Re: Buoyancy is easy
Ships float on the ocean by displacing a volume of water with equal mass. On mars you would have to fill the balloon with a gas that is lighter than Mars' atmosphere (pretty tough), and then displace a volume of atmosphere equal to the mass of the balloon, fill gas, and payload. I am not certain that it would work on Mars. In fact I'm surprised they didn't have to use rockets to land the Pathfinder.