At present, NASA is planning a mission to Europa, to try and assess its habitability. An argument might also be made for Enceladus, however. Let's imagine you had to pick one of those moons to send an uncrewed (i.e. robotic) mission to. The spacecraft will probably orbit the giant planet (Jupiter in the case of Europa, Saturn in the case of Enceladus), and make many flybys of the moon you're studying. Which moon would you pick, and why?