The European Jupiter Ganymed Orbiter would focuss on Ganymed, the largest moon in the solar system and the only one with a significant magnetic field, which it will investigate.
The American Jupiter Europa Orbiter would instead focus on Europa, and especially on the composition of the ice field, as well as map the moon thoroughly, to identify possible future landing sites. Especially points where the ice sheeth is thinner.
It will also try to map the internal structure of the moon, which is up to now pure guesswork based on a few results. The orbiter would carry an ice penetrating radar, which could be used to estimate the thickness of the ice.
Although Europa is relatively far a way from the Sun it is kept warm by tidal heat. It's orbit around Jupiter is slightly eliptical, meaning that it is stretcehd and squashed by Jupiter's gravitational forces, which creates heat and 500 meter high tides in its interior. According to several experts life on Europa could well exist even beyond single cell microorganisms although even just finding simple bacteria would be a sensation.
|Europa. The darker regions are where the water ice has a higher mineral content. (Credit: NASA)|
The two orbiters will be indipendently developed by both space agencies, and each would be a significant mission in their own right. ESA is expected to name this their flagship mission for the next few years, meaning that financial support on the european side is assured. NASA has also promised support for the mission but has a larger number of other missions to pay for.