Well, you have low pressure and low gravity working both for you and against you in this case. It doesn't have to be very strong, because atmospheric pressure

Well, you have low pressure and low gravity working both for you and against you in this case. It doesn't have to be very strong, because atmospheric pressure is so low. But also, the lifting power is also very low, so it would have to be far less dense than a vacuum container would have to be to work on earth. Low gravity means you need less lift, but low gravity also means the weight of the surrounding air is less so you need more volume. I guess that cancels out. I suspect it's not practical in atmosphere as thin as Mars, but maybe with a light enough or strong enough material it's possible? At any rate, you'd need an enormous volume just to lift relatively small payloads. And, just filling a balloon with hydrogen would probably make more sense in almost any situation. At least one good thing Mars has going for it in terms of airships is that CO2 is more dense than Earth's atmosphere. Even nitrogen would be a weak lifting gas on Mars (though it's not available in abundance there as far as I know). Wikipedia has some useful background information: https://en.wikipedia.org/wiki/Vacuum_airship https://en.wikipedia.org/wiki/Vacuum_airship