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First ideas about possible physical influences of the Sun on Earth other than by electromagnetic (EM) radiation were scientifically discussed more seriously after Richard Carrington’s famous observation of a spectacular white-light flare in 1859 and the subsequent conclusion that this flash of EM radiation was connected with the origin of strong perturbations of the Earth’s outer magnetic field, commonly referred to as geomagnetic storms, which were recorded about 24 hours after the solar flare. Tentatively significant correlations of the number of geomagnetic storms and aurorae with the varying number of sunspots seen on the visible solar disk were found in the long-term with respect to the roughly 11-year periodicity of the solar activity cycle. Although theories of sporadic solar eruptions were postulated soon after the Carrington observations, the physical mechanism of the transfer of energy from the Sun to the Earth remained unknown. Early in the 20 century Chapman and Ferraro proposed the concept of huge clouds of charged particles emitted by the Sun as the triggers of geomagnetic storms. Based on the inference of the existence of a solar magnetic field, magnetized plasma clouds were subsequently introduced. Eugene Parker derived theoretical evidence for a continuous stream of ionized particles, the solar wind, leading to continuous convection of the Sun’s magnetic field into interplanetary space. The existence of the solar wind was confirmed soon after the launch of the first satellites. Since then the Sun is known to be a permanent source of particles filling interplanetary space. However, it was still thought that the Sun’s outer atmosphere, the solar corona, is a static rather than a dynamic object, undergoing only long-term structural changes in phase with the Sun’s activity cycle. This view completely changed after space borne telescopes provided extended series of solar images in the EUV and soft X-ray range of the EM spectrum, invisible to ground-based observers. The remote-sensing observations undertaken by Yohkoh, followed by multi-wavelength movies from SoHO (Solar Heliospheric Observatory) and high resolution EUV imaging by TRACE (Transition Region and Coronal Explorer) have revealed to date that the Sun’s atmosphere is highly dynamic and never at rest. Solar eruptions have been tracked into space in unprecedented detail. In combination with near-Earth satellites, their interplanetary and geo-space effects could be investigated in depth, having provided the roots for space weather forecasts. This chapter summarizes the discoveries about the origin and evolution of solar storms and their space weather effects, providing a comprehensive picture of the most important links in the Sun-Earth system. It finally provides an outlook to future research in the field of space weather.

Mercury may provide answers to questions regarding the formation and evolution of our Solar System. This article reviews what is known about Mercury from the Mariner 10 flybys of 1974 and 1975 and from thirty years of ground-based telescopic observations with ever improving instrumentation. Many new discoveries, such as possible water ice at Mercury’s polar regions, make the anticipation of the arrival of the MESSENGER spacecraft for its first flyby of Mercury in 2008 even more intense.

As the Earth’s nearest planetary neighbour, Venus has been studied by ground-based observers for centuries, and has been visited by more than 20 spacecraft. However, in the last decade and a half Venus research has been relatively neglected, despite the fact that a great many major questions about its atmosphere, surface and interior remain unanswered. Several of these questions relate to the unique position of Venus as the Earth’s near twin, in terms of size, density and proximity to the Sun, which led early astronomers to expect an Earth-like environment on the planet, possibly one fit for human habitation, and perhaps even the seat of indigenous life. The picture which has emerged from missions to the planet is quite different, raising questions about the evolution and stability of terrestrial planet environments that are both intriguing and possibly of practical relevance to global change problems on the Earth. This chapter reviews the scientific issues, and goes on to describe two new missions to Venus, the European Venus Express and the Japanese Planet-C orbiters, which will take place in the next few years to address some of them in depth. Other questions will remain unanswered, and further missions will be required, including landing on Venus and sample return.

The origin of the Moon 4.56 Gyr ago, its subsequent evolution, and the implications of both relative to the Earth remain subject to lively debate. Because the internal geochemistry and geophysics of the Moon does not appear consistent with an origin by the giant impact of a Mars-sized asteroid on the Earth, this hypothesis is challenged by one that proposes the capture of an independently formed planetesimal. The Moon—s internal structure also indicates that it and all the terrestrial planets initially had relatively cool, chondritic proto-cores prior to formation of metallic cores. Evidence exists that these proto-cores delayed formation of metallic cores for periods that correlate with the final mass of a planet.

The impact history of the inner solar system has been broadly outlined by the modern investigation of the Moon. First, soon after the formation of a coherent lunar crust and against an intense background of smaller cratering events, the Moon was subjected to extremely large impacts that formed basins up to 3200km in diameter. On Earth, the melt sheets from these continental-scale impacts may have been responsible for the formation of the first continental crust at ∼4.4 Gyr. Second, ∼50 impact basins >300 km in diameter formed between 4.5 and 3.8 Gyr probably by pulses of impactors produced during the migration and interaction of the giant planets within a structured solar disk of planetesimal rings. The last of these pulses at about 3.85 Gyr, producing the ∼14 mascon basins, resurfaced most of the Moon, and suggested an apparent “cataclysm” at that time. This period of 700 Myr may have been one of “punctuated cataclysm” as one or more giant planets encountered separate planetesimal rings and gaps during outward orbital migration. Finally, the implication of this violent impact history in the inner solar system prior to 3.8 Gyr relative to the surfaces of the hydrous terrestrial planets, that is, Earth, Mars and probably early Venus, is that clays were the dominant mineral species. These clays, as well as volcanic sulfides, may have provided the templates for the formation of complex organic precursors that made up the first living cells.

One of the most striking differences between the present day climates of Earth and Mars is the ubiquitous and abundant presence of liquid water on Earth and the extremely dry atmosphere and surface of Mars. Features on the surface of Mars, discovered by early spacecraft missions in the 1970s and apparently caused by flowing water on the surface in the past, have lead to much speculation concerning the early Martian climate and the possibility that the planet was once relatively warm and wet. Such speculation is fuelled by the search for life on Mars, either in the present or as a fossil record. Until recent missions, however, there has been little direct evidence for the existence of large water deposits, other than in the form of ice, largely around the northern polar cap. During the past 2 years, however, NASA’s Mars Odyssey and ESA’s Mars Express spacecraft have discovered evidence for considerable amounts of ice lying at relatively shallow depths in the Martian regolith. The NASA Mars Rovers have also found considerable evidence for ancient water in the nearby rocks and landscape.

It still seems unclear, despite various attempts to model the ancient Martian climate, whether Mars had a sustained warm, wet climate, with liquid water flowing on the surface, or whether it has remained mostly in a frozen state, interrupted by occasional melting events for short periods of time. Climate change on more recent timescales (10–10 years BP) has perhaps been less dramatic, but more amenable to systematic modelling. There is strong evidence of changes in Mars’ climate on these timescales in the polar-layered deposits, associated with the obliquity cycle, and Mars GCMs have started to make progress in modelling climate change associated with varying astronomical parameters. We will briefly review such studies as well as the limited observational evidence for more dramatic climate change since the early epochs of the planet’s history.

Data collected from recent spacecraft missions to Mars are making it possible to scientifically evaluate whether habitable environments are present, or existed in the past, on the planet. Determining where, when, and how such environments occur will play a crucial role in guiding future exploration efforts. Mars appears to possess the critical elements for life as we know it, including the required chemical elements, energy sources, and the presence ofwater. Identification of potential habitats for life therefore depends on the convergence of these factors under favorable circumstances. Spacecraft observations are more clearly defining the history of water on Mars, and appear to indicate that significant amounts of liquid water have been present at the surface in the past. Most of this water has been lost over time or retreated underground, although there is some evidence for periodic near-surface water in recent eras. If Mars did indeed have a warmer, wetter past, the possibilities for habitable environments were substantial and diverse. On the other hand, the inhospitable nature of the present surface indicates that potential habitats for extant life are much more limited. Nevertheless, habitable environments might exist in the subsurface or at the surface during more favorable climatic periods.

The discovery of the first extrasolar planet, 51 Pegasi, in 1995 has opened up a new and exciting area of planetary astronomy. Using a number of different techniques 170 extrasolar planets have now been discovered, and this number is rapidly increasing. Due partly to observational biases, just over half of these newly discovered planets are ‘Hot Jupiters’ — Jupiter sized planets orbiting within 1 AU of their parent stars. However, the number of giant planets discovered in more distant orbits, as seen in our own Solar System, is increasing all the time as detection techniques improve and longer time-series are analysed. In this paper, the characteristics of the extrasolar planets discovered to date will be reviewed, together with the implications these characteristics have upon theories of the formation of planetary systems in general and our own Solar System in particular. Current and future methods for detecting extrasolar planets are also reviewed, together with prospects for detecting terrestrial planets and perhaps even searching for the signs of life itself.

The Galilean satellites formed in a nebula of dust and gas that surrounded Jupiter toward the end of the formation of the giant planet itself. Their diverse initial compositions were determined by conditions in the circum-jovian nebula, just as the planets’ initial properties were governed by their formation within the circum-solar nebula. The Galilean satellites subsequently evolved under the complex interplay of orbital and geophysical processes, which included the effects of orbital resonances, tides, internal differentiation, and heat. The history and character of the satellites can be inferred from consideration of the formation of planets and the satellites, from studies of their plausible orbital evolution, from measurements of geophysical properties, especially gravitational and magnetic fields, from observations of the compositions and geological structure of their surfaces, and from geophysical modeling of the processes that can relate these lines of evidence. The three satellites with large water-ice components, Europa, Ganymede, and Callisto are very different from one another as a result of the ways that these processes have played out in each case. Europa has a deep liquid-water ocean with a thin layer of surface ice, Ganymede and Callisto likely have relatively thin liquid water layers deep below their surfaces, and Callisto remains only partially differentiated, with rock and ice mixed through much of its interior. A tiny inner satellite, Amalthea, also appears to be largely composed of ice. Each of these moons is fascinating in its own right, and the ensemble provides a powerful set of constraints on the processes that led to their formation and evolution.

The international Cassini-Huygens Mission is a joint mission of the National Aeronautics and Space Administration and the European Space Agency, with the Italian Space Agency as a major partner. The spacecraft was launched from Cape Canaveral, Florida, on October 15, 1997, and was inserted into orbit around the planet Saturn on July 1, 2004. The Huygens Probe separated from the Cassini Orbiter on December 26, 2004 and coasted to a descent by parachute through the atmosphere of Titan to the surface on January 14, 2005. This chapter summarizes the primary science results to date from both the orbiter and the probe. Many of the analyses are preliminary in nature, and some may be revised as additional data enables better characterization of Saturn and its system of rings, magnetosphere and satellites. The initial results summarized in this chapter include images and other data from a close encounter of Phoebe prior to Saturn orbit insertion, from ring studies and magnetospheric studies during orbit insertion and the first year in orbit, from studies of Titan during the first several flybys and from the Huygens Probe mission, and from close flybys of several of Saturn’s icy satellites. The results also include discovery of several new satellites and a new radiation belt between the rings and Saturn’s atmosphere.

The current state of knowledge of the Ice Giants, Uranus and Neptune, is presented. The changing appearance of the atmosphere of Uranus is discussed, and its current cloud patterns and zonal winds are reviewed. Highlights of recent uranian ring and satellite observations are presented, along with a brief discussion of the ionosphere as deduced from ground-based observations. For the Neptune system, the rapidly evolving atmosphere is assessed, with a discussion of the longterm record to put recent observations into context. Also discussed are advances in characterizing the clumpy ring system of Neptune. Remarkable changes in the atmosphere of Neptune’s moon Triton are described, and the ever-growing number of smaller satellites is reported. Concluding remarks include a synopsis of future exploration of these dynamic planetary systems.