I know Titan is technically a moon, but I still consider that the Huygens Probe landed on another planet and sent back images that to my untrained eye look a lot like Mars, only with more atmosphere.
The probe's mission was another fine scientific joint venture between the USA and Europe. Back when I was a rocket scientist, I worked on the launches of IRAS, which was another spectacular joint venture, and on EXOSAT, which was all European except for the launch, which was switched to Delta (which is what I worked on) from Ariane at the last minute because of an Ariane launch failure. I had a great time working with the Europeans on EXOSAT, and it just goes to show that in those endevors where Europe can pull its own weight (or more!) they make fine partners.
