The >14 kW m −2 solar thermal input and the large velocity change required to reach Mercury orbit make this a very challenging mission from thermal and mass perspectives. MESSENGER overcomes these challenges with innovative applications of existing technologies and materials. Seven miniaturized instruments, along with the spacecraft telecommunications system, satisfy all scientific objectives of the mission. The Mercury flybys are preceded by gravity assists at the Earth (2005) and at Venus (2006 and 2007). This paper offers an overview of mission history and objectives, spacecraft and payload, current status, and data acquisition plans.

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.

The MESSENGER mission to Mercury: spacecraft and mission design

A Mercury orbiter mission is challenging from thermal and mass perspectives. The Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) mission overcomes these challenges while avoiding esoteric technologies by using an innovative approach with commonly available materials, minimal moving parts, and maximum heritage. This approach yields a spacecraft with good margins in all categories and low technical risk. The key concepts are a ceramic-cloth sunshade, an integrated lightweight structure and high- performance propulsion system, and a solar array incorporating optical solar reflectors (OSRs). The sunshade maintains the spacecraft at room temperature. The integrated structure and propulsion system provides ample mass margin. The solar array with OSRs, which has already undergone significant testing, provides thermal margin even if the panels are inadvertently pointed directly at the Sun at 0.3 AU. 0.3 AU .

The MESSENGER mission to Mercury: scientific objectives and implementation

Mercury holds answers to several critical questions regarding the formation and evolution of the terrestrial planets. These questions include the origin of Mercury's anomalously high ratio of metal to silicate and its implications for planetary accretion processes, the nature of Mercury's geological evolution and interior cooling history, the mechanism of global magnetic field generation, the state of Mercury's core, and the processes controlling volatile species in Mercury's polar deposits, exosphere, and magnetosphere. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission has been designed to fly by and orbit Mercury to address all of these key questions. After launch by a Delta 2925H-9.5, two flybys of Venus, and two flybys of Mercury, orbit insertion is accomplished at the third Mercury encounter. The instrument payload includes a dual imaging system for wide and narrow fields-of-view, monochrome and color imaging, and stereo; X-ray and combined gamma-ray and neutron spectrometers for surface chemical mapping; a magnetometer; a laser altimeter; a combined ultraviolet–visible and visible-near-infrared spectrometer to survey both exospheric species and surface mineralogy; and an energetic particle and plasma spectrometer to sample charged species in the magnetosphere. During the flybys of Mercury, regions unexplored by Mariner 10 will be seen for the first time, and new data will be gathered on Mercury's exosphere, magnetosphere, and surface composition. During the orbital phase of the mission, one Earth year in duration, MESSENGER will complete global mapping and the detailed characterization of the exosphere, magnetosphere, surface, and interior.