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I think he's interested in going to Mars to use it as a test. He already knows he can get to the moon. That's an easy task for him. His goal is interplanetary life in as a whole so anything that expands our reach in a positive way he will be on board with.

Going to Mars is not a bad choice as Saturn is much far away. But in the future, 100 or 200 years from now maybe, Titan could be a good choice.