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 first results from the Herschel-HIFI mission

This paper contains a summary of the results from the first years of observations with the HIFI instrument onboard ESA’s Herschel space observatory. The paper starts by outlining the goals and possibilities of far-infrared and submillimeter astronomy, the limitations of the Earth’s atmosphere, and the scientific scope of the Herschel-HIFI mission. The presentation of science results from the mission follows the life cycle of gas in galaxies as grouped into five themes: Structure of the interstellar medium, First steps in interstellar chemistry, Formation of stars and planets, Solar system results and Evolved stellar envelopes. The HIFI observations paint a picture where the interstellar medium in galaxies has a mixed, rather than a layered structure; the same conclusion may hold for protoplanetary disks. In addition, the HIFI data show that exchange of matter between comets and asteroids with planets and moons plays a large role. The paper concludes with an outlook to future instrumentation in the far-infrared and submillimeter wavelength ranges.     

China's Deep-space Exploration to 2030

Focusing on the key scientific questions of deep space exploration which include the origin and evolution of the solar system and its planets, disastrous impact on the Earth by the solar activities and small bodies, extraterrestrial life, this paper put forward a propose about the roadmap and scientific objectives of China's Deep-space Exploration before 2030.     

Radio Wave Emission from the Outer Planets Before Cassini

We review observations and theories of radio wave emissions from the outer planets. These include radio emissions from the auroral regions and from the radiation belts, low-frequency electromagnetic emissions, and atmospheric lightning. For each of these emissions, we present in more details our knowledge of the Saturn counterpart, as well as expectations for Cassini. We summarize the capabilities of the radio instrument onboard Cassini, observations performed during the Jupiter flyby, and first (remote) observations of Saturn. Open questions are listed along with the specific observations that may bring responses to them. The coordinated observations (from the ground and from space) that would be valuable to perform in parallel to Cassini measurements are briefly discussed. Finally, we outline future missions and perspectives.     

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.     

Exoplanetary searches with gravitational microlensing: Polarization issues

There are different methods for finding exoplanets such as radial spectral shifts, astrometrical measurements, transits, timing etc. Gravitational microlensing (including pixel-lensing) is among the most promising techniques with the potentiality of detecting Earth-like planets at distances about a few astronomical units from their host star or near the so-called snow line with a temperature in the range 0–100 ^°C on a solid surface of an exoplanet. We emphasize the importance of polarization measurements which can help to resolve degeneracies in theoretical models. In particular, the polarization angle could give additional information about the relative position of the lens with respect to the source.     

Geology of the Icy Satellites

In the last 25 years, the explorations of the Voyager and Galileo missions have resulted in an entirely new view of the icy worlds orbiting the giant outer planets. These objects show a huge diversity in their characteristics, resulting from their formation histories, internal processes and interactions with their space environments. This paper will review the current state of knowledge about the icy satellites and discuss the exciting prospects for the upcoming Cassini/Huygens mission as it begins a new era of exploration of the Saturn satellite system.     

Neutral Atmospheres of the Giant Planets: An Overview of Composition Measurements

Measurements of the chemical composition of the giant planets provide clues of their formation and evolution processes. According to the currently accepted nucleation model, giant planets formed from the initial accretion of an icy core and the capture of the protosolar gas, mosly composed of hydrogen and helium. In the case of Jupiter and Saturn (the gaseous giants), this gaseous component dominates the composition of the planet, while for Uranus and Neptune (the icy giants) it is only a small fraction of the total mass. The measurement of elemental and isotopic ratios in the giant planets provides key diagnostics of this model, as it implies an enrichment in heavy elements (as well as deuterium) with respect to the cosmic composition. Neutral atmospheric constituents in the giant planets have three possible sources: (1) internal (fromthe bulk composition of the planet), (2) photochemical (fromthe photolysis ofmethane) and(3) external (from meteoritic impacts, of local or interplanetary origin). This paper reviews our present knowledge about the atmospheric composition in the giant planets, and their elemental and istopic composition. Measurements concerning key parameters, like C/H, D/H or rare gases in Jupiter, are analysed in detail. The conclusion addresses open questions and observations to be performed in the future.     

Comparative Study of the Dynamics of the Outer Planets

The two classes of outer planets, Gas Giants and Ice Giants, have distinctly different global circulation patterns and internal structure. Ongoing ground-based observations of the Ice Giants provide clues to better understanding and Galileo and Cassini data will generate constraints for Gas Giant modeling. The composition below the cloud levels, the depths to which the winds penetrate and the processes that sustain the zonal winds and weather systems are not understood. Basic questions concerning the structure, composition and atmospheric dynamics that are sustained on the four giants could be answered by a combination of orbiters and probes. Future missions that could answer these questions are not currently under development.