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3045 results in Planetary systems and astrobiology

Page 30 of 153

  • 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. 

  • 12 - Mercury’s Hollows

  • 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 324-345

  • Summary

    Images from the MESSENGER spacecraft show that irregular, flat-floored depressions with high-reflectance interiors and haloes are common on the surface of planet Mercury. These landforms, called hollows, are among Mercury's youngest non-impact features and may be forming today. Hollows are unique to Mercury, with no close equivalent on other planetary bodies. Clues to understanding hollows come from consideration of morphological features associated with ice-bearing surfaces on Mars and icy satellites, and of processes leading to loss of sulfur from asteroids. Evidence suggests that hollows form when sublimation or destruction of a volatile-bearing phase weakens the host rock, causing collapse and scarp retreat. The phase susceptible to loss may be a sulfide mineral or graphite. Loss of the volatile component could be driven by solar heating, exposure to solar ultraviolet radiation, exposure to the solar wind, sputtering by magnetospheric ions, and micrometeoroid bombardment. The depth to which hollows grow may be controlled by accumulation of a protective lag deposit. The volatile-bearing phase that is lost appears to be a pervasive component of the host rock, but in some cases the hollow-forming phase may have been concentrated by volcanic processes or differentiation of impact melts. 

  • 12 - Mercury’s Hollows

  • 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 324-345

  • Summary

    Images from the MESSENGER spacecraft show that irregular, flat-floored depressions with high-reflectance interiors and haloes are common on the surface of planet Mercury. These landforms, called hollows, are among Mercury's youngest non-impact features and may be forming today. Hollows are unique to Mercury, with no close equivalent on other planetary bodies. Clues to understanding hollows come from consideration of morphological features associated with ice-bearing surfaces on Mars and icy satellites, and of processes leading to loss of sulfur from asteroids. Evidence suggests that hollows form when sublimation or destruction of a volatile-bearing phase weakens the host rock, causing collapse and scarp retreat. The phase susceptible to loss may be a sulfide mineral or graphite. Loss of the volatile component could be driven by solar heating, exposure to solar ultraviolet radiation, exposure to the solar wind, sputtering by magnetospheric ions, and micrometeoroid bombardment. The depth to which hollows grow may be controlled by accumulation of a protective lag deposit. The volatile-bearing phase that is lost appears to be a pervasive component of the host rock, but in some cases the hollow-forming phase may have been concentrated by volcanic processes or differentiation of impact melts. 

Exploring Planetary Climate
  • A History of Scientific Discovery on Earth, Mars, Venus and Titan
  • Ralph D. Lorenz
  • Published online:
    14 December 2018
    Print publication:
    03 January 2019
  • This book chronicles the history of climate science and planetary exploration, focusing on our ever-expanding knowledge of Earth's climate, and the parallel research underway on some of our nearest neighbours: Mars, Venus and Titan. From early telescopic observation of clouds and ice caps on planetary bodies in the seventeenth century, to the dawn of the space age and the first robotic planetary explorers, the book presents a comprehensive chronological overview of planetary climate research, right up to the dramatic recent developments in detecting and characterising exoplanets. Meanwhile, the book also documents the discoveries about our own climate on Earth, not only about how it works today, but also how profoundly different it has been in the past. Highly topical and written in an accessible and engaging narrative style, this book provides invaluable historical context for students, researchers, professional scientists, and those with a general interest in planetary climate research.

Saturn in the 21st Century
  • Edited by Kevin H. Baines, F. Michael Flasar, Norbert Krupp, Tom Stallard
  • Published online:
    13 December 2018
    Print publication:
    06 December 2018
  • The Cassini Orbiter mission, launched in 1997, has provided state-of-the-art information into the origins and workings of Saturn. Drawing from new discoveries and scientific insight from the mission, this book provides a detailed overview of the planet as revealed by Cassini. Chapters by eminent planetary scientists and researchers from across the world comprehensively review the current state of knowledge regarding Saturn's formation, interior, atmosphere, ionosphere, thermosphere and magnetosphere. Specialised chapters discuss the planet's seasonal variability; the circulation of strong zonal winds; the constantly changing polar aurorae; and the Great Storm of 2010–2011, the most powerful convective storm ever witnessed by humankind. Documenting the latest research on the planet, from its formation to how it operates today, this is an essential reference for graduate students, researchers and planetary scientists.

Mercury
  • The View after MESSENGER
  • Edited by Sean C. Solomon, Larry R. Nittler, Brian J. Anderson
  • Published online:
    10 December 2018
    Print publication:
    20 December 2018
  • Observations from the first spacecraft to orbit the planet Mercury have transformed our understanding of the origin and evolution of rocky planets. This volume is the definitive resource about Mercury for planetary scientists, from students to senior researchers. Topics treated in depth include Mercury's chemical composition; the structure of its crust, lithosphere, mantle, and core; Mercury's modern and ancient magnetic field; Mercury's geology, including the planet's major geological units and their surface chemistry and mineralogy, its spectral reflectance characteristics, its craters and cratering history, its tectonic features and deformational history, its volcanic features and magmatic history, its distinctive hollows, and the frozen ices in its polar deposits; Mercury's exosphere and magnetosphere and the processes that govern their dynamics and their interaction with the solar wind and interplanetary magnetic field; the formation and large-scale evolution of the planet; and current plans and needed capabilities to explore Mercury further in the future.

  • 5 - The Mysterious Periodicities of Saturn

  • Edited by Kevin H. Baines, University of Wisconsin, Madison, F. Michael Flasar, NASA-Goddard Space Flight Center, Norbert Krupp, Tom Stallard, University of Leicester
  • Book:
    Saturn in the 21st Century
    Published online:
    13 December 2018
    Print publication:
    06 December 2018, pp 97-125

  • Summary

    The rotation rate of a planet is a fundamental parameter, no less than its mass or composition, and planetary investigators require this rate to assess various other phenomena such as planetary wind speeds, internal and atmospheric models, ring dynamics and so forth. Saturn presents a conundrum, however, because none of its various planetary periods indicates the “true” rotation of the planet. Thus, although the planet displays an abundance of periodicities near 10.7 hours, the exact rotation period of Saturn is unknown. In the magnetosphere, “planetary-period oscillations” (PPOs) appear in charged particles, magnetic fields, energetic neutral atoms, radio emissions and motions of the plasma sheet and magnetopause. In Saturn’s rings, the spoke phenomenon can exhibit periodicities near 10.7 hours, and ring phenomena themselves may be related to the interior rotation of the planet. In the high-latitude ionosphere, modulations near this period appear in auroral motions and intensities. In the upper atmosphere, some cloud features rotate near this period, although wind speeds are generally faster, and the well-known polar hexagon rotates with a period close to 10.7 hours. Some of the magnetospheric/ionospheric oscillations differ in the northern and southern hemispheres and their periods do not remain constant, sometimes varying on long time scales of a year or longer and sometimes on much shorter time scales. These variations in the period argue against a cause related to changes interior to Saturn, and because the magnetic and spin axes of Saturn are reported to be axisymmetric (unlike those of any other known planet), Saturn’s periodicities cannot be explained as “wobble” caused by a geometric tilt or by a nondipolar magnetic anomaly. Several models have been proposed to account for the observed periodicities, including rotating atmospheric vortices, periodic plasma releases and a flapping magnetodisk, but none can satisfactorily explain all of Saturn’s periodicities nor their common origin, and none can determine the exact rotation rate of the planet. This chapter reviews Saturn’s periodicities, theories thereof, and how they might be used to determine the elusive rotation rate of the planet.

  • 3 - The Interior of Saturn

  • Edited by Kevin H. Baines, University of Wisconsin, Madison, F. Michael Flasar, NASA-Goddard Space Flight Center, Norbert Krupp, Tom Stallard, University of Leicester
  • Book:
    Saturn in the 21st Century
    Published online:
    13 December 2018
    Print publication:
    06 December 2018, pp 44-68

  • Summary

    We review our current understanding of the interior structure and thermal evolution of Saturn, with a focus on recent results in the Cassini era. There has been important progress in understanding physical inputs, including equations of state of planetary materials and their mixtures, physical parameters like the gravity field and rotation rate, and constraints on Saturnian free oscillations. At the same time, new methods of calculation, including work on the gravity field of rotating fluid bodies, and the role of interior composition gradients, should help to better constrain the state of Saturn’s interior, now and earlier in its history. However, a better appreciation of modeling uncertainties and degeneracies, along with a greater exploration of modeling phase space, still leave great uncertainties in our understanding of Saturn’s interior. Further analysis of Cassini data sets, as well as precise gravity field measurements from the Cassini Grand Finale orbits, will further revolutionize our understanding of Saturn’s interior over the next few years.

Page 30 of 153