As NASA, along with the European Space Agency, plan a return mission to either Jupiter or Saturn, two prominent astronomers discuss which planet we should return to.The gas giants Jupiter and Saturn are magnificent and mysterious worlds, surrounded by moons which still perplex the astronomers with their strange appearance and incredible diversity. Europa, Jupiter's moon, offers the best chance yet of finding life elsewhere in our solar system, while Saturn's moon Titan, with its complex organic chemical lakes and a shore line uncannily like Cornwall, is now believed to be like the primeval Earth.