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17002 results

Page 122 of 851

  • 12 - Conclusions

  • Ralph D. Lorenz, The Johns Hopkins University
  • Book:
    Exploring Planetary Climate
    Published online:
    14 December 2018
    Print publication:
    03 January 2019, pp 265-270

  • Summary

    Review of developments in Mars, Venus and Titan climate studies, and relations between these efforts and terrestrial climate and exoplanets. Ongoing evolution of Earth's carbon dioxide abundance.

  • 5 - First Contact: The Dawn of Planetary Exploration

  • Ralph D. Lorenz, The Johns Hopkins University
  • Book:
    Exploring Planetary Climate
    Published online:
    14 December 2018
    Print publication:
    03 January 2019, pp 83-110

  • Summary

    Describes the dawn of planetary exploration: Mariner II measures Venus temperatures by microwave; Mariner 5 uses radio methods to measure the density of the Mars atmosphere, much smaller than had been expected. Development of early global circulation models, application of energy balance models to explore ice–albedo climate instability of Earth by Budyko and Sellers, and to explore Martian climate by Leighton and Murray. First in-situ exploration of Venus by Soviet Venera probes, calculations of Venus greenhouse effect by Sagan, Pollack and others. Lovelock originates Gaia Hypothesis.

  • 11 - Worlds Beyond: Exoplanet Climate

  • Ralph D. Lorenz, The Johns Hopkins University
  • Book:
    Exploring Planetary Climate
    Published online:
    14 December 2018
    Print publication:
    03 January 2019, pp 249-264

  • Summary

    Calculation of the habitable zone around other stars. Detection of light and heat from extrasolar planets. First detection of atmospheres on exoplanets. Numerical modeling of extreme climates: many exoplanets tidally locked with permanent night- and dayside. Multiple planet systems discovered, and a planet in the habitable zone of Proxima Centauri, our nearest star.

  • 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.  

  • 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.

  • 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. 

  • 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%. 

  • 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. 

  • 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. 

Page 122 of 851