The discovery of four large moons orbiting around Jupiter by Galileo Galilei 
four hundred years ago spurred the Copernican Revolution and forever changed 
our view of the Solar System and universe. Today, Jupiter is seen as the 
archetype for giant planets in our Solar System as well as a paradigm for the 
numerous giant planets known to orbit other stars. In many respects, and in 
all their complexities, Jupiter and its diverse satellites form a mini-Solar 
System.In the framework of the ESA "Cosmic Vision" programme, the JUpiter ICy 
moons Explorer (JUICE) mission has been selected as the L-class mission to be 
launched in 2022, with an arrival at Jupiter planned in 2030. JUICE is an 
ESA-led mission arising from the reformulation of the EJSM-Laplace mission, 
which was instead an ESA-NASA project. Here we discuss the main scientific 
aspects of the current JUICE mission concept. 
The overarching theme for JUICE is: "The emergence of habitable worlds around 
gas giants". The focus of JUICE is to characterise the conditions that may 
have led to the emergence of habitable environments among the Jovian icy 
satellites, with special emphasis on the three ocean-bearing worlds, 
Ganymede, Europa, and Callisto. Ganymede is identified for detailed 
investigation since it provides a natural laboratory for analysis of the 
nature, evolution and potential habitability of icy worlds in general, but 
also because of the role it plays within the system of Galilean satellites, 
and its unique magnetic and plasma interactions with the surrounding Jovian 
environment. For Europa, where two targeted flybys are planned, the focus 
will be on the chemistry essential to life, including organic molecules, 
and on understanding the formation of surface features and the composition 
of the non water-ice material, leading to the identification and 
characterisation of candidate sites for future in situ exploration. 
Furthermore, JUICE will provide the first subsurface observations of this icy 
moon, including the first determination of the minimal thickness of the icy 
crust over the most recently active regions. The study of the diversity of 
the satellite system, important also to shed light on their origins, will be 
completed with twelve targeted flybys of Callisto and additional information 
gathered remotely on Io and smaller moons. 
Focused studies of Jupiter's atmosphere and magnetosphere, and their 
interaction with the Galilean satellites, will further enhance our 
understanding of the evolution and dynamics of the Jovian system. 
The circulation, meteorology, chemistry and structure of Jupiter will 
be studied from the cloud tops to the thermosphere. These observations 
will be attained over a sufficiently long temporal baseline with broad 
latitudinal coverage to investigate evolving weather systems and the 
mechanisms of transporting energy, momentum and material between the 
different layers. The focus in Jupiter's magnetosphere will include an 
investigation of the three dimensional properties of the magnetodisc and 
in-depth study of the coupling processes within the magnetosphere, 
ionosphere and thermosphere.