Mercury Flyby Research Articles

Determination of an instantaneous Laplace plane for Mercury’s rotation

Mercury is the target of two space missions: MESSENGER, which carried out its first and second flybys of Mercury on January 14, 2008 and October 6, 2008, and the ESA/JAXA space mission BepiColombo, scheduled to arrive at Mercury in 2020. The preparation of these missions requires a good knowledge of the rotation of Mercury. This paper presents studies performed by the MORE/ROMEO Team of BepiColombo mission. In particular, we show here an analytical study of the rotation of Mercury, and the determination of a suitable Laplace plane that can be considered as a reference inertial plane for the duration of the mission. Our main results are a prediction of the short-period perturbations that should be detected in the longitudinal rotational motion of Mercury, and an easy method to define an instantaneous Laplace plane taking into account the very small precession of Mercury’s orbit. This plane is afterward chosen as constant reference plane. An estimation of the (small) error induced by this choice is also computed.

Emplacement and tectonic deformation of smooth plains in the Caloris basin, Mercury

MESSENGER's first flyby of Mercury provided the first complete view of the Caloris basin and its interior smooth plains. Multispectral imaging shows that the interior plains are spectrally distinct from surrounding terrain, with a reflectance about 15–20% higher and a steeper spectral slope than the global average. The Caloris basin rim massifs and hummocky material, in contrast, have lower reflectance and a shallower spectral slope than the global average and extend around the entire basin. Isolated, relatively red patches along the margins of Caloris each surround irregularly shaped, scalloped-edged, rimless depressions that are interpreted to be volcanic vents, some with associated pyroclastic deposits. A nearly continuous annulus of smooth plains exterior to the basin displays spectral characteristics that contrast sharply with those of the basin-interior smooth plains. The exterior smooth plains are lower in reflectance and spectrally similar to the Caloris rim massifs and the hummocky materials. The rims and floors of larger impact craters in the interior plains expose low-reflectance material. There is widespread evidence of contractional and extensional tectonic deformation of the Caloris interior smooth plains. A set of linear radial graben, Pantheon Fossae, radiates outward from a zone near the basin center, and some of these graben extend to and intersect a set of generally basin-concentric graben distributed along the outer margin of the Caloris basin floor. The average areal extensional strain expressed by the mapped system of graben is ~ 0.08%. Wrinkle ridges also deform the interior smooth plains and predate both basin-radial and basin-concentric graben. The average areal contractional strain expressed by the mapped wrinkle ridges is ~ 0.07%. The density of wrinkle ridges on the western interior smooth plains is less than on the eastern interior plains, in contrast to the more even distribution of graben. A similar difference is seen between the density of wrinkle ridges on the western and eastern exterior smooth plains, with far fewer wrinkle ridges per unit area on the western annular plains. Superposition relations, spectral contrast, and the occurrence of volcanic vents and possible pyroclastic deposits all support a volcanic origin for the Caloris interior plains. Crater counts indicate that the Caloris exterior plains and hummocky material are younger than the Caloris rim and interior plains and are therefore also likely volcanic in origin. The contrast in spectral properties between the interior and exterior plains indicates a difference in the composition of the two expanses of volcanic material. Models for extensional tectonic deformation of the Caloris basin interior inferred from the distribution of graben viewed by Mariner 10 must be reevaluated to account for the discovery of Pantheon Fossae.

Volcanism on Mercury: Evidence from the first MESSENGER flyby for extrusive and explosive activity and the volcanic origin of plains

The first MESSENGER flyby of Mercury obtained images of 21% of the surface not seen by Mariner 10, including the center and western half of the Caloris basin and regions near the terminator that show details of the nature of smooth and intercrater plains. These new data have helped to address and resolve a series of longstanding questions on the existence and nature of volcanism on Mercury and the distribution of volcanic materials. Data from the Mercury Dual Imaging System (MDIS) on the MESSENGER spacecraft have shown the following: (1) Numerous volcanic vents, in the form of irregularly shaped rimless depressions, are concentrated around the interior edge of the Caloris basin. (2) These vents appear to be sources for effusive volcanism that in one case built a shield in excess of 100 km in diameter and in some cases formed bright haloes around the vents that are interpreted to represent pyroclastic eruptions. (3) Lobate margins of plains units, seen previously in Mariner 10 data, are documented in MESSENGER images with more clarity and are often distinctive in morphology and color properties, supporting the interpretation that these features are the edges of lava flow units. (4) The interior of the Caloris basin is filled with plains units spectrally distinctive from the rim deposits, and comparison with the lunar Imbrium basin and superposed impact crater stratigraphy provide evidence that these units are volcanic in origin; detailed differences in the mineralogy of lava flow units, so prominent in Imbrium, are not seen in the Caloris interior. (5) Some of the smooth plains surrounding the exterior of the Caloris basin show distinct differences in color and morphological properties, supporting a volcanic origin. (6) Some smooth and intercrater plains units distant from the Caloris basin show evidence of flooding and embayment relations unrelated to Caloris ejecta emplacement; local and regional geological and color relationships support a volcanic origin for these plains. (7) Large impact craters show a sequence of embayment of interior floor and exterior ejecta deposits that supports a volcanic origin for the embayment and filling processes. (8) Crater embayment and flooding relationships in selected areas suggest volcanic plains thicknesses of many hundreds of meters and local thicknesses inside impact craters of up to several kilometers. (9) Impact crater size–frequency distributions for Caloris exterior deposits, including the facies of the Caloris Group and relatively high- and low-albedo smooth plains, show that they are younger than plains interior to Caloris and thus must be dominantly the product of post-Caloris volcanism. These new data provide evidence that supports and confirms earlier hypotheses from Mariner 10 data that volcanism was important in shaping the surface of Mercury. The emerging picture of the volcanic style of Mercury is similar to that of the Moon, the other small, one-plate planetary body: there are no major shield volcanoes (e.g., comparable to Tharsis Montes on Mars), shallow magma reservoirs are rare, and there is little evidence for surface deformation or long-lived volcanic sources related to sites of upwelling mantle. The close association of volcanic plains and surface deformation features suggests that future observations and analyses can help document the relation between the volcanic flux and the evolving state and magnitude of stress in the lithosphere of Mercury.

Evidence for intrusive activity on Mercury from the first MESSENGER flyby

Images from MESSENGER's first flyby of Mercury have shown convincing evidence for surface volcanism. Here we report on evidence in the new data for several features that are characterized by fractures and graben — rare features on a planet dominated by contractional deformation — that may be linked to intrusive activity. These features include: (1) A floor-fractured crater, interpreted to have been the site of laccolith-like sill intrusions; the feature is similar to some floor-fractured craters on the Moon and shows evidence for individual fractured dome-like uplifts on the floor. (2) A concentric complex of graben, observed inside the peak ring on the floor of the ~ 250-km-diameter Raditladi basin and associated with dark plains and possibly embayed by them; the feature may represent an unusual type of floor-fracturing associated with deeper intrusions and related ring dikes or cone sheets, or the graben may instead be the product of non-magmatic uplift of the basin floor. (3) A large radial graben swarm, Pantheon Fossae, located near the center of the Caloris basin, thus far unique on Mercury, and characterized by hundreds of individual graben segments ranging from ~ 5 km to ~ 110 km in length. In the nexus, graben crosscut one another and produce a local polygonal pattern; others curve away from the center as the nexus is approached. Two scales of graben length are observed; the radius of the dense radially symmetric plexus of graben is ~ 175 km, and a few graben extend to greater radial distances to the north and southwest out to distances that intersect with a ring of generally concentric graben around the outer basin floor. Two width scales of graben are observed; a large graben about 8 km wide emerges from the nexus and extends for ~ 100 km; most graben are less than half this width. Some graben walls appear cuspate, with convex-outward wall segments that resemble crater chain segments. One crater chain with distinctive raised rims parallels nearby graben. Locally, some graben appear in en echelon patterns, and smaller graben sometimes show cross-cutting (superposition) relationships. Abundant impact craters, the most prominent being Apollodorus, and secondary crater clusters and chains are superposed on the graben system; there is little evidence that craters greater than 5 km in diameter have been cut by a graben. This relation implies that the graben swarm formed soon after the emplacement of the Caloris floor plains. These graben are interpreted to be the surface expression of a radial dike swarm emanating from a subsurface magma reservoir. Similar features, in which the dikes contribute to a near-surface stress field that favors radial graben, are known on the Earth, Venus, and Mars. The location of Pantheon Fossae in the center of the Caloris basin suggests that formation of the radial graben structure is linked to basin evolution.

Modeling Mercury's internal magnetic field with smooth inversions

Three-component vector magnetic field observations from MESSENGER's first flyby of Mercury have confirmed the presence of an internal field, along with external fields due to magnetospheric current systems. We use techniques from inverse theory to investigate structure in Mercury's internal magnetic field permitted by the Mariner 10 and MESSENGER flyby data and recoverable from orbit. We correct the flyby observations using external fields predicted by a parameterized magnetospheric model, and we include noise contributions from long-wavelength uncertainties in the external field and from un-modeled short-wavelength features. The internal field is represented by a spherical harmonic expansion to degree and order 8, with regularization constraints applied to the power spectrum. Latitudinal and longitudinal structure in the field is required in order to fit the data. Enhanced radial magnetic field in the region of the Mariner 10 and MESSENGER flyby latitudes is seen. Contributions to the internal field predicted by Aharonson and others for a long-wavelength crustal field are present, but the field is dominated by the g 0 1 term rather than the predicted g 0 3 term. Observations from MESSENGER's second and third flybys will provide additional low-altitude coverage at equatorial latitudes, while the orbital phase of the mission will provide high-latitude, northern-hemisphere coverage. We investigate recovery of three simulated core fields using the cumulative data coverage that will be obtained from the three MESSENGER flybys and MESSENGER's orbital phase, under the assumption that long-wavelength external fields can be modeled and removed. This provides a best-case scenario for internal field modeling. The results show excellent recovery of the dipole field and of field structure at mid to high northern latitudes with spatial scales equivalent to degree and order 10, providing encouraging results for the identification and characterization of core fields.

The tectonics of Mercury: The view after MESSENGER's first flyby

During its first flyby of Mercury, MESSENGER imaged many tectonic landforms, most of which are contractional in nature and include lobate scarps, high-relief ridges, and wrinkle ridges. Wrinkle ridges are found on the smooth plains that partially fill the interior and surround the exterior of the Caloris basin and also on smooth plains that fill the interiors of smaller impact basins and larger craters. MESSENGER revealed a radial graben complex, Pantheon Fossae, nearly co-centered with the Caloris basin. Pantheon Fossae and a network of mostly basin-concentric graben in the outer portions of the Caloris basin interior form a pattern of extension not seen elsewhere on Mercury. The first clear example of extensional faults outside of the Caloris basin has been documented on smooth plains inside the peak ring of the relatively young Raditladi basin. A map of the distribution of tectonic landforms imaged by MESSENGER and Mariner 10 shows that lobate scarps are the most widespread type of feature. Estimates of the horizontal shortening associated with lobate scarps that crosscut and overthrust small impact craters imaged by MESSENGER range from ~ 1 to 3 km. Previously unrecognized lobate scarps detected in areas imaged by Mariner 10 indicate that earlier estimates of contractional strain are low. A new estimate of the average areal contractional strain (~ 0.06%) accommodated by scarps is at least one third greater than comparable previous estimates and corresponds to a decrease in Mercury's radius of at least 0.8 km since the end of heavy impact bombardment of the inner solar system. These figures are lower bounds because of the likelihood that not all lobate scarps have been identified even in areas imaged to date. Crosscutting and embayment relations revealed by MESSENGER suggest that lobate scarps formed before the end of smooth plains emplacement and continued to be active after the emplacement of the youngest smooth plains deposits. Relatively recent activity on lobate scarps may be the expression of slow but continuous radial contraction that accompanied cooling of Mercury's interior and the growth of the planet's solid inner core.

Sodium‐ion pickup observed above the magnetopause during MESSENGER's first Mercury flyby: Constraints on neutral exospheric models

Single‐particle tracings of sodium pickup ions launched upstream of Mercury's magnetopause are used to investigate the access of these ions to the magnetosphere and set limits on the distribution of neutral sodium about the planet during the first MESSENGER flyby. The transport of pickup ions is modeled using flow velocity and magnetic fields from a three‐dimensional magnetohydrodynamic simulation. Extensive penetration of pickup ions into the magnetosphere is found in the post‐noon and dusk local time sectors due to the northward interplanetary magnetic field at the time of MESSENGER's first flyby. It is concluded that: (1) pickup of magnetosheath photoions may be an important source of hot planetary ions within the magnetosphere; and (2) the sodium ions in the magnetosheath observed by MESSENGER must originate from an extended neutral exosphere due to ion sputtering and/or to a partially escaping distribution generated by photon‐stimulated desorption with yields modified by regolith trapping.

Narrow-band ultra-low-frequency wave observations by MESSENGER during its January 2008 flyby through Mercury's magnetosphere

[1] During MESSENGER's first flyby of Mercury, numerous narrow-band ultra-low-frequency (ULF) waves and their harmonics were detected between closest approach (CA) and the outbound magnetopause (MP) crossing. The fundamental mode was at frequencies between the He+ (fcHe+) and the H+ (fcH+) cyclotron frequencies. A boundary layer (BL) was detected before the MP crossing. The ULF frequency and amplitude increased from CA to the edge of the BL. In the BL the frequency dropped by a factor of 2, and the amplitude increased by an order of magnitude. There was a large variation in the wave-normal angle (Ψ), with a slight tendency for Ψ to be perpendicular to the ambient magnetic field near CA and parallel away from CA. Near CA the parallel power tended to be larger than the perpendicular power, while away from CA the perpendicular power dominated. Large variations in wave polarization properties were observed.

MESSENGER observations of Mercury's magnetosphere during northward IMF

MESSENGER's January 14, 2008, flyby of Mercury has provided new observations of the planet's magnetosphere for northward interplanetary magnetic field (IMF). The dusk magnetopause was located inward from the mean magnetopause surface, possibly due to reduced tail magnetic flux content for IMF Bz > 0 and/or the pressure of planetary pickup ions as they respond to the dawnward – v × B electric field in the magnetosheath. Within the plasma sheet rotations of the magnetic field are observed consistent with, Kelvin–Helmholtz vortices ∼1 RM in diameter (RM is Mercury's radius). MESSENGER exited through a 1,000 km‐wide boundary layer bordered by inner and outer current sheets that resemble rotational and tangential discontinuities, respectively. The total magnetic field change across this layer is consistent with the predicted solar wind ram pressure at Mercury during the MESSENGER flyby.

Mercury's Exosphere: Observations During MESSENGER's First Mercury Flyby

During MESSENGER's first Mercury flyby, the Mercury Atmospheric and Surface Composition Spectrometer measured Mercury's exospheric emissions, including those from the antisunward sodium tail, calcium and sodium close to the planet, and hydrogen at high altitudes on the dayside. Spatial variations indicate that multiple source and loss processes generate and maintain the exosphere. Energetic processes connected to the solar wind and magnetospheric interaction with the planet likely played an important role in determining the distributions of exospheric species during the flyby.

Spectroscopic Observations of Mercury's Surface Reflectance During MESSENGER's First Mercury Flyby

During MESSENGER's first flyby of Mercury, the Mercury Atmospheric and Surface Composition Spectrometer made simultaneous mid-ultraviolet to near-infrared (wavelengths of 200 to 1300 nanometers) reflectance observations of the surface. An ultraviolet absorption (<280 nanometers) suggests that the ferrous oxide (Fe2+) content of silicates in average surface material is low (less than 2 to 3 weight percent). This result is supported by the lack of a detectable 1-micrometer Fe2+ absorption band in high-spatial-resolution spectra of mature surface materials as well as immature crater ejecta, which suggests that the ferrous iron content may be low both on the surface and at depth. Differences in absorption features and slope among the spectra are evidence for variations in composition and regolith maturation of Mercury's surface.

The Structure of Mercury's Magnetic Field from MESSENGER's First Flyby

During its first flyby of Mercury, the MESSENGER spacecraft measured the planet's near-equatorial magnetic field. The field strength is consistent to within an estimated uncertainty of 10% with that observed near the equator by Mariner 10. Centered dipole solutions yield a southward planetary moment of 230 to 290 nanotesla RM3 (where RM is Mercury's mean radius) tilted between 5 degrees and 12 degrees from the rotation axis. Multipole solutions yield non-dipolar contributions of 22% to 52% of the dipole field magnitude. Magnetopause and tail currents account for part of the high-order field, and plasma pressure effects may explain the remainder, so that a pure centered dipole cannot be ruled out.

Return to Mercury: An Overview of the MESSENGER Spacecraft Thermal Control System Design and Up-to-Date Flight Performance

Launched on August 3, 2004, MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) will be the first spacecraft to orbit the planet Mercury. Designed and built by The Johns Hopkins University Applied Physics Laboratory in conjunction with the Carnegie Institution of Washington, MESSENGER will study Mercury during a 1-year orbital phase that will begin in March 2011. Currently the spacecraft is in the middle of a 7-year cruise phase that so far has included a flyby of the Earth (August 2005), two flybys of Venus (October 2006 and June 2007), and the first of three flybys of Mercury (January 2008). The January 2008 Mercury flyby marked the first spacecraft visit since Mariner 10 (1975) and made MESSENGER the first spacecraft to encounter Mercury when near the planet’s perihelion. This paper will provide an overview of the thermal design challenges for both the cruise and orbital phases, the solutions implemented to resolve those challenges, and the flight temperature and power data that verify the performance of the thermal control subsystem over the mission to date.

Scientific objectives and instrumentation of Mercury Plasma Particle Experiment (MPPE) onboard MMO

Mercury is one of the least explored planets in our solar system. Until the recent flyby of Mercury by MESSENGER, no spacecraft had visited Mercury since Mariner 10 made three flybys: two in 1974 and one in 1975. In order to elucidate the detailed plasma structure and dynamics around Mercury, an orbiter BepiColombo MMO (Mercury Magnetospheric Orbiter) is planned to be launched in 2013 as a joint mission between ESA and ISAS/JAXA. Mercury Plasma Particle Experiment (MPPE) was proposed in order to investigate the plasma/particle environment around Mercury. MPPE is a comprehensive instrument package for plasma, high-energy particle and energetic neutral atom measurements. It consists of seven sensors: two Mercury electron analyzers (MEA1 and MEA2), Mercury ion analyzer (MIA), Mercury mass spectrum analyzer (MSA), high-energy particle instrument for electron (HEP-ele), high-energy particle instrument for ion (HEP-ion), and energetic neutrals analyzer (ENA). Since comprehensive full three-dimensional simultaneous measurements of low to high-energy ions and electrons around Mercury as well as measurements of energetic neutral atoms will not be realized before BepiColombo/MMO's arrival at Mercury, it is expected that many unresolved problems concerning the Mercury magnetosphere will be elucidated by the MPPE observation.

The MESSENGER mission to Mercury: Status after the Venus flybys

NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, launched on 3 August 2004, is well into its voyage to initiate a new era in our understanding of the terrestrial planets. The mission, spacecraft, and payload are designed to answer six fundamental questions regarding the innermost planet during three flybys and a one-year-long, near-polar-orbital observational campaign. The cruise phase to date has been used to commission the spacecraft and instruments and begin the transition to automated use of the instruments with on-board, time-tagged commands. An Earth flyby one year after launch, a large propulsive maneuver in December 2005, and Venus flybys in October 2006 and June 2007 began the process of changing MESSENGER's heliocentric motion. The second Venus flyby was also used to complete final rehearsals for Mercury flyby operations in January 2008 while coordinating observations with the European Space Agency's Venus Express mission. The upcoming Mercury flyby will be the first since that of Mariner 10 in 1975. Along with the second and third MESSENGER flybys in October 2008 and September 2009, that flyby will provide images of the hemisphere of Mercury never seen before by spacecraft as well as the first high-resolution information on Mercury's surface mineralogy. These three flybys, interspersed with deep space maneuvers, finish adjusting the spacecraft motion sufficiently for Mercury orbit injection to follow in March 2011. In the orbital phase, the spacecraft's nominal periapsis latitude of 60∘N will gradually drift northward as the periapsis altitude of 200km gradually drifts upward due to solar gravitational perturbations. The 12-h period and 80∘ inclination are maintained while the altitude is readjusted downward to 200km every Mercury revolution about the Sun. The profile enables mapping of the entire planet and acquiring detailed elemental and topographic data over the northern hemisphere. After conclusion of the nominal mission in March 2012, an additional year of data analysis and archiving is planned before the conclusion of the MESSENGER project. To broaden scientific participation in the mission, NASA has established a Participating Scientist Program, bringing a full complement of international scientific researchers into the mission. The MESSENGER team is also continuing its informal interaction with members of the BepiColombo project to maximize the overall scientific return from both missions.

Particle Acceleration in Mercury’s Magnetosphere

This paper is devoted to the problem of particle acceleration in the closest to the Sun Hermean magnetosphere. We discuss few available observations of energetic particles in Mercury environment made by Mariner-10 in 1974-1975 during Mercury flyby's and by Helios in 1979 upstream of the Hermean bow shock. Typically ions are non-adiabatic in a very dynamic and compact Mercury magnetosphere, so one may expect that particle acceleration will be very effective. However, it works perfectly for electrons, but for ions the scale of magnetosphere is so small that it allows their acceleration only up to 100 keV. We present comparative analysis of the efficiency of various acceleration mechanisms (in- ductive acceleration, acceleration by the centrifugal impulse force, stochastic acceleration in a turbulent magnetic fields, wave-particle interactions and bow shock energization) in the magnetospheres of the Earth and Mercury. Finally we discuss several points which need to be addressed in a future Hermean 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.

MESSENGER Mission Design and Navigation

Nearly three decades after the Mariner 10 spacecraft’s third and final targeted Mercury flyby, the 3 August 2004 launch of the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft began a new phase of exploration of the closest planet to our Sun. In order to ensure that the spacecraft had sufficient time for pre-launch testing, the NASA Discovery Program mission to orbit Mercury experienced launch delays that required utilization of the most complex of three possible mission profiles in 2004. During the 7.6-year mission, the spacecraft’s trajectory will include six planetary flybys (including three of Mercury between January 2008 and September 2009), dozens of trajectory-correction maneuvers (TCMs), and a year in orbit around Mercury. Members of the mission design and navigation teams optimize the spacecraft’s trajectory, specify TCM requirements, and predict and reconstruct the spacecraft’s orbit. These primary mission design and navigation responsibilities are closely coordinated with spacecraft design limitations, operational constraints, availability of ground-based tracking stations, and science objectives. A few days after the spacecraft enters Mercury orbit in mid-March 2011, the orbit will have an 80° inclination relative to Mercury’s equator, a 200-km minimum altitude over 60°N latitude, and a 12-hour period. In order to accommodate science goals that require long durations during Mercury orbit without trajectory adjustments, pairs of orbit-correction maneuvers are scheduled every 88 days (once per Mercury year).

Mercury's exosphere: A possible source for Na

Many ground-based observations of Na in Mercury's surface-bounded exosphere have been made and continued to be made in an effort to understand the sources, sinks, and distribution of Na around Mercury. These time consuming and costly efforts are made to better understand the physical processes on and around Mercury. A big step would be to discover an actual source of the Na from Mercury's crust because it is already known that meteorites and comets provide Na to the exosphere through impact. We provide ground-based CCD imagery obtained with small ground-based telescopes that show bright albedo features at locations coincident with enhanced Na emissions in Mercury's exosphere. We suggest these locations are sources for Na. We also provide a mechanism to test this hypothesis using in situ observations by instruments on the MESSENGER spacecraft during the three fly bys of Mercury that will occur in 2008 and 2009, and during the orbital mission which begins in 2011. It is necessary to prove that Na is delivered to the exosphere from one or more crustal source regions before exospheric Na can be used as a measure of the volatile content of Mercury used to infer formation and evolution from the primitive solar nebula. The same applies to other elements such as K which is known to be in Mercury's exosphere and S which is postulated to be present. We expound on the impact that the discovery of one or more source regions from Mercury's crust would have on our ability to discern between the three leading models of Mercury's formation and crustal evolution.

Trajectory Design and Maneuver Strategy for the MESSENGER Mission to Mercury

Destined to become the first spacecraft to orbit the planet Mercury, the MESSENGER spacecraft was launched on 3 August 2004. The 6.6-year ballistic trajectory to Mercury will utilize six gravity-assist flybys of Earth (one), Venus (two), and Mercury (three). With three trajectory correction maneuvers completed by mid-December 2005, many more maneuvers will be necessary during the journey to Mercury and the subsequent 1-year duration Mercury orbit phase. The spacecraft's design and operational capability will enable real-time monitoring of every course-correction maneuver. A complex mission plan will provide multiple opportunities to obtain observational data that will help fulfill the mission's scientific objectives. Soon after entering Mercury orbit in mid-March 2011, the initial primary science orbit will have an 80-deg orbit inclination relative to Mercury's equator, 200-km periapsis altitude, 60°N subspacecraft periapsis latitude, and a 12-h orbit period. With science goals requiring infrequent orbit-phase trajectory adjustments, pairs of orbit-correction maneuvers occur at about the same time every Mercury year, or every 88 days. For the first time, the spacecraft's orbit design at Mercury accounts for the best available Mercury gravity model, small solar pressure perturbations due to changes in the solar array tilt angle, and an improved strategy for performing orbit correction maneuvers.