Planetary Dynamos from a Solar Perspective

Direct numerical simulations of the geodynamo and other planetary dynamos have been successful in reproducing the observed magnetic fields. We first give an overview on the fundamental properties of planetary magnetism. We review the concepts and main results of planetary dynamo modeling, contrasting them with the solar dynamo. In planetary dynamos the density stratification plays no major role and the magnetic Reynolds number is low enough to allow a direct simulation of the magnetic induction process using microscopic values of the magnetic diffusivity. The small-scale turbulence of the flow cannot be resolved and is suppressed by assuming a viscosity far in excess of the microscopic value. Systematic parameter studies lead to scaling laws for the magnetic field strength or the flow velocity that are independent of viscosity, indicating that the models are in the same dynamical regime as the flow in planetary cores. Helical flow in convection columns that are aligned with the rotation axis play an important role for magnetic field generation and forms the basis for a macroscopic α-effect. Depending on the importance of inertial forces relative to rotational forces, either dynamos with a dominant axial dipole or with a small-scale multipolar magnetic field are found. Earth is predicted to lie close to the transition point between both classes, which may explain why the dipole undergoes reversals. Some models fit the properties of the geomagnetic field in terms of spatial power spectra, magnetic field morphology and details of the reversal behavior remarkably well. Magnetic field strength in the dipolar dynamo regime is controlled by the available power and found to be independent of rotation rate. Predictions for the dipole moment agree well with the observed field strength of Earth and Jupiter and moderately well for other planets. Dedicated dynamo models for Mercury, Saturn, Uranus and Neptune, which assume stably stratified layers above or below the dynamo region, can explain some of the unusual field properties of these planets.     

Updated Review of Planetary Atmospheric Electricity

This paper reviews the progress achieved in planetary atmospheric electricity, with focus on lightning observations by present operational spacecraft, aiming to fill the hiatus from the latest review published by Desch et al. (Rep. Prog. Phys. 65:955–997, 2002). The information is organized according to solid surface bodies (Earth, Venus, Mars and Titan) and gaseous planets (Jupiter, Saturn, Uranus and Neptune), and each section presents the latest results from space-based and ground-based observations as well as laboratory experiments. Finally, we review planned future space missions to Earth and other planets that will address some of the existing gaps in our knowledge.     

Large-Scale Structure and Dynamics of the Magnetotails of Mercury,Earth, Jupiter and Saturn

Spacecraft observations have established that all known planets with an internal magnetic field, as part of their interaction with the solar wind, possess well-developed magnetic tails, stretching vast distances on the nightside of the planets. In this review paper we focus on the magnetotails of Mercury, Earth, Jupiter and Saturn, four planets which possess well-developed tails and which have been visited by several spacecraft over the years. The fundamental physical processes of reconnection, convection, and charged particle acceleration are common to the magnetic tails of Mercury, Earth, Jupiter and Saturn. The great differences in solar wind conditions, planetary rotation rates, internal plasma sources, ionospheric properties, and physical dimensions from Mercury’s small magnetosphere to the giant magnetospheres of Jupiter and Saturn provide an outstanding opportunity to extend our understanding of the influence of such factors on basic processes. In this review article, we study the four planetary environments of Mercury, Earth, Jupiter and Saturn, comparing their common features and contrasting their unique dynamics.     

The Interior Structure, Composition, and Evolution of Giant Planets

We discuss our current understanding of the interior structure and thermal evolution of giant planets. This includes the gas giants, such as Jupiter and Saturn, that are primarily composed of hydrogen and helium, as well as the “ice giants,” such as Uranus and Neptune, which are primarily composed of elements heavier than H/He. The effect of different hydrogen equations of state (including new first-principles computations) on Jupiter’s core mass and heavy element distribution is detailed. This variety of the hydrogen equations of state translate into an uncertainty in Jupiter’s core mass of 18M _⊕ . For Uranus and Neptune we find deep envelope metallicities up to 0.95, perhaps indicating the existence of an eroded core, as also supported by their low luminosity. We discuss the results of simple cooling models of our solar system’s planets, and show that more complex thermal evolution models may be necessary to understand their cooling history. We review how measurements of the masses and radii of the nearly 50 transiting extrasolar giant planets are changing our understanding of giant planets. In particular a fraction of these planets appear to be larger than can be accommodated by standard models of planetary contraction. We review the proposed explanations for the radii of these planets. We also discuss very young giant planets, which are being directly imaged with ground- and space-based telescopes.     

Photochemistry in Outer Solar System Atmospheres

The photochemistries of the H₂-He atmospheres of the gas giants Jupiter, Saturn and ice giants Uranus and Neptune and Titan’s mildly reducing N₂ atmosphere are reviewed in terms of general chemical and physical principles. The thermochemical furnace regions in the deep atmospheres and the photochemical regions of the giant planets are coupled by vertical mixing to ensure efficient recyling of photochemical products. On Titan,mass loss of hydrogen ensures photochemical evolution of methane into less saturated hydrocarbons. A summary discussion of major dissociation paths and essential chemical reactions is given. The chapter ends with a overview of vertical transport processes in planetary atmospheres.     

Planetary Magnetic Fields: Achievements and Prospects

The past decade has seen a wealth of new data, mainly from the Galilean satellites and Mars, but also new information on Mercury, the Moon and asteroids (meteorites). In parallel, there have been advances in our understanding of dynamo theory, new ideas on the scaling laws for field amplitudes, and a deeper appreciation on the diversity and complexity of planetary interior properties and evolutions. Most planetary magnetic fields arise from dynamos, past or present, and planetary dynamos generally arise from thermal or compositional convection in fluid regions of large radial extent. The relevant electrical conductivities range from metallic values to values that may be only about one percent or less that of a typical metal, appropriate to ionic fluids and semiconductors. In all planetary liquid cores, the Coriolis force is dynamically important. The maintenance and persistence of convection appears to be easy in gas giants and ice-rich giants, but is not assured in terrestrial planets because the quite high electrical conductivity of an iron-rich core guarantees a high thermal conductivity (through the Wiedemann-Franz law), which allows for a large core heat flow by conduction alone. This has led to an emphasis on the possible role of ongoing differentiation (growth of an inner core or “snow”). Although planetary dynamos mostly appear to operate with an internal field that is not very different from (2ρΩ/σ)^1/2 in SI units where ρ is the fluid density, Ω is the planetary rotation rate and σ is the conductivity, theoretical arguments and stellar observations suggest that there may be better justification for a scaling law that emphasizes the buoyancy flux. Earth, Ganymede, Jupiter, Saturn, Uranus, Neptune, and probably Mercury have dynamos, Mars has large remanent magnetism from an ancient dynamo, and the Moon might also require an ancient dynamo. Venus is devoid of a detectable global field but may have had a dynamo in the past. Even small, differentiated planetesimals (asteroids) may have been capable of dynamo action early in the solar system history. Induced fields observed in Europa and Callisto indicate the strong likelihood of water oceans in these bodies. The presence or absence of a dynamo in a terrestrial body (including Ganymede) appears to depend mainly on the thermal histories and energy sources of these bodies, especially the convective state of the silicate mantle and the existence and history of a growing inner solid core. As a consequence, the understanding of planetary magnetic fields depends as much on our understanding of the history and material properties of planets as it does on our understanding of the dynamo process. Future developments can be expected in our understanding of the criterion for a dynamo and on planetary properties, through a combination of theoretical work, numerical simulations, planetary missions (MESSENGER, Juno, etc.) and laboratory experiments.     

Giant Planet Ionospheres and Thermospheres: The Importance of Ion-Neutral Coupling

Planetary upper atmospheres-coexisting thermospheres and ionospheres-form an important boundary between the planet itself and interplanetary space. The solar wind and radiation from the Sun may react with the upper atmosphere directly, as in the case of Venus. If the planet has a magnetic field, however, such interactions are mediated by the magnetosphere, as in the case of the Earth. All of the Solar System’s giant planets have magnetic fields of various strengths, and interactions with their space environments are thus mediated by their respective magnetospheres. This article concentrates on the consequences of magnetosphere-atmosphere interactions for the physical conditions of the thermosphere and ionosphere. In particular, we wish to highlight important new considerations concerning the energy balance in the upper atmosphere of Jupiter and Saturn, and the role that coupling between the ionosphere and thermosphere may play in establishing and regulating energy flows and temperatures there. This article also compares the auroral activity of Earth, Jupiter, Saturn and Uranus. The Earth’s behaviour is controlled, externally, by the solar wind. But Jupiter’s is determined by the co-rotation or otherwise of the equatorial plasmasheet, which is internal to the planet’s magnetosphere. Despite being rapid rotators, like Jupiter, Saturn and Uranus appear to have auroral emissions that are mainly under solar (wind) control. For Jupiter and Saturn, it is shown that Joule heating and “frictional” effects, due to ion-neutral coupling can produce large amounts of energy that may account for their high exospheric temperatures.     

Formation of the Outer Planets

Models of the origins of gas giant planets and ‘ice’ giant planets are discussed and related to formation theories of both smaller objects (terrestrial planets) and larger bodies (stars). The most detailed models of planetary formation are based upon observations of our own Solar System, of young stars and their environments, and of extrasolar planets. Stars form from the collapse, and sometimes fragmentation, of molecular cloud cores. Terrestrial planets are formed within disks around young stars via the accumulation of small dust grains into larger and larger bodies until the planetary orbits become well enough separated that the configuration is stable for the lifetime of the system. Uranus and Neptune almost certainly formed via a bottom-up (terrestrial planet-like) mechanism; such a mechanism is also the most likely origin scenario for Saturn and Jupiter.     

The significance of atmospheric measurements for interior models of the major planets

We present a discussion of proposed models for interior processes in Jupiter and Saturn, and discuss how these models can be tested by atmospheric measurements by space vehicles. The importance of measurements at Uranus and Neptune is also discussed.This conclusion follows directly from consideration of the mass, radius, and oblateness of Jupiter and Saturn (DeMarcus 1958; Peebles, 1964; Hubbard, 1970): the point is also discussed in the papers by Cameron and Lewis, this issue, pp. 383–400 and 401–411.This is one of the publications by the Science Advisory Group.     

Formation of the outer planets

A discussion is given of a number of physical processes which were probably important during the formation of the outer planets if these formed from a gaseous solar nebula in which magnetic effects were not important. Arguments are given that large-scale gravitational instabilities in the solar nebula did not occur. Qualitative consideration is given to the conditions in which dynamical capture of gas onto a planetary core may take place; this may have played a major role in the formation of Jupiter and Saturn. Because of the great difficulty of fractionating hydrogen from helium in the assembly of the outer planets, it is argued that a new approach should be made to the construction of planetary models. Conditions which may lead to the formation of the regular satellite systems are discussed, and the associated problem of removal of primordial angular momentum from Jupiter, Saturn, and Uranus.This is one of the publications by the Science Advisory Group.     

The ionospheres of the major planets

Physical and chemical processes which affect the equilibrium distribution of ionization in the atmospheres of Jupiter, Saturn, Uranus and Neptune are reviewed. Current models imply readily detectable ionospheres for all four planets and suggest that protons should represent the dominant positive ion. Attention is directed to the probable importance of dissociative ionization of H₂ as a source of H⁺. A number of potentially important loss mechanisms for H⁺ are discussed including a possible reaction of H⁺ with vibrationally excited H₂. Protons may be removed efficiently at lower altitudes by reaction with CH₄ and this process may offer a simple remote means for location of the turbopause.This is one of the publications by the Science Advisory Group.     

Characteristics of Icy Surfaces

The surfaces of the Solar System’s icy satellites show an extraordinary variety of morphological features, which bear witness to exchange processes between the surface and subsurface. In this paper we review the characteristics of surface features on the moons of Jupiter, Saturn, Uranus and Neptune. Using data from spacecraft missions, we discuss the detailed morphology, size, and topography of cryovolcanic, tectonic, aeolian, fluvial, and impact features of both large moons and smaller satellites.     

A survey of the outer planets Jupiter,Saturn, Uranus,Neptune, Pluto,and their satellites

A survey of current knowledge about Jupiter, Saturn, Uranus, Neptune, Pluto, and their satellites is presented. The best available numerical values are given for physical parameters, including orbital and body properties, atmospheric composition and structure, and photometric parameters. The more acceptable current theories of these bodies are outlined with thorough referencing offering access to the details. The survey attempts to cover the literature through May 1, 1972. Prepared Under Contract No. NAS7-100 National Aeronautics and Space Administration.     

The Magnetic Field of Mercury

The magnetic field strength of Mercury at the planet’s surface is approximately 1% that of Earth’s surface field. This comparatively low field strength presents a number of challenges, both theoretically to understand how it is generated and observationally to distinguish the internal field from that due to the solar wind interaction. Conversely, the small field also means that Mercury offers an important opportunity to advance our understanding both of planetary magnetic field generation and magnetosphere-solar wind interactions. The observations from the Mariner 10 magnetometer in 1974 and 1975, and the MESSENGER Magnetometer and plasma instruments during the probe’s first two flybys of Mercury on 14 January and 6 October 2008, provide the basis for our current knowledge of the internal field. The external field arising from the interaction of the magnetosphere with the solar wind is more prominent near Mercury than for any other magnetized planet in the Solar System, and particular attention is therefore paid to indications in the observations of deficiencies in our understanding of the external field. The second MESSENGER flyby occurred over the opposite hemisphere from the other flybys, and these newest data constrain the tilt of the planetary moment from the planet’s spin axis to be less than 5°. Considered as a dipole field, the moment is in the range 240 to 270 nT-R _M ³ , where R _M is Mercury’s radius. Multipole solutions for the planetary field yield a smaller dipole term, 180 to 220 nT-R _M ³ , and higher-order terms that together yield an equatorial surface field from 250 to 290 nT. From the spatial distribution of the fit residuals, the equatorial data are seen to reflect a weaker northward field and a strongly radial field, neither of which can be explained by a centered-dipole matched to the field measured near the pole by Mariner 10. This disparity is a major factor controlling the higher-order terms in the multipole solutions. The residuals are not largest close to the planet, and when considered in magnetospheric coordinates the residuals indicate the presence of a cross-tail current extending to within 0.5R _M altitude on the nightside. A near-tail current with a density of 0.1 μA/m² could account for the low field intensities recorded near the equator. In addition, the MESSENGER flybys include the first plasma observations from Mercury and demonstrate that solar wind plasma is present at low altitudes, below 500 km. Although we can be confident in the dipole-only moment estimates, the data in hand remain subject to ambiguities for distinguishing internal from external contributions. The anticipated observations from orbit at Mercury, first from MESSENGER beginning in March 2011 and later from the dual-spacecraft BepiColombo mission, will be essential to elucidate the higher-order structure in the magnetic field of Mercury that will reveal the telltale signatures of the physics responsible for its generation.     

Meteoric Layers in Planetary Atmospheres

Metallic ions coming from the ablation of extraterrestrial dust, play a significant role in the distribution of ions in the Earth’s ionosphere. Ions of magnesium and iron, and to a lesser extent, sodium, aluminium, calcium and nickel, are a permanent feature of the lower E-region. The presence of interplanetary dust at long distances from the Sun has been confirmed by the measurements obtained by several spacecrafts. As on Earth, the flux of interplanetary meteoroids can affect the ionospheric structure of other planets. The electron density of many planets show multiple narrow layers below the main ionospheric peak which are similar, in magnitude, to the upper ones. These layers could be due to long-lived metallic ions supplied by interplanetary dust and/or their satellites. In the case of Mars, the presence of a non-permanent ionospheric layer at altitudes ranging from 65 to 110 km has been confirmed and the ion Mg⁺?CO₂ identified. Here we present a review of the present status of observed low ionospheric layers in Venus, Mars, Jupiter, Saturn and Neptune together with meteoroid based models to explain the observations. Meteoroids could also affect the ionospheric structure of Titan, the largest Saturnian moon, and produce an ionospheric layer at around 700 km that could be investigated by Cassini.     

Voyager mission description

The Voyager Project, managed by the Jet Propulsion Laboratory, involves the lauching of two advanced spacecraft to explore the Jovian and Saturnian systems, as well as interplanetary space. The one-month lauch period opens on August 20, 1977, with arrivals at Jupiter in March and July of 1979, and at Saturn in November of 1980 and August of 1981. Gravity-assist swingbys of Jupiter are utilized in order to reduce the lauch energy demands needed to reach Saturn. In addition, a gravity-assist targeting option at Saturn will be maintained on the second-arriving Voyager for a possible continuation on to Uranus, with arrival in January of 1986. Flight through the Jovian and Saturnian systems will achieve close to moderate flyby encounters with several of the natural satellites, including special flyby geometry conditions for Io and Titan, as well as an Earth occultation of the spacecraft's radio signal by the rings of Saturn. The purpose of this paper is to describe the Voyager mission characteristics in order to establish a framework upon which to better understand the objectives and goals of the eleven scientific investigations which are described in subsequent papers.     

Environments in the Outer Solar System

The outer planets of our solar system Jupiter, Saturn, Uranus, and Neptune are fascinating objects on their own. Their intrinsic magnetic fields form magnetic environments (so called magnetospheres) in which charged and neutral particles and dust are produced, lost or being transported through the system. These magnetic environments of the gas giants can be envisaged as huge plasma laboratories in space in which electromagnetic waves, current systems, particle transport mechanisms, acceleration processes and other phenomena act and interact with the large number of moons in orbit around those massive planets. In general it is necessary to describe and study the global environments (magnetospheres) of the gas giants, its global configuration with its large-scale transport processes; and, in combination, to study the local environments of the moons as well, e.g. the interaction processes between the magnetospheric plasma and the exosphere/atmosphere/magnetosphere of the moon acting on time scales of seconds to days. These local exchange processes include also the gravity, shape, rotation, astrometric observations and orbital parameters of the icy moons in those huge systems. It is the purpose of this chapter of the book to describe the variety of the magnetic environments of the outer planets in a broad overview, globally and locally, and to show that those exchange processes can dramatically influence the surfaces and exospheres/atmospheres of the moons and they can also be used as a tool to study the overall physics of systems as a whole.     

Voyager imaging experiment

The overall objective of this experiment is exploratory reconnaissance of Jupiter, Saturn, their satellites, and Saturn's rings. Such reconnaissance, at resolutions and phase angles unobtainable from Earth, can be expected to provide much new data relevant to the atmospheric and/or surface properties of these bodies. The experiment also has the following specific objectives:Observe and characterize the global circulation of the atmospheres of Jupiter and Saturn;Determine the horizontal and vertical structure of the visible clouds and establish their relationship to the belted appearance and dynamical properties of the planetary atmospheres;Determine the vertical structure of high, optically-thin, scattering layers on Jupiter and Saturn;Determine the nature of anomalous features such as the Great Red Spot, South Equatorial Belt disturbances, etc.;Characterize the nature of the colored material in the clouds of Jupiter and Saturn, and identify the nature and sources of chromophores on Io and Titan;Perform comparative geologic studies of many satellites at less than 15-km resolution;Map and characterize the geologic structure of several satellites at high resolution (1 km);Investigate the existence and nature of atmospheres on the satellites;Determine the mass, size, and shape of many of the satellites by direct measurement;Determine the direction of the spin axes and periods of rotation of several satellites, and establish coordinate systems for the larger satellites;Map the radial distribution of material in Saturn's rings at high resolution;Determine the optical scattering properties of the primaries, rings, and satellites at several wavelengths and phase angles;Search for novel physical phenomena, e.g., phenomena associated with the Io flux tube, meteors, aurorae, lightning, or satellite shadows.Team leader.Deputy team leader.     

An Overview of Earth’s Global Electric Circuit and Atmospheric Conductivity

The Earth’s global atmospheric electric circuit depends on the upper and lower atmospheric boundaries formed by the ionosphere and the planetary surface. Thunderstorms and electrified rain clouds drive a DC current (～1 kA) around the circuit, with the current carried by molecular cluster ions; lightning phenomena drive the AC global circuit. The Earth’s near-surface conductivity ranges from 10^?7 S?m^?1 (for poorly conducting rocks) to 10^?2 S?m^?1 (for clay or wet limestone), with a mean value of 3.2 S?m^?1 for the ocean. Air conductivity inside a thundercloud, and in fair weather regions, depends on location (especially geomagnetic latitude), aerosol pollution and height, and varies from ～10^?14 S?m^?1 just above the surface to 10^?7 S?m^?1 in the ionosphere at ～80 km altitude. Ionospheric conductivity is a tensor quantity due to the geomagnetic field, and is determined by parameters such as electron density and electron–neutral particle collision frequency. In the current source regions, point discharge (coronal) currents play an important role below electrified clouds; the solar wind-magnetosphere dynamo and the unipolar dynamo due to the terrestrial rotating dipole moment also apply atmospheric potential differences. Detailed measurements made near the Earth’s surface show that Ohm’s law relates the vertical electric field and current density to air conductivity. Stratospheric balloon measurements launched from Antarctica confirm that the downward current density is ～1 pA m^?2 under fair weather conditions. Fortuitously, a Solar Energetic Particle (SEP) event arrived at Earth during one such balloon flight, changing the observed atmospheric conductivity and electric fields markedly. Recent modelling considers lightning discharge effects on the ionosphere’s electric potential (～+250 kV with respect to the Earth’s surface) and hence on the fair weather potential gradient (typically ～130 V?m^?1 close to the Earth’s surface. We conclude that cloud-to-ground (CG) lightning discharges make only a small contribution to the ionospheric potential, and that sprites (namely, upward lightning above energetic thunderstorms) only affect the global circuit in a miniscule way. We also investigate the effects of mesoscale convective systems on the global circuit.     

Magnetic field experiment for Voyagers 1 and 2

The magnetic field experiment to be carried on the Voyager 1 and 2 missions consists of dual low field (LFM) and high field magnetometer (HFM) systems. The dual systems provide greater reliability and, in the case of the LFM's, permit the separation of spacecraft magnetic fields from the ambient fields. Additional reliability is achieved through electronics redundancy. The wide dynamic ranges of ± 0.5 G for the LFM's and ± 20 G for the HFM's, low quantization uncertainty of ± 0.002 ( = 10^–5 G) in the most sensitive (± 8 ) LFM range, low sensor RMS noise level of 0.006 , and use of data compaction schemes to optimize the experiment information rate all combine to permit the study of a broad spectrum of phenomena during the mission. Objectives include the study of planetary fields at Jupiter, Saturn, and possibly Uranus; satellites of these planets; solar wind and satellite interactions with the planetary fields; and the large-scale structure and microscale characteristics of the interplanetary magnetic, field. The interstellar field may also be measured.