The planets of our Solar System are divided in two main classes: the terrestrial planets, populating the inner Solar System, and the giant planets, which dominate the outer Solar System. The giant planets, in turn, can be divided between the gas giants Jupiter and Saturn, whose mass is mostly constituted by H and He, and the ice giants Uranus and Neptune, whose bulk composition is instead dominated by the combination of the astrophysical ices H2O, NH3 and CH4 with metals and silicates. While in the case of the gas giants H and He constitutes more than 90% of their masses, in the case of the ice giants these gaseous envelopes are more limited, amounting to only 1-4 Earth masses (De Pater and Lissauer 2010). The terrestrial planets and the gas giants have been extensively studied with ground-based observations and with a large numbers of dedicated space missions. The bulk of the data on the ice giants, on the contrary, has been supplied by the Voyager 2 mission, which performed a fly-by of Uranus in 1986 followed by one of Neptune in 1989.
The giant planets appeared extremely early in the history of the Solar System, forming across the shot time-span when the Sun was still surrounded by a circumstellar disk of gas and dust and therefore predating the terrestrial planets. The role of the giant planets in shaping the formation and evolution of the young Solar System was already recognized in the pioneering works by Oort and Safronov in 1950-1960. In particular, Safronov (1969) suggested that the formation of Jupiter would inject new material, in the form of planetesimals scattered by the gas giant, in the formation regions of Uranus and Neptune. More recently, the renewed understanding of planetary formation we obtained by the study of extrasolar planetary systems gave rise to the idea that the Solar System could have undergone a much more violent evolution than previously imagined (e.g. the Nice Model for the Late Heavy Bombardment, Tsiganis et al. 2005), in which the giant planets played the role of the main actors in shaping the current structure of the the Solar System.
The purpose of this document is to discuss the scientific case of a space mission to the ice giants Uranus and Neptune and their satellite systems and its relevance to advance our understanding of the ancient past of the Solar System and, more generally, of how planetary systems form and evolve. As a consequence, the leading theme of this proposal will be the first scientific theme of the Cosmic Vision 2015-2025 program:
In pursuing its goals, the present proposal will also address the second and third scientific theme of the Cosmic Vision 2015-2025 program, i.e.:
The mission concept we will illustrate in the following will be referred to through the acronym ODINUS, this acronym being derived from its main fields of scientific investigation: Origins, Dynamics and Interiors of Neptunian and Uranian Systems. As the name suggests, the ODINUS mission is based on the use of two twin spacecrafts to perform the exploration of the ice giants and their regular and irregular satellites with the same set of instruments. This will allow to perform a comparative study of these two systems so similar and yet so different and to unveil their histories and that of the Solar System.