The biggest unanswered question is how Venus went so terribly wrong and became such a hellish place for life.
We hope to identify these mysterious compounds -- compare Venus to Earth and Mars to see how Venus went so badly wrong.
If Pluto was discovered today, it would not be a planet.
Venus has become somewhat of a 'forgotten planet' with the emphasis on Mars exploration in the U.S.. But Venus can tell us about Earth's future and about planets around other stars -- I expect we will discover a number of surprises from this most capable new mission.