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.     

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.     

Surface, Subsurface and Atmosphere Exchanges on the Satellites of the Outer Solar System

The surface morphology of icy moons is affected by several processes implicating exchanges between their subsurfaces and atmospheres (if any). The possible exchange of material between the subsurface and the surface is mainly determined by the mechanical properties of the lithosphere, which isolates the deep, warm and ductile ice material from the cold surface conditions. Exchanges through this layer occur only if it is sufficiently thin and/or if it is fractured owing to tectonic stresses, melt intrusion or impact cratering. If such conditions are met, cryomagma can be released, erupting fresh volatile-rich materials onto the surface. For a very few icy moons (Titan, Triton, Enceladus), the emission of gas associated with cryovolcanic activity is sufficiently large to generate an atmosphere, either long-lived or transient. For those moons, atmosphere-driven processes such as cryovolcanic plume deposition, phase transitions of condensable materials and wind interactions continuously re-shape their surfaces, and are able to transport cryovolcanically generated materials on a global scale. In this chapter, we discuss the physics of these different exchange processes and how they affect the evolution of the satellites’ surfaces.     

Quiet,Discrete Auroral Arcs—Observations

Space Science Reviews - The icy moons of the outer Solar System harbor potentially habitable environments for life, however, compared to the terrestrial biosphere, these environments are...     

Experiences in Managing the Prometheus Project

Congress authorized NASA's Prometheus Project in February 2003, with the first Prometheus mission slated to explore the icy moons of Jupiter. The project had two major objectives: 1) to develop a nuclear reactor that would provide unprecedented levels of power and show that it could be processed safely and operated reliably in space for long-duration, deep-space exploration; and 2) to explore the three icy moons of Jupiter - Callisto, Ganymede, and Europa - and return science data that would meet the scientific goals as set forth in the Decadal Survey Report of the National Academy of Sciences. Early in project planning, it was determined that the development of the Prometheus nuclear-powered spaceship would be complex and require the intellectual knowledge residing at numerous organizations across the country. In addition, because of the complex nature of the project and the multiple partners, approaches beyond those successfully used to manage a typical JPL project would be needed. This describes the key experiences in managing Prometheus, which should prove useful for future projects of similar scope and magnitude.     

Exobiology and Planetary Protection of icy moons

The outer solar system is an important area of investigation for exobiology, the study of life in the universe. Several moons of the outer planets involve processes and structures comparable to those thought to have played an important role in the emergence of life on Earth, such as the formation and exchange of organic materials between different reservoirs. The study of these prebiotic processes on, and in, outer solar system moons is a key goal for exobiology, together with the question of habitability and the search for evidence of past or even present life. This chapter reviews the aspects of prebiotic chemistry and potential presence of life on Europa, Enceladus and Titan, based on the most recent data obtained from space missions as well as theoretical and experimental laboratory models. The habitability of these extraterrestrial environments, which are likely to include large reservoirs of liquid water in their internal structure, is discussed as well as the particular case of Titan’s hydrocarbon lakes. The question of planetary protection, especially in the case of Europa, is also presented.     

Magnetic Fields of the Satellites of Jupiter and Saturn

This paper reviews the present state of knowledge about the magnetic fields and the plasma interactions associated with the major satellites of Jupiter and Saturn. As revealed by the data from a number of spacecraft in the two planetary systems, the magnetic properties of the Jovian and Saturnian satellites are extremely diverse. As the only case of a strongly magnetized moon, Ganymede possesses an intrinsic magnetic field that forms a mini-magnetosphere surrounding the moon. Moons that contain interior regions of high electrical conductivity, such as Europa and Callisto, generate induced magnetic fields through electromagnetic induction in response to time-varying external fields. Moons that are non-magnetized also can generate magnetic field perturbations through plasma interactions if they possess substantial neutral sources. Unmagnetized moons that lack significant sources of neutrals act as absorbing obstacles to the ambient plasma flow and appear to generate field perturbations mainly in their wake regions. Because the magnetic field in the vicinity of the moons contains contributions from the inevitable electromagnetic interactions between these satellites and the ubiquitous plasma that flows onto them, our knowledge of the magnetic fields intrinsic to these satellites relies heavily on our understanding of the plasma interactions with them.     

Separation of the Magnetic Field into External and Internal Parts

The magnetic field of a planet or a planetary moon contains contributions from a variety of sources in the environment of the body (external sources) and its interior (internal sources). This chapter describes different methods that have been developed for the separation of external and internal source contributions, and their application to selected planets and one of Jupiter’s moons, Ganymede.     

Formation and Evolution of Titan’s Atmosphere

The origin and evolution of Titan’s enigmatic atmosphere is reviewed. Starting with the present-day volatile inventory, the question of what was the original composition on Titan and how a satellite of similar size to other Galilean moons managed to acquire and hold on to the required material is discussed. In particular the possible sources and sinks of the main mother molecules (nitrogen, methane and oxygen) are investigated in view of the most recent models and laboratory experiments. The answers expected to be provided by the instruments aboard the Cassini-Huygens mission to some of the most prominent current questions regarding Titan’s atmosphere are defined.     

The New Horizons Kuiper Belt Extended Mission

The central objective of the New Horizons prime mission was to make the first exploration of Pluto and its system of moons. Following that, New Horizons has been approved for its first extended mission, which has the objectives of extensively studying the Kuiper Belt environment, observing numerous Kuiper Belt Objects (KBOs) and Centaurs in unique ways, and making the first close flyby of the KBO 486958 2014 MU₆₉. This review summarizes the objectives and plans for this approved mission extension, and briefly looks forward to potential objectives for subsequent extended missions by New Horizons.     

Dynamo Models for Planets Other Than Earth

Observations from planetary spacecraft missions have demonstrated a spectrum of dynamo behaviour in planets. From currently active dynamos, to remanent crustal fields from past dynamo action, to no observed magnetization, the planets and moons in our solar system offer magnetic clues to their interior structure and evolution. Here we review numerical dynamo simulations for planets other than Earth. For the terrestrial planets and satellites, we discuss specific magnetic field oddities that dynamo models attempt to explain. For the giant planets, we discuss both non-magnetic and magnetic convection models and their ability to reproduce observations of surface zonal flows and magnetic field morphology. Future improvements to numerical models and new missions to collect planetary magnetic data will continue to improve our understanding of the magnetic field generation process inside planets.     

Modeling and Simulating Flowing Plasmas and Related Phenomena

Simulation has become a valuable tool that compliments more traditional methods used to understand solar system plasmas and their interactions with planets, moons and comets. The three popular simulation approaches to studying these interactions are presented. Each approach provides valuable insight to these interactions. To date no one approach is capable of simulating the whole interaction region from the collisionless to the collisional regimes. All three approaches are therefore needed. Each approach has several implicit physical assumptions as well as several numerical assumptions depending on the scheme used. The magnetohydrodynamic (MHD), test-particle/Monte-Carlo and hybrid models used in simulating flowing plasmas are described. Special consideration is given to the implicit assumptions underlying each model. Some of the more common numerical methods used to implement each model, the implications of these numerical methods and the resulting limitations of each simulation approach are also discussed.     

Electromagnetic Induction Effects and Dynamo Action in the Hermean System

Embedded in a large mass density and strong interplanetary magnetic field solar wind environment and equipped with a magnetic field of minor strength, planet Mercury exhibits a small magnetosphere vulnerable to severe solar wind buffeting. This causes large variations in the size of the magnetosphere and its associated currents. External fields are of far more importance than in the terrestrial case and of a size comparable to any internal, dynamo-generated field. Induction effects in the planetary interior, dominated by its huge core, are thought to play a much more prominent role in the Hermean magnetosphere compared to any of its companions. Furthermore, the external fields may cause planetary dynamo amplification much as discussed for the Galilean moons Io and Ganymede, but with the ambient field generated by the dynamo and its magnetic field-solar wind interaction.     

Data mining as a foundation for science-enabling autonomy

In-situ exploration by spacecraft and planetary rovers will increasingly require knowledge "on demand" in the future as downlinlk constraints limit the amount of information that can be transmitted from these platforms back to Earth. Several on-board processing methods have the potential to significantly enhance scientific results in these settings. They include automatic detection Of natural satellites of planetary bodies, investigation of possible surface motions on planets and planetary moons, and directed acquisition Of scientific data by planetary rovers. The key ingpredient in all three cases is the need to process scientific data directly on-board, so that information can be rapidly provided to an automated spacecraft ex~ecutive and/or to ground-based Principal Inesiators (pis). We discuss, herein, recent developments in data mining technology that were designed initially for ground-based scientific data analysis. We then outline how these ideas can be migrated to on-board platforms to dramatically enhance the scientific capabilities of autonomous spacecraft.     

Water and the Interior Structure of Terrestrial Planets and Icy Bodies

Water content and the internal evolution of terrestrial planets and icy bodies are closely linked. The distribution of water in planetary systems is controlled by the temperature structure in the protoplanetary disk and dynamics and migration of planetesimals and planetary embryos. This results in the formation of planetesimals and planetary embryos with a great variety of compositions, water contents and degrees of oxidation. The internal evolution and especially the formation time of planetesimals relative to the timescale of radiogenic heating by short-lived ²⁶Al decay may govern the amount of hydrous silicates and leftover rock–ice mixtures available in the late stages of their evolution. In turn, water content may affect the early internal evolution of the planetesimals and in particular metal-silicate separation processes. Moreover, water content may contribute to an increase of oxygen fugacity and thus affect the concentrations of siderophile elements within the silicate reservoirs of Solar System objects. Finally, the water content strongly influences the differentiation rate of the icy moons, controls their internal evolution and governs the alteration processes occurring in their deep interiors.     

The Cassini/Huygens Mission to the Saturnian System

The international Cassini/Huygens mission consists of the Cassini Saturn Orbiter spacecraft and the Huygens Titan Probe that is targeted for entry into the atmosphere of Saturn's largest moon, Titan. From launch on October 15, 1997 to arrival at Saturn in July 2004, Cassini/Huygens will travel over three billion kilometers. Once in orbit about Saturn, Huygens is released from the orbiter and enters Titan's atmosphere. The Probe descends by parachute and measures the properties of the atmosphere. If the landing is gentle, the properties of the surface will be measured too. Then the orbiter commences a four-year tour of the Saturnian system with 45 flybys of Titan and multiple encounters with the icy moons. The rings, the magnetosphere and Saturn itself are all studied as well as the interactions among them. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Galileo magnetic field investigation

The Galileo Orbiter carries a complement of fields and particles instruments designed to provide data needed to shed light on the structure and dynamical variations of the Jovian magnetosphere. Many questions remain regarding the temporal and spatial properties of the magnetospheric magnetic field, how the magnetic field maintains corotation of the embedded plasma and the circumstances under which corotation breaks down, the nature of magnetic perturbations that transport plasma across magnetic shells in different parts of the system, and the electromagnetic properties of the Jovian moons and how they interact with the magnetospheric plasma. Critical to answering these closely related questions are measurements of the dc and low-frequency magnetic field. The Galileo Orbiter carries a fluxgate magnetometer designed to provide the sensitive measurements required for this purpose. In this paper, the magnetometer is described. The instrument has two boom-mounted, three-axis sensor assemblies. Flipper mechanisms are included in each sensor assembly for the purpose of offset calibration. The microprocessor controlled data handling system produces calibrated despun data that can be used directly without further processing. A memory system stores data for those periods when the spacecraft telemetry is not active. This memory system can also be used for storing high time-resolution snapshots of data.     

Composition and Measurement of Charged Atmospheric Clusters

Atmospheric charged clusters are formed in a series of rapid chemical reactions after ionisation, leaving a central ion X⁺ or X^? clustered with n ligands (Y) _n . In solar system tropospheres and stratospheres there are two distinct cluster regimes: the terrestrial planets contain largely hydrated clusters (i.e. Y=H₂O), whereas the gas planets and their moons have organic or nitrogenated cluster species. These classifications are largely based on model predictions, since hardly any measurements are available. The few existing composition measurements are reviewed, including the recent detection of massive charged particles in Titan’s upper atmosphere. Technologies for both remote sensing and in situ measurements of atmospheric charged clusters are discussed. Preliminary measurements in the terrestrial atmosphere are presented indicating that ambient charged cluster species interact with downwelling infra-red radiation at 9.15 μm, even in the presence of cloud. This supports the possibility of future infrared detection of charged clusters.     

Rheological and Thermal Properties of Icy Materials

Laboratory measurements of physical properties of planetary ices generate information for dynamical models of tectonically active icy bodies in the outer solar system. We review the methods for measuring both flow properties and thermal properties of icy planetary materials in the laboratory, and describe physical theories that are essential for intelligent extrapolation of data from laboratory to planetary conditions. This review is structured with a separate and independent section for each of the two sets of physical properties, rheological and thermal. The rheological behaviors of planetary ices are as diverse as the icy moons themselves. High-pressure water ice phases show respective viscosities that vary over four orders of magnitude. Ices of CO₂, NH₃, as well as clathrate hydrates of CH₄ and other gases vary in viscosity by nearly ten orders of magnitude. Heat capacity and thermal conductivity of detected/inferred compositions in outer solar system bodies have been revised. Some low-temperature phases of minerals and condensates have a deviant thermal behavior related to paramount water ice. Hydrated salts have low values of thermal conductivity and an inverse dependence of conductivity on temperature, similar to clathrate hydrates or glassy solids. This striking behavior may suit the dynamics of icy satellites.     

Implications of Rotation, Orbital States, Energy Sources, and Heat Transport for Internal Processes in Icy Satellites

Internal processes in icy satellites, e.g. the exchange of material from the subsurface to the surface or processes leading to volcanism and resurfacing events, are a consequence of the amount of energy available in the satellites’ interiors. The latter is mainly determined shortly after accretion by the amount of radioactive isotopes incorporated in the silicates during the accretion process. However, for satellites—as opposed to single objects—important contributions to the energy budget on long time-scales can come from the interaction with other satellites (forcing of eccentricities of satellites in resonance) and consequently from the tidal interaction with the primary planet. Tidal evolution involves both changes of the rotation state—usually leading to the 1:1 spin orbit coupling—and long-term variations of the satellite orbits. Both processes are dissipative and thus connected with heat production in the interior. The way heat is transported from the interior to the surface (convection, conduction, (cryo-) volcanism) is a second main aspect that determines how internal processes in satellites work. In this chapter we will discuss the physics of heat production and heat transport as well as the rotational and orbital states of satellites. The relevance of the different heat sources for the moons in the outer solar system are compared and discussed.