SM wrote: 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.

Chris writes: The major component of the Martian atmosphere is CO2. Lots of gasses are lighter that it!

SM wrote: I am not certain that it would work on Mars.

Chris writes: Helium, Hydrogen, any number of other gasses lighter than martian air or even a Montgolfiere balloon (hot air)will works fine.

SM wrote: In fact I'm surprised they didn't have to use rockets to land the Pathfinder.

Chris writers: Pathfinder used airbags not balloons!!! Pathfinder had airbags to absorb shock not for lift! There is no relationship between airbags and balloons other than they can be round and hold gas.

References:
Pioneer Astronautics - Mars Balloon work:
http://www.pioneerastro.com/Mmb/mmb.html

JPL sites Balloons on Mars:
http://mars.jpl.nasa.gov/technology/balloons/

Airbags use on Mars:
http://www.space.com/scienceastronomy/solarsystem/mars_airbags_000707.html

--Chris Vancil