My two favorites:
"How many worlds harbor life in our solar system?"
"Did Mars really ever have oceans -- rather than just lakes, rivers, and rains"
The questions that Alan poses are ones around which future planetary missions, or series of missions can be planned.
Alan list does not have any questions on Venus, so I'll add a couple of my own:
- Did Venus ever support life?
- What drives the geological cycle on Venus, which replaced almost the entire surface within the last few hundred million years, work?