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17002 results

Page 467 of 851

  • 1 - Classical Hamiltonian formulation of General Relativity

  • Thomas Thiemann, Max-Planck-Institut für Gravitationsphysik, Germany
  • Book:
    Modern Canonical Quantum General Relativity
    Published online:
    04 August 2010
    Print publication:
    13 September 2007, pp 39-73

  • Summary

    In this chapter we provide a self-contained exposition of the classical Hamiltonian formulation of General Relativity. It is mandatory to know all the details of this classical work as it lays the ground for the interpretation of the theory, the understanding of the problem of time and its implication for the interpretation of quantum mechanics, the meaning of observables, the relation between spacetime diffeomorphisms and gauge transformations and finally (Poincaré) symmetries versus gauge transformations. It also defines the platform on which the quantum theory is based. Only a solid knowledge of topology and differential geometry is necessary for this chapter, of which we give an account in Chapters 18 and 19.

    The ADM action

    The contents of this section were developed by Arnowitt et al. [206]. Modern treatments can be found in the beautiful textbooks by Wald [207] (especially appendix E and chapter 10) and by Hawking and Ellis [208]. We will treat only the vacuum case. Matter and cosmological terms can be treated the same way.

    What we are going to do in what follows seems to be a dangerous enterprise in a generally covariant theory: we will split the spacetime manifold into space and time. While this is necessary in a canonical approach, as otherwise we cannot define velocities and hence momenta conjugate to the configuration variables, this seems to break diffeomorphism invariance. However, this is not the case because we do not fix the split into space and time, rather we keep it arbitrary, we do not fix a coordinate system. The arbitrariness in fact exhausts the full diffeomorphism group.

  • 1 - History of astrobiological ideas

  • from PART I - History
  • Edited by Woodruff T. Sullivan, III, University of Washington, John Baross, University of Washington
  • Book:
    Planets and Life
    Published online:
    28 May 2018
    Print publication:
    13 September 2007, pp 9-45

  • Summary

    Overview

    Why history?

    The core questions of astrobiology are not new. They have always been asked and are central to Western intellectual history. How did life begin? How has it changed? What is the relation of humans to other species? Does life exist elsewhere? If so, where might it be and what is it like? Although these questions are ancient, what is new are the tools at hand to search for answers, ranging from robotic spacecraft to genome sequencing, from electron microscopes to radio telescopes. These tools and other factors (see the Prologue and Chapter 2) appear to have brought astrobiology to a point where it is gelling into something qualitatively different – our first sound attack on these questions. But is this so? Or is today no different from any other time in the past few centuries?

    In every era, including our own, scientists can do no more than tackle questions with the best tools available, apply the best insight they can muster, and struggle to fashion a consensus as to the nature of the world. In this manner our understanding has progressed, for example, from the “animalcules” that van Leeuwenhoek described three hundred years ago to the richness of contemporary microbiology. To understand such a thread as it meanders through history, we need to document more than the accumulation of facts. When evaluating a given episode, historians of science look carefully at evidence of not only the science itself, but also of the larger enveloping context.

  • 19 - Europa

  • from PART V - Potentially habitable worlds
  • Edited by Woodruff T. Sullivan, III, University of Washington, John Baross, University of Washington
  • Book:
    Planets and Life
    Published online:
    28 May 2018
    Print publication:
    13 September 2007, pp 388-423

  • Summary

    Introduction

    Europa, one of the four large satellites of Jupiter, is nearly the size of Earth's Moon. Tidal flexing driven by Jupiter's gravity and sustained by an orbital resonance with two other jovian satellites, Io and Ganymede, results in significant heat dissipation within Europa. Calculations indicate that this tidal heating is sufficient to maintain liquid water beneath Europa's ice crust. Moreover, observational evidence suggests that it is indeed probable, but not yet completely certain, that Europa harbors a subsurface ocean of liquid water whose volume is about twice that of Earth's oceans. The likely presence of abundant liquid water places Europa among the highest priority targets for astrobiology.

    To support life, Europa would also require an inventory of biogenic elements and a source of sufficient free energy. The ability to support life does not of course guarantee that the origin of life took place on Europa, or that life is present today; answering these questions will require further exploration. This chapter considers Europa in an astrobiological context, distinguishing among what is known, what is supported by evidence but still uncertain, and what remains more speculative. We conclude with a discussion of future missions that will be needed to address current geological and astrobiological questions.

    Jupiter and its satellites

    The planet Jupiter, orbiting the Sun at 5.2 AU, is more massive than the other planets in the Solar System combined; it is 3.3 times more massive than Saturn and 318 times more than the Earth.

  • 22 - How to search for life on other worlds

  • from PART VI - Searching for extraterrestrial life
  • Edited by Woodruff T. Sullivan, III, University of Washington, John Baross, University of Washington
  • Book:
    Planets and Life
    Published online:
    28 May 2018
    Print publication:
    13 September 2007, pp 461-472

  • Summary

    Introduction and summary

    In the original Star Trek series, Episode 26 (“The Devil in the Dark”), Mr. Spock uses a hand-held tricoder to remotely detect a silicon-based lifeform known as a “Horta.” Unfortunately, NASA engineers have not yet invented the tricoder to aid in our own search for life on Mars or Europa. In fact they are not even close to understanding the principle by which a tricoder is able to distinguish lifeforms, either carbon or silicon, from non-living matter. Even The Physics of Star Trek (Krauss, 1995) is notably silent on the operating physics behind the tricoder. How then do we achieve the Prime Mission of Astrobiology: to boldly go and seek out new lifeforms on distant worlds? The answer, not surprisingly, is that we base our search for life elsewhere on what we know about life on Earth. The basic elements of this approach can be summarized as follows.

  • There is no specific definition of life that usefully contributes to the search for life (see Chapter 5). Don't wait for one.

  • In searching for either extant or extinct life the most useful guide is the short list of the ecological requirements for life. These are: (a) energy, (b) carbon, (c) liquid water, and (d) other elements such as N, P, and S. On the planets of our Solar System liquid water is the limiting factor.

  • Life is composed of, and produces, organic (carbon-based) matter. Forget silicon-based life for now.

  • Organic matter of biological origin can be differentiated from other organic matter because life preferentially selects and uses a few specific organic molecules. This selectivity is probably a general feature of life.

  • […]

  • 17 - Mass extinctions

  • from PART IV - Life on Earth
  • Edited by Woodruff T. Sullivan, III, University of Washington, John Baross, University of Washington
  • Book:
    Planets and Life
    Published online:
    28 May 2018
    Print publication:
    13 September 2007, pp 335-354

  • Summary

    Introduction

    The frequency of animal life in the Universe must be some function of how often it arises, and then how long it survives after evolving. Both of these factors may be significantly influenced by the frequency and intensity of mass extinctions, brief intervals of time when significant proportions of a planet's biota are killed off. They are killed by one or some combination of too much heat or cold, not enough food or nutrients, too little (or too much) water, oxygen, or carbon dioxide, excess radiation, incorrect acidity in the environment, or environmental toxins. Based on the history of Earth's life, mass extinctions seemingly have the potential to end life on any planet where it has arisen.

    Mass extinctions do more than threaten biota. They may also play a large part in evolutionary novelty. On Earth there have been about 15 such episodes during the last 500 Myr, five of which eliminated more than half of all species then inhabiting our planet. These events significantly affected the evolutionary history of Earth's biota: for example, if the dinosaurs had not been suddenly killed off following a comet collision with the Earth 65 Ma, there probably would not have been an Age of Mammals, since mammals were held in evolutionary check so long as dinosaurs existed. The wholesale evolution of mammalian diversity took place only after the dinosaurs were swept from the scene. Mass extinctions are thus both instigators as well as foils to evolution and innovation.

  • 18 - Volcanism on Io: a post-Galileo view

  • Ashley Gerard Davies
  • Book:
    Volcanism on Io
    Published online:
    05 October 2014
    Print publication:
    09 August 2007, pp 287-293

  • Summary

    By the end of the Galileo mission, many characteristics of Io's volcanism had been identified for the first time. Processes were observed that continue to yield insight into the way volcanic activity helped shape the Earth and other terrestrial planets. This chapter summarizes the post-Galileo view of Io.

    Volcanism and crustal structure

    Galileo revealed a world dominated by silicate volcanism, putting to rest a major point of contention that dated back to Voyager. Galileo discovered many active volcanic centers on Io (Appendix 1), considerably more than were detected by Voyager. The global distribution of activity and volcanic features favors the heating of Io by tidal dissipation taking place, in large part, in the aesthenosphere – the partially molten upper mantle.

    The upper few kilometers of Io's crust appear to be mostly mafic silicates interbedded with deposits of sulphur, sulphur dioxide, and silicate pyroclasts. The thickness of this volatile-rich layer probably varies from location to location but provides plenty of material that is easily mobilized by thermal interaction with mafic magma. Ultramafic magmas may be present, but the wide error bars ascribed to the data do not show this conclusively. If recent data analyses (Keszthelyi et al., 2004a, 2005a) are confirmed, ultramafic volcanism may not be present or widespread, and the presence of a global magma ocean (e.g., Keszthelyi et al., 1999) may not be required.

  • 2 - Between Voyager and Galileo: 1979–1995

  • Ashley Gerard Davies
  • Book:
    Volcanism on Io
    Published online:
    05 October 2014
    Print publication:
    09 August 2007, pp 27-38

  • Summary

    After the Voyager encounters, analysis of the data continued. More hot spots were identified in IRIS data (see McEwen et al., 1996; also see Appendix 1). Early post-Voyager models of Io incorporated a sulphur-rich crust with an ocean of sulphur beneath (e.g., Sagan, 1979; Smith et al., 1979b, 1979c). The sulphur was kept liquid and mobile through interaction with hot or molten silicates within the crust (e.g., Lunine and Stevenson, 1985). This model was supported by the abundance of sulphur compounds on Io's surface and sulphur in the Io plasma torus and neutral clouds, and by the fact that silicates had not been detected on the surface. The occasional eruption of silicate lava was still possible, but Io's volcanism was dominated by sulphur lavas.

    Silicate volcanism on Io?

    The case for ubiquitous silicate activity was made by Carr (1986), who argued on the basis of crustal strength that the “sulphur ocean” postulated to lie close to the surface was unlikely, based on the strength of the crust needed to support the steep walls of many paterae, where temperatures in excess of 650 K would literally melt the caldera walls. Heat transport from Io's interior to the surface was via silicate volcanism. To support the observed mountains, Io required a predominantly silicate lithosphere at least as thick as the mountains were high (>15 km in some cases).

  • Preface

  • Ashley Gerard Davies
  • Book:
    Volcanism on Io
    Published online:
    05 October 2014
    Print publication:
    09 August 2007, pp xi-xii

  • Summary

    I have always been fascinated by volcanoes, and especially by Io, a tiny moon that beyond any expectation turned out to be the most volcanically active body in the Solar System. Now that the NASA Galileo mission is over and initial data analyses have been completed, this is an appropriate time to assess the “state of the satellite” and review what has been learned about Io over the past few decades.

    A fascination with volcanoes is understandable, but I am also inspired to understand, through modeling of volcanic processes, how volcanoes work. Such motivation was instilled in me as a post-graduate student by Lionel Wilson and Harry Pinkerton at Lancaster University in the UK.

    In this book, therefore, I have endeavored not only to describe what Galileo saw, but also to provide the necessary background for understanding the physical, volcanological processes taking place on Io, and to demonstrate how remote-sensing data of volcanic activity can be used to peel back the layers of a planet to reveal interior processes and structure. To put the majestic scale of volcanism on Io into proper context, comparison is made wherever possible with volcanic activity on Earth.

    It has taken nearly two years to write this book. Along the way, I have had a great deal of help from friends, family, and colleagues.

  • 1 - Io, 1610–1979

  • Ashley Gerard Davies
  • Book:
    Volcanism on Io
    Published online:
    05 October 2014
    Print publication:
    09 August 2007, pp 7-26

  • Summary

    This chapter reviews the history of Io observations up to and including the Voyager encounters. The material in this chapter is drawn primarily from Satellites of Jupiter (Morrison, 1982) and Satellites (Burns and Matthews, 1986), both published by the University of Arizona Press, and Time-Variable Phenomena in the Jovian System, NASA Special Publication 494 (Belton et al., 1989).

    Io before Voyager

    The study of Io dates from the very beginning of telescope-based astronomy. In his observation notes for January 7, 1610, Galileo Galilei wrote: “when I was viewing the heavenly bodies with a spyglass, Jupiter presented itself to me; and because I had prepared a very excellent instrument for myself I perceived that beside the planet there were three little stars, small indeed, but very bright.” Subsequent observations revealed a fourth “little star.”

    Thus were discovered the Galilean satellites, named by Simon Marius (a contemporary of Galileo) Io, Europa, Ganymede, and Callisto. Subsequently, little attention was paid to the satellite system except as a means of measuring the speed of light (Roemer's method). Not until the nineteenth century did physical observations become important. In 1805, Laplace used the orbital resonant properties to estimate satellite masses. New refracting telescopes at Lick and Yerkes measured satellite sizes, and bulk densities were obtained.

Page 467 of 851