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  • 3 - Mercury’s Crust and Lithosphere: Structure and Mechanics

  • Edited by Sean C. Solomon, Larry R. Nittler, Carnegie Institution of Washington, Washington DC, Brian J. Anderson
  • Book:
    Mercury
    Published online:
    10 December 2018
    Print publication:
    20 December 2018, pp 52-84

  • Summary

    The outermost “crust” and an underlying, compositionally distinct, and denser layer, the “mantle,” constitute the silicate portion of a terrestrial planet. The “lithosphere” is the planet’s high-strength outer shell. The crust records the history of shallow magmatism, which along with temporal changes in lithospheric thickness, provides information on a planet’s thermal evolution. We focus on the basic structure and mechanics of Mercury’s crust and lithosphere as determined primarily from gravity and topography data acquired by the MESSENGER mission. We first describe these datasets: how they were acquired, how the data are represented on a sphere, and the limitations of the data imparted by MESSENGER’s highly eccentric orbit.  We review different crustal thickness models obtained by parsing the observed gravity signal into contributions from topography, relief on the crust–mantle boundary, and density anomalies that drive viscous flow in the mantle. Estimates of lithospheric thickness from gravity–topography analyses are at odds with predictions from thermal models, thus challenging our understanding of Mercury’s geodynamics. We show that, like those of the Moon, Mercury's ellipsoidal shape and geoid are far from hydrostatic equilibrium, possibly the result of Mercury's peculiar surface temperature distribution and associated buoyancy anomalies and thermoelastic stresses in the interior.  

  • 9 - Impact Cratering of Mercury

  • Edited by Sean C. Solomon, Larry R. Nittler, Carnegie Institution of Washington, Washington DC, Brian J. Anderson
  • Book:
    Mercury
    Published online:
    10 December 2018
    Print publication:
    20 December 2018, pp 217-248

  • Summary

    Impact craters are the dominant landform on Mercury and range from the largest basins to the smallest young craters.  Peak-ring basins are especially prevalent on Mercury, although basins of all forms are far undersaturated, probably the result of the extensive volcanic emplacement of intercrater plains and younger smooth plains between about 4.1 and 3.5 Ga.  This chapter describes the geology of the two largest well-preserved basins, Caloris and Rembrandt, and the three smaller Raditladi, Rachmaninoff, and Mozart basins.  We describe analyses of crater size–frequency distributions and relate them to populations of asteroid impactors (Late Heavy Bombardment in early epochs and the near-Earth asteroid population observable today during most of Mercury’s history), to secondary cratering, and to exogenic and endogenic processes that degrade and erase craters.  Secondary cratering is more important on Mercury than on other solar system bodies and shaped much of the surface on kilometer and smaller scales, compromising our ability to use craters for relative and absolute age-dating of smaller geological units.  Failure to find “vulcanoids” and satellites of Mercury suggests that such bodies played a negligible role in cratering Mercury.  We describe an absolute cratering chronology for Mercury’s geological evolution as well as its uncertainties. 

  • 16 - Structure and Configuration of Mercury’s Magnetosphere

  • Edited by Sean C. Solomon, Larry R. Nittler, Carnegie Institution of Washington, Washington DC, Brian J. Anderson
  • Book:
    Mercury
    Published online:
    10 December 2018
    Print publication:
    20 December 2018, pp 430-460

  • Summary

    Mercury is the only terrestrial planet other than Earth that possesses a global magnetic field, and the unique solar wind environment of the inner heliosphere has profound consequences for both the structure and dynamics of its magnetosphere. The first in situ observations of Mercury and its space environment made four decades ago by the Mariner 10 spacecraft revealed a magnetic field that is sufficiently strong to stand off the solar wind and form a magnetosphere. Many new insights into Mercury’s magnetosphere were enabled by data returned by the MESSENGER spacecraft. The extensive magnetic field and particle observations allowed detailed characterization of the magnetospheric structure and configuration. MESSENGER magnetic field observations definitively determined the orientation, moment, and location of the internal planetary magnetic dipole field. Furthermore, these observations established the configuration of the magnetopause, bow shock, and magnetospheric current systems. Plasma observations revealed the distribution and composition of plasma in the magnetosphere. We review the geometry and dominant physical processes of Mercury’s unique magnetosphere inferred from MESSENGER data, including the solar wind environment, the shape and location of magnetospheric boundaries, and the fundamental regions and configuration of the magnetosphere and transport and heating of plasma therein. 

  • 15 - Understanding Mercury’s Exosphere: Models Derived from MESSENGER Observations

  • Edited by Sean C. Solomon, Larry R. Nittler, Carnegie Institution of Washington, Washington DC, Brian J. Anderson
  • Book:
    Mercury
    Published online:
    10 December 2018
    Print publication:
    20 December 2018, pp 407-429

  • Summary

    Mercury is surrounded by a tenuous exosphere in which particles travel on ballistic trajectories under the influence of a combination of gravity and solar radiation pressure. The densities are so small that the surface forms the exobase, and particles in the exosphere are more likely to collide with it rather than with each other. During the three flybys of Mercury by the Mariner 10 spacecraft in 1974–1975, the probe's Ultraviolet Spectrometer made measurements of hydrogen and helium and a tentative detection of oxygen. These observations were followed a decade later by discoveries with Earth-based telescopes of exospheric sodium and potassium, and still later of calcium, aluminum, and iron. In addition to characterizing sodium, calcium, and hydrogen in Mercury’s exosphere, the Mercury Atmospheric and Surface Composition Spectrometer instrument on the MESSENGER spacecraft detected magnesium, ionized calcium, aluminum, and manganese. Thus, the total inventory of confirmed exospheric neutral species now includes H, He, Na, K, Ca, Mg, Al, Fe, and Mn. This chapter summarizes both ground-based and space-based observations of Mercury’s exosphere that have been made from its discovery by Mariner 10 through the four Earth years of nearly continuous orbital observations by the MESSENGER spacecraft.

  • 6 - The Geologic History of Mercury

  • Edited by Sean C. Solomon, Larry R. Nittler, Carnegie Institution of Washington, Washington DC, Brian J. Anderson
  • Book:
    Mercury
    Published online:
    10 December 2018
    Print publication:
    20 December 2018, pp 144-175

  • Summary

    We assess Mercury’s geologic history, focusing on the distribution and origin of terrain types and an overview of Mercury’s evolution from the pre-Tolstojan through the Kuiperian Period. We review evidence for the nature of Mercury’s early crust, including the possibility that a substantial portion formed by the global eruption of lavas generated by partial melting during and after overturn of the crystalline products of magma ocean cooling, whereas a much smaller fraction of the crust may have been derived from crystal flotation in such a magma ocean. The early history of Mercury may thus have been similar to that of the other terrestrial planets, with much of the crust formed through volcanism, in contrast to the flotation-dominated crust of the Moon. Small portions of Mercury’s early crust may still be exposed in a heavily modified and brecciated form; the majority of the surface is dominated by intercrater plains (Pre-Tolstojan and Tolstojan in age) and smooth plains (Tolstojan and Calorian) that formed through a combination of volcanism and impact events. As effusive volcanism waned in the Calorian, explosive volcanism continued at least through the Mansurian Period; the Kuiperian Period was dominated by impact events and the formation of hollows. 

  • 1 - The MESSENGER Mission: Science and Implementation Overview

  • Edited by Sean C. Solomon, Larry R. Nittler, Carnegie Institution of Washington, Washington DC, Brian J. Anderson
  • Book:
    Mercury
    Published online:
    10 December 2018
    Print publication:
    20 December 2018, pp 1-29

  • Summary

    MESSENGER was the first spacecraft to visit the planet Mercury in more than three decades and the first to orbit the solar system’s innermost planet, and it provided the first global observations of Mercury’s surface, interior, exosphere, magnetosphere, and heliospheric environment. This chapter begins with summaries of the objectives for the MESSENGER mission and the design of the spacecraft, payload instruments, and orbit selected to achieve those objectives. We then describe the procedures adopted to optimize the scientific return from the complex series of orbital data acquisition operations that MESSENGER followed. An overview is given next of the primary MESSENGER mission, including the three Mercury flybys prior to orbit insertion and the first year of Mercury orbital observations. We then outline the rationale for and accomplishments of MESSENGER’s first extended mission, conducted over the second year of orbital operations, and MESSENGER’s second extended mission, conducted over the final two years of orbital operations. The second extended mission included a distinctive low-altitude campaign completed at the culmination of the mission. A concluding section briefly introduces the other chapters of this book.

  • 2 - The Chemical Composition of Mercury

  • Edited by Sean C. Solomon, Larry R. Nittler, Carnegie Institution of Washington, Washington DC, Brian J. Anderson
  • Book:
    Mercury
    Published online:
    10 December 2018
    Print publication:
    20 December 2018, pp 30-51

  • Summary

    The chemical composition of a planetary body reflects its starting conditions modified by numerous processes during its formation and geological evolution. Measurements by X-ray, gamma-ray, and neutron spectrometers on the MESSENGER spacecraft revealed Mercury’s surface to have surprisingly high abundances of the moderately volatile elements sodium, sulfur, potassium, and chlorine, and a low abundance of iron. This composition rules out some formation models for which high temperatures are expected to have strongly depleted volatiles and indicates that Mercury formed under conditions much more reducing than the other rocky planets of our solar system. Through geochemical modeling and petrologic experiments, the planet’s mantle and core compositions can be estimated from the surface composition and geophysical constraints. The bulk silicate composition of Mercury is likely similar to that of enstatite or metal-rich chondrite meteorites, and the planet’s unusually large core is most likely Si rich, implying that in bulk Mercury is enriched in Fe and Si (and possibly S) relative to the other inner planets. The compositional data for Mercury acquired by MESSENGER will be crucial for quantitatively testing future models of the formation of Mercury and the Solar System as a whole, as well as for constraining the geological evolution of the innermost planet.

  • 14 - Observations of Mercury’s Exosphere: Composition and Structure

  • Edited by Sean C. Solomon, Larry R. Nittler, Carnegie Institution of Washington, Washington DC, Brian J. Anderson
  • Book:
    Mercury
    Published online:
    10 December 2018
    Print publication:
    20 December 2018, pp 371-406

  • Summary

    Mercury is surrounded by a tenuous exosphere in which particles travel on ballistic trajectories under the influence of a combination of gravity and solar radiation pressure. The densities are so small that the surface forms the exobase, and particles in the exosphere are more likely to collide with it rather than with each other. During the three flybys of Mercury by the Mariner 10 spacecraft in 1974–1975, the probe's Ultraviolet Spectrometer made measurements of hydrogen and helium and a tentative detection of oxygen. These observations were followed a decade later by discoveries with Earth-based telescopes of exospheric sodium and potassium, and still later of calcium, aluminum, and iron. In addition to characterizing sodium, calcium, and hydrogen in Mercury’s exosphere, the Mercury Atmospheric and Surface Composition Spectrometer instrument on the MESSENGER spacecraft detected magnesium, ionized calcium, aluminum, and manganese. Thus, the total inventory of confirmed exospheric neutral species now includes H, He, Na, K, Ca, Mg, Al, Fe, and Mn. This chapter summarizes both ground-based and space-based observations of Mercury’s exosphere that have been made from its discovery by Mariner 10 through the four Earth years of nearly continuous orbital observations by the MESSENGER spacecraft.

  • 17 - Mercury’s Dynamic Magnetosphere

  • Edited by Sean C. Solomon, Larry R. Nittler, Carnegie Institution of Washington, Washington DC, Brian J. Anderson
  • Book:
    Mercury
    Published online:
    10 December 2018
    Print publication:
    20 December 2018, pp 461-496

  • Summary

    MESSENGER’s exploration of Mercury has led to many important discoveries and a global perspective on its magnetosphere, exosphere, and interior as a coupled system. Mercury’s proximity to the Sun, weak planetary magnetic field, electrically conducting core, and sodium-dominated exosphere give rise to a highly dynamic magnetosphere unlike that of any other planet. The strong interplanetary magnetic fields so close to the Sun result in a high rate of energy transfer from the solar wind into Mercury’s magnetosphere. Surprisingly, direct solar wind impact on the surface during coronal mass ejection impact has been found to be infrequent.  Electric currents induced in Mercury’s highly conducting interior buttress the weak planetary magnetic field against direct impact for all but the strongest solar events. Kinetic effects associated with the large orbits of planetary ions about the magnetic field and the small dimensions of the magnetosphere are observed to significantly affect some fluid instabilities such as Kelvin-Helmholtz waves along the magnetopause. As at Earth, magnetic reconnection, dipolarization fronts, and plasmoid ejection are closely associated with substorms in Mercury’s magnetosphere, and MESSENGER frequently observed energetic electrons with energies of tens of to several hundred thousand electron volts. However, no “Van Allen” radiation belts with durable trapping are present.

  • 8 - Spectral Reflectance Constraints on the Composition and Evolution of Mercury’s Surface

  • Edited by Sean C. Solomon, Larry R. Nittler, Carnegie Institution of Washington, Washington DC, Brian J. Anderson
  • Book:
    Mercury
    Published online:
    10 December 2018
    Print publication:
    20 December 2018, pp 191-216

  • Summary

    MESSENGER characterized the spectral reflectance of Mercury using the Mercury Dual Imaging System wide-angle camera and the Mercury Atmospheric and Surface Composition Spectrometer. Compared with other differentiated silicate bodies, Mercury lacks the 1-µm crystal-field absorption due to ferrous iron in silicate yet is unusually low in reflectance. Spectral modeling suggests that the likely darkening phase is graphite, and surficial carbon has been confirmed with data from MESSENGER's Neutron Spectrometer. Control of reflectance by this minor opaque phase, rather than by the abundance of iron in silicates as on the Moon, prevents the correlation of spectral reflectance and major element composition as on the Moon. Variations in reflectance and color nevertheless serve as markers for the structure of the upper crust, revealing that at least 5 km of volcanic plains overlie carbon-enriched low-reflectance material. The one definitive absorption due to oxidized iron, an oxygen-metal charge transfer (OMCT) band in the ultraviolet observed in bright, pyroclastic deposits, may originate by oxidation of darkening carbon and sulfides, reducing sufficient iron to metal to unsaturate the OMCT band. The content of ferrous iron implied by the presence of this feature and the lack of a 1-µm feature is between 0.1 and 1 wt%. 

  • 11 - The Volcanic Character of Mercury

  • Edited by Sean C. Solomon, Larry R. Nittler, Carnegie Institution of Washington, Washington DC, Brian J. Anderson
  • Book:
    Mercury
    Published online:
    10 December 2018
    Print publication:
    20 December 2018, pp 287-323

  • Summary

    Mercury is a volcanic world: the planet has experienced a geological history that included partial melting of the interior and the transport of magma to, and eruption onto, the surface. In this chapter, we review Mercury’s volcanic character, first in terms of effusive volcanism (as characterized by lava plains, erosional landforms, and spectral characteristics), next in regard to the planet’s explosive volcanic activity, and then from the perspective of intrusive magmatism. We also visit the planet’s ancient yet spatially expansive intercrater plains and the prospect that they, too, are volcanic. We combine the observations of and inferences for Mercury’s smooth and intercrater plains to propose a model for the planet’s crustal stratigraphy. The extent of our understanding of the petrology of surface materials on Mercury is then discussed, including compositions and lithologies, mineral assemblages, physicochemical properties, and volatile contents. We then describe in broad terms the history of effusive and explosive volcanism on the planet, before addressing the influence that the planet’s lithospheric properties and tectonic evolution have played on volcanism. We finish by listing some major outstanding questions pertaining to the volcanic character of Mercury, and we suggest how those questions might best be addressed. 

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