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17002 results

Page 124 of 851

  • 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.

  • 5 - Mercury’s Internal Magnetic Field

  • 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 114-143

  • Summary

    The MESSENGER mission provided a wealth of discoveries regarding Mercury’s present and past magnetic field and completed the first-order characterization of the magnetic fields of the solar system’s inner planets. MESSENGER demonstrated that Mercury is the only inner planet other than Earth to possess a global magnetic field generated by fluid motions in its liquid iron core. The field possesses some similarities to that of Earth, particularly its dipolar nature, but it is more than a factor of 100 weaker at the surface and unlike Earth’s field is highly asymmetric about the geographic equator. This structure constrains the dynamo process that generates the field and in turn the compositional and thermal structure of Mercury’s interior. Measurements made by MESSENGER less than 100 km above the planetary surface revealed signatures of crustal magnetization, at least some of which were acquired in a very ancient global magnetic field. Electric currents flow in the planet’s interior as a result of the dynamic interactions of the global magnetic field with the solar wind. These currents provide information on the radius of Mercury’s electrically conductive core, as well as the conductivity structure of the crust and mantle, which in turn reflects interior composition and temperature. 

  • 10 - The Tectonic 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 249-286

  • Summary

    Mercury is a tectonic world: the planet has experienced a long and complicated history of deformation, recorded by its preserved tectonic landforms. As the study of tectonics naturally intersects with volcanology, chemistry, interior structure, and thermal evolution, understanding the tectonic character of Mercury is a crucial means by which to more fully comprehend the planet’s geological history. In this chapter, we seek to tie together the various strands of observational and analytical studies of the tectonics of Mercury conducted since the first Mercury flyby of the MESSENGER mission. We describe the shortening and extensional structures on the innermost planet, as well as an enigmatic set of long-wavelength topographic warps that may have been tectonically driven, before reviewing our understanding of the structure and properties of Mercury's lithosphere. The mechanisms for tectonic deformation are next discussed, and we then explore the other major aspect of Mercury's tectonics – when deformation took place – as we work to describe at least in broad terms the tectonic history of the planet. The influence of tectonics on Mercury's volcanic activity is then addressed. Finally, we list some major questions regarding Mercury’s tectonics that remain open and suggest how they might yet be answered. 

  • 7 - The Geochemical and Mineralogical Diversity 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 176-190

  • Summary

    Geochemical measurements from the MESSENGER mission indicate distinct geochemical terranes on the surface of Mercury. We report chemical compositions and derived mineralogy for four geochemical terranes, as well as Mercury’s average surface composition. The geochemical terranes share higher Mg and S, and lower Al, Ca, and Fe, than terrestrial oceanic basalts. The low Fe and high S concentrations suggest that all terranes formed under highly reducing conditions. All terranes are enriched in plagioclase. Heating melted the silicate shell of Mercury and produced a global magma ocean in which stratification developed during crystallization, with basal ultramafic material grading to incompatible-element-enriched material near the surface. Later differentiation began with partial melting as result of mantle convection and heating from the decay of radioactive elements. These high-Mg, high-temperature partial melts were exceptionally fluid and produced thin, laterally extensive flows. The largest impacts excavated into the upper layers of the mantle and deposited distinctive material, including remnants of a graphite-rich flotation crust from the magma ocean, at the top of the crust. Smooth plains deposits originated as laterally extensive flood basalts that efficiently covered pre-existing layers. Distinct source compositions suggest that convection was insufficient to homogenize the mantle at ~3.8–3.9 Ga.

  • 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.

  • 18 - The Elusive Origin 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 497-515

  • Summary

    An objective of the MESSENGER mission was to learn the physical processes that determined Mercury’s high bulk ratio of metal to silicate. In the course of addressing that objective, the mission discovered multiple anomalous characteristics about the innermost planet. The lack of FeO and the reduced oxidation state of Mercury’s silicate crust and mantle are more extreme than nearly all other known materials in the solar system. In contrast, moderately volatile elements are present in abundances comparable to or greater than those of the other terrestrial planets. No single process during Mercury’s formation is able to account for all of these observations. Here, we review the current ideas for the origin of Mercury’s distinctive characteristics. Gaps in understanding the innermost regions of the early solar nebula limit the testing of different hypotheses. Even so, all proposed models are incomplete and need further development in order to unravel Mercury’s remaining secrets. 

  • 20 - Future Missions: Mercury after MESSENGER

  • 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 544-569

  • Summary

    Missions to Mercury are challenging because of the planet’s proximity to the Sun, as close as one-third the mean Earth–Sun distance. This location imparts a stressing thermal environment because of intense solar illumination, as well as major propulsion requirements because of the energy gained by a spacecraft descending from Earth into the Sun’s gravity well. Although Mercury has been a primary exploration target since the 1960s, it was not until the discovery of gravity-assist trajectories to Mercury that robotic exploration became feasible. The Mariner 10 flybys in the 1970s revealed many of Mercury's characteristics and whetted the appetite of the science community for an orbiter mission. Enabled by multiple planetary gravity assists and innovations in spacecraft and instrumentation, MESSENGER successfully orbited Mercury from 2011 into 2015 and revolutionized our understanding of the planet. New questions raised by the MESSENGER results motivate the much larger, dual-spacecraft BepiColombo mission, scheduled to arrive at Mercury in late 2025. Even after BepiColombo, many key questions central to understanding Mercury’s formation will likely require a Mercury lander mission, potentially enabled by sufficiently large launch vehicles. The return of samples from Mercury to Earth may long remain an aspiration for future generations of scientists and engineers. 

  • 19 - Mercury’s Global Evolution

  • 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 516-543

  • Summary

    MESSENGER’s exploration of Mercury has revealed a rich and dynamic geological history and provided constraints on the processes that control the planet’s internal evolution. That history includes resurfacing by impacts and volcanism prior to the end of the late heavy bombardment and a subsequent rapid waning of effusive volcanism. MESSENGER also revealed a global distribution of thrust faults that collectively accommodated a decrease in Mercury’s radius far greater than thought before the mission. Measurements of elemental abundances on Mercury’s surface indicate the planet is strongly chemically reduced, helping to characterize the composition and manner of crystallization of the metallic core. The discovery of a northward offset of the weak, axially aligned internal magnetic field, and of crustal magnetization in the planet’s ancient crust, places new limits on the history of the core dynamo and the entire interior. Models of Mercury’s thermochemical evolution subject to these observational constraints indicate that mantle convection may persist to the present but has been incapable of significantly homogenizing the mantle. These models also indicate that Mercury’s dynamo generation is influenced by both a static layer at the top of the core and convective motions within the core driven by compositional buoyancy.

  • 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. 

  • 13 - Mercury’s Polar Deposits

  • 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 346-370

  • Summary

    Two and a half decades ago, the discovery of Mercury’s polar deposits from Earth-based radar observations provided the first tantalizing, but limited, evidence for the possibility of water ice on the solar system’s innermost planet. Identifying the materials in Mercury’s polar deposits was one of the six major science questions that originally motivated the MESSENGER mission. In the course of the mission’s more than four Earth years of operations in orbit about Mercury, MESSENGER produced multiple datasets to investigate Mercury’s polar deposits: determinations of regions of permanent shadow, neutron spectrometer observations, laser altimeter reflectance measurements, thermal model results, and direct images of the deposits. These datasets provided compelling evidence that in addition to substantial amounts of water ice stored in Mercury’s polar deposits, there are also other volatile materials, postulated to be dark, organic-rich compounds that bury the water ice deposits. This chapter reviews MESSENGER’s investigations of Mercury’s polar deposits and discusses the resulting implications for the origin and evolution of Mercury’s polar water ice. 

  • 7 - The Geochemical and Mineralogical Diversity 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 176-190

  • Summary

    Geochemical measurements from the MESSENGER mission indicate distinct geochemical terranes on the surface of Mercury. We report chemical compositions and derived mineralogy for four geochemical terranes, as well as Mercury’s average surface composition. The geochemical terranes share higher Mg and S, and lower Al, Ca, and Fe, than terrestrial oceanic basalts. The low Fe and high S concentrations suggest that all terranes formed under highly reducing conditions. All terranes are enriched in plagioclase. Heating melted the silicate shell of Mercury and produced a global magma ocean in which stratification developed during crystallization, with basal ultramafic material grading to incompatible-element-enriched material near the surface. Later differentiation began with partial melting as result of mantle convection and heating from the decay of radioactive elements. These high-Mg, high-temperature partial melts were exceptionally fluid and produced thin, laterally extensive flows. The largest impacts excavated into the upper layers of the mantle and deposited distinctive material, including remnants of a graphite-rich flotation crust from the magma ocean, at the top of the crust. Smooth plains deposits originated as laterally extensive flood basalts that efficiently covered pre-existing layers. Distinct source compositions suggest that convection was insufficient to homogenize the mantle at ~3.8–3.9 Ga.

  • 18 - The Elusive Origin 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 497-515

  • Summary

    An objective of the MESSENGER mission was to learn the physical processes that determined Mercury’s high bulk ratio of metal to silicate. In the course of addressing that objective, the mission discovered multiple anomalous characteristics about the innermost planet. The lack of FeO and the reduced oxidation state of Mercury’s silicate crust and mantle are more extreme than nearly all other known materials in the solar system. In contrast, moderately volatile elements are present in abundances comparable to or greater than those of the other terrestrial planets. No single process during Mercury’s formation is able to account for all of these observations. Here, we review the current ideas for the origin of Mercury’s distinctive characteristics. Gaps in understanding the innermost regions of the early solar nebula limit the testing of different hypotheses. Even so, all proposed models are incomplete and need further development in order to unravel Mercury’s remaining secrets. 

  • 20 - Future Missions: Mercury after MESSENGER

  • 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 544-569

  • Summary

    Missions to Mercury are challenging because of the planet’s proximity to the Sun, as close as one-third the mean Earth–Sun distance. This location imparts a stressing thermal environment because of intense solar illumination, as well as major propulsion requirements because of the energy gained by a spacecraft descending from Earth into the Sun’s gravity well. Although Mercury has been a primary exploration target since the 1960s, it was not until the discovery of gravity-assist trajectories to Mercury that robotic exploration became feasible. The Mariner 10 flybys in the 1970s revealed many of Mercury's characteristics and whetted the appetite of the science community for an orbiter mission. Enabled by multiple planetary gravity assists and innovations in spacecraft and instrumentation, MESSENGER successfully orbited Mercury from 2011 into 2015 and revolutionized our understanding of the planet. New questions raised by the MESSENGER results motivate the much larger, dual-spacecraft BepiColombo mission, scheduled to arrive at Mercury in late 2025. Even after BepiColombo, many key questions central to understanding Mercury’s formation will likely require a Mercury lander mission, potentially enabled by sufficiently large launch vehicles. The return of samples from Mercury to Earth may long remain an aspiration for future generations of scientists and engineers. 

  • 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.

  • 4 - Mercury’s Internal 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 85-113

  • Summary

    We describe the current state of knowledge about Mercury's interior structure. We review the available observational constraints, including mass, radius, density, gravity field, spin state, composition, and tidal response. These data enable the construction of models that represent the distribution of mass inside Mercury. In particular, we infer radial profiles of the pressure, density, and gravitational acceleration in the core, mantle, and crust. We also examine Mercury's rotational dynamics and the influence of an inner core on the spin state and the determination of the moment of inertia. Finally, we discuss the wide-ranging implications of Mercury's internal structure on its thermal evolution, surface geology, capture into a distinctive spin-orbit resonance, and magnetic field generation. 

  • 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. 

Page 124 of 851