A Comparative Study of the Outer Planets before the Exploration of Saturn by Cassini/Huygens: Introduction

This volume, number 19 in the “Space Sciences Series of ISSI,” presents the proceedings of the workshop on “A comparative study of the outer planets before the exploration of Saturn by Cassini-Huygens” which was held at ISSI in Bern on January 12–16, 2004. The purpose of this workshop was to bring together representatives of several scientific communities, such as planetary scientists, astronomers, space physicists, chemists and astrobiologists, to review our knowledge on four major themes: (1) the study of the formation and evolution processes of the outer planets and their satellites, beginning with the formation of compounds and planetesimals in the solar nebula, and the subsequent evolution of the interiors of the outer planets, (2) a comparative study of the atmospheres of the outer planets and Titan, (3) the study of the planetary magnetospheres and their interactions with the solar wind, and (4) the formation and properties of satellites and rings, including their interiors, surfaces, and their interaction with the solar wind and the magnetospheres of the outer planets. At present, the study of the outer planets is particularly motivated by the fact that the Saturn system is being investigated by the Cassini-Huygens mission which will last until 2008 and possibly beyond. Ground-based and space observations of the giant planets over the past decade give evidence that each system has unique characteristics. Jupiter has been extensively studied over the past ten years by the Galileo mission, which, for instance, has measured a global enrichment of heavy elements as compared to hydrogen, with respect to the solar values, showing evidence for a solar composition of the icy planetesimals which formed Jupiter; Galileo has also revealed the unexpected internal dynamics of the Jovian satellites. Comparisons among the giant planets’ satellites have provided clues to our understanding of the major processes driving the evolution of Earth-like planets. Jupiter has also been explored at the time of the Cassini flyby, while all four giant planets have been studied by HST, ISO and ground-based observations. The following key questions were addressed at the workshop: What will we explore on Saturn and Titan with Cassini-Huygens, and what do we expect to find? Which coordinated ground-based observations should be made to complement and