The Origin of Mercury’s Internal Magnetic Field

Mariner 10 measurements proved the existence of a large-scale internal magnetic field on Mercury. The observed field amplitude, however, is too weak to be compatible with typical convective planetary dynamos. The Lorentz force based on an extrapolation of Mariner 10 data to the dynamo region is 10⁻⁴ times smaller than the Coriolis force. This is at odds with the idea that planetary dynamos are thought to work in the so-called magnetostrophic regime, where Coriolis force and Lorentz force should be of comparable magnitude. Recent convective dynamo simulations reviewed here seem to resolve this caveat. We show that the available convective power indeed suffices to drive a magnetostrophic dynamo even when the heat flow though Mercury’s core–mantle boundary is subadiabatic, as suggested by thermal evolution models. Two possible causes are analyzed that could explain why the observations do not reflect a stronger internal field. First, toroidal magnetic fields can be strong but are confined to the conductive core, and second, the observations do not resolve potentially strong small-scale contributions. We review different dynamo simulations that promote either or both effects by (1) strongly driving convection, (2) assuming a particularly small inner core, or (3) assuming a very large inner core. These models still fall somewhat short of explaining the low amplitude of Mariner 10 observations, but the incorporation of an additional effect helps to reach this goal: The subadiabatic heat flow through Mercury’s core–mantle boundary may cause the outer part of the core to be stably stratified, which would largely exclude convective motions in this region. The magnetic field, which is small scale, strong, and very time dependent in the lower convective part of the core, must diffuse through the stagnant layer. Here, the electromagnetic skin effect filters out the more rapidly varying high-order contributions and mainly leaves behind the weaker and slower varying dipole and quadrupole components (Christensen in Nature 444:1056–1058, 2006). Messenger and BepiColombo data will allow us to discriminate between the various models in terms of the magnetic fields spatial structure, its degree of axisymmetry, and its secular variation.     

The Surface of Mercury as Seen by Mariner 10

The Mariner 10 spacecraft made three flyby passes of Mercury in 1974 and 1975. It imaged a little less than half of the surface and discovered Mercury had an intrinsic magnetic field. This paper briefly describes the surface of Mercury as seen by Mariner 10 as a backdrop to the discoveries made since then by ground-based observations and the optimistic anticipation of new discoveries by MESSENGER and BepiColombo spacecraft that are scheduled for encounter in the next decade.     

The Geology of Mercury: The View Prior to the MESSENGER Mission

Mariner 10 and Earth-based observations have revealed Mercury, the innermost of the terrestrial planetary bodies, to be an exciting laboratory for the study of Solar System geological processes. Mercury is characterized by a lunar-like surface, a global magnetic field, and an interior dominated by an iron core having a radius at least three-quarters of the radius of the planet. The 45% of the surface imaged by Mariner 10 reveals some distinctive differences from the Moon, however, with major contractional fault scarps and huge expanses of moderate-albedo Cayley-like smooth plains of uncertain origin. Our current image coverage of Mercury is comparable to that of telescopic photographs of the Earth’s Moon prior to the launch of Sputnik in 1957. We have no photographic images of one-half of the surface, the resolution of the images we do have is generally poor (∼1 km), and as with many lunar telescopic photographs, much of the available surface of Mercury is distorted by foreshortening due to viewing geometry, or poorly suited for geological analysis and impact-crater counting for age determinations because of high-Sun illumination conditions. Currently available topographic information is also very limited. Nonetheless, Mercury is a geological laboratory that represents (1) a planet where the presence of a huge iron core may be due to impact stripping of the crust and upper mantle, or alternatively, where formation of a huge core may have resulted in a residual mantle and crust of potentially unusual composition and structure; (2) a planet with an internal chemical and mechanical structure that provides new insights into planetary thermal history and the relative roles of conduction and convection in planetary heat loss; (3) a one-tectonic-plate planet where constraints on major interior processes can be deduced from the geology of the global tectonic system; (4) a planet where volcanic resurfacing may not have played a significant role in planetary history and internally generated volcanic resurfacing may have ceased at ∼3.8 Ga; (5) a planet where impact craters can be used to disentangle the fundamental roles of gravity and mean impactor velocity in determining impact crater morphology and morphometry; (6) an environment where global impact crater counts can test fundamental concepts of the distribution of impactor populations in space and time; (7) an extreme environment in which highly radar-reflective polar deposits, much more extensive than those on the Moon, can be better understood; (8) an extreme environment in which the basic processes of space weathering can be further deduced; and (9) a potential end-member in terrestrial planetary body geological evolution in which the relationships of internal and surface evolution can be clearly assessed from both a tectonic and volcanic point of view. In the half-century since the launch of Sputnik, more than 30 spacecraft have been sent to the Moon, yet only now is a second spacecraft en route to Mercury. The MESSENGER mission will address key questions about the geologic evolution of Mercury; the depth and breadth of the MESSENGER data will permit the confident reconstruction of the geological history and thermal evolution of Mercury using new imaging, topography, chemistry, mineralogy, gravity, magnetic, and environmental data.     

Radar Imaging of Mercury

Earth-based radar has been one of the few, and one of the most important, sources of new information about Mercury during the three decades since the Mariner 10 encounters. The emphasis during the past 15 years has been on full-disk, dual-polarization imaging of the planet, an effort that has been facilitated by the development of novel radar techniques and by improvements in radar systems. Probably the most important result of the imaging work has been the discovery and mapping of radar-bright features at the poles. The radar scattering properties of these features, and their confinement to permanently shaded crater floors, is consistent with volume backscatter from a low-loss volatile such as clean water ice. Questions remain, however, regarding the source and long-term stability of the putative ice, which underscores the need for independent confirmation by other observational methods. Radar images of the non-polar regions have also revealed a plethora of bright features, most of which are associated with fresh craters and their ejecta. Several very large impact features, with rays and other bright ejecta spreading over distances of 1,000 km or more, have been traced to source craters with diameters of 80–125 km. Among these large rayed features are some whose relative faintness suggests that they are being observed in an intermediate stage of degradation. Less extended ray/ejecta features have been found for some of the freshest medium-size craters such as Kuiper and Degas. Much more common are smaller (<40 km diameter) fresh craters showing bright rim-rings but little or no ray structure. These smaller radar-bright craters are particularly common over the H-7 quadrangle. Diffuse areas of enhanced depolarized brightness have been found in the smooth plains, including the circum-Caloris planitiae and Tolstoj Basin. This is an interesting finding, as it is the reverse of the albedo contrast seen between the radar-dark maria and the radar-bright cratered highlands on the Moon.     

Missions to Mercury

Mercury is a very difficult planet to observe from the Earth, and space missions that target Mercury are essential for a comprehensive understanding of the planet. At the same time, it is also difficult to orbit because it is deep inside the Sun’s gravitational well. Only one mission has visited Mercury; that was Mariner 10 in the 1970s. This paper provides a brief history of Mariner 10 and the numerous imaginative but unsuccessful mission proposals since the 1970s for another Mercury mission. In the late 1990s, two missions—MESSENGER and BepiColombo—received the go-ahead; MESSENGER is on its way to its first encounter with Mercury in January 2008. The history, scientific objectives, mission designs, and payloads of both these missions are described in detail.     

The Mercury Laser Altimeter Instrument for the MESSENGER Mission

The Mercury Laser Altimeter (MLA) is one of the payload science instruments on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, which launched on August 3, 2004. The altimeter will measure the round-trip time of flight of transmitted laser pulses reflected from the surface of the planet that, in combination with the spacecraft orbit position and pointing data, gives a high-precision measurement of surface topography referenced to Mercury’s center of mass. MLA will sample the planet’s surface to within a 1-m range error when the line-of-sight range to Mercury is less than 1,200 km under spacecraft nadir pointing or the slant range is less than 800 km. The altimeter measurements will be used to determine the planet’s forced physical librations by tracking the motion of large-scale topographic features as a function of time. MLA’s laser pulse energy monitor and the echo pulse energy estimate will provide an active measurement of the surface reflectivity at 1,064 nm. This paper describes the instrument design, prelaunch testing, calibration, and results of postlaunch testing.     

Interior Evolution of Mercury

The interior evolution of Mercury—the innermost planet in the solar system, with its exceptional high density—is poorly known. Our current knowledge of Mercury is based on observations from Mariner 10’s three flybys. That knowledge includes the important discoveries of a weak, active magnetic field and a system of lobate scarps that suggests limited radial contraction of the planet during the last 4 billion years. We review existing models of Mercury’s interior evolution and further present new 2D and 3D convection models that consider both a strongly temperature-dependent viscosity and core cooling. These studies provide a framework for understanding the basic characteristics of the planet’s internal evolution as well as the role of the amount and distribution of radiogenic heat production, mantle viscosity, and sulfur content of the core have had on the history of Mercury’s interior. The existence of a dynamo-generated magnetic field suggests a growing inner core, as model calculations show that a thermally driven dynamo for Mercury is unlikely. Thermal evolution models suggest a range of possible upper limits for the sulfur content in the core. For large sulfur contents the model cores would be entirely fluid. The observation of limited planetary contraction (∼1–2 km)—if confirmed by future missions—may provide a lower limit for the core sulfur content. For smaller sulfur contents, the planetary contraction obtained after the end of the heavy bombardment due to inner core growth is larger than the observed value. Due to the present poor knowledge of various parameters, for example, the mantle rheology, the thermal conductivity of mantle and crust, and the amount and distribution of radiogenic heat production, it is not possible to constrain the core sulfur content nor the present state of the mantle. Therefore, it is difficult to robustly predict whether or not the mantle is conductive or in the convective regime. For instance, in the case of very inefficient planetary cooling—for example, as a consequence of a strong thermal insulation by a low conductivity crust and a stiff Newtonian mantle rheology—the predicted sulfur content can be as low as 1 wt% to match current estimates of planetary contraction, making deep mantle convection likely. Efficient cooling—for example, caused by the growth of a crust strongly in enriched in radiogenic elements—requires more than 6.5 wt% S. These latter models also predict a transition from a convective to a conductive mantle during the planet’s history. Data from future missions to Mercury will aid considerably our understanding of the evolution of its interior.     

Surface-Exosphere-Magnetosphere System Of Mercury

Mercury is a poorly known planet, since the only space-based information comes from the three fly-bys performed in 1974 by the Mariner 10 spacecraft. Ground-based observations also provided some interesting results, but they are particularly difficult to obtain due to the planet’s proximity to the Sun. Nevertheless, the fact that the planet’s orbit is so close to the Sun makes Mercury a particularly interesting subject for extreme environmental conditions. Among a number of crucial scientific topics to be addressed, Mercury’s exosphere, its interaction with the solar wind and its origin from the surface of the planet, can provide important clues about planetary evolution. In fact, the Hermean exosphere is continuously eroded and refilled by these interactions, so that it would be more proper to consider the Hermean environment as a single, unified system – surface-exosphere-magnetosphere. These three parts are indeed strongly linked to each other. In recent years, the two missions scheduled to explore the iron planet, the NASA MESSENGER mission (launched in March 2004) and the ESA cornerstone mission (jointly with JAXA) BepiColombo (to be launched in 2012), have stimulated new interest in the many unresolved mysteries related to it. New ground-based observations, made possible by new technologies, have been obtained, and new simulation studies have been performed. In this paper some old as well as the very latest observations and studies related to the surface-exosphere-magnetosphere system are reviewed, outlining the investigations achievable by the planned space-based observations. This review intends to support the studies, in preparation of future data, and the definition of specific instrumentation.     

Electromagnetic Induction Effects and Dynamo Action in the Hermean System

Embedded in a large mass density and strong interplanetary magnetic field solar wind environment and equipped with a magnetic field of minor strength, planet Mercury exhibits a small magnetosphere vulnerable to severe solar wind buffeting. This causes large variations in the size of the magnetosphere and its associated currents. External fields are of far more importance than in the terrestrial case and of a size comparable to any internal, dynamo-generated field. Induction effects in the planetary interior, dominated by its huge core, are thought to play a much more prominent role in the Hermean magnetosphere compared to any of its companions. Furthermore, the external fields may cause planetary dynamo amplification much as discussed for the Galilean moons Io and Ganymede, but with the ambient field generated by the dynamo and its magnetic field-solar wind interaction.     

The Mercury Atmospheric and Surface Composition Spectrometer for the MESSENGER Mission

The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) is one of seven science instruments onboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft en route to the planet Mercury. MASCS consists of a small Cassegrain telescope with 257-mm effective focal length and a 50-mm aperture that simultaneously feeds an UltraViolet and Visible Spectrometer (UVVS) and a Visible and InfraRed Spectrograph (VIRS). UVVS is a 125-mm focal length, scanning grating, Ebert-Fastie monochromator equipped with three photomultiplier tube detectors that cover far ultraviolet (115–180 nm), middle ultraviolet (160–320 nm), and visible (250–600 nm) wavelengths with an average 0.6-nm spectral resolution. It will measure altitude profiles of known species in order to determine the composition and structure of Mercury’s exosphere and its variability and will search for previously undetected exospheric species. VIRS is a 210-mm focal length, fixed concave grating spectrograph equipped with a beam splitter that simultaneously disperses the spectrum onto a 512-element silicon visible photodiode array (300–1050 nm) and a 256-element indium-gallium-arsenide infrared photodiode array 850–1,450 nm. It will obtain maps of surface reflectance spectra with a 5-nm resolution in the 300–1,450 nm wavelength range that will be used to investigate mineralogical composition on spatial scales of 5 km. UVVS will also observe the surface in the far and middle ultraviolet at a 10-km or smaller spatial scale. This paper summarizes the science rationale and measurement objectives for MASCS, discusses its detailed design and its calibration requirements, and briefly outlines observation strategies for its use during MESSENGER orbital operations around Mercury.     

The Formation Of The First Stars In The Universe

In this review, I survey our current understanding of how the very first stars in the universe formed, with a focus on three main areas of interest: the formation of the first protogalaxies and the cooling of gas within them, the nature and extent of fragmentation within the cool gas, and the physics – in particular the interplay between protostellar accretion and protostellar feedback – that serves to determine the final stellar mass. In each of these areas, I have attempted to show how our thinking has developed over recent years, aided in large part by the increasing ease with which we can now perform detailed numerical simulations of primordial star formation. I have also tried to indicate the areas where our understanding remains incomplete, and to identify some of the most important unsolved problems.     

The Mercury Dual Imaging System on the MESSENGER Spacecraft

The Mercury Dual Imaging System (MDIS) on the MESSENGER spacecraft will provide critical measurements tracing Mercury’s origin and evolution. MDIS consists of a monochrome narrow-angle camera (NAC) and a multispectral wide-angle camera (WAC). The NAC is a 1.5° field-of-view (FOV) off-axis reflector, coaligned with the WAC, a four-element refractor with a 10.5° FOV and 12-color filter wheel. The focal plane electronics of each camera are identical and use a 1,024×1,024 Atmel (Thomson) TH7888A charge-coupled device detector. Only one camera operates at a time, allowing them to share a common set of control electronics. The NAC and the WAC are mounted on a pivoting platform that provides a 90° field-of-regard, extending 40° sunward and 50° anti-sunward from the spacecraft +Z-axis—the boresight direction of most of MESSENGER’s instruments. Onboard data compression provides capabilities for pixel binning, remapping of 12-bit data into 8 bits, and lossless or lossy compression. MDIS will acquire four main data sets at Mercury during three flybys and the two-Mercury-solar-day nominal mission: a monochrome global image mosaic at near-zero emission angles and moderate incidence angles, a stereo-complement map at off-nadir geometry and near-identical lighting, multicolor images at low incidence angles, and targeted high-resolution images of key surface features. These data will be used to construct a global image base map, a digital terrain model, global maps of color properties, and mosaics of high-resolution image strips. Analysis of these data will provide information on Mercury’s impact history, tectonic processes, the composition and emplacement history of volcanic materials, and the thickness distribution and compositional variations of crustal materials. This paper summarizes MDIS’s science objectives and technical design, including the common payload design of the MDIS data processing units, as well as detailed results from ground and early flight calibrations and plans for Mercury image products to be generated from MDIS data.     

Processes that Promote and Deplete the Exosphere of Mercury

It has been speculated that the composition of the exosphere is related to the composition of Mercury’s crustal materials. If this relationship is true, then inferences regarding the bulk chemistry of the planet might be made from a thorough exospheric study. The most vexing of all unsolved problems is the uncertainty in the source of each component. Historically, it has been believed that H and He come primarily from the solar wind (Goldstein, B.E., et al. in J. Geophys. Res. 86:5485–5499, 1981), Na and K come from volatilized materials partitioned between Mercury’s crust and meteoritic impactors (Hunten, D.M., et al. in Mercury, pp. 562–612, 1988; Morgan, T.H., et al. in Icarus 74:156–170, 1988; Killen, R.M., et al. in Icarus 171:1–19, 2004b). The processes that eject atoms and molecules into the exosphere of Mercury are generally considered to be thermal vaporization, photon-stimulated desorption (PSD), impact vaporization, and ion sputtering. Each of these processes has its own temporal and spatial dependence. The exosphere is strongly influenced by Mercury’s highly elliptical orbit and rapid orbital speed. As a consequence the surface undergoes large fluctuations in temperature and experiences differences of insolation with longitude. Because there is no inclination of the orbital axis, there are regions at extreme northern and southern latitudes that are never exposed to direct sunlight. These cold regions may serve as traps for exospheric constituents or for material that is brought in by exogenic sources such as comets, interplanetary dust, or solar wind, etc. The source rates are dependent not only on temperature and composition of the surface, but also on such factors as porosity, mineralogy, and space weathering. They are not independent of each other. For instance, ion impact may create crystal defects which enhance diffusion of atoms through the grain, and in turn enhance the efficiency of PSD. The impact flux and the size distribution of impactors affects regolith turnover rates (gardening) and the depth dependence of vaporization rates. Gardening serves both as a sink for material and as a source for fresh material. This is extremely important in bounding the rates of the other processes. Space weathering effects, such as the creation of needle-like structures in the regolith, will limit the ejection of atoms by such processes as PSD and ion-sputtering. Therefore, the use of laboratory rates in estimates of exospheric source rates can be helpful but also are often inaccurate if not modified appropriately. Porosity effects may reduce yields by a factor of three (Cassidy, T.A., and Johnson, R.E. in Icarus 176:499–507, 2005). The loss of all atomic species from Mercury’s exosphere other than H and He must be by non-thermal escape. The relative rates of photo-ionization, loss of photo-ions to the solar wind, entrainment of ions in the magnetosphere and direct impact of photo-ions to the surface are an area of active research. These source and loss processes will be discussed in this chapter.     

The Geophysics of Mercury: Current Status and Anticipated Insights from the MESSENGER Mission

Current geophysical knowledge of the planet Mercury is based upon observations from ground-based astronomy and flybys of the Mariner 10 spacecraft, along with theoretical and computational studies. Mercury has the highest uncompressed density of the terrestrial planets and by implication has a metallic core with a radius approximately 75% of the planetary radius. Mercury’s spin rate is stably locked at 1.5 times the orbital mean motion. Capture into this state is the natural result of tidal evolution if this is the only dissipative process affecting the spin, but the capture probability is enhanced if Mercury’s core were molten at the time of capture. The discovery of Mercury’s magnetic field by Mariner 10 suggests the possibility that the core is partially molten to the present, a result that is surprising given the planet’s size and a surface crater density indicative of early cessation of significant volcanic activity. A present-day liquid outer core within Mercury would require either a core sulfur content of at least several weight percent or an unusual history of heat loss from the planet’s core and silicate fraction. A crustal remanent contribution to Mercury’s observed magnetic field cannot be ruled out on the basis of current knowledge. Measurements from the MESSENGER orbiter, in combination with continued ground-based observations, hold the promise of setting on a firmer basis our understanding of the structure and evolution of Mercury’s interior and the relationship of that evolution to the planet’s geological history.     

MESSENGER: Exploring Mercury’s Magnetosphere

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission to Mercury offers our first opportunity to explore this planet’s miniature magnetosphere since the brief flybys of Mariner 10. Mercury’s magnetosphere is unique in many respects. The magnetosphere of Mercury is among the smallest in the solar system; its magnetic field typically stands off the solar wind only ∼1000 to 2000 km above the surface. For this reason there are no closed drift paths for energetic particles and, hence, no radiation belts. Magnetic reconnection at the dayside magnetopause may erode the subsolar magnetosphere, allowing solar wind ions to impact directly the regolith. Inductive currents in Mercury’s interior may act to modify the solar wind interaction by resisting changes due to solar wind pressure variations. Indeed, observations of these induction effects may be an important source of information on the state of Mercury’s interior. In addition, Mercury’s magnetosphere is the only one with its defining magnetic flux tubes rooted beneath the solid surface as opposed to an atmosphere with a conductive ionospheric layer. This lack of an ionosphere is probably the underlying reason for the brevity of the very intense, but short-lived, ∼1–2 min, substorm-like energetic particle events observed by Mariner 10 during its first traversal of Mercury’s magnetic tail. Because of Mercury’s proximity to the sun, 0.3–0.5 AU, this magnetosphere experiences the most extreme driving forces in the solar system. All of these factors are expected to produce complicated interactions involving the exchange and recycling of neutrals and ions among the solar wind, magnetosphere, and regolith. The electrodynamics of Mercury’s magnetosphere are expected to be equally complex, with strong forcing by the solar wind, magnetic reconnection, and pick-up of planetary ions all playing roles in the generation of field-aligned electric currents. However, these field-aligned currents do not close in an ionosphere, but in some other manner. In addition to the insights into magnetospheric physics offered by study of the solar wind–Mercury system, quantitative specification of the “external” magnetic field generated by magnetospheric currents is necessary for accurate determination of the strength and multi-polar decomposition of Mercury’s intrinsic magnetic field. MESSENGER’s highly capable instrumentation and broad orbital coverage will greatly advance our understanding of both the origin of Mercury’s magnetic field and the acceleration of charged particles in small magnetospheres. In this article, we review what is known about Mercury’s magnetosphere and describe the MESSENGER science team’s strategy for obtaining answers to the outstanding science questions surrounding the interaction of the solar wind with Mercury and its small, but dynamic, magnetosphere.     

Mercury’s Atmosphere: A Surface-Bounded Exosphere

The existence of a surface-bounded exosphere about Mercury was discovered through the Mariner 10 airglow and occultation experiments. Most of what is currently known or understood about this very tenuous atmosphere, however, comes from ground-based telescopic observations. It is likely that only a subset of the exospheric constituents have been identified, but their variable abundance with location, time, and space weather events demonstrate that Mercury’s exosphere is part of a complex system involving the planet’s surface, magnetosphere, and the surrounding space environment (the solar wind and interplanetary magnetic field). This paper reviews the current hypotheses and supporting observations concerning the processes that form and support the exosphere. The outstanding questions and issues regarding Mercury’s exosphere stem from our current lack of knowledge concerning the surface composition, the magnetic field behavior within the local space environment, and the character of the local space environment.     

Mercury’s Interior Structure, Rotation, and Tides

This review addresses the deep interior structure of Mercury. Mercury is thought to consist of similar chemical reservoirs (core, mantle, crust) as the other terrestrial planets, but with a relatively much larger core. Constraints on Mercury’s composition and internal structure are reviewed, and possible interior models are described. Large advances in our knowledge of Mercury’s interior are not only expected from imaging of characteristic surface features but particularly from geodetic observations of the gravity field, the rotation, and the tides of Mercury. The low-degree gravity field of Mercury gives information on the differences of the principal moments of inertia, which are a measure of the mass concentration toward the center of the planet. Mercury’s unique rotation presents several clues to the deep interior. From observations of the mean obliquity of Mercury and the low-degree gravity data, the moments of inertia can be obtained, and deviations from the mean rotation speed (librations) offer an exciting possibility to determine the moment of inertia of the mantle. Due to its proximity to the Sun, Mercury has the largest tides of the Solar System planets. Since tides are sensitive to the existence and location of liquid layers, tidal observations are ideally suited to study the physical state and size of the core of Mercury.     

Earth-Based Visible and Near-IR Imaging of Mercury

New planned orbiter missions to Mercury have prompted renewed efforts to investigate the surface of Mercury via ground-based remote sensing. While the highest resolution instrumentation optical telescopes (e.g., HST) cannot be used at angular distances close to the Sun, advanced ground-based astronomical techniques and modern analytical and software can be used to obtain the resolved images of the poorly known or unknown part of Mercury. Our observations of the planet presented here were carried out in many observatories at morning and evening elongation of the planet. Stacking the acquired images of the hemisphere of Mercury, which was not observed by the Mariner 10 mission (1974–1975), is presented. Huge features found there change radically the existing hypothesis that the “continental” character of a surface may be attributed to the whole planet. We present the observational method, the data analysis approach, the resulting images and obtained properties of the Mercury’s surface.     

Evolution of the Protosolar Nebula and Formation of the Giant Planets

The formation of planetary systems is intimately tied to the question of the evolution of the gas and solid material in the early nebula. Current models of evolution of circumstellar disks are reviewed here with emphasis on the so-called “alpha models” in which angular momentum is transported outward by turbulent viscosity, parameterized by an dimensionless parameter α. A simple 1D model of protoplanetary disks that includes gas and embedded particles is used to introduce key questions on planetesimal formation. This model includes the aerodynamic properties of solid ice and rock grains to calculate their migration and growth. We show that the evolution of the nebula and migration and growth of its solids proceed on timescales that are generally not much longer than the timescale necessary to fully form the star-disk system from the molecular cloud. Contrary to a widely used approach, planet formation therefore can neither be studied in a static nebula nor in a nebula evolving from an arbitrary initial condition. We propose a simple approach to both account for sedimentation from the molecular cloud onto the disk, disk evolution and migration of solids. Giant planets have key roles in the history of the forming Solar System: they formed relatively early, when a significant amount of hydrogen and helium were still present in the nebula, and have a mass that is a sizable fraction of the disk mass at any given time. Their composition is also of interest because when compared to the solar composition, their enrichment in elements other than hydrogen and helium is a witness of sorting processes that occured in the protosolar nebula. We review likely scenarios capable of explaining both the presence of central dense cores in Jupiter, Saturn, Uranus and Neptune and their global composition. This revised version was published online in August 2006 with corrections to the Cover Date.